Lasers in the Conservation of Artworks: Proceedings of the International Conference Lacona VII, Madrid, Spain, 17 - 21 September 2007

LASERS IN THE CONSERVATION OF ARTWORKS

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE LACONA VII, MADRID, SPAIN, 17–21 SEPTEMBER 2007

Lasers in the Conservation of Artworks Editors Marta Castillejo Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

Pablo Moreno Laser Facility, University of Salamanca, Salamanca, Spain

Mohamed Oujja Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

Roxana Radvan National Institute of Research and Development for Optoelectronics, Bucharest, Rumania

Javier Ruiz Department of Applied Physics I, University of Málaga, Málaga, Spain

CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2008 Taylor & Francis Group, London, UK Typeset by Charon Tec Ltd (A Macmillan Company), Chennai, India Printed and bound in Great Britain by Cromwell Press Ltd, Towbridge, Wiltshire All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publishers. Although all care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. Published by:

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ISBN 13: 978-0-415-47596-9

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Table of Contents

Preface

XI

Committees

XIII

Sponsors

XV

Photonic restoration of marine artifacts and vessels of New Spain J.F. Asmus

1

Innovative Approaches in Laser Cleaning and Analysis Towards the restoration of darkened red lead containing mural paintings: A preliminary study of the β-PbO2 to Pb3 O4 reversion by laser irradiation S. Aze, P. Delaporte, J.M. Vallet, V. Detalle, O. Grauby & A. Baronnet

11

Prospective and applications of two-photon fluorescence in archaeology and art conservation D. Artigas, L. Serrado, I.G. Cormack, S. Psilodimitrakopoulos & P. Loza-Alvarez

15

Fast spectral optical coherence tomography for monitoring of varnish ablation process M. Góra, P. Targowski, A. Kowalczyk, J. Marczak & A. Rycyk

23

Study of laccaic acid and other natural anthraquinone dyes by Surface-Enhanced Raman Scattering spectroscopy M.V. Cañamares & M. Leona Potential of THz-Time Domain Spectroscopy in object inspection for restoration M. Panzner, Th. Grosse, S. Liese, U. Klotzbach, E. Beyer, M. Theuer, W. Köhler & H. Leitner Femtosecond laser cleaning of paintings: Modifications of tempera paints by femtosecond laser irradiation S. Gaspard, M. Oujja, M. Castillejo, P. Moreno, C. Méndez, A. García & C. Domingo Cleaning of paint with high repetition rate laser: Scanning the laser beam A.V. Rode, D. Freeman, N.R. Madsen, K.G.H. Baldwin, A. Wain, O. Uteza & P. Delaporte Removal of unwanted material from surfaces of artistic value by means of Nd:YAG laser in combination with Cold Atmospheric-Pressure Plasma C. Pflugfelder, N. Mainusch, W. Viöl & J. Ihlemann

29 35

41 49

55

Analytical Techniques Optical coherence tomography for structural imaging of artworks P. Targowski, M. Góra, T. Bajraszewski, M. Szkulmowski, M. Wojtkowski, A. Kowalczyk, B. Rouba, L. Tymi´nska-Widmer & M. Iwanicka Atmospheric Pressure Laser Desorption Mass Spectrometry based methods for the study of traditional painting materials M.P. Licciardello, R. D’Agata, G. Grasso, S. Simone & G. Spoto

V

61

67

Study of chromophores of Islamic glasses from Al-Andalus (Murcia, Spain) N. Carmona, M. García-Heras, M.A. Villegas, P. Jiménez & J. Navarro

73

Polychromed sculptures of Mercadante and Millán analysed by XRF non-destructive technique A. Križnar, M.A. Respaldiza, M.V. Muñoz, F. de la Paz & M. Vega

79

Litharge and massicot: Thermal decomposition synthetic route for basic lead(II) carbonate and Raman spectroscopy analysis M. San Andrés, J.M. De la Roja, S.D. Dornheim & V.G. Baonza Contamination identification on historical paper by means of the NIR spectroscopic technique M. Sawczak & A. Kaminska

89 95

Three dimensional survey of paint layer profile measurements E. Pampaloni, R. Fontana, M.C. Gambino, M. Mastroianni, L. Pezzati, P. Carcagnì, R. Piccolo, P. Pingi, R. Bellucci & A. Casaccia

101

Use of LA-ICP-MS technique with SEM-EDS analysis in the study of finishing layers L. Rampazzi, B. Rizzo, C. Colombo, C. Conti & M. Realini

109

Compositional depth profiles of gilded wood polychromes by means of LIBS A.J. López, A. Ramil, M.P. Mateo, C. Álvarez & A. Yáñez

115

Classification of archaeological ceramics by means of Laser Induced Breakdown Spectroscopy (LIBS) and Artificial Neural Networks A. Ramil, A.J. López, M.P. Mateo & A. Yáñez

121

Laser ablation- and LIBS-ranging by webcam and image processing during laser cleaning M. Lentjes, J. Hildenhagen & K. Dickmann

127

LIBS analysis of metal artefacts from Sucevita Monastery, Romania M. Oujja, M. Castillejo, W. Maracineanu, M. Simileanu, R. Radvan, V. Zafiropulos & D. Ferro

133

Comparative study of historic stained glass by LIBS and SEM/EDX K. Szelagowska, M. Szymonski, F. Krok, M. Walczak, P. Karaszkiewicz & J.S. Prauzner-Bechcicki

141

Portable Laser Systems for Remote and On-Site Applications Scanning hyperspectral lidar fluorosensor for fresco diagnostics in laboratory and field campaigns F. Colao, L. Caneve, R. Fantoni, L. Fiorani & A. Palucci

149

A lidar experiment for the characterization of photoautotrophic and heterotrophic biodeteriogens by means of remote sensed autofluorescence spectra V. Raimondi, L. Palombi, D. Lognoli, G. Cecchi & I. Gomoiu

157

Design and development of a new high speed performance fluorescence imaging lidar for the diagnostics of indoor and outdoor cultural heritage V. Raimondi, L. Palombi, D. Lognoli, G. Cecchi & L. Masotti

163

Remote fluorescence lidar imaging of monuments: The Coliseum and the Lateran baptistery in Rome J. Hällström, K. Barup, V. Raimondi, L. Palombi, D. Lognoli, G. Cecchi, R. Grönlund, A. Johansson, S. Svanberg & C. Conti Portable spectroscopic analysis of nitrates affecting to cultural heritage materials M. Maguregui, I. Martinez-Arkarazo, M. Angulo, K. Castro, L.A. Fernández & J.M. Madariaga A study of laser cleaning parameters using a portable system on a gargoyle of the Torres de Serranos in Valencia, Spain B. Sáiz & M. Iglesias

VI

169

177

183

Laser Cleaning of Monuments and Sculptures Castle of Quart, Aosta Valley: Laser uncovering of medieval wall paintings S. Siano, L. Appolonia, A. Piccirillo & A. Brunetto

191

Colour changes in Galician granitic stones induced by UV Nd:YAG laser irradiation A. Ramil, A.J. López, M.P. Mateo, C. Álvarez & A. Yáñez

199

Arch-collegiate church in Tum: Laser renovation of priceless architectural decorations A. Koss, J. Marczak & M. Strzelec

203

Laser cleaning of the Nickerson Mansion: The first building in the US entirely cleaned using laser ablation A. Dajnowski Laser cleaning of a set of 18th century ivory statues of Twelve Apostles A. Koss, D. Dre´scik, J. Marczak, R. Ostrowski, A. Rycyk & M. Strzelec

209 215

Laser Cleaning of Paintings and Polychromes Investigating the use of the Nd:YAG laser to clean ancient Egyptian polychrome artefacts C. Korenberg, M. Smirniou & K. Birkholzer

221

Laser cleaning as a more culturally appropriate treatment option for Native American pictographs and pictograms M. Abraham & C. Dean

227

The Arca Scaligera of Cansignorio della Scala by Bonino da Campione: Cleaning of the polychrome and gilded decorations V. Fassina, G. Gaudini, S. Siano & R. Cavaletti

231

Assessment of laser cleaning on a polychrome Islamic ceramic B. Sáiz, E. Aura, M.T. Domenech & A. Domenech

237

Laser cleaning of stucco’s fragments from an early middle age bas-relief A. Sansonetti, C. Colombo, M. Realini, M. Palazzo & M. De Marchi

243

Soot removal from artificial fresco models by KrF excimer laser J. Hildenhagen, K. Dickmann, W. Maracineanu & R. Radvan

249

The interaction of laser radiation at 2.94 µm with azurite and malachite pigments M. Camaiti, M. Matteini, A. Sansonetti, J. Striová, E. Castellucci, A. Andreotti, M.P. Colombini, A. deCruz & R. Palmer

253

Conservation of medieval polychromed wooden sculpture of Madonna and Child K. Chmielewski, A. Koss, M. Mazur, J. Marczak & M. Strzelec

259

Advanced laser renovation of old paintings, paper, parchment and metal objects J. Marczak, M. Strzelec, R. Ostrowski, A. Rycyk, A. Sarzy´nski, W. Skrzeczanowski, A. Koss, R. Szambelan, R. Salimbeni, S. Siano, J. Kolar, M. Strlic, Z. Márton, I. Sánta, I. Kisapáti, Z. Gugolya, Z. Kántor, S. Barcikowski, P. Engel, M. Pires, J. Guedes, A. Hipólito, S. Santos, A.S. Dement’ev, V. Švedas, E. Murauskas, N. Slavinskis, K. Jasiunas & M. Trtica

263

Comparative study of laser varnish removal from historical paintings Z. Márton, I. Sánta, É. Galambos, C. Dobai, Á. Dics˝o & Z. Kántor

271

VII

Laser Cleaning of Metal Objects Laser interactions with copper, copper alloys and their corrosion products used in outdoor sculpture in the United Kingdom M. Froidevaux, P. Platt, M. Cooper & K. Watkins

277

Investigating the laser cleaning of archaeological copper alloys using different laser systems C. Korenberg, A.M. Baldwin & P. Pouli

285

Investigating and optimising the laser cleaning of corroded iron C. Korenberg, A.M. Baldwin & P. Pouli

291

Nd:YAG laser cleaning of heavily corroded archaeological iron objects and evaluation of its effects J. Chamón, J. Barrio, E. Catalán, M. Arroyo & A.I. Pardo

297

Laser as a cleaning tool for the treatment of large-scale bronze monuments A. Dajnowski

303

Experimental study on the use of laser cleaning of silver plating layers in Roman coins A.A. Serafetinides, E. Drakaki, I. Zergioti, C. Vlachou-Mogire & N. Boukos

309

Morphological and colorimetric changes induced by UV laser radiation on metal leaves S. Acquaviva, E. D’Anna, M.L. De Giorgi, A. Della Patria & L. Pezzati

317

Application of Ion Beam Analysis (IBA) techniques for the assessment of laser cleaning on gilded copper M. Barrera, C. Escudero, M.D. Ynsa & A. Climent-Font Laser cleaning: Influence of laser beam characteristics M. Pires, C. Curran, W. Perrie & K. Watkins

323 329

Laser Cleaning of Documents and Textiles Study of laser cleaning of ancient fabric with femtosecond pulses C. Escudero, M.A. Martínez, P. Moreno, A. García, C. Méndez, C. Prieto & A. Sanz

337

Monitoring of the laser cleaning process of artificially soiled paper S. Pentzien, A. Conradi, J. Krüger & R. Wurster

345

Systematic case study on common cleaning problems on paper and parchment by using Nd:YAG laser (ω, 2ω, 3ω) J. Hildenhagen, M. Lentjes, K. Dickmann & B. Geller Use of laser and optical diagnostic techniques on paper: The Pomelnic from Sucevita Monastery (Romania) M. Strlic, J. Kolar, G. Pajagic Bregar, V. Ljubic, R. Radvan, M. Simileanu, W. Maracineanu, J. Hildenhagen, M. Castillejo, M. Oujja, W. Kautek, V. Zafiropulos, T. Sinigalia, O. Boldura & N. Melniciuc Laser cleaning of 19th century papers and manuscripts assisted by digital image processing G.M. Bilmes, C.M. Freisztav, N. Cap, H. Rabal & A. Orsetti Laser reduction of stamps from paper to avoid migration to the recto side: Case study based on illustrations from Jan Heesters M. Lentjes, K. Dickmann & P. van Dalen Laser cleaning and multi-method diagnostics of textile pieces of art W. Kautek, M. Oujja, M. Castillejo, J. Hildenhagen, V. Ljubic, M. Simileanu, W. Maracineanu, R. Radvan, V. Zafiropulos, N. Melniciuc, G. Pajagic Bregar & M. Strlic

VIII

353

357

361

367 371

Study of the effects of laser cleaning on historic fabrics: Review and results eight years after applications A. Martínez & C. Escudero

375

Structural Diagnosis and Monitoring Multifunctional encoding system for assessment of movable cultural heritage V. Tornari, E. Bernikola, W. Osten, R.M. Groves, M. George, T. Cedric, G.M. Hustinx, E. Kouloumpi, A. Moutsatsou, M. Doulgeridis, S. Hackney & T. Green

381

Digital preservation, documentation and analysis of heritage with active and passive sensors F. Remondino

387

Monitoring of changes in the surface movement of model panel paintings following fluctuations in relative humidity: Preliminary results using Digital Holographic Speckle Pattern Interferometry E. Bernikola, V. Tornari, A. Nevin & E. Kouloumpi

393

Integrated digital speckle based techniques for artworks monitoring D. Ambrosini, D. Paoletti & G. Galli

399

Laser-based structural diagnosis: A museum’s point of view E. Kouloumpi, A.P. Moutsatsou, M. Trompeta, J. Olafsdottir, C. Tsaroucha, A.V. Terlixi, R.M. Groves, M. Georges, G.M. Hustinx & V. Tornari

407

High-resolution 3D laser digitisation of the Maiano terracotta roundels for documentation and condition monitoring K. Hallett, Z. Roberts, S. Julien-Lees & A. Geary

413

An SLDV/GPR/IR-T integrated approach for structural and frescoes investigation in the medieval monasteries of Moldavia E. Esposito, A. Agnani, M. Feligiotti, A. del Conte & S. Goncalves Tavares

419

Development of an impact assessment procedure for artwork using shearography as a measurement tool R.M. Groves, W. Osten, S. Hackney, E. Kouloumpi & V. Tornari

427

Imaging and Documentation Ultra high-resolution 3D laser color imaging of paintings: The Mona Lisa by Leonardo da Vinci F. Blais, J. Taylor, L. Cournoyer, M. Picard, L. Borgeat, G. Godin, J.A. Beraldin, M. Rioux & C. Lahanier

435

Characterization and virtual reconstruction of polychromed alabaster sculptures A. Sarmiento, K. Castro, M. Angulo, I. Martinez-Arkarazo, L.A. Fernández, J.M. Madariaga, J.M. Gonzalez-Cembellín & M. Urrutikoetxea Barrutia

441

ITR: A laser rangefinder for cultural heritage conservation applications with multi-sensor data integration capabilities R. Ricci, M. Ferri De Collibus, G. Fornetti, M. Francucci, M. Guarneri & E. Paglia

447

Multispectral and multi-modal imaging data processing for the identification of painting materials A. Pelagotti, A. Del Mastio & V. Cappellini

453

Recovering colour and volume from relics in restoration tasks J. Finat, J.I. San José, J.J. Fernández, J.D. Pérez-Moneo, J. Martínez, F. Gutiérrez-Baños, L. Giuntini & F.M. Morillo

IX

459

Multi IR reflectography R. Fontana, M. Greco, M. Mastroianni, M. Materazzi, E. Pampaloni, L. Pezzati & P. Carcagnì

465

Miscellaneous Imaging and mass spectrometry of microparticles generated during surface decontamination of an ancient parchment sample by laser radiation R. Wurster, S. Pentzien & W. Kautek

473

Laser for removing remains of carbonated matrices from Pleistocene fossils L. López-Polín, A. Ollé, J. Chamón & J. Barrio

477

Environmental optical sol-gel sensors for preventive conservation of cultural heritage N. Carmona, E. Herrero, M.A. Villegas & J. Llopis

483

Author index

489

X

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Preface

Materials and technologies used in the creative artistic process or in the manufacture of historically relevant objects evolve with time reflecting the knowledge and uses of the society at the historic moment in which these works were created. Scientific research on Cultural Heritage, both for the study of its material aspects and for designing restoration and conservation strategies, faces a diversity of challenges due to the complexity and intrinsic value of the substrates and objects and their past history of exposure to degrading agents and to the passage of time. Fast developing laser systems and advanced optical techniques offer new solutions for Conservation scientists and provide answers to the challenges of Conservation Science. In this field, the international conference series LACONA, Lasers in the Conservation of Artworks, has been established as a reference meeting point for conservators, end users and scientists. LACONA series started in 1995 in Heraklion, (Greece) being followed every two years by editions in Liverpool (Great Britain), Florence (Italy), Paris (France), Osnabrük (Germany) and Vienna (Austria). The last edition, the 7th International Conference on Lasers in the Conservation of Artworks, LACONA VII, was celebrated in Madrid (Spain), 17–21 September, 2007 at the headquarters of Consejo Superior de Investigaciones Científicas (CSIC) the main research institution in Spain. This Volume of Proceedings of LACONA VII presents a selection of contributions on both emerging and well established applications of laser systems and techniques to real Conservation problems. Innovative Approaches in Laser Cleaning and Analysis are presented together with Analytical Techniques and Portable Laser Systems for Remote and On-Site Applications. A substantial number of contributions to this Volume deal with Laser Cleaning of monuments and sculptures, paintings and polychromes, metal objects and documents and textiles. Also included are developments on Structural Diagnosis, Monitoring, Imaging and Documentation of artworks. I would like to express my gratitude to the Co-Editors of this Volume, Pablo Moreno, Mohamed Oujja, Roxana Radvan and Javier Ruiz, for their enthusiasm and efficient work. Also, the organization of LACONA VII and the elaboration of this Volume of Proceedings has greatly benefited from the cooperative spirit of the Permanent Scientific Committee. Finally, I want to thank the sponsoring of LACONA VII by CSIC and its Thematic Network on Cultural Heritage, by the Instituto de Química Física Rocasolano, by the Spanish Ministry of Science and Education and by Consejería de Educación de la Comunidad de Madrid. Besides the official Institutions, I have very much appreciated the support of a numerous group of private sponsors and the competent technical assistance of Fase 20 Congresos.

Marta Castillejo, LACONA VII Chair, Madrid, May 2008.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Committees

PERMANENT SCIENTIFIC COMMITTEE Margaret Abraham, Los Angeles County Museum of Art, Los Angeles, USA John Asmus IPAPS, University of California, San Diego, USA Gerd v. Bally, Laboratory of Biophysics, University of Münster, Germany Giorgio Bonsanti, University of Florence and Centro Europeo di Ricerche sul Restauro (CERR) di Siena, Italy Marta Castillejo, Instituto de Química Física Rocasolano, CSIC, Madrid, Spain Martin Cooper, The Conservation Centre, National Museums Liverpool, United Kingdom Klaus Dickmann, Laserzentrum FH Münster, Germany Costas Fotakis, Foundation for Research and Technology Hellas, IESL, Heraklion, Crete, Greece Wolfgang Kautek, University of Vienna, Department of Physical Chemistry, Austria Eberhard König, Freie Universität Berlin, Germany Mauro Matteini, Istituto per la Conservazione e Valorizzazione dei Beni Culturali, CNR, Florence, Italy Johann Nimmrichter, Bundesdenkmalamt, Austrian Federal Office for the Care of Monuments, Centre of Art Conservation, Vienna, Austria Roxana Radvan, National Institute of Research and Development for Optoelectronics, Bucharest, Romania Renzo Salimbeni, Istituto di Fisica Applicata Nello Carrara CNR, Florence, Italy Manfred Schreiner, Academy of Fine Arts, Vienna, Austria Véronique Vergès-Belmin, Laboratoire de Recherche des Monuments Historiques, Champs-sur-Marne, France Kenneth Watkins, Department of Engineering, University of Liverpool, United Kingdom Vassilis Zafiropulos, Technological Educational Institute of Crete & Center for Technological Research – Crete, Sitia, Crete, Greece

ORGANIZING COMMITTEE Chair: Marta Castillejo, Instituto de Química Física Rocasolano, CSIC, Madrid Mónica Álvarez de Buergo, Instituto de Geología Económica, CSIC, Universidad Complutense de Madrid, Madrid Rocio Bruquetas, Spanish Group of IIC, Instituto del Patrimonio Histórico Español, Madrid Noemí Carmona, Centro Nacional de Investigaciones Metalúrgicas, CSIC, Madrid Manuela Casado, Instituto de Química Física Rocasolano, CSIC, Madrid Solenne Gaspard, Instituto de Química Física Rocasolano, CSIC, Madrid Fernando Guerra-Librero, CORESAL, Madrid Ana Laborde, Spanish Group of IIC, Instituto del Patrimonio Histórico Español, Madrid

XIII

Margarita Martín, Instituto de Química Física Rocasolano, CSIC, Madrid Pablo Moreno, Laser Facility, University of Salamanca, Salamanca Mohamed Oujja, Instituto de Química Física Rocasolano, CSIC, Madrid Javier Ruiz, Department of Applied Physics I, University of Málaga, Málaga Malgorzata Walczak, Instituto de Química Física Rocasolano, CSIC, Madrid

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Sponsors

OFFICIAL SPONSORS Consejo Superior de Investigaciones Científicas (CSIC)

CSIC Thematic Network on Cultural Heritage

Instituto de Química Física Rocasolano, CSIC

Ministerio de Educación y Ciencia

Consejería de Educación, Comunidad de Madrid

XV

PRIVATE SPONSORS Faro Technologies Inc., www.faro.com nub3d S.L., www.nub3d.com El.En Group, www.elengroup.com Lasertech Ibérica S.L., www.lasertechib.com CTS, www.ctseurope.com Erich Pummer Atelier, www.atelier-pummer.at Innova Sci., www.innovasci.com Renishaw plc, www.renishaw.com Lasing, www.lasing.com Linsinger, www.linsinger.at Iberlaser, www.iberlaser.com Clar, www.clar.es International Institute for Conservation of Historic and Artistic Works – Grupo Español GE-IIC, http://ge-iic.com/ Arespa, www.arespaph.com Geocisa, www.geocisa.es Artemon, www.artemon.es

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Photonic restoration of marine artifacts and vessels of New Spain J.F. Asmus University of California, San Diego, La Jolla, CA, USA

ABSTRACT: On September 28, 1524 Juan Rodríguez Cabrillo moored his vessels, San Salvador, Victoria, and San Miguel, at what is now Ballast Point at the entrance to San Diego Bay. The crew spent six days at this location performing naval maintenance and studying the customs of the natives, the marine life, and marine fossils embedded in the adjoining cliffs. When the Scripps Institution of Oceanography (SIO) was founded at the dawn of the 20th Century, Ballast Point was selected as the port for the research vessels. In contrast to Cabrillo’s crew, we had the opportunity to investigate in-situ underwater radiation ablation divestment to remove marine fouling from vessel surfaces and reactivate antifouling coatings. Cabrillo’s tradition of marine animal study and environmental science were continued and enhanced through the radiation cleaning of research-aquarium windows, fossils, native and Spanish artifacts, as well as the laser branding of whales.

1

INTRODUCTION

a Venetian holographic feasibility study was proposed. The objective was to demonstrate that even large (meters) Venetian statues could be recorded in holograms, in situ. Thus, a high-quality visual archival library would be produced of sculptural patrimony before further deterioration took place. In November 1971 Ente Nazionale Idrocaburi (ENI) funded the feasibility study (winter 1971–72) and more than 50 large-object large-format holograms of Venetian artworks were recorded and placed on display in the Academia Museum. In the course of the three-month investigation two unexpected discoveries were made. First, it was found that double-exposure holographic interferograms are able to reveal hidden defects in artworks and assess the success of subsequent conservation repairs. Second, a recent laser publication (Ready 1971) inspired the team to the discovery that pulsed-laser ablation could be adapted to advance art conservation technology in the attainment of very high-quality surface divestment. In a very few weeks it was found that self-limiting divestment of sulfation from stone, minerals from pottery, glue from canvas, corrosion from metals, fungi from leather, and soils from textiles was feasible (Asmus 1972). The conservation journal, “Studies in Conservation” immediately accepted a manuscript describing the holography. However, a manuscript describing the divestment results was rejected with the terse observation: “Laser cleaning is too hypothetical to be taken seriously” (even though 70 items had been laser cleaned at that point). Consequently, the SIO team returned to San Diego and addressed the divestment issues of foremost concern at the home facility, the

The introduction of laser technology into the field of art conservation began in January 1972 with the collaboration of three oceanographic institutions. For some years the Scripps Institution of Oceanography (SIO) of the University of California had been cooperating with a CNR Laboratory (Laboratorio per lo Studio della Dinamica delle Grandi Masse: Palazzo Papadopoli, Venice, Italy) and the Istituto di Geodesia e Geofisica of the University degli Studi di Trieste, Italy, in the computer modelling of the tides and currents of the Adriatic Sea and the Venetian Lagoon. The objective was guidance in the design of lagoon closure gates with the goal of alleviating the severity of “acqua alta” events. As the interdisciplinary team of theoretical and experimental scientists witnessed the progressive and accelerating deterioration of the city and its patrimony (Lorenzetti 1961), it became increasingly obvious that more immediate preservation measures were required due to the vigour and severity of the environmental assault. This observation was especially germane in light of the protracted time line for the approval, design, construction, and installation of the floodgates. The only proposal that was brought forward from the emergency deliberations was to record the overall appearance and geometry of the artistic patrimony with the highest possible fidelity. As pulsed ruby-laser in-situ 3D diffraction-limited holography had been demonstrated in the laboratory (and as enthusiasm for holography was running high in Italy because a resident of Tuscany, Dennis Gabor, was expected to win the Nobel prize for this discovery),

1

Figure 1. Depiction of Cabrillo’s landing at Ballast Point. Figure 2. Caption monument at Point Loma (left) and depiction of Kumeyaay native resident of the region (right).

Marine Physical Laboratory (MPL), at “Ballast Point”, Point Loma, San Diego. Whereas the radiation divestment studies reported in the following were performed during the interval 1975–78 at MPL, “Studies in Conservation” continued to reject the submissions and only fragments of the work were published in physics, chemistry, engineering, metallurgical, and oceanographic journals. A summary of the radiation divestment work performed at the Ballast Point marine facility is presented in the following sections. 2

HISTORICAL BACKGROUND Figure 3. Entrance channel to San Diego Harbour showing the MPL naval facility of SIO at Ballast Point in the foreground.

The crossbowman Juan Rodríguez Cabrillo arrived in the Americas in 1520 and joined Hernán Cortés in the conquest of the Aztec capital of Tenochtitlan. In gratitude for Cabrillo’s service the Viceroy of New Spain gave him command of three galleons and a commission to discover a route to Asia and the Spice Islands. On September 28, 1542 this flotilla entered a harbour that Cabrillo describes as “un puerto cerrado y muy bueno”. The vessels, San Salvador, Victoria, and San Miguel, were moored at a natural spit near the entrance to that harbour (known, today, as “Ballast Point”, within San Diego Bay). Before continuing to sail up the coast of North America, Cabrillo’s crew spent six days at this location (Fig. 1) performing naval maintenance and studying the customs of the local Kumeyaay natives (Fig. 2), the marine life, and the marine fossils embedded in the adjoining cliffs. When the Scripps Institution of Oceanography was founded at the dawn of the 20th Century, Ballast Point was selected as the port for its research vessels, bearing in mind the same considerations that lead Cabrillo to that site. Today, as in the 16th Century, there are no dry-dock facilities at Ballast Point, and routine maintenance of ship hulls is performed in-situ and/or underwater. In contradistinction to Cabrillo’s crew, SIO personnel had the opportunity to investigate underwater radiation ablation divestment to remove marine fouling such as barnacles and algae from vessel surfaces and marine research facilities as well as historic artifacts excavated from the adjoining terrain

(Fig. 3). It was found that ruby laser and Nd:YAG laser radiation as well as xenon flashlamp light have comparable cleaning effectiveness whether in air or underwater. Whereas seawater attenuates the laserbeam intensity reaching the surface being cleaned, a “witness plate” effect was found to enhance the strength of the interaction.

3

RADIATION ABLATIVE DIVESTMENT OF ARCHAEOLOGICAL ARTIFACTS AT BALLAST POINT

The excavations and dredging (60–70 years ago) that prepared Ballast Point for modern naval port activities uncovered prehistoric and native artifacts as well as items of New Spain dating from the 16th Century. When MPL was established neither museum nor conservation program existed, and the excavated artifacts were simply stored. The encouraging laser-divestment tests in Venice with the SIO lasers suggested pursuing a continuation of those conservation studies with those marine artifacts in storage. Figure 4 shows the results of laser divestment of a silver Spanish coin from the era of Cabrillo or one of his followers. Those areas where the marine fouling was removed with the ruby

2

Figure 6. Fossil of a whale vertebrate bone showing original condition (right) and laser-cleaned area (left).

Figure 4. Spanish silver coin covered with marine deposits (left) and laser cleaned (right). The bright areas on the cleaned coin resulted from CO2 TEA laser and the areas with a light grey patina were cleaned with a Q-switched ruby laser.

Figure 5. Laser removal (top) of a protective coating on wood.

Figure 7. Laser-cleaned (top) and dollar fossil.

holographic laser retained a pale grey patina, whereas the radiation from a CO2 TEA laser yielded a bright metallic finish. Wooden remains were also recovered from Ballast Point. Some may be the remains of ship repair work conducted by the Spanish explorers. When such materials were discovered during the construction of the port facility they were simply coated with polyurethane and placed in storage. Years later it was deemed desirable to remove the coating in order to study and display these pieces. Several laser types were tested in order to ablate the thick coating. Eventually it was learned that high-energy (50 J) normal-mode pulses from a Nd:glass laser soften the polyurethane and aerodynamic forces from the Laser-Induced Combustion (LIC) wave eject the coating residue in large molten droplets. It was determined that the laser ejection of condensed material is more efficient than radiation ablation (Fig. 5). Deeper excavations at Ballast Point site yielded marine fossils from earlier epochs. Figure 6 reveals a vertebrae fossil from a whale. Figure 7 is a sand dollar fossil, and Figure 8 shows the fossil of a dinosaur bone.

Figure 8. Fossil of a dinosaur bone. A laser cleaned the left side and upper portion.

4

LASER RESTORATION OF SIO SHIPS AND NAVAL HARDWARE

The apparent success of the exploratory laser cleaning tests on the marine archaeological specimens (Section 3) led to an expansion of the probative effort to

3

Figure 9. Barnacle removal from the steel hull of a ship (1000 ea. Ruby laser pulses at pulse energy of 10 J).

Figure 11. Laser removal of rust (left) from a portion of a marine engine-control linkage (steel).

Figure 10. Cylinder heads of a marine diesel engine. Carbon buildup is shown on left. Laser removal of carbon on right.

Figure 12. Deep ocean SIO submersible research vehicle.

include surface preparation of relevance to ship maintenance. Many of the conventional surface-cleaning methods employed in shipyards utilize or produce toxic substances that harm the environment and/or jeopardize worker health. Examples are the organic solvents commonly used to remove paints and greases. High-pressure blasting (abrasive, air, and/or water) is also widely employed in shipyard maintenance and frequently presents problems of debris containment. Figure 9 shows an example of laser barnacle removal from a ship hull. Figure 10 displays laser carbon removal for marine engine maintenance, Figure 11 is an engine-control linkage where corrosion removal by laser was demonstrated without resorting to the use of acids, toxic organic solvents, or blasting with abrasive media. Whereas these results have demonstrated the technical feasibility of laser surface preparation for ship maintenance, cost effectiveness, and utility are not assured. Particulate and vapour debris control may

be easier with laser divestment than with highpressure blasting. However, this supposition has yet to be proven. Further, at the time of these probative investigations (1975) laser performance (viz., power, efficiency, and reliability) had not reached levels commensurate with practical surface-coverage rates. In addition, even today, it is questionable as to whether the physical and optical durability of fibre optic beam delivery systems are able to meet the logistical requirements demanded by these applications. The submersibles in the SIO fleet (Fig. 12) present a number of extremely challenging maintenance problems. Because these vehicles dive to enormous depths, the most difficult of these issues is the adhesion of rubber insulating sheets on the exterior hull. Figure 13 shows high-power flashlamp irradiation of a typical rubber sheet in order to clean and etch the surface prior to the application of the adhesive and subsequent attachment to the submersible’s hull.

4

Figure 15. Pointing to area of coupon divested of marine fouling, underwater.

Figure 13. Xenon flashlamp system in use to clean and etch a rubber insulation panel from a deep-ocean submarine.

Figure 16. 1 MHz quartz ultrasonic transducer used to measure the stress wave amplitude under water and in air.

Figure 14. Water tank (white box) used for underwater flashlamp irradiation. The waterproof flashlamp/reflector housing is attached to the end of the immersion pole is lying on the tank. An algae-covered test coupon is leaning against the tank.

The logistics of marine-vessel maintenance could be further improved by performing the radiation ablation divestment and/or cleaning in situ (underwater). The initial submersible flashlamp test apparatus is shown in Figure 14. Subsequent underwater removal of marine fouling (algae) and reactivation of a spent organic copper antifouling coating was performed in bay water under an MPL pier (Fig. 15). Figures 16 and 17 show an ultrasonic transducer and its signals for laser irradiation both underwater and in air: with

Figure 17. Oscilloscope traces of laser-generated stress waves at a water interface (top) and when the target is in air (lower).

5

Figure 18. Photographs of freely expanding ablation blow-off plasma in air (top) and confined in underwater surface irradiation (bottom).

Figure 19. Aquarium observation window covered with a thin buildup of marine algae. An underwater xenon flash dislodged the algae on the left. A flash on the right from the airside went through the glass and dislodged the algae from behind.

pure water, a UV-grade fused silica window, and KrF excimer radiation (600 mJ/cm2 ). Figure 18 flashes show the manner in which underwater irradiation inhibits the expansion of the ejected plasma. This leads to a higher peak stress for such an underwater irradiation. The underwater stress wave was, on average, 52% stronger than that generated in air. Presumably, this is a consequence of the classic witness-plate effect, whereby the water blanket inhibits the free expansion of the blow off plasma. The peak stresses in water and air were 6.1 and 4.0 bars, respectively. 5

RESEARCH AQUARIUM FACILITIES

MPL’s seaside facility has large numbers of aquariums for the study of marine life. In addition there are numerous pools for marine mammals (viz., whales and dolphins). With a strong propensity for marine growth to inundate these facilities, it is a constant struggle to keep them clean and free of parasites and diseases. Figure 19 shows a view of one of these aquariumpool underwater glass windows. The waterproof xenon flashlamp head shown in Figure 14 was placed against the air side of the glass and flashed once. It was next placed against the algae on the water side. After the xenon flashes the algae debris could be seen drifting away from the glass. The circulation system eventually captured the residue in its filters. The flashlamp had a 10 cm arc length and a 7 mm bore. It was operated near its explosion limit at an electrical energy of 3 kJ. No flow tube or water jacket was employed as most of the irradiation of test specimens was performed underwater. Whereas this underwater xenon flashlamp approach to aquarium maintenance was a technical success, lamp life at the required electrical loading was only a few thousand shots. Consequently, a new lamp was

Figure 20. Aluminium aircraft wing panel used to demonstrate coating (paint) removal by means of a high-power flashlamp.

needed for each window (which negated any logistical or cost advantage). 6 AIRCRAFT MAINTENANCE There is an historic airfield across the channel from MPL. Some seventy years ago Charles Lindberg’s craft, “The Spirit of St. Louis” (Fig. 20, inset), was built and tested at that site. The flight to the start of Lindberg’s nonstop solo trans-Atlantic crossing began at this field on San Diego Bay. Thus, there is a well-established tradition of aviation research and engineering at Ballast point. Environmental concerns have complicated the routine maintenance of aircraft. Paint must be stripped regularly in order to inspect for stress-corrosion

6

Subsequently, the spectral camera images the scanned area. Next, the laser rescans the area; however, laser pulses are inhibited at those pixel locations having the character of the substrate composite spectrum. During the scan the laser only fires at pixel locations showing the character of the coating. This cycle repeats until the entire area is clear of the coating. Figure 22 shows the system in operation on an aircraft radome. This system functions by rotating the radome in order to achieve continuous surface scanning while the robot maintains a constant distance between the laser optics and the work surface as well as the lateral scan.

Figure 21. Feedback-controlled laser stripping of grey epoxy coating from advanced aerospace carbon composite test panel. The stripped area is the dark rectangle at the centre.

7

PHOTOCHEMICAL SURFACE PASSIVATION OF STEEL

Steel is the most frequently found material in naval vessels. Most of the effort that is expended in ship repair has to do with the removal of rust. An environmentally friendly approach to the removal of rust from steel and iron is citric acid chelation. Due to its non-toxicity, ease of handling, low cost, and ready biodegradability, hot aqueous solutions of citric acid have conventionally been used to effectively remove rust while maintaining the integrity of the base metal. The thermal mechanism by which it dissolves an iron oxide layer in solution is not complex. The acidity of the solution increases the solubility of the deposit followed by citrate anion chelation of the iron. Agitation and high temperatures accelerate the process. Solution additives (ammonia, triethanolamine) are needed to prevent precipitation of the iron-citrate salt. As effective as citric acid is, it still retains a number of drawbacks. The requirement of large solution volumes at high temperatures (95◦ C) poses constraints. Surfaces cleaned in hot citric acid baths demonstrate little passivation against flash rusting following the final rinse. The most serious drawback of this cleaning technique is that it is slow. As the rate of citric acid chelation is temperature dependent, a new experimental approach was undertaken to utilize superheated citric acid produced by high intensity light pulses. The hybrid photonic flash chelation process begins with the “painting” of the rust with a 6% aqueous citric acid solution. Then the moist rust is superheated with a xenon flashlamp pulse at an optical fluence of 3 J/cm2 . Early in the temporal evolution of the flashlamp pulse the surface temperature rises leading to gaseous release and material blowoff. Both oxygen vapour and hydration water from the rust are explosively removed. Simultaneously, dissolution of the iron oxide layer from the surface and iron-citrate anion chelation occurs. The mechanisms by which these processes progress (photochemical, photolytic, thermal, or some combination) are greatly accelerated by the high transient flashlamp-induced surface temperature. Subsequent sublimation and/or

Figure 22. Robot laser paint stripper removing epoxy coating from an aircraft radome fabricated from a composite material.

cracks. Unfortunately, chemical strippers as well as abrasive procedures (e.g., wet or dry blasting) can pollute the environment and present health hazards to workers. The xenon “flashblaster” shown in Sections 4 and 5 was adapted (through the addition of vacuum and air jet-accessories) to ablative paint stripping. Figure 20 shows the result of the flashlamp removal of the topcoat from an aluminium aircraft wing panel. The selective removal of coatings from aluminium is uniquely favourable due to its high optical reflectively and thermal conductivity. In contrast advanced composites are increasingly utilized in modern aircraft. Unfortunately, in contradistinction to aluminium, composites are low in thermal conductivity and in optical reflectivity. It was found that successful divestment of carbon composites calls for real-time process control. Figure 21 presents a carbon composite test panel that was stripped of its grey epoxy coating (central rectangle) with a pulsed CO2 laser. This was accomplished through system process control by means of multispectral imaging feedback to the scanner/laser machine controller. In this approach the laser performs a raster scan across the designated area.

7

Figure 24. Pacific dolphin named “Aquarius” that was the test animal for the laser branding experiments.

Figure 23. Citric acid treatment with flash rerust (left), flashlamp/citric-acid treatment (centre), original rust (right).

evaporation of the iron-citrate complex serves to etch oxide deposits away. As the surface temperature rises to its final value late in the pulse, a black oxide layer is formed by the reduction and dehydration of the initial layer of atmospheric rust (X-ray diffractometry has shown this black substance to be magnetite, a mixed Fe(II)-Fe(III) oxide). Upon conclusion of the flash pulse, a fresh layer of citric acid solution is applied to the surface and again flash treated. This cycle is repeated as many times as necessary to remove the rust completely. Figure 23 shows the comparative results of citric acid and citric acid plus flashlamp cleaning. Perhaps, the most far-reaching aspect of flashlamp/citric acid cleaning is its passivation. 8

Figure 25. Laser marking test on dorsal fin of a Pacific dolphin. The laser “brands” are the series of small white spots near the top of the figure. The large white spot near the centre of the frame is an earlier liquid nitrogen “freeze brand”.

MARINE MAMMAL RESEARCH AND CONSERVATION

One facility at MPL is the marine mammal research and training complex of large pools for large animals. The study and conservation of California grey and bowhead whales are two of the major activities. The identification and counting of individual whales during their annual migrations is a principal responsibility. The most challenging problem associated with the program is the identification of specific individuals. Today, identification depends upon an observer noting characteristics (size, colour, injuries, colour, and scars) of an animal in the wild (Fig. 24). This is difficult, labour intensive, and limited in reliability. For decades researchers have tested marking (or branding) techniques in order to enhance the individuality of whales and to facilitate easier counting. MPL assigned one of the captive dolphins for laser branding tests (Fig. 24). Figure 25 shows the tests being conducted on the dorsal fin of the animal. The laser radiation bleaches the melanin in the external tissue and may inactivate the malanocytes so that the brand

will not fade with time. When implemented, the visibility of the laser brands should permit counting from the Scripps Pier. 9

CONCLUSIONS

It is ironic that entirely new laser technologies for art conservation emerged from oceanographic research. That irony is compounded by the fact that these same technologies migrated back to oceanography. REFERENCES Asmus, J., Munk, W. & Wuerker, R. 1972. Lasers and Holography in Art Preservation and Restoration. IEEE NEREM Proceedings (November): 17–20. Lorenzetti, G. 1961. Venice and its Lagoon. Trieste: Lint. Ready, J. 1971. Effects of High-Power Laser Radiation. New York: Academic Press.

8

Innovative Approaches in Laser Cleaning and Analysis

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Towards the restoration of darkened red lead containing mural paintings: A preliminary study of the β-PbO2 to Pb3 O4 reversion by laser irradiation S. Aze & P. Delaporte LP3, Marseille, France

J.M. Vallet CICRP, Marseille, France

V. Detalle LRMH, Champs sur Marne, France

O. Grauby & A. Baronnet CRMCN, Marseille, France

ABSTRACT: Red lead pigment darkening in paintings is generally caused by the pigment alteration into plattnerite (β-PbO2 ). Plattnerite reversion into minium may be achieved by heating it over 375◦ C. This reaction may occur by using continuous-wave (cw) laser irradiation. Laser-induced photo-thermal reduction of plattnerite into minium was investigated through irradiation tests on both pure plattnerite powder samples and darkened red lead paint samples taken from experimental wall paintings. The effects of both visible (514 nm) and near infrared (1064 nm) sources were investigated. This study shows the high potentiality of an innovative technique based on a continuous wave-laser irradiation for the restoration of darkened red lead-containing paintings.

1

INTRODUCTION

Restoring blackened red lead containing paintings may consist in recovering the original red colour. In most cases, removing the black layer is inconceivable, considering the low amount of remaining pigment. The most sustainable solution might be based on the reversion of the alteration products into red lead. The main component of traditional red lead pigment is similar to minium, the mineral mixed valence lead oxide of formula Pb3 O4 . However, as a consequence of the manufacture process, significant amounts of lead monoxide may remain in the pigment. Both crystalline forms, i.e. orthorhombic (massicot) or tetragonal (litharge), have been identified in red lead-containing artworks. Either chemical or physical routes may achieve reversion of plattnerite into minium. Due to the complexity of the Pb-O system, which includes a number of distinct minerals, a chemical reduction of β-PbO2 into Pb3 O4 is challenging. Lead dioxide, however, is able to undergo thermal reduction through successive oxygen losses, which lead to the formation of lower oxides. In particular, plattnerite evolution into minium is spontaneous over ca. 375◦ C. This thermal transformation can be attained by using laser irradiation (Burgio et al. 2001). In order to profit from this property of a possible reversion of

Applications of laser beam techniques in the field of Art and Archaeology have been widely developed since a few decades. Both analytical and imaging methods, such as micro-Raman Spectroscopy, Laser Induced Breakdown Spectroscopy (LIBS), Laser Radar, Interferometry and Holography, provide useful information related to the object composition, conservation state and defects. Laser-based restoration techniques which mainly consist of surface cleaning (varnish removal from paintings, elimination of gypsum crusts from the rock materials) use either continuous wave or pulsed Q-switched laser radiations. This work attempts to apply continuous wave (cw) laser radiation for the restoration of darkened red leadcontaining paintings. Such an alteration phenomenon is commonly observed in mural paintings. Previous works showed that red lead darkening often result from the pigment transformation into black lead dioxide (plattnerite, β-PbO2 ). The degradation process is mainly controlled by both environmental factors, such as humidity, light, temperature or sulphur-containing pollutants. Besides, influence of intrinsic parameters, including composition and grain structure of the pigment, has been stated.

11

Table 1. Visual effects of Ar+ irradiation tests on pure plattnerite powder samples as a function of the laser power (P) and irradiation time (I). (R: reddishing, Y: yellowing).

darkened red lead, a series of laser irradiation tests were carried out. 2 2.1

P(W)

EXPERIMENTAL Samples

The effects of laser irradiation on plattnerite were investigated on both raw samples (powdered β-PbO2 , Merck, 99.95%) and blackened red lead-containing samples taken from experimental wall paintings (Morineau & Stefanaggi 1995, Aze 2005). Details related to both mural alterations and red lead discoloration have been described in previous papers (Aze 2006, Aze et al. 2007). 2.2

I(s)

0.75

0.90

1.00

1.10

1.25

1 5 10 30 60 120

None None None None None None

None Slight R Slight R Slight R Slight R Slight R

R R R R R R

R Y Y Y Y Y

Y Y Y Y Y Y

Methodology

Irradiation tests were carried out with both visible and near-infrared radiation, using a cw-Ar+ laser (514 nm) and a cw-Nd:YAG laser (1064 nm), respectively. The laser beams were set-up using a set of optical devices, including both converging and diverging lenses, diaphragms and mirrors. The resulting laser beam size, measured with a CCD device, was approximately of 1.8 mm2 . The effect of the main irradiation parameters, namely, irradiation times and laser power density was investigated. The result of each irradiation test on the plattnerite samples was estimated through observations of the sample surface by means of optical microscopy. 2.3

Figure 1. Raman spectra obtained from (a) the red areas, (b) the yellow areas resulting from plattnerite irradiation using Ar+ laser. The spectra match with the reference spectra of Pb3 O4 (minium) and β-PbO (massicot), respectively.

Characterisation of the method

Local analyses of both sample surface and crosssections were carried out using a micro-Raman spectrometer. Spectra of ca. 1 µm2 areas were recorded using a Renishaw inVia system equipped with a Spectra Physics Ar+ Laser (514.5 nm, 20 mW) and a Renishaw GaAs diode Laser (785 nm, 300 mW) calibrated using the 520.5 cm−1 line of a silicon wafer. Laser power, optical magnification and irradiation times were selected so that no degradation occurs due to photo-thermal effects. Spectral separation of the scattered photons was performed using both a Notch filter and grating monochromators (1800 l/mm with 514.5 nm, 1200 l/mm with 785 nm). Photons were collected over the 100–3000 cm−1 range with a spectral resolution of 1 cm−1 using a Peltier-cooled charge-coupled device (CCD) detector. 3

A slight reddishing of the plattnerite grains was observed in these conditions of irradiation longer than 5 seconds. In the case of longer irradiation times (up to 120 seconds), no visible increase of the reddishing intensity was observed. At higher laser powers, the reddishing was noticeably as being more intense, until a strong yellowing took place. For irradiation times over 5 seconds, this phenomenon occurred at laser powers higher than 1.10 W. Micro-Raman spectroscopic analyses of the red and yellow phases showed the reduction of plattnerite grains into Pb3 O4 (minium) and β-PbO (massicot), respectively (Fig. 1). 3.2

Nd:YAG laser irradiation tests

A slight reddishing of the plattnerite powder samples occurred at 0.15 W (Table 2). For long irradiation times (t > 5 seconds), the reddishing was more intense for laser powers between 1.0 and 3.6 W. According to microscopic observations, all visible plattnerite grains were transformed for laser powers higher than 2 W.

RESULTS

3.1 Ar+ laser irradiation tests Visible effects of the laser irradiation appeared for a laser power higher than 0.90 W (Table 1).

12

a consequence, the laser power density threshold for minium reduction into massicot is much higher with Nd:YAG irradiation than with Ar+ irradiation. Influence of the irradiation time appears to be negligible, over few seconds. We thus suppose that local temperature of the irradiated material quickly rises until it reaches a maximum corresponding to a stable thermal regime.

Table 2. Visual effects of Nd:YAG irradiation tests on pure plattnerite powder samples as a function of the laser power (P) and irradiation time (I). P(W) I(s)

0.10

0.15

1.00

3.00

3.60

1 5 10 30 60 120

None None None None None None

Slight R Slight R Slight R Slight R Slight R Slight R

Slight R R R R R R

R R R R R R

Y Y Y Y Y Y

5

CONCLUSION

Plattnerite irradiation using Ar+ laser (514 nm) leads to the formation of pure minium within a slight laser power density range. Due to the absorption of laser light by minium, massicot is readily produced. On the contrary, Nd:YAG irradiation (1064 nm) produces minium over a large power density range. Such irradiation tests show the high potentiality of continuous wave Nd:YAG laser for the possible reversion of blackened red lead pigment in paintings. ACKNOWLEDGEMENTS The authors wish to acknowledge the French Ministry of Culture and Communication for financial support, and the PLANI group from the LILM laboratory (CEA) which supplied the experimental set up.

Figure 2. Raman spectrum of the red phase obtained from plattnerite irradiated with cw-Nd:YAG laser at 2 W, compared to the reference Raman spectrum of minium Pb3 O4 .

REFERENCES

Yellowing of the plattnerite sample was observed over 3.60 W. According to micro-Raman analyses of the red phase obtained at 3.0 W, most of plattnerite grains have been reduced into minium (Fig. 2). The presence of an additional band near 342 cm−1 may be attributed to the lead sesquioxyde of formula Pb2 O3.33 (Aze 2005). 4

Aze, S. 2005. Altérations chromatiques des pigments au plomb dans les oeuvres du Patrimoine, PhD thesis, University of Marseille. Aze, S., Vallet, J.-M., Baronnet, A., Grauby, O. 2006. The fading of red lead pigment in wall paintings: tracking the physico-chemical transformations by means of complementary micro-analysis techniques. European Journal of Mineralogy 18: 835–843. Aze, S., Vallet, J.-M., Pomet, M., Baronnet, A., Grauby, O. 2007. Red lead darkening in wall paintings: natural ageing of experimental wall paintings versus artificial ageing tests. European Journal of Mineralogy 19: 883–890. Burgio, L., Clark, R. J. H., Firth, S. 2001. Raman spectroscopy as a means for the identification of plattnerite (PbO2 ), of lead pigments and of their degradation products. Analyst 126: 222–227. Ciomartan, D.A., Clark, R.J.H., McDonald, L.J., Odlyha, M. 1996. Studies on the thermal decomposition of basic lead (II) carbonate by Fourier-transform Raman spectroscopy, X-ray diffraction, and thermal analysis. Journal of Chemical Society – Dalton Transactions 18: 3639–3645. Clark, G. L., Schieltz, N. C., Quirke, T. T. 1937. A New Study of the Preparation and Properties of the Higher Oxides of Lead. Journal of the American Chemical Society 59: 2305–2308. Morineau, A., Stefanaggi, M. 1995. A statistical approach to the problem of frescoes:The French experience. Statistical Methods and Applications 4: 37–53.

DISCUSSION

Plattnerite reduction into minium spontaneously takes place over 375◦ C (Clark et al. 1937). On the other hand, minium itself is reduced into massicot over 512◦ C (Ciomartan et al. 1996). Depending on the irradiation parameters, plattnerite may thus be transformed into either minium or massicot. Irradiation of plattnerite by Ar+ laser (514 nm) initially leads to the formation of minium. Over a certain power density threshold, minium is reduced into massicot, due to the high absorption of the green laser light. When irradiated by near-IR laser beam (Nd:YAG, 1064 nm), a similar phenomenon occurs. Minium produced by plattnerite reduction, however, has a relatively low absorptivity in the IR spectral domain. As

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Prospective and applications of two-photon fluorescence in archaeology and art conservation D. Artigas & L. Serrado Department of Signal Theory and Communications, Universitat Politècnica de Catalunya, Barcelona, Spain

I.G. Cormack, S. Psilodimitrakopoulos & P. Loza-Alvarez ICFO-Institut de Ciències Fotòniques, Castelldefels, Spain

ABSTRACT: Two-Photon Excited Fluorescence (TPEF) is proposed as a new technique in archaeology and art conservation. In this paper, the fundamental basis of the technique and its main characteristics are explained. As an example of its use in archaeology, a recent work on the recovery of missing writings from an archaeological object is reviewed. In this case, the small quantity of paint and the necessity of ensuring low photodamage made TPEF the best choice. We also propose the use of TPEF, to perform optical sectioning in conservation studies for three dimensional characterization of the layer-distribution depth into the paint of an artwork.

1

INTRODUCTION

ideal technique in live science microscopy. However, the need of expensive ultrashort pulsed laser sources, usually a Titanium:sapphire oscillator and the relative complexity of these systems has, so far, restricted its wider application. The situation is changing rapidly. A new generation of cheap diode-pumped solid-state femtosecond oscillators is now under development (Droum et al. 2007). As a result, compact, low cost systems based on Yb:KYW crystals are being commercialized. This is making the use of TPEF a more attractive technique in biomedical applications than the conventional fluorescence techniques, which are currently routinely used. The objectives here are to understand how the materials were made and applied and to determine its origin and trade pattern. These objectives are normally achieved using spectroscopy techniques (Clark 2002), such as laser induced fluorescence (LIF) (Anglos et al. 1996, Weibring et al. 2001), Raman microscopy (Clark et al. 1998; Smith et al. 2004) or laser induced breakdown spectroscopy (LIBS) (Anglos 2001, Muller 2003). These are very powerful techniques which can be extensively used and combined in a number of situations (Castillejo et al. 2001). In addition to these techniques, we devise an application where the use of TPEF in archaeology and art conservation is considered to be more advantageous than the conventional techniques. A first application of TPEF is in those situations where the quantity of material to be identified is present in small amounts. Signals obtained with Raman microscopy from very low concentration of

Two photon absorption was first predicted by Maria Goeppert-Mayer in 1931 as a single quantum event. Here two photons with equal or different wavelengths are simultaneously absorbed by a molecule, thereby, exciting it from its ground state to an excited estate. This nonlinear effect is associated with the imaginary part of the third order susceptibility, and therefore, its absorption efficiency scales with the light intensity (Boyd 1992). The high intensity is usually reached by using mode-locked lasers, which provides the required high peak powers while at the same time maintains an energy low enough to preserve the sample. When the relaxation to the ground state is radiative the process is known as two-photon excited fluorescence (TPEF). Two-photon microscopy is one of the more interesting applications of two-photon fluorescence (Denk 1990). It offers some advantages over the widely used confocal fluorescence microscopy. In a two-photon microscope, the highest intensity is reached at the focal volume of the objective and TPEF is therefore generated only at this point. This provides optical sectioning without the need of a confocal aperture. In addition, the small confined excitation volume also minimizes the total photobleaching and photodamage. Since the excitation is produced with photons with half the frequency of the energy gap between the ground and the excited state of the molecule, laser sources in the near infrared are used. This further reduces the possibility of photodamage and increases the penetration depth in the sample. All these advantages make TPEF the

15

the target sample would be indistinguishable from the background signal. In LIF, high energy UV photons are needed for excitation. These can damage the valuable artwork material. Finally, LIBS is an intrinsically semi-destructive method, and therefore, not suitable for these purposes: low photodamage probability and photobleaching afforded by TPEF makes the technique highly suitable to address such problems. A second advantage of this technique is the potential to provide information about the depth of material within the sample surface, i.e., the painting layer morphology. This is possible due to the intrinsic optical sectioning of the nonlinear effect, providing 3D resolution, for both imaging and characterization, and to the use of IR light, which increases the penetration depth. In this paper we discus the potential of TPEF by analyzing these two applications. To do that, the remaining of the paper is organized as follows. In section 2, TPEF and other nonlinear techniques that have been successfully applied to microscopy are described. This is followed, in section 3, by an example of the use ofTPEF to detect painting in an archaeological sample. The use of 3D characterization of paints and determination of the layer morphology is proposed in section 4. Finally, in section 5, discussion and conclusions are presented.

Figure 1. Electronic energy level scheme showing the different excitation processes: one-photon (linear) absorption and two-photon (nonlinear) absorption.

to non-radiative relaxation between vibrational levels. However, the radiated wavelength is still much shorter than the long wavelength used to produce two-photon absorption. This large Stokes shift makes filtering of the near-IR excitation wavelengths from the fluorescent signal wavelengths very effective. The excitation of TPEF in microscopy requires a laser source able to provide the required high intensity wile at the same time, maintaining a low enough averaged power (Pav ) to preserve the sample. This can be achieved by using mode-locked lasers such as Titanium:sapphire laser. What makes this laser interesting for biological applications is its wide wavelength tuning range, λ = 720–920 nm. This spectral range allows pumping most of the dyes with linear absorption region in the green-blue-UV were specifically developed for fluorescence microscopy. Examples are: rodamin-B, fluorescein, coumarin, and the green-fluorescent protein present in genetically modified organisms (Xu 1996). A standard commercial system can deliver ultrashort pulses with duration in the range  tp = 50–200 fs, repetition rate around fR = 80 MHz, and average powers up to 2W. This results in peak powers above Pp = 100 KW. When the IR beam is focused into the sample, most light is mainly absorbed at the focus. The number of absorbed photons can be estimated by the following equation:

2 TWO-PHOTON FLUORESCENCE AND NONLINEAR MICROSCOPY 2.1 Two photon fluorescence Two-photon absorption is a nonlinear effect associated to the presence of the third-order susceptibility, χ(3) , which is present in any material (Boyd 1992). This effect can occur when the energy difference between electronics energy levels is similar to the added energy of two photons. In order to observe this effect, the two photons must coincide at the same position within the material. In addition, since the process lasts only for a very short period, in the femtosecond-attosecond scale, the two photons must effectively coincide also in the time domain. As a consequence, the absorption probability, and thus the efficiency, depend on the photon density, i.e., on the light intensity. The efficiency also depends on the wavelength (absorption bandwidth) and the material capability to perform two-photon absorption, features that are characterized by the two-photon absorption cross-section σ2p . Once an atom or molecule has been excited by two photons, the relaxation to the ground state is identical to the situation with one-photon (linear) excitation. When the process is in part radiative, the result is the expected fluorescence (see Fig. 1). Usually, the energy of the emitted photon is lower compared to the sum of the energy of the absorbed photons. This is due

where NA is the objective’s numerical aperture, h is the Plank constant and c the speed of light. The most important differences between this and the one-photon (linear) absorption are the quadratic dependence on the average power (linear in one-photon) and the

16

Figure 2. Volume at which fluorescence (white) is generated. (a) In one-photon fluorescence, it is generated mainly at the surface since the power of the excitation beam (at λ = 354 nm in this example) is depleted during propagation due to the linear absorption. (b) In two-photon fluorescence, it is generated only at the focal volume where the highest intensity is reached. Here, there is not absorption outside the focal volume and therefore there is not power depletion (in this case at λ = 708 nm the medium is transparent).

Figure 3. Scheme of an inverted nonlinear microscope simultaneously operating with TPEF (epi-detection), SHG and THG (forward detection). A CARS microscope would require at least an additional laser source and extra modifications.

different points of a three dimensional object with a precision that can be smaller than 1 µm3 . As mentioned before, virtually all the detected fluorescence arrives from the focal point. The entire generated signal therefore contributes to the detection, allowing the use of moderate to low excitation average powers. On the contrary, in confocal microscopy, due to the use of a pinhole lower detected signal are usually compensated by higher excitation powers, compromising the integrity of the sample. Another important advantage of TPEF in microscopy is the higher penetration depth of IR light. Rayleigh scattering is the main limitation to penetration length. It scales with the inverse of the wavelength, λ−n , with a power n that varies between 2.2 and 4, depending on the material. Therefore, scattering is in general an order of magnitude lower with TPEF than that the equivalent one-photon excitation in the visible-UV spectrum. Finally, photodamage and photobleaching in TFP is reduced to the focal volume, whereas in one-photon fluorescence the upper and lower regions of the excitation light cone are affected. This two negative factor are further reduced due to the use of a comparative lower intensity (since there is no pinhole) and the lower energy of the IR photons to produce the same excitation.

power-of-four dependence on the NA of the microscope objective (quadratic in one-photon). The result is that with standard values of the NA, around 80% of the photons are absorbed at the focus. This means that the fluorescence is mainly generated at this point, as shown schematically in Figure 2b, providing what is known as intrinsic optical sectioning for 3D imaging. On the other hand, with one-photon (linear) absorption, the fluorescence, is generated mainly at the surface, decreasing along the beam path (as shown in Fig. 2a). Therefore, optical sectioning can only be achieved by using a confocal pinhole.

2.2 Two-photon fluorescence scanning microscopy A two-photon fluorescent scanning microscope is depicted in Figure 3. It consists on the laser source, a scanning system (usually two galvanometric mirrors) in one of the inverted-microscope ports and a hot mirror leading the beam to the microscope objective. The fluorescence is then collected by the same objective and directed (crossing the hot mirror) to the detection system, coupled in one of the microscope output ports. Additional filters (BG39) can be used to stop the remaining IR light. For imaging purposes, the detection system is a standard photomultiplier. The excitation beam is then scanned at the sample plane using the two galvanometric mirrors. The generated signal is recorded and used to computer-reconstruct the 3D image. In spectroscopy applications, a spectrograph and a CCD camera can be coupled to the output port. This allows recording the fluorescence spectra at

2.3 Other nonlinear effects applied to microscopy Two-photon scanning microscopy belongs to a broader family of microscopy techniques making use of nonlinear effects. This family of techniques is known as nonlinear or multiphoton microscopy. In this subsection, for completeness, we briefly comment on these techniques. The best known multiphoton techniques are second-harmonic generation (SHG) microscopy

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(Gannaway 1978) and third-harmonic generation (THG) microscopy (Barad 1997). Another interesting nonlinear technique is also commented: coherent anti-stokes Raman scattering (CARS) microscopy (Zumbusch 1999). Since all these techniques are based on nonlinear processes they share one of the main characteristics of the TPEF microscopy: intrinsic optical sectioning. The first modality, SHG-microscopy is associated with the second order nonlinear susceptibility, χ(2) . This nonlinearity appears in non-centrosymmetric molecules or crystalline structures and is responsible for a variety of two wave mixing processes (Boyd 1992). SHG can also be generated in surfaces (that break the centrosymmetry) and nanocrystals (Wang 1996) and can be enhanced in structured patterns due to quasi-phasematching. Here, the generated SHG light can be used to distinguish material with large χ(2) or areas with a definite-ordered structure from the surrounding materials. THG is associated with the real part of χ(3) susceptibility. This nonlinear effect is present in all materials. However, due to the phase change in a Gaussian beam along the focus (Gouy phase) the THG signal is reconverted back to the fundamental wave after the focus. This THG signal only appears when the focus is in an interface between two media with different refractive indices. Then, a fraction of the generated signal before the focus is not reconverted. As a consequence, by detecting this signal, the corresponding interface can be imaged. Finally CARS again relays on the χ(3) nonlinear susceptibility. In this case it makes use of two laser sources, the first corresponding to the pump and the second to the Stokes wavelength, that interacts generating coherent anti-Stokes light. The generated light is resonantly enhanced when the difference in photon energy between the pump and the Stokes beam coincides with the energy difference between two vibrational states (a Raman resonance). The observed CARS signal is then related to the presence (and concentration) of species which resonantly react at the pump-stokes frequency difference.

Figure 4. Amphora under study, above (a), enlarged image showing the state of the paint (consular date), below (b).

war. At some moment they decided to take control of the country starting this foundation program. Among the first founded cities there were Empúries, Beatulo (Badalona) and Iluro (Mataró), together with others settlements not so well known as Iesso (Guissona) (Guitart et al. 2006). The exact foundation date of these cities is however still not known. Recently, in the archaeological site of Iesso, a piece of an amphora of the type Dressel A1 was found (Fig. 4a). In this piece there was some painted writing corresponding to the consular date (the name of the two consuls ruling in the year of wine production), a sign of good quality wine. In this case, it is believed that this amphora was brought by the first Roman Legions arriving to Iesso and that it was consumed during some ritual ceremony related with the city foundation (Guitard et al. 2004). The consular date in the amphora could therefore provide a terminus post quem for the city foundation of Iesso and indirectly, the starting of the Roman colonization of Hispania Citerior. Unfortunately, the past of time has deteriorated the painted area in the amphora (see Fig. 4b). At the moment of the discovery it was still possible to recognize some letters:

3 AN EXAMPLE OF PAINT DETECTION IN ARCHAEOLOGICAL ARTIFACTS 3.1 The archaeological problem Although the Roman colonization of Hispania is a well known process, there are still some events which have not been clearly determined. One of these cases is the starting of a city foundation program that took place at the end of the II century BC in Hispania Citerior. Previously to this foundation program, the Romans remained in Tarraco (Tarragona) as the only Roman possession since their arrival during the Second Punic

The first group of letters corresponds to the initials of the first consul, Quintus Fabius. The second group means consolibus (being consuls). The missing letters in the area between the two groups of letters correspond to the second consul. By examining the list of Roman consuls at the end of the 2nd and beginning of the 1st centuries BC, there were two occasions at which a consul had the name Quintus Fabius. The first

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one, Quintus Fabius Maximus Allobrogicus was consul in the years 121 BC, together with Lucius Opimius. The second possibility was Quintus Fabius Maximus Eburnus, who was consul in the year 116 BC, together with Caius Licinius Geta. Therefore, the objective was to recover the missing letters by mapping the paint distribution using a laser scanning technique and to identify through fluorescence which of the two possible options was written in the amphora (Cormack et al. 2007). This would determine the year for the wine harvest contained in the amphora and thus provide a better indication of the city foundation date. 3.2

Paint detection

Figure 5. (a) Direct two-photon fluorescent image obtained after scanning the area shown in figure 4(b), (b) result after performing a Gaussian convolution. Squares show the area that could correspond to the Q and the F letters of the first consul Quintus Fabius, (c) equalization histogram of the same area. Squares show areas where paint could be observed by eye inspection resulting in no fluorescence.

One possibility to uncover the missing name is to detect small amounts of pigments and/or agglutinant to form an image of the missing letters. A method based on detecting the fluorescence generated by the paint was sought. As commented, TPEF was chosen to recover the missing letters in the amphora, since the use of IR excitation (when compared to UV excitation) helped to prevent damaging the paint. Thus, Cormack et al. (2007) used a Titanium:sapphire laser operating at 830 nm, delivering pulses of 150 fs at a 76 MHz repetition rate to excite the TPEF. Here, as high resolution was not required, a 125 mm focal length lens was used as the microscope’s objective (spot size 25 µm). Some preliminary experiments with fragments of amphora found in the same archaeological site showed that the only source of fluorescence was from painted areas. The main assumption was that the fluorescence would probably be generated by the agglutinant while the pigment (probably red ochre) would inhibit it. The collected fluorescence was centered at 570 nm with 80 nm bandwidth. This fluorescence was considerably shifted to the yellow-orange range, rather unusual for organic binding media (Nevin et al. 2006). It was therefore not possible to identify the agglutinant. These preliminary experiments also allowed determination of the averaged power thresholds for both, the fluorescence emission (20 mW) and damage (80–90 mW) measured at the sample plane. In addition, the reliability of the imaging system and some basic image processing techniques was tested using these fragments of amphora. After these preliminary results, the area corresponding to the consular date (Fig. 4b) was scanned. To do that, the total area was divided into three sections to better adapt to the curvature of the amphora. The scan was performed at three different depths to maximize the fluorescence. Unfortunately, the resulting image, shown in Figure 5a, did not give enough information to identify the missing letters. To do that, some more sophisticated image processing techniques and the use of historical information was required.

3.3 Identification of the missing letters As commented, in the preliminary experiments an image processing technique was tested. This was based on a Gaussian convolution and binarization that allowed determination of the painted area. Figure 5b shows the result after a Gaussian convolution. Here some regions at the beginning of the analyzed area could be associated with the letter Q and F of Quintus Fabius, the first consul. Nevertheless, the results were poor and no information was obtained from the second consul. A more sophisticated image processing technique based on equalization histogram (Castleman, 1996) was then used.Although the first attempt shown in Figure 5c was not a success, it allowed us to obtain very valuable information. First, it showed that areas in the amphora where the paint was visible by eye inspection resulted in black color (no fluorescence) areas while areas in white (larger fluorescence) were obtained outside painted areas. This is in agreement with previous reported results showing that the fluorescence observed in paint based upon metallic compounds is generated by the agglutinant, while the pigment actually inhibits the generation of fluorescence (Miyoshi, 1988). Second, we noticed that large fluorescence obtained from well preserved areas was decreasing the contrast in the region where the name of the second consul was. With the previous information, the equalization histogram was performed in areas corresponding to the size of a letter and changing the grey scale so that the white color corresponds either to a maximum or to a minimum of fluorescence. The results corresponding to the direct equalization histogram are shown in

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surface of the paint. X-ray radiography (Schreiner 2004) and infrared reflectography (Van Asperen de Boer 1969) are extensively used to see below the surface, however they reduce the 3D information to a 2D plane, loosing information about the localization of a special feature in the vertical direction. The traditional procedure to solve this problem requires the extraction of a sample from the painting to be analyzed using different methods as can be scanning electron microscopy-energy dispersive X-ray spectroscopy (Ciliberto et al. 2000). Recently, two non destructive different alternatives have been proposed. The first one, optical coherent tomography (OCT), has been applied to analyze both archaeological samples (Yang et al. 2004) and in conservation studies (Targowski et al. 2004). This technique can accurately provide information of multilayer paint morphology and it is especially well suited to measure varnish thickness. However, it lacks the required characterization capability. The second technique is based on a sophisticated confocal X-ray fluorescence microscope able to characterize different layers with a total thickness around 500 µm (Woll 2006) with an axial precision up to 5 µm. LIF confocal microscopy, has also been proposed as alternative technique (Hogan 2007), although to our knowledge, no practical demonstration has been performed. In principle, this would be a more simple technique to characterize different paint layers. However, the use of visible-UV light would greatly reduce the penetration depth due to the high absorption and scattering at these wavelengths. This is the case in industrial application for polymeric paint analysis, where the poor penetration depth and photobleaching, have limited the scope of it use. For industrial applications, two alternative techniques have been proposed. The first one is a confocal aperture within a Raman microscope that can effectively characterize paint deep within the sample (Tabaksblat 1992). In this case, similarly to a fluorescent confocal microscope, only a small fraction of the generated signal (the one passing through the pinhole) is captured by the detector, reducing the sensitivity of the system. The second alternative is based on a TPEF microscope. Here, the entire signal is originated at the focus, making the detection system more simple and efficient, although it lacks the characterization specificity of Raman-based techniques. Two-photon microscopy has been used to characterize fluorophore-doped polymer blocks with submicron resolution, in multilayer structures (Bhawalkar 1997). In this work, the lower scattering and the greater transparency at IR wavelengths allowed characterizing structures up to a depth of 200 µm. The previous evidence shows the viability of twophoton microscopy to characterize the composition and structure of buried layers in artwork painting. The

Figure 6. (a) Histogram equalization corresponding to the letters forming the initials of the second consul, (b) result after performing a Gaussian convolution and a binarization to figure (a), (c) first option for the second consul: Lucius Opimius and in (d) second option, Caius Licinius Geta. In both cases, capitalis rustica letterform was used.

Figure 6a. In addition, after the equalization histogram, a Gaussian convolution and a posterior binarization was performed to help the eye, resulting in the image in Figure 6b. This figure shows clear, well defined areas where the original paint distribution can be identified. However, the immediate identification of the missing letters was still not possible and it was necessary to compare with the two possible options.The first option, OPI, corresponding to the surname for Lucius Opinius and the second option, LIC for Caius Licinius Geta, are written in Figure 6c, and 6d respectively. For a better evaluation, in both cases, the surnames initials were written using capitalis rustica, the informal cursive capital letterform used in those times (Lowe 1972). Comparing Figures 6b and 6c, the first letter can be identified as an O while the top part of the letter P in capitalis rustica clearly resembles the central panel in Figure 6b. Finally the last letter could be an I.Therefore it was assumed that the name of the second consul was Lucius Opinius, and the wine in the amphora was from the 121 BC harvest. With the results showed in that work, the archeologist must now determine the wine aging to more precisely determine the foundation date of Iesso and the starting of the roman colonization of Hispania Citerior. 4

PROSPECT OF TWO-PHOTON EXCITED FLUORESCENCE FOR PAINT MORPHOLOGY EXAMINATION

There are situations in which the knowledge of an artwork condition, the extent of a previous restoration or the working method of an artist requires a layer-by-layer measurement of the sample. LIF, Raman microscopy and LIBS give information mostly on the

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Clark, R. J. H. 2002. Pigment identification by spectroscopic means: an arts/science interface, C. R. Chimie 5; 7–20. Cormack, I. G., Loza-Alvarez, P., Sarrado, L., Tomas. S., Amat-Roldan, I., Torner, L., Artigas, D., Guitart, J., Pera, J. & Ros, J. 2007. Lost writing uncovered by laser two-photon fluorescence provides a terminus post quem for Roman colonization of Hispania Citerior. J. Archaeol. Sci. 34: 1594–1600. Denk, W., Strickler, J. H., & Webb, W. W. 1990. Two-photon Laser Scanning Fluorescence Microscopy, Science 248: 73–76. Droum, F., Balembois, F. & George, P. 2007. C.R. Physique 8: 153–164. Gannaway, J.N. & Sheppard, C. 1978. 2nd-harmonic imaging in scanning optical microscope, Opt. Quantum Electron. 10: 435–439. Guitart, J., Pera, J. & Carreras, C. 2004. La presència del vi itàlic a les fundacions urbanes del principi del segle I aC a l’interior de Catalunya: l’exemple de Iesso. In J. Guitart & J. Pera (eds.), El vi a l’antiguitat. Econòmia, producció i comerç¸ al Mediterrani Occidental, Badalona (1998). Iesso I. Miscellània arqueològica, Barcelona-Guissona. Guitart, J., 2006. Iluro, Baetulo and Iesso and the establishment of the Roman town model in the territory of present-day Catalonia. In: Early roman towns in Hispania Tarraconense (2nd century BCe1st century AD), Portsmouth. Hogan, H., 2007. Photonics for art’s sake, Photonics Spectra 41: 46–53. Lowe, E.A., 1972. Codices Latini antiquiores.A Palaeographical Guide to Latin Manuscripts Prior to the Ninth Century. Oxford: The Clarendon Press. Miyoshi, T. 1988. Fluorescence from colours used for Japanese painting Ander N2 laser excitation. Jpn. J. Appl. Phys 27: 627–630. Muller, K. 2003. Evaluation of the analytical potential of laser-induced breakdown spectrometry (libs) for the analysis of historical glasses, Archaeometry 45: 421–433. Nevin, A., Cather, S., Anglos, D., Fotakis, C., 2006. Analysis of protein-based binding media found in paintings using laser induced fluorescence spectroscopy, Anal. Chim. Acta 573: 341–346. Schreiner, M., Frühmann, B., Jembrih-Simbürger, D. & Linke, R., 2004. X-rays in art and archaeology: An overview, Powder Diffraction 19: 3–11. Smith, G. D. & Clark, R. J. H. 2004. Raman microscopy in archaeological sciences. J. Archaeol. Sci. 31: 1137–1160. Tabaksblat R., Meier, R. J. & Kip, B. J. 1992. Confocal Raman microspectroscopy – Theory and application to thin polymer samples. Applied Spectroscopy 46: 60–68. Targowski, P., Rouba, B., Wojtkowski, M. & Kowalczyk, A. 2004. The application of optical coherence tomography to non-destructive examination of museum objects. Studies in Conservation 49: 107–114. VanAsperen de Boer, J.R.J. 1969. Reflectography of paintings using an infra-red vidicon television system. Studies in Conservation 14: 96–118. Wang, H., Yan, E.C.Y., Borguet, E. & Eisenthal, K.B. 1996. Second harmonic generation from the surface of centrosymmetric particles in bulk solution. Chem. Phys. Lett. 259: 15–20.

technique would consist of a spectrometer coupled to the output port of a nonlinear microscope. Then by scanning the sample and observing the emitted fluorescence spectrum, information about the paint structure and composition deep in the sample can be obtained.

5

DISCUSSION AND CONCLUSIONS

TPEF is a more sophisticated variant of the well known LIF technique widely used in archaeology and art conservation. In most of the standard situations, TPEF provides the same characterization features than LIF, but a more complicated ultrashort pulse laser source is required. Nevertheless, there are two situations where its unique characteristics makeTPEF the ideal option.The first one is when an extreme care is required and any kind of damage is not allowed. Here the use of low energy IR photons increases the photodamage threshold and at the same time, and in case this is produced it only occurs at the focal point. A second application has been proposed in those cases where information about the material depth in the sample or the paint layer structure is required. However, to demonstrate the viability of this technique, systematic studies are required to properly evaluate its properties, advantages and disadvantages for its application in cultural heritage conservation. REFERENCES Anglos, D., Solomidou, M., Zergioti, I., Zafiropulos, V., Papazoglou, T. G. & Fotakis, C. 1996. Laser-induced fluorescence in artwork diagnostics:An application in pigment analysis. Applied Spectroscopy 50; 1331–1334. Anglos, D. 2001. Laser-induced breakdown spectroscopy in art and archaeology. Applied Spectroscopy. 55; 186A– 205A. Barad,Y., Eisenberg, H., Horowitz, M. & Silberberg,Y. 1997. Nonlinear scanning laser microscopy by third harmonic generation. Appl. Phys. Lett. 70: 922–924. Boyd, R. W. 1992. Nonlinear Optics. Boston:Academic Press. Castillejo, M., Martin, M., Oujja, M., Silva, D., Torres, R., Domingo, C., Garcia-Ramos, JV. & Sanchez-Cortes, S. 2001. Spectroscopic analysis of pigments and binding media of polychromes by the combination of optical laserbased and vibrational techniques. Applied Spectroscopy 55; 992–998. Castleman, K.R. 1996. Digital Image Processing. New Jersey: Prentice May. Ciliberto, E., Spoto, G. 2000, Modern Analytical Methods in Art and Archaeology. New York: Wiley. Clark, R. J. H. & Gibbs, P. J. 1998. Analysis of 16th Century Qazwını Manuscripts by Raman Microscopy and Remote Laser Raman Microscopy. J. Archaeol. Sci. 25: 621–629.

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Weibring, P., Johansson, T,. Edner, H,. Svanberg, S,. Sundnér, B., Raimondi, V., Cecchi, G. & Pantani, L. 2001. Fluorescence lidar imaging of historical monuments. Appl. Opt. 40: 6111–6120. Woll, A.R., Mass, J., Bisulca, C., Huang, R., Bilderback, D.H., Gruner, S. & Gao N. 2006. Development of confocal X-ray fluorescence (XRF) microscopy at the Cornell high energy synchrotron source. Appl. Phys. A 83: 235–238. Xu, C. & Webb, W., W. 1996. Measurement of two-photon

excitation cross sections of molecular fluorophores with data from 690 to 1050 nm. J. Opt. Soc. Am. B; 481–491. Yang, M.L., Lu, C.W., Hsu, I.J. & Yang, C.C., 2004. The use of optical coherence tomography for monitoring the subsurface morphologies of archaic jades. Archaeometry 46: 171–182. Zumbusch, A., Holtom, G.R., & Xie X.S. 1999. Threedimensional vibrational imaging by coherent anti-Stokes Raman scattering. Phys. Rev. Lett. 82: 4142–4145.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Fast spectral optical coherence tomography for monitoring of varnish ablation process M. Góra, P. Targowski & A. Kowalczyk Institute of Physics, Nicolaus Copernicus University, Toru´n, Poland

J. Marczak & A. Rycyk Institute of Optoelectronics, Military University of Technology, Warszawa, Poland

ABSTRACT: Optical Coherence Tomography (OCT) is a new, fast-growing technique for non-contact and non-destructive imaging of semi-transparent objects. The application of OCT to monitor varnish ablation with the fourth harmonic of Nd:YAG laser is presented. It is shown that OCT may be used for recovering parameters like ablation rate and for in situ tracking of the process. Frames from OCT tomographic movies demonstrating dynamic processes during ablation are also presented.

1

INTRODUCTION

investigation, but it seems to be a promising one in certain circumstances. At present, laser techniques are well established in several branches of art conservation, e.g. for cleaning of stone sculptures and architectonic details (Bartoli et al. 2006). The choice of laser source, essential to the success of the treatment, mostly depends on the absorption properties of the removed layer. Since all varnishes absorb well both infrared and ultraviolet radiation, lasers emitting in these two spectral ranges are utilised for this purpose. In the ultraviolet range, excimer lasers (λ = 157 nm, 193 nm and 248 nm) and Q-switched Nd:YAG laser (fourth harmonic) are mostly tested (CRAFT 2001, Bordalo et al. 2006). The infrared radiation at λ = 2936 nm emitted by the Er:YAG (Erbium-doped Yttrium-AluminiumGarnet, Er : Y3Al5 O12 ) laser is also considered as a very promising one (de Cruz et al. 1999, 2000). Since the varnish layer is very inhomogeneous in thickness and often its susceptibility to ablation varies from point to point, it is important to monitor online and control in situ the process. For this purpose Laser Induced Breakdown Spectroscopy (LIBS) (Anglos et al. 1997) is generally used. With this technique, traces of elements from the underlying paint layer occurring in the plasma ablation plume are detected (CRAFT 2001). This method is fast and sensitive, but a small damage to the underlying paint layer cannot be avoided. Additionally, it cannot be used if partial removal (to a certain fraction of the original thickness) of varnish is desirable. On the contrary, Optical Coherence Tomography (OCT), that enables non-contact in situ continuous direct measurement of varnish layer thickness, should

OCT provides an art conservator with information about the structure of varnish, glaze and paint layers in a non-contact and non-invasive way (Targowski et al. 2004, Liang et al. 2005, Targowski et al. 2006). In this low-coherence interferometric technique a narrow beam of light is sent to the object. The light scattered and/or back reflected on its structural elements carries information about their locations within the object. The application of the technique is thus limited to imaging of transparent and semitransparent layers. So far, it is well suited for varnish layers imaging, especially in old pictures where the layer is thick enough to clearly distinguish boundaries even with moderate resolution OCT systems. Examinations of modern varnish layers often require a higher axial resolution. A varnish layer plays an important, protective role to the underlying paint layers. As a surface layer it also enhances colours of the paint. Unfortunately, it changes very often with time due to the long-term exposure to light or external pollutants and must be removed and replaced by a new one. It should also be removed in case it covers old retouches and the conservation process must be repeated. The traditional conservation methods utilize mechanical and chemical means. The mechanical method is the safest, but unfortunately, it is also timeconsuming. Chemical methods based on toxic solvents are very difficult to control, because of possible penetration of the solvent into the paint layer and different (often very limited) solubility of the old varnishes. The third method – laser ablation – is still under

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of 18 s. The latter is limited by the size of the operation memory of the computer utilised. In case of real time tracking, the data processing must be performed in real time and the image quality is limited by the performance of the computer. For a 1.8 GHz system it is possible to imagine 4 B-scans/s, each composed of 200 A-scans. Despite of this low line density, the quality of images is sufficient for current process control. In all OCT tomograms presented here, the intensity of the light scattered and/or reflected from the internal structures within the sample is coded in a greyscale. Dark shades of grey indicate a high scattering level of the probing light, while the light ones indicate low scattering levels. In all presented tomograms light is coming from the top, the interface between air and varnish is always the uppermost line, varnish-paint layer interface is below.

allow continuous control of the remaining varnish layer during its removal. In this contribution, the applicability of OCT for monitoring the ablation process induced by UV radiation is discussed. The dynamical processes like ablation and detachment of varnish layer are investigated. In former reports (Góra et al. 2006, Targowski et al. 2007a) the implementation of OCT to monitor varnish ablation by Er:YAG laser was described. In the present study, the fourth harmonic of Nd:YAG laser was utilised. 2 2.1

INSTRUMENTATION Nd:YAG laser system

The fourth harmonic of Nd:YAG laser (λ = 266 nm) is strongly absorbed by the varnish and the penetration of light into the irradiated medium is very small. For the purpose of this study a ReNOVALaser 5 was chosen. It is a Q-switched system, generating pulses of duration 12 ns and output energy up to 120 mJ. The repetition rate of pulses was adjusted from 2 Hz to 10 Hz. During experiments, the output energy was kept between 10 and 55 mJ. Laser radiation was focused on the sample with a quartz lens (f = 50 cm) to achieve fluences from 1 to 7 J/cm2 . 2.2

3

EXPERIMENTS

3.1 Samples The determination of the ablation threshold and the studies of the process of laser ablation were performed with the specially prepared samples. For the purposes of the present study three different varnishes were tested: Maimeri Dammar Matt varnish, Talens Acrylic varnish (matt) 115 and Talens Picture varnish (ketone, glossy) 002. All samples were prepared with a cardboard support. For the first sample the support was covered with an acrylic paint, the others were made with an oil paint. When the paint layer dried out, all samples were covered with the respective varnish. Just before testing, all samples were sprayed with a thin layer of matt Schminke varnish, to reduce mirror reflections.

SOCT instrument

The Spectral OCT system used to obtain the data presented in this contribution is a prototype instrument constructed in our laboratory. It is described in detail elsewhere in this volume (Targowski et al. 2007b). Briefly, to illuminate the sample a superluminescent diode (SLD) with central wavelength of 840 nm and bandwidth of 50 nm, emitting high spatial but low temporal coherence light is used. The light is launched into a fibre optic Michelson interferometer. It consists of reference and object arm. In the latter, light is scanned across the object by galvoscanners. The optical power of the beam incident at the surface of the object is 800 µW. The in-depth (axial) resolution of the system is limited mostly by the bandwidth of the light source. In the system used in this study it is estimated to be 6 µm in varnish. The transverse resolution depends on the optical properties of the system and is kept below 20 µm. For the purpose of the study, the acquisition speed of the system is a crucial parameter. The exposure time per A-scan is 40 µs and the acquisition rate is 20000 Ascans/s. The instrument may work in two modes: registration and real time tracking. In the former, raw data are quickly stored and processed afterwards to create an OCT movie. To achieve high quality of imaging, every cross-sectional image (B-scan) must be composed at least of 1200 A-scans. This leads to a recording rate of 16 B-scans/s and a recording time

3.2 Experimental setup To monitor the varnish layer removal, the OCT system was combined with the Nd:YAG laser (Fig. 1). Both instruments were focused to the same spot, with the sample surface perpendicular to the OCT◦scanning beam. The laser beam was tilted at about 40 . For each sample, two kinds of experiments were performed. First, the ablation rate under different fluences and pulse repetition rates was determined by examining ablation craters created by the laser beam focused at the varnish layer. During this experiment the sample was held in a steady position and irradiated with a series of laser pulses. The energy of these pulses was determined by means of a power-meter. Simultaneously, the OCT cross-sections were recorded in the place of the ablation crater, as described in the preceding section. Finally, the position of the bottom of the ablation crater was recovered from these images as

24

to the OCT system

Nd:YAG Laser

Power meter Figure 1. The experiment setup, a top view.

Figure 3. Relative position of the bottom of ablation crater in Talens Acrylic varnish after cumulative number of laser pulses, fluence = 6.7 J/cm2 .

Figure 2. a) Determination of the position of the bottom of crater. b) Determination of the surface of the ablation crater.

shown in Figure 2a. To estimate the fluence, the diameter of the ablation crater was determined from the surface image (200 × 200 pixels, 4 × 4 mm) obtained with the same OCT system (Fig. 2b). Once the ablation rate and ablation threshold were determined, progressive ablation of the varnish layer was performed. For this purpose, the sample was shifted continuously with constant velocity. In all these experiments the diameter of the laser beam spot was approximately 2 mm and the scanning speed was 0.4 mm/s. Therefore, for a repetition rate of 2 Hz, each point on the sample was irradiated 10 times. If repetition rate is increased, the number of accumulated pulses increased accordingly. The ablation process was registered with the OCT device by recording the OCT movie as described in the previous section.

4

Figure 4. Frames from OCT movie registered during ablation of Acrylic varnish. Numbers in parenthesis show the amount of laser pulses accumulated. Arrow points to the exfoliated layer.

Results shown in Figure 3 were measured on frames extracted from the OCT movie, registered with 16 frames per second as described in Section 2.2. Selected frames from this movie are presented in Figure 4. Normal ablation process is illustrated in Figure 4a and 4b. When the remaining varnish layer becomes thin the exfoliation process starts – a separate layer is visible just above the paint layer (Fig. 4c, arrow). Then, immediately after the next pulse, the whole remaining varnish is exfoliated (Fig. 4d) and removed by the two following pulses (Fig. 4e, f).

RESULTS

4.1 Talens Acrylic varnish In this case, the ablation rate was measured for a fluence of 6.7 J/cm2 , well above the ablation threshold. It is worth noting that the varnish layer for a similar sample was only exfoliated rather than ablated when it was treated with an IR laser (λ = 2.936 µm, fluence of 2.2 J/cm2 ) (Targowski et al. 2007a). This was probably due to insufficient absorption of IR laser radiation by the varnish layer. On the contrary, in case of UV treatment, the layer is effectively ablated and the rate may be estimated to 4.3 µm/pulse (Fig. 3).

4.2 Talens Ketone varnish The results obtained for Talens Ketone varnish are shown in Figure 5. As in the previous case, data were collected with repetition rate 2 Hz. The ablation rate may be estimated to 6 µm/pulse.

25

Figure 5. Relative position of the bottom of ablation crater in Talens Ketone varnish after cumulative number of laser pulses, fluence = 3.6 J/cm2 . Figure 7. Frames from the OCT movies, showing tomograms of the Maimeri Dammar varnish layer ablated with laser repetition rate (a) 2 Hz and (b) 8 Hz. Other laser parameters were the same. Laser pulses were applied at the centre of the image and during the process the sample was moved to the right.

phenomenon was observed. During ablation with low repetition rate, an additional volumetric damage to the varnish layer occurred, well visible in Figure 7a. This additional scattering diminishes the transparency of the remaining layer and thus should be avoided. Similar effects were observed with OCT for IR ablation as well (Góra et al. 2006). When the repetition rate was increased to 8 Hz (Fig. 7b) this effect was not present. Interestingly, attentive inspection of frames from this movie revealed that, at the beginning of the laser treatment, when the number of pulses accumulated was small, the same destruction as for 2 Hz was present. But after accumulation of the following pulses, the trace of internal damage vanished, probably by local melting/resolidifying process.

Figure 6. Relative position of the bottom of ablation crater in Maimeri Dammar varnish after cumulative number of laser pulses obtained for 2 and 4 Hz pulse repetition rate. Fluences were 4.5 J/cm2 and 5 J/cm2 respectively.

4.3

Maimeri Dammar varnish

If a layer is significantly resistive to the laser treatment it may be convenient to increase the repetition rate. In Figure 6, the ablation rate of the Dammar varnish is presented for 2 different repetition rates. In both cases, the pulse energy was the same. Ablation rates were 3.2 µm/pulse and 3.7 µm/pulse respectively. Taking into account the slightly higher fluence in the second case, one may assume that the ablation rate does not depend on the repetition rate of the laser and indeed the thickness of ablated layer may be conveniently controlled this way. After determination of the ablation rates, the experiments simulating a cleaning process were performed (Fig. 7). As it can be seen, increasing the repetition rate allows effective control of the ablation depth. However, for this specific varnish another interesting

5

CONCLUSIONS

Optical Coherence Tomography can be conveniently used for imaging transparent varnish layers. This technique permits fast and non-invasive direct assessment of the condition (e. g. transparency) of the remaining layer and direct determination of its thickness. In this contribution, we present an application of the OCT to the monitoring of the varnish ablation process. First, the OCT technique may be used for this purpose as a tool for adjustment of the process parameters, like ablation rate. Second, fast modalities of OCT, like Spectral and Sweep Source OCT may be used for in

26

CRAFT Final Report project ENV4-CT98-0787. 2001. Advanced workstation for controlled laser cleaning of artworks. http://www.art-innovation.nl/. deCruz, A., Hauger, S.A. & Wolbarsht, M.L. 1999. The role of lasers in fine arts conservation and restoration. Optics and Photonics News 10: 36–40. deCruz, A. Wolbarsht, M.L. & Hauger, S.A. 2000. Laser removal of contaminants from painted surfaces, Journal of Cultural Heritage 1: 173–180. Góra, M., Targowski, P., Rycyk, A. & Marczak, J. 2006. Varnish ablation control by Optical Coherence Tomography. Laser Chemistry doi:10.1155/2006/10647, http://www.hindawi.com/journals/lc/. Liang, H., Cid, M., Cucu, R., Dobre, G., Podoleanu, A., Pedro, J. & Saunders, D. 2005. En-face optical coherence tomography–a novel application of non-invasive imaging to art conservation. Optics Express 13: 6133–6144. Targowski, P., Rouba, B., Wojtkowski, M & Kowalczyk, A. 2004. The application of optical coherence tomography to non-destructive examination of museum objects. Studies in conservation 49: 107–114. Targowski, P., Góra, M. & Wojtkowski, M. 2006. Optical Coherence Tomography for Artwork Diagnostics. Laser Chemistry doi:10.1155/2006/35373 http://www.hindawi. com/journals/lc/. Targowski, P., Marczak, J., Góra, M., Rycyk,A. & Kowalczyk, A. 2007a. Optical Coherence Tomography for Varnish Ablation Monitoring. Proc. of SPIE 6618: 661803-1– 661803-7. Targowski, P., Góra, M., Bajraszewski, T., Szkulmowski, M., Wojtkowski, M., Kowalczyk, A., Rouba, B., Tymi´nskaWidmer L. & Iwanicka M. 2007b. Optical coherence tomography for structural imaging of artworks. In this volume.

situ tracking of the process. Specifically, it is possible to estimate in real time the thickness of the ablated and remaining layers. The OCT method also allows evaluation of the quality of the remaining varnish layer. Collecting images every 60 ms allows monitoring of fast, dynamical processes like creation of exfoliations and melting resolidification inside the remaining layer. Since properties of paintings vary significantly, the adjustment of ablation conditions must be performed separately for every object. Moreover, in certain cases the choice of the wavelength and pulse duration may not be obvious. Therefore, the OCT monitoring may increase significantly the safety and efficiency of using lasers for varnish ablation and contribute this way to the propagation of this technique in future. REFERENCES Anglos, D., Couris, S. & Fotakis, C. 1997. Laser Diagnostics of Painted Artworks: Laser-Induced Breakdown Spectroscopy in Pigment Identification. Applied Spectroscopy 51: 1025–1030. Bartoli, L., Siano, S., Salimbeni, R., Pouli, P. & Fotakis, C. 2006. Characterization of laser cleaning of pollution encrustation on stonework by Nd:YAG lasers with different pulse duration, Laser Chemistry doi:10.1155/2006/81750, http://www.hindawi.com/ journals/lc/. Bordalo, R., Morais, P.J., Gouveia, H. & Young Ch. 2006. Laser Cleaning of Easel Paintings: An Overview. Laser Chemistry doi:10.1155/2006/90279, http://www.hindawi. com/journals/lc/.

27

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Study of laccaic acid and other natural anthraquinone dyes by Surface-Enhanced Raman Scattering spectroscopy M.V. Cañamares & M. Leona The Metropolitan Museum of Art, New York, NY, USA

ABSTRACT: The red dyes of natural origin, such as lac dye, cochineal and madder, are of great importance in the history of early textiles and lake pigments. The main colouring substances of these dyes (laccaic acid, carminic acid and alizarin, respectively) are similar in chemical composition: all of them are based on a hydroxyanthraquinonic structure. In this work, we present the Surface Enhanced Raman Scattering spectroscopy (SERS) study of laccaic acid at 514.5 nm. The SERS spectra were obtained at different pH values and on differently prepared Ag nanoparticles, by means of chemical reduction of silver nitrate with tri-sodium citrate and hydroxylamine hydrochloride. The aim of this work is to find the best experimental conditions for the vibrational study and detection of lac dye. A comparison between the SERS spectra of laccaic acid and other anthraquinone dyes (carminic acid and alizarin) was also carried out.

1

INTRODUCTION

Rubiaceae family, common to Europe and the Middle East (Scheweppe & Winter 1998). Madder dye was extensively used in Asia for dyeing materials since ancient times. In Egypt, however, it was employed in the 16th century for dyeing. Alizarin (AZ, Fig. 1c), the simplest anthraquinone dye, is the main colouring component of madder. The high fluorescence of these anthraquinone dyes seriously limits the application of normal Raman spectroscopy to study these molecules. However, FT-Raman and SERS can be successfully used to characterize this dye in aqueous media. The applicability of Fourier Transform-Raman spectroscopy to the study of LA is limited to the pure dye in the solid phase and in highly concentrated solutions, as the Raman effect is very weak at near-infrared excitation. SERS, on the other hand, can be successfully used to study very diluted aqueous solutions, and has the additional advantage of quenching the fluorescence due to the presence of a metal surface (Creighton 1988, Moskovits 1985). Recent articles have reported SERS identification and/or adsorption behaviour of several anthraquinone dyes (Cañamares et al. 2004, 2006, Chen et al. 2006a, b, Shadi et al. 2006, Whitney et al. 2006). In this work we present a SERS study of LA on silver nanoparticles prepared by chemical reductions with citrate and hydroxylamine as well as at different pH values. The aim of this study was to determine the best experimental conditions for detecting low concentrations of LA in samples from artifacts, such

The red dyes of natural origin are of great importance in the history of early textiles and lake pigments. Their identification on ancient and historical textiles can be crucial to answer questions of dating, provenance, types of degradation, original colour and appearance of the artwork (Hofenk de Graaff 2004). The main colouring matters of the most important red natural dyes, such as lac dye, cochineal and madder, are based on a hydroxyanthraquinonic chromophore structure. Lac dye is a resinous secretion developed by the insect Laccifer lacca, native to Bengal, Siam, southern India and the Moluccas. This dye was used in India and Japan since antiquity, and it seems to have been extensively used in European dyeing practice only in the late 18th century. The main colouring components of lac dye are laccaic acid A (LA, Fig. 1a) (71–96%) and B (0–20%) (Hofenk de Graaff 2004). Cochineal, another dye of scale origin, is extracted from the female beetles of Dactylopius coccus L. Costa, native to Mexico, Central America and regions of South America. Aztecs used this dye for dyeing and painting. It was brought to Europe in the 16th century following the Spanish conquest. It superseded kermes, the European red dye (Scheweppe & RoosenRunge 1986). The main colouring matter is carminic acid (CA, Fig. 1b), whose structure is similar to that of LA, as can be seen in Figure 1. Madder, a vegetal dye, is extracted from the roots of Rubia tinctorum L. and various other plants of the

29

CH2CH2NHCOCH3 OH

O

OH COOH CO OH

OH HO

OH OH

HO HO

OH

O

O

OH

O

CH3

OH

COO H OH OH

HO

O

OH

(a)

(b)

O

O

(c)

Figure 1. Structure of laccaic acid (a), carminic acid (b) and alizarin (c).

conditions. The colloidal suspension containing the dye was placed in a 1 cm-path cell.

as textiles or paintings. A comparison of the SERS spectra of LA, CA and AZ on the silver nanoparticles was also carried out.

3 2 2.1

EXPERIMENTAL

RESULTS AND DISCUSSION

3.1 SERS spectra of laccaic acid with different pH and Ag nanoparticles

Materials

The SERS spectra of LA obtained at different pH values on SH and SC nanoparticles at 514.5 nm excitation are shown in Figure 2. Excitation at this wavelength is expected to result in the enhancement of the bands corresponding to the acidic species of LA (result not shown). The SERS spectrum obtained at pH 5 on SH colloid was too intense, therefore, the power had to be reduced to half of the value normally used (∼1 mW). For this reason, the equivalent global intensity of the spectra at pH 5 is approximately twice as large as that shown. A decrease in SERS intensity results from the pH changing from acidic to alkaline, the decrease in the resonance Raman effect and the increase of the electrostatic repulsion between LA and the Ag surface. As seen in Figure 2, the decrease in SERS intensity of LA upon increasing the pH of SC nanoparticles is higher than increasing that of the SH colloid. This is due to the higher negative zeta potential of the SC nanoparticle surface (Munro et al. 1995). The spectra undergo significant changes as the pH increased from 5 to 7. In the case of the SH colloids (Fig. 2c) there is a large increase in the intensity of the bands at 1532, 1344, 1291 and 1191 cm−1 and a shift of the bands at 1633, 1579, 1462 and 1227 to 1620, 1569, 1454 and 1233 cm−1 . On the other hand, when using SC nanoparticles (see Fig. 2d), an enhancement of the bands at 1534, 1350, 1300 and 1227 cm−1 takes place, together with a shift of the bands at 1670, 1585, 1462, and 1227 to 1659, 1571, 1453 and 1233 cm−1 . These observed differences can be attributed to the deprotonation of the hydroxyl and carboxylic groups. When an ionization process occurs, the charge distribution in the molecule is modified, leading to the changes observed in the relative intensities of the bands. As the chemical structure of LA, CA, and AZ is based on an anthraquinone skeleton, the first thought

LA was purchased from Tokyo Kaisei Co. (98%), CA from Sigma (96%) and AZ from Acros (97%). Stock solutions of the dyes (10−3 M) were prepared in water. All the reagents employed were of analytical grade and purchased from Sigma and Merck. The aqueous solutions were prepared by using 18 M ultra pure water (Millipore MilliQ). Silver colloids were prepared by reduction of silver nitrate with tri-sodium citrate dihydrate (SC) and hydroxylamine hydrochloride (SH) according to previously described procedures (Cañamares et al. 2005). Before adding the dye solutions, the citrate and hydroxylamine Ag colloids were activated by mixing 0.5 ml of each colloid with 12 and 20 µl of 0.5 M potassium nitrate solution, respectively. This activation is a prerequisite for observing SERS spectra at higher intensities, as it was demonstrated (Sanchez-Cortes et al. 1994, 1995). After activating the colloids, 55 µl of the dyes aqueous solution were added to 0.5 ml of each Ag colloid in order to reach a concentration of 10−4 M. Nitric acid and sodium hydroxide were employed to control the pH.

2.2 Instrumentation In a micro-Raman Renishaw RM1000 instrument, the SERS spectra were recorded at 514.5 nm with an Ar+ laser and a 50X objective. The maximum laser power at the sample was 1 mW. The resolution was set at 4 cm−1 and the geometry of micro-Raman measurements was 180◦ . Each spectrum was obtained by recording a single 30 s scan. The measurements of LA and CA were conducted with a micro-Raman setup where the laser beam was focused on a drop of the dye-nanoparticle suspension deposited on the surface of a glass slide. On the other hand, the SERS spectra of AZ were recorded in macro

30

1191

HO

-

O

O Ag

(d) SC pH 7

1300

1194

(c) SH pH 7

1427

1620

Figure 3. Adsorption mechanism of AZ on Ag surfaces.

(f) SC pH 11

1300

1087

x 1/2

(c) SH pH 7

(b) SC pH 5 1076

x 1/4 (a) SH pH 5 454

is to conclude that all of these dyes interact with the Ag surface in the same way. This interaction would be through the keto and hydroxyl groups, as shown in Figure 3 (Cañamares et al. 2004, 2006). The large increase of the band at 1344 cm−1 , assigned to the OH group adjacent to the C = O, supports this suggestion. However, LA could present an electrostatic interaction with the Cl− ions surrounding the silver nanoparticles by means of the NH+ of the amide group. This mechanism gains support from the vibration at 1620 cm−1 observed at neutral pH, which can be assigned to the amide group (Socrates 2001). On increasing the pH to 11 (Figs. 2e, f ) a decrease of the relative intensity of the bands at 1569, 1454, 1233 and 1010 cm−1 is observed. We suggest that these changes are due to another deprotonation process of LA. The downward shift undergone by higher frequency bands and the upward shift of the lower ones, originated as a result of the higher electronic delocalization in the molecule, supports this fact. The intensity of the SERS spectra obtained with the SH colloid is higher than with SC at every pH value. Thus, we can conclude that the SH colloid should be preferably used for the analysis of textiles suspected to contain lac dye. The existence of chloride ions on

1449

1640 1645

Figure 2. SERS spectra of LA (10−4 M) on SC and SH colloids at acidic, neutral and alkaline pH. Excitation at 514.5 nm. Spectrum (a) was obtained using half the laser power of the other ones.

(d) SC pH 7

1227

500

Wavenumber /cm -1

1327

x 1/2

1457

1000

SERS intensity

(a) SH pH 5

412

1011

(b) SC pH 5

454

1227

(e) SH pH 11

(e) SH pH 11

1100

1453

1347 1403 1350 1344 1300 1297 1291 1191 1194 1191 1101 1100 1013

1454

1368 1288 1462 1333

1625

1532

1606 1534

1659

1620 1569 1670 1585 1633 1579

SERS intensity

1500

O (f) SC pH 11

1500

1000

x 1/4

500

Wavenumber / cm −1 Figure 4. SERS spectra of CA (10−4 M) on SC and SH colloids at acidic, neutral and alkaline pH. Excitation at 514.5 nm. Spectra (a), (b) and (d) were obtained using 1/4, 1/4 and 1/2 of the laser power of the other ones, respectively.

the surface of the SH colloid would be responsible for the larger enhancement of the Raman bands if an electrostatic interaction between the Cl− and the NH+ of the amide group takes place. In the case of SC nanoparticles, the electrostatic interaction would happen between the citrate ions and the NH+ . This interaction is weaker than the Cl− -NH+ , so the SERS enhancement obtained would be weaker as well. 3.2 SERS spectra of carminic acid on different Ag nanoparticles SERS spectra of CA at pH 5 (on both Ag colloids, Figs. 4a, b) and 7 (on CT nanoparticles, Fig. 4d)

31

1422

1322 1290

1601

900

1047

1160

1500

1000

343

833

(a) SH pH 6.5 1012

1185

1287

1324

1452 1627 1557

SERS intensity

(b) SC pH 6.5

500

Wavenumber / cm−1 Figure 5. SERS spectra of AZ (10−4 M) on SH and SC nanoparticles at pH 6.5. Excitation at 514.5 nm.

Figure 6. Comparison of the SERS spectra on SH nanoparticles of 10−4 M solution of CA and LA at various pH. Spectrum (b) was obtained using 1/2 of the laser power of the other ones.

were too intense. As a result, laser power had to be reduced to 1/4 and 1/2 of the value normally used (∼1 mW), respectively. For this reason, the equivalent global intensity of those spectra is approximately 4 and 2 times larger than shown. The study to determine the best type of Ag nanoparticles to analyze CA in low concentration (10−4 M) by SERS is shown in Figure 4. The 514.5 nm excitation line was used. The intensity of the Raman bands slightly decreases when going from acidic (Figs. 4a, b) to neutral pH (Figs. 4c, d), while the SERS enhancement remains practically unchanged when going to an alkaline pH (Figs. 4e, f ). A high fluorescence background is seen in Figure 4, even at neutral pH. This shows that CA is weakly adsorbed on the Ag surface. In contrast to LA, the SERS spectra of CA show that the type of Ag nanoparticles used in the measurements is not especially significant in the detection of CA, as it interacts very weakly with the SERS substrate.

3.3

were obtained at acidic (Figs. 5a, b), neutral (Figs. 5c, d) and alkaline (Figs. 5e, f) pH on the SH colloid. The SERS spectra of LA are more intense than those of the CA in the pH range studied, which shows that the interaction of LA with the Ag surface is stronger. This is due to the existence of an electrostatic interaction between the Cl− ions that surrounds the SH surface and the NH of the amide group. Such an interaction is much stronger than the adsorption that occurs between CA and the silver nanoparticles. Consequently, if a mixture of LA and CA is found in a real sample, only LA features will be observed in the SERS spectra. Furthermore, there are some interesting differences between the SERS spectra pattern of LA and CA. The band at 1344 cm−1 shows a large increase in the LA spectrum at pH 7 and 11; yet is not present in the CA spectrum; the band at 1454 cm−1 decreases in the LA spectrum less than in the CA spectrum at pH 11; and finally, the three bands at 1010, 1058 and 1100 cm−1 are unique characteristics of LA.These features permit an easy differentiation of CA and LA by SERS, despite their similar chemical structures (Fig. 1). Figure 6 shows the SERS spectra of AZ obtained in macro conditions. Because the spectra of the other

Comparison of SERS spectra of laccaic acid with other anthraquinone dyes

Figure 5 shows a comparison of the SERS spectra of various 10−4 M aqueous solutions of LA and CA employing the 514.5 nm excitation line. The spectra

32

acknowledged. We are also indebted to the National Institute of Justice (Department of Justice Award #2006-DN-BX-K034) and the City University Collaborative Incentive program (#80209).

anthraquinone dyes were measured in micro conditions, these spectra of AZ are presented in a separate graph. As it was seen with CA, no significant differences appear between the enhancement produced by SC and SH nanoparticles. This fact could be related to the type of interaction that takes place between these dyes and the surface. Thus, as the enhancement of the LA bands produced by the SH colloid is much higher than that gathered from SC, the electrostatic interaction proposed for LA can be supported. However, the different SERS profiles obtained show that the interaction of AZ with the Ag surface is much stronger than the CA one. 4

REFERENCES Cañamares, M. V., Garcia-Ramos, J. V., Domingo, C. & Sanchez-Cortes, S. 2004. Journal of Raman Spectroscopy 35: 921–927. Cañamares, M. V., Garcia-Ramos, J. V., Gomez-Varga, J. D., Domingo, C. & Sanchez-Cortes S. 2005. Langmuir 21: 8546–8553. Cañamares, M. V., Garcia-Ramos, J. V., Domingo, C. & Sanchez-Cortes, S. 2006. Vibrational Spectroscopy 40: 161–167. Chen, K., Leona, M., Vo-Dinh, K. C., Yan, F., Wabuyele, M. B. & Vo-Dinh, T. 2006a. Journal of Raman Spectroscopy 37: 520–527. Chen, K., Vo-Dinh, K. C., Yan, F., Wabuyele, M. B. & Vo-Dinh, T. 2006b. Analytica Chimica Acta 569: 234–237. Creighton, J. A. 1988. Selection Rules for Surface-Enhanced Raman Spectroscopy. In R. J. H. Clark (ed.), Spectroscopy of Surfaces. Chichester: Wiley. Hofenk de Graaff, J. H. 2004. The Colorful Past. Origins, Chemistry and Identification of Natural Dyestuffs. Riggisberg, London: Abegg-Stiftung and Archetype Publications. Moskovits, M. 1985. Review of Modern Physics 57: 783–826. Munro, C. H., Smith, W. E., Garner, M., Clarkson, J. & White, P. C. 1995 Langmuir 11: 3712–3720. Sanchez-Cortes, S., Garcia-Ramos, J. V. & Morcillo, G. 1994. Journal of Colloid and Interface Science 167: 428–436. Sanchez-Cortes, S, Garcia-Ramos, J. V., Morcillo, G. & Tinti, A. 1995. Journal of Colloid and Interface Science 175: 358–368. Scheweppe, H. & Roosen-Runge, H. 1986. Carmine. In R.L. Feller (ed.), Artists’ Pigments. A Handbook of their History and Characteristics, vol 1: 255–283. Cambridge: Cambridge University Press. Scheweppe, H. & Winter, J. 1998. Madder and Alizarin. In E.W. FitzHugh (ed.), Artists’ Pigments. A Handbook of their History and Characteristics, vol 3: 109–142. Oxford: Oxford University Press. Shadi, I. T., Chowdhry, B. Z., Snowden, M. J. & Withnall, R. 2004. Journal of Raman Spectroscopy 35: 800–807. Socrates, G. 2001. Infrared and Raman Characteristic Group Frequencies:Tables and Charts. Chichester: John Wiley & Sons, Ltd. Whitney, A. V., Van Duyne, R. P. & Casadio, F. 2006. Journal of Raman Spectroscopy 37: 993–1002.

CONCLUSIONS

A Surface-Enhanced Raman Scattering spectroscopy (SERS) study of lac dye was carried out using Ag nanoparticles. The surface enhancement of lac dye is strongly dependent on the pH and the nature of the nanoparticles used. When exciting at 514.5 nm, SERS spectra obtained at a pH above 5 show a decrease in the SERS enhancement. This is due to the decrease of the resonance Raman effect and the increase of the electrostatic repulsion between LA and the nanoparticles. Upon transitioning from acid to alkaline pH, successive deprotonation processes occur on the molecule and lead to changes in the charge distribution. However, when the SH colloid is employed, the enhancement of the SERS spectra of LA is higher overall than with SC nanoparticles. This is true to all pH values studied. Thus, the SH colloid is better than the SC one for detecting low concentration solutions of the dye. Lac dye interacts with Ag nanoparticles more strongly than carminic acid in the pH 3–11 interval. As a result, only laccaic acid will be detected when using SERS spectroscopy if a mixture of both red dyes is found in a textile. ACKNOWLEDGEMENTS Grants from the NSF (DMR-0526926), the Andrew W. Mellon Foundation and the David H. Koch Family Foundation in support of scientific research at The Metropolitan Museum of Art are gratefully

33

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Potential of THz-Time Domain Spectroscopy in object inspection for restoration M. Panzner, Th. Grosse, S. Liese, U. Klotzbach & E. Beyer Fraunhofer Institute Material and Beam Technology, Dresden, Germany

M. Theuer Fraunhofer Institute for Physical Measurement Techniques, Kaiserslautern, Germany

W. Köhler Labor Köhler, Potsdam, Germany

H. Leitner Hochschule für bildende Künste, Dresden, Germany

ABSTRACT: Teraherz-Time Domain Spectroscopy enables a parallel determination of tomographic and spectroscopic information. The electromagnetic pulses used for this method have pulse durations of approximately one picosecond. Thus, runtime measurements can be used to investigate the layered structure of materials with accuracy higher than geo-radar methods. While passing the material, the pulse shape changes because of dispersion. Rotational and vibrational resonances of polar molecular groups appearing as peaks in the Fourier spectrum serve as a useful means of identification of substances, as it was demonstrated with Lindane and Pentachlorophenol. THz-images can be made by scanning the object step by step as well. The method is tested on an oil painting. E(t) – and E(ω) – movies were calculated from the data at each point of the measuring grid. This method allows hidden paintings to be revealed. 1

INTRODUCTION

1.1 Teraherz-Technology In the last years, Teraherz (THz)-technology has developed to become a powerful tool for non-destructive material investigation (Bäumer 2002, Hoffmann et al. 2005, Mansner et al. 2007). Tomographic information as well as spectroscopic information can be deduced from experimental results of THz-Time Domain Spectroscopy (THz-TDS) by the runtime and spectrum of electromagnetic picosecond THz pulses. Such pulses can be generated by high acceleration of free electrons in a semiconductor. The free electrons are produced by photoelectric effect due to femtosecond laser irradiation. A voltage of about 100 V applied to electrodes on the semiconductor with a small distance in the micron range generates a strong electric field in the case of an Auston Switch emitter (Dorney et al. 2000, Shur 2005). For the acceleration of electrons, internal fields at the semiconductor surface (photoDember effect) can be used as well. Figure 1 shows a scheme of an experimental setup for THz-TDS. The spectrum of the picosecond THz-pulse (Fig. 2a) can be calculated by Fourier transformation (Fig. 2b).

Figure 1. Experimental setup used for THz-Time Domain Spectroscopy.

The width of the spectra is in the range of 3 to 10 THz, depending on the applied emitters and detectors. Figure 3 shows the comparison of a detected picosecond THz pulse with and without sample in

35

the beam line. Three characteristic differences can be seen: – delay of the sample pulse because of different signal velocities (real part of the material refraction index); – decay of the peak amplitude because of losses (imaginary part of the material refraction index); – changes in the pulse shape (broadening of the pulse, modulations) caused by the dispersion within the material and resonances of polar molecular groups. A wide variety of materials has been investigated by means of THz radiation (Roskos et al. 2004, Gorenflo et al. 2005, Smith 2005). The work in this field is fostered by promising applications in industry and specific problems of safety engineering. THz radiation is transmitted without important attenuation by many non-polar substances. This allows investigation of objects covered by such materials. Our work is focused on the utilization of these potentials for the investigation of artworks. 1.2 Detection of fungicides and insecticides In the past many objects of wooden and textile art were contaminated by such substances to avoid the invasion by different pests. Today the poisonousness of these substances is a well known issue and the objects have to be decontaminated. To apply a suitable technique, the type of substance has to be determined. For this identification, non-destructive methods are needed. Because of the low photon energy and the low power of the THzradiation used for THz-TDS, this method could solve the identification problem. This contribution is presenting the first THz-TDS measurements on Lindane and Pentachlorophenol (PCP).

Figure 2. (a) Electrical field of a THz picosecond pulse as a function of time. (b) Fourier transformation of the same electrical field.

1.3 Visualization of paintings by THz radiation Paintings and murals are valuable genuine pieces of the cultural heritage of mankind and efforts are devoted to its conservation (Möhlenkamp 2002, Pursche 2002). Up to now, revealing the images hidden below without damaging the likewise valuable top layers has been successfully done only in exceptional cases (Stewart 1991, Leitner 1994). Infrared light and X-rays are usually applied for this purpose (Humphries 2001, Mairinger 2003). The success of these techniques is restricted by the inherent limits to penetration and depth resolution. The use of THz radiation could solve this problem because of its interesting properties. As a first step, the visualization of a painting by THz-TDS in the transmission mode is presented. 2

EXPERIMENTAL

The experiments were carried out with the setup shown in the scheme of Figure 1. The picosecond THzpulses were generated by an In As-semiconductor

Figure 3. THz-pulses, before and after penetrating a sample of Lindane. E is the decay of the electrical field peak and t the delay of the pulse through the sample.

36

(photo-Dember effect) that was fired by a Ti: Sapphire laser “MaiTai” from Spectra Physics (pulse duration: 100 fs, repetition rate: 80 MHz, average power: 1.5 W). The THz-pulses were detected by an Auston Switch. The THz-beam was focused on the samples with metal mirrors to a diameter of about 1.5 mm. The fungicide Pentachlorophenol (PCP) – C6 Cl5 OH – and the insecticide Lindane – C6 H6 Cl6 – were available as powders in plastic bags. These bags were placed between two Teflon plates to homogenise the sample surface and to compress the powder. The sample used for the scanning experiments was a piece of an oil painting on canvas. During the measurement this segment (10 × 10 cm2 ) was fixed to a frame. The sample was systematically rastered through the focus of the THz beam in steps of 2.2 mm in X and Z direction. At each point, an E(t)-curve was measured by the described equipment and stored in a single file. So far, 2116 files containing 12801 data pairs each had to be considered for the calculation of graphics. 3

Figure 4. Structural formulae of the insecticide Lindane (left) and the fungicide Pentachlorophenol (right).

RESULTS

3.1

Spectroscopic investigation of Lindane and Pentachlorophenol

The molecules of both Lindane and Pentachlorophenol show aromatic structures (Fig. 4). The THz spectra were calculated by Fourier transformation of the E(t) data. The E(ω) curves of Lindane and Pentachlorophenol exhibit different minima caused by molecular resonances (Figs. 5, 6). This allows a fingerprint-like identification of these substances. Additionally, the spectra of the reference pulse (without sample) and that with the empty sample container are shown. 3.2

Figure 5. THz-spectrum of Lindane.

Investigation of paintings

To visualize the image of the painting, the E(t)-curves of each measured spot were used to calculate pictures containing the THz information. A complex Matlab routine was programmed to generate images of: – – – –

Figure 6. THz-spectrum of Pentachlorophenol.

the value of the electrical field in the maximum; the value of the electrical field in the minimum; the difference of maximum and minimum; the pulse delay with regard to the reference pulse.

at certain intervals of time and frequency. Figure 7 shows such characteristic frames of movie sequences for the E(t)- and the E(ω)-mode. The painting is only faintly reproduced by the THz pictures. The blurring is due to variations of thickness, complex refraction index, modulation by resonances, etc, on the THz transmission signals from the layer stack of oil painting. This includes effects of the surface and regions below the surface as well as the varying characteristics of the canvas. For solving the related problems, further investigations on samples with well defined materials

Additionally the program allows: – the calculation of an E(t)-movie using all values of E(t) at the measuring grid (x, y) as a function of time and, – the generation of an E(ω)-movie by Fourier transformation of E(t) at each point (x,y) of the scan. Particularly, by the movie procedure, image information could be visualized through structures seen

37

spot distance. The homogeneity as well as the resolution of the pictures could be improved by a finer measuring grid, which is, however, limited by the large focal diameter of 1–2 mm. A reduction of the focal diameter is limited by the rather long wavelength of THz radiation.

4

CONCLUSIONS

The THz-TDS measurements of the fungicide PCP and the insecticide Lindane demonstrate the existence of characteristic resonances within the THz spectral range. This allows a fingerprint-like identification of these substances. The present preliminary tests of the technique were done by penetrating 1–2 mm through the substance. For identification of biocides on contaminated art objects, experience has to be gathered with measurements on samples with lower concentration. The sensitivity of this method will be determined in forthcoming stages of our research. As an advantage of the THz-TDS technique, textile and thin wooden objects can transmit THz radiation without deterioration, which enables information to be obtained from inside without any damage. One of the next challenges of our work is the detection of different substances in the reflection mode for the case of non-THz-transparent objects like thicker wooden objects. The THz images of the oil painting represent partial information of the real image. This is promising to extend this method to reflective measurements. Hence, our ongoing investigations are focused on scanning of paintings with THz-TDS in the reflective mode. In case of success, “visualization of hidden wall paintings” could become real. A further advantage of the reflection method results from possible runtime measurements of THz-pulses. Thus, tomographic information, e.g. layer thickness, could also be deduced from these investigations. The spectroscopic analysis of the E(ω)-movie offers the possibility to display the distribution of substances. To get the frame showing the distribution of a certain substance, one has to stop the movie at the minimum of the spectrum which is related to the substance. This THz analysis method is not restricted to the investigation of paintings. Other artworks can be investigated even if they are hidden behind THz-transparent materials.

Figure 7. Selected frames of the E(t)-movie (top) and the E(ω)-movie (bottom) of the painting segment in the middle.

ACKNOWLEDGEMENTS as well as investigations on the individual materials and substances of paintings must be carried out. The images shown in Figure 7 are characterized by comparably large pixels due to the large measuring

We gratefully acknowledge Prof. Unger (RathgenForschungslabor) for helpful discussions and sample supply.

38

REFERENCES

Möhlenkamp, A., Kuder, U. & Albrecht, U. 2002. Geschichte in Schichten. Wand und Deckenmalerei im städtischen Wohnbau des Mittelalters und der frühen Neuzeit Int. Symposium 200 in Lübeck, Lübeck: Möhlenkamp, A., Kuder, U. & Albrecht, U. Pursche, J. 2002. Freilegen oder Verdecken? Erfahrungen aus Jahrzehnten. Geschichte in Schichten. Wand und Deckenmalerei im städtischen Wohnbau des Mittelalters und der frühen Neuzeit, Int. Symposium 200 in Lübeck: 204–219. Lübeck: S. Möhlenkamp, A., Kuder, U. & Albrecht, U. Roskos, H. & Löffler, T. 2004. Kurze Wellen, lange Wellen, Terawellen. Forschung aktuell (3–4): 45–48. Shur, M. 2005.Terahertz technology: devices and applications. Proceedings of ESSCIRC: 13–21. Smith, P. 2005. Pharmaceutical Analysis using Terahertz Spectroscopy. Innovation in Pharmaceutical Technology: 73–76. Stewart, S. 1991. The uncovering of wall paintings: Ethics and methods. Unpublished Diploma Thesis. London: Courtauld Institute of Art.

Bäumer, K. Terahertz durch dick und dünn – bei anderem Licht besehen. EMVU und Technik: 7–10. Dorney, T. et al. 2000. Imaging with Thz pulses: 763–767. Rice University, Houston. Gorenflo, S. et al. 2005. Terahertz – Time – Domain – Spektroskopie mit einem leistungsoptimierten elektrooptischen Detektionsverfahren. Technisches Messen 72: 435–437. Hoffmann, S. & Hofmann, M. 2005. Terahertz – Strahlung entgeht nichts. Rubin 1: 42–48. Humphries, H. 2001. Infrared and Thermal Testing for Conservation of Fine Art. Infrared and thermal testing. ASNT Non-destructive Testing Handbook 3 (3). Leitner, H. 1994. Die Freilegung der Landkartengalerie der erzbischöflichen Residenz in Salzburg, unveröffentlichter Restaurierbericht. Mairinger, F. 2003. Strahlenuntersuchung an Kunstwerken, Leipzig. Manser,A. & Battaglia, C. 2007. Prozessnah und berührungslos. MQ Management und Qualität 06: 37–39.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Femtosecond laser cleaning of paintings: Modifications of tempera paints by femtosecond laser irradiation S. Gaspard, M. Oujja & M. Castillejo Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

P. Moreno, C. Méndez & A. García Servicio Láser, Universidad de Salamanca, Salamanca, Spain

C. Domingo Instituto de Estructura de la Materia, CSIC, Madrid, Spain

ABSTRACT: Laser cleaning of paintings aims at removing oxidized varnish layers or overpaints. Removal of these coatings has to be performed without degradation of the light-sensitive paint layers. Femtosecond (fs) laser irradiation is being explored as a cleaning technique for different types of substrates due to the reduced thermal load as compared with nanosecond (ns) irradiation. In this work we present a study of the modifications induced by fs Ti:Sapphire laser pulses (795 nm, 120 fs, 1 KHz) on model unpigmented (egg yolk based binder) and pigmented tempera samples (azurite and zinc white). Irradiation with fluences below and above the ablation threshold, leads to various degrees of discoloration. Several analytical techniques were used to characterize the chemical and physical effects induced by fs laser irradiation including colorimetry, optical microscopy, laser induced fluorescence (LIF) at 266 nm and FT Raman at the laser excitation of 1064 nm.

1

INTRODUCTION

fluence at the specified wavelength (Chappé et al. 2003, Andreotti et al. 2007). In the last years, material processing with femtosecond (fs) laser pulses has attracted increasing attention. Compared to ns pulses, the main advantages of fs pulses are the reduction of the thermal diffusion and consequently the thermal degradation of the target, the reduction of shielding induced by the plasma plume and the improvement of the morphology of the ablated surface. In particular, fs laser cleaning can be advantageous for the treatment of light-sensitive substrates as artistic paintings (Fotakis et al. 2007 & Pouli et al. 2007). The study presented in this work aims to explore possible advantages that could offer the use of fs pulses for the cleaning of egg yolk based tempera paints by examining the physical and chemical effects taking place upon direct exposure of unvarnished samples to laser irradiation. Unvarnished aged model temperas of unpigmented, azurite and zinc white paints were irradiated by a Ti:Sapphire laser at 795 nm with pulses of 120 fs. We determined ablation thresholds of the systems and the effects induced by laser irradiation; physical and chemical modifications, were assessed by colorimetry, optical microscopy, laser induced fluorescence and Fourier transform FT-Raman. The results

Laser cleaning of paintings and polychromes was introduced as a novel conservation method few years ago (Teule et al. 2002, Castillejo et al. 2002, 2003, Gordon Sobott et al. 2003, Bordalo et al. 2006, Nevin et al. 2007). Different studies have been carried out on the laser removal of degraded varnishes or contamination layers from paintings and polychromes. These studies used pulses of nanosecond (ns) duration delivered by UV excimer (Georgiou et al. 1998), Q-switched Nd:YAG (Hildenhagen et al. 2003) and Erbium YAG lasers (Bracco et al. 2003). Due to the sensitivity to light of the components of pictorial artworks, careful studies are required to characterize the effects of laser irradiation on pigments, binders or varnishes. Investigation is necessary to identify the possible alterations induced by laser and several multianalytical investigations have been performed (Fotakis et al. 2005) to understand the mechanisms operating in the interaction with laser pulses, responsible of discoloration and degradation (Zafiropoulos et al. 2003, Pouli et al. 2003). In all, previous studies on the laser cleaning of paintings have highlighted the importance of the optimization of laser parameters, specifically pulse duration and

41

obtained are compared with those of a previous study performed with a KrF excimer laser at 248 nm, 25 ns pulses (Castillejo et al. 2002). 2 2.1

EXPERIMENTAL Samples

The samples of tempera paint on wood panels consist in a single paint layer of around 100 µm thickness applied on primed panels by using a stopping knife, (Castillejo et al. 2002). The pigments were previously mixed with egg yolk using a spatula and a glass plate. The samples were naturally aged for a period of four years in the dark. In this work, we present the results obtained in azurite (basic copper carbonate, 2CuCO3 · Cu(OH)2 ) and zinc white (zinc oxide, ZnO). Samples of unpigmented paint (egg yolk) were used as reference to study the modifications of the binding medium upon laser irradiation. 2.2

Figure 1. Pictures of tempera samples and schemes of the different irradiated zones. Conditions of irradiation are given in Table 1: a) unpigmented, b) zinc white and c) azurite. The irradiated areas are of 1 cm2 .

Laser treatment

Laser ablation was carried out in air using a commercial Ti:Sapphire oscillator (Tsunami, Spectra Physics) and a regenerative amplifier system (Spitfire, Spectra Physics) based on the chirped pulse amplification technique. The system produces linearly polarized 120 fs pulses at 795 nm with a repetition rate of 1 kHz. The pulse energy can reach a maximum of 1.1 mJ which is controlled by means of neutral density filters and measured with a powermeter. The transversal mode is gaussian TEM00. The beam was focused perpendicularly onto the target surface which was placed on a motorized XYZ translation stage. The pulses were focused on the surface by a cylindrical lens with focal length of 75 mm providing a spot size 6 × 9000 µm2 (1/e2 criterion). Homogeneous irradiation of the sample within a 1 cm2 , was achieved by using a squared mask placed on the surface of the sample and by scanning the sample along the direction of the smallest spot dimension of the beam with a scanning speed of 780 µm/s. At this speed, the pulses overlapped at an intensity of 87% of the maximum. After processing the whole square length, the sample was moved in the transverse direction by steps of 1500 µm, 2000 µm or 3000 µm, resulting in an overlap of 94.6, 90.6 and 82.6%. Motion in the Z axis helped to accurately focus the laser beam on the material surface. After determination of the ablation threshold fluences (Fth ) for each system (see 3.1), different irradiated zones were prepared with fluences below and above the threshold (Fig. 1, Table 1). For the unpigmented paint, seven zones were irradiated with fluences between 0.30 J/cm2 (F/Fth = 0.31) and 0.80 J/cm2 (F/Fth = 0.82). The azurite sample was irradiated with fluences between 0.28 J/cm2

Table 1.

Irradiation conditions of tempera paints.

Zones∗ Unpigmented tempera 1 (1500), 2 (2000) 3 (1500), 4 (2000) 5 (1500), 6 (2000) 7 (1500) Azurite tempera 1 (1500) 2 (1500) 3 (1500), 4 (3000), 5 (2000) 6 (1500), 7 (2000) 8 (1500), 9 (2000) 10 (1500), 11 (2000) Zinc white tempera 1 (1500), 2 (2000) 3 (1500), 4 (2000)

Fluence J/cm2

F/Fth

0.80 0.60 0.40 0.30

0.82 0.62 0.41 0.31

1.26 0.99 0.90 0.56 0.37 0.28

2.38 1.87 1.70 1.06 0.70 0.53

0.65 0.53

1.67 1.00

∗ The displacement given in µm, is reported in parentheses (see text).

(F/Fth = 0.53) and 1.26 J/cm2 (F/Fth = 2.38). Finally, different areas were irradiated in zinc white tempera with fluences of 0.53 J/cm2 (F/Fth = 1.00) and 0.65 J/cm2 (F/Fth = 1.67). 2.3 Analytical techniques To record the reflectance spectra and characterize the chromatic properties and changes induced by laser irradiation, we used a Minolta CM-2500d

42

Table 2. Ablation thresholds and incubation factors. Ablation thresholds J/cm2

Unpigmented Azurite Zinc White

1 pulse

5 pulses

10 pulses

100 pulses

Incubation factor

1.5 ± 0.2 0.8 ± 0.1 0.57 ± 0.09

1.1 ± 0.2 0.58 ± 0.09 0.42 ± 0.06

0.90 ± 0.08 0.50 ± 0.04 0.37 ± 0.03

0.54 ± 0.04 0.31 ± 0.02 0.24 ± 0.02

0.78 ± 0.01 0.79 ± 0.02 0.81 ± 0.02

by an aperture. The diffraction pattern (Airy disk and rings) is focused on the target surface. The material is damaged where the laser fluence is above the threshold value. The diameters of the craters were determined by optical microscopy (ZeissAxio Imager Z1m) and scanning electron microscopy (Zeiss DSM940) (Moreno et al. 2006). Thresholds measured for 1, 5, 10 and 100 pulses and incubation factors ξ are reported in Table 2. The incubation factor is determined by Fth (N) = Fth (1) × N(ξ−1) , with Fth (1) and Fth (N) being the threshold fluences for 1 and N pulses respectively. It is important to underline that we clearly observed discoloration in the irradiated zones of the unpigmented sample upon irradiation with fluences below the ablation threshold. Thresholds of unpigmented system are higher than those of pigmented paints. In fact, in presence of pigment, the effective multiphoton absorption of the paints increases. The threshold drops for a higher number of pulses per spot. This can be correlated with incubation phenomena, however the high incubation factors calculated, similar in all systems studied (∼0.8), indicate a weak incubation effect and therefore a relative chemical stability of the systems upon multiple pulse irradiation (Krüger & Kautek 2004). As during the processing, a scanning speed of 780 µm/s was used and the pulses overlapped at an intensity of 87% of its maximum, 7.6 pulses participate in the formation of one crater. Therefore, ablation thresholds used therein are calculated by interpolation for this number of pulses.

portable spectrophotometer. The parameters were a standard illuminant D65 (average daylight) and a 10◦ standard observer, on an observation area of 1 cm diameter. Three spectra were recorded in each irradiated zone, and averaged to obtain one data point. Changes in the reflectance spectra were determined with the CIEL∗ a∗ b∗ colorimetric procedure. L∗ indicates lightness and a∗ and b∗ are the chromaticity coordinates. Colour changes are given in a three dimensions space (L∗ : + lighter, − darker; a∗ : + redder, − greener; b∗ : + yellower, − bluer). The magnitude of the colour change is given by E ∗ = [(L∗ )2 + (a∗ )2 + (b∗ )2 ]1/2 . Laser induced fluorescence (LIF) measurements of the virgin and irradiated areas were carried out using laser excitation at 266 nm (Q-switched Nd:YAG laser, 4th harmonic, 6 ns pulse) and a 0.30 m spectrograph with a 300 lines/mm grating (TMc300 Bentham)intensified charged coupled detector (2151 Andor Technologies) system. The temporal gate was operated at zero time delay and at a temporal width of 3 µs. The sample was illuminated at an incidence angle of 45◦ at a laser energy of about 0.1 mJ/pulse. For the results presented here, a 300 nm cutoff filter was installed in front of the spectrograph. Each spectrum resulted from the average of 20 measurements in 5 different points of each irradiated zone. FT-Raman spectra were recorded with an RFS 100/S-G Bruker spectrometer. The excitation source consisted of a Nd:YAG laser emitting at 1064 nm. Low laser power outputs, in the range of 10–20 mW, were used. Only 1/3 of this power illuminated the sample surface, preventing damage or laser-induced degradation of the samples during measurements. The light scattered from a surface of 0.01 cm2 was collected in backscattering (or 180◦ ) geometry. The wavenumber resolution was 8 cm−1 . Each data point was the result of the accumulation of 200 scans. 3

3.2 Colorimetric measurements Colorimetric measurements were performed on virgin and irradiated areas of the samples to characterize the discoloration resulting from laser irradiation of the tempera paint. Virgin zones of the samples were characterized in the CIEL∗ a∗ b∗ colorimetric space and the coordinates are reported in Table 3. In Figure 2, values of E ∗ are represented as a function of the irradiation conditions (F/Fth ). We previously reported (Gaspard et al., in press) an important degree of discoloration of the unpigmented sample upon irradiation with fluences below the ablation threshold (E ∗ = 46 at a fluence of 0.80 J/cm2 ).

RESULTS AND DISCUSSION

3.1 Ablation thresholds Ablation thresholds, for irradiation with fs pulses, were calculated using the method described by Dumitru et al. (2002) based on the diffraction of a laser beam

43

Table 3. CIEL∗ a∗ b∗ colorimetric coordinates of the virgin zones of the samples. Tempera paint

L∗

a∗

b∗

Unpigmented Azurite Zinc white

76.3 31.4 89.5

9.8 −8.6 −0.5

55.6 1.1 15.9 Figure 3. Optical microscopy pictures of the unpigmented tempera surface: (a) non irradiated zone and (b) irradiated zone at 0.80 J/cm2 .

Figure 2. Values of E∗ as a function of the irradiation conditions for the three tempera paints. Values correspond to zones irradiated with a displacement of 1500 µm.

Figure 4. LIF spectra at the excitation wavelength of 266 nm of virgin and irradiated zones of azurite tempera paint, with fluences of 0.28 and 0.90 J/cm2 (zones 10 and 3 respectively, see Table 1).

The main colour shift observed was due to changes in b∗ (−22, shift to bluer) and L∗ (−15, shift to darker). Furthermore, the observation of the irradiated zones of the unpigmented tempera by optical microscopy reveals the formation of bubbles below the ablated surface. (Fig. 3) Interaction of the laser radiation with the azurite system results in two different behaviours. Below the ablation threshold (F/Fth < 1), the colour of the sample was not altered. Above the ablation threshold, irradiation results in discoloration and the pigment acquires a white colour. A maximum E ∗ value of 9.6 at 1.26 J/cm2 is observed with L∗ (+ 8.2, shift to lighter) being the main factor of discoloration. Zinc white sample reacts differently, as this system remain practically unaltered upon laser irradiation with fluences above the threshold value. A maximum value of E ∗ = 1.8 at 0.65 J/cm2 was observed (Figs. 1–2). 3.3

the aromatic amino acids of the proteins tyrosine and tryptophan at 333 nm, the phospholipids in the 520– 570 range and derived crosslinked products of egg yolk (Wisniewski et al. 2007, Gaspard et al. 2008a). These products of photo-oxidation, combination and modification of amino acids, such as dityrosine, 3,4 dihydroxyphenylalanine (DOPA) or N-formylkynurenine (NFK) and kynurenine display fluorescence in the 400–500 nm region (Nevin et al. 2006). Other products of cross linking reactions between amino acids and sugar or lipids that are present in egg yolk can also contribute to the broad emission in the 400–650 nm region. LIF spectra of azurite paint are similar to those of the binder. They consist of two broad bands centred at 330 nm and at 470 nm with a shoulder at 445 nm (Fig. 4). In this case, the emission is narrower than the one of the binder and the emission from phospholipids is not observed. LIF spectra of zinc white tempera consist of a very intense and narrow band at 385 nm, attributed to the luminescence of semiconductor ZnO and a very broad band in the 400–500 nm region attributed to photodegradation products of the binder (Fig. 5).

Laser Induced Fluorescence

LIF spectra were recorded on virgin and irradiated zones of the samples upon excitation at 266 nm. We previously reported LIF analysis on the unpigmented sample (Gaspard et al. in press). At 266 nm, the spectrum of the virgin zone consists of two broad bands centred at 333 nm and 520 nm with a shoulder at 450 nm. The emissions observed have their origin in

44

has been adequately subtracted. For pigmented samples, spectra of the virgin zones show bands of both pigment and binder and agree with the previously reported spectra for azurite and zinc white (Burgio et al. 2001). We previously reported the most relevant and characteristic bands of the FT Raman spectra of the unpigmented tempera (Gaspard et al. in press). In particular, we observed the C-H stretching region from 2700 to 3100 cm−1 , the C = O stretching at 1741 cm−1 , the Amide I and Amide III bands of the proteins backbone at 1653 and 1263 cm−1 respectively and the methylene groups of lipids at 1445 and 1302 cm−1 . From amino acids, only the phenylalanine band at 1003 cm−1 could be clearly identified. After irradiation, the binder bands showed no appreciable changes in the 1700–500 cm−1 region. New bands indicating a change in the chemical composition of the azurite pigment appear around 143 and 640 cm−1 as a result of laser irradiation. These two bands can be assigned to the reddish semiconductor cuprite Cu2 O (Castillejo et al. 2002). A very low shift to red (b∗ = 3 at 1.26 J/cm2 ) is observed by colorimetric measurements in the irradiated zone 1 of the sample. No changes were observed in the spectra of irradiated zones of zinc white paint. After irradiation, all spectra show an increase of the intensity in the C-H stretching region (not shown). This change related to the increase of CH3 groups is attributed to the degradation of lipids and proteins, also observed by LIF.

Figure 5. Normalized LIF spectra at the excitation wavelength of 266 nm of virgin and irradiated zones of zinc white tempera paint, with fluences of 0.53 and 0.65 J/cm2 (zones 1 and 3 respectively, see Table 1).

4

The modifications induced by 120 fs laser pulses at 795 nm were examined in unpigmented, azurite and zinc white tempera paints. Irradiation results in various degrees of discoloration and chemical changes as monitored by LIF and FT Raman. Results presented above can be discussed in relation with previous studies by some of us on KrF excimer laser irradiation (248 nm, 25 ns pulses) of similar tempera paint samples (Castillejo et al. 2002, 2003). Regarding colour changes, the degree of discoloration of the unpigmented tempera is significant below the ablation threshold while azurite tempera colour remains stable below the threshold and shows some shift to white above this value. In comparison with measurements under ns KrF laser treatment at equivalent irradiation fluences, a higher degree of discoloration (higher values of E ∗ ) is observed under fs irradiation. Nevertheless, in the presence of a pigment, the inverse phenomenon is observed. Zinc white tempera remains unaltered in the explored fluence range whereas the paint turned to black at 248 nm, 25 ns irradiation. LIF and Raman measurements allow the discussion of the chemical alterations induced in the binder and

Figure 6. FT Raman spectra of the azurite tempera paint sample in virgin and irradiated zone 1 (see Table 1) at 1.26 J/cm2 . Azurite bands are indicated by vertical bars.

LIF spectra recorded in the irradiated areas of the unpigmented and azurite tempera paints revealed an overall decrease of the fluorescence signal. A relative increase of the shoulder band at 450 nm was observed and attributed to enhanced contribution of photodegradation products of proteins participating in the composition of egg yolk which emissions are predominant in this region. For the zinc white paint, a comparable relative increase of the broad band in the 400–500 nm region was observed upon irradiation (see normalized spectra in Fig. 5). 3.4

CONCLUSIONS

FT-Raman spectra

FT Raman spectra were recorded in virgin and irradiated zones of the temperas. Spectra of azurite sample are shown in Figure 6, once the spectrum of the panel

45

the pigments under fs laser irradiation. LIF spectral modifications observed upon irradiation indicate an enhancement of photodegradation compounds of proteins and lipids which are present in the egg yolk based binder. The effect of laser irradiation on the pigment itself is extremely dependent of the pigment composition, as illustrated in this work, in the comparison characteristic LIF and Raman results for azurite and zinc white pigments. Whitening of the azurite system, accompanied by formation of cuprite, is in contrast with unaltered colour and absence of chemical changes in zinc white paint. Excimer laser ablation thresholds for unpigmented, azurite and zinc white systems, 0.17, 0.24 and 0.34 J/cm2 respectively (Castillejo et al. 2002, 2003) are lower than those reported here for 795 nm, 120 fs irradiation (Table 2) which indicates a higher stability of tempera systems upon fs irradiation. In all cases no build-up of extra bands of amorphous carbon (indicative of carbonization or charring) takes place, in contrast with previous observations upon irradiation with 248 nm, 25 ns pulses (Castillejo et al. 2002, 2003). The differences with these previous studies illustrate the participation of mechanisms of diverse origin in the ns and fs domains. Work is in progress to characterize the mechanisms involved in the interaction of fs laser pulses with paints, aiming at establishing the advantages, related to the high degree of control over the induced modifications, offered by ultrashort laser pulses for the cleaning of paintings. ACKNOWLEDGEMENTS Funding from MEC (Project CTQ2007-60177-C0201/PPQ) is gratefully acknowledged. S.G. thanks EU for a Marie Curie contract (MESTCT-2004-513915). We thank R. Hesterman (Hesterman Restauratie Atelier voor Schilderijen, The Netherlands) for the preparation of the samples and M.I. Sanchez Rojas (Instituto Eduardo Torroja, CSIC) for the use of the spectrophotometer. We also acknowledge the support of the Red Temática de Patrimonio Histórico y Cultural, CSIC. REFERENCES Andreotti, A., Colombini, M. P., Nevin, A., Melessanaki, K., Pouli, P. & Fotakis, C. 2007. Multianalytical Study of Laser Pulse Duration Effects in the IR Laser Cleaning of Wall Paintings from the Monumental Cemetery of Pisa. Laser Chemistry Article ID 39046. Bordalo, R., Morais, P. J., Gouveia, H. & Young, C. 2006. Laser Cleaning of Easel Paintings: An Overview. Laser Chemistry Article ID 90279. Bracco, P., Lanterna, G., Matteini, M., Nakahara, K., Sartiani, O., de Cruz,A., Wolbarsht, M. L.,Adamkiewicz, E. & Colombini, M. P. 2003. Er:YAG laser: an innovative tool for controlled cleaning of old paintings: testing and evaluation. Journal of Cultural Heritage 4: 202s–208s.

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Burgio, L. & Clark, R. J. H. 2001. Library of FT-Raman spectra of pigments, minerals, pigment media and varnishes, and supplement to existing library of Raman spectra of pigments with visible excitation. SpectrochimicaActa Part A 57: 1491–1521. Castillejo, M., Martín, M., Oujja, M., Silva, D., Torres, R., Manousaki A., Zafiropulos, V., Van den Brink, O. F., Heeren, R. M. A., Teule, R., Silva, A. & Gouveia, H. 2002. Analytical study of the chemical and physical changes induced by KrF laser cleaning of tempera paints. Analytical Chemistry 74: 4662–4671. Castillejo, M., Martín, M., Oujja, M., Santamaría, J., Silva, D., Torres, R., Manousaki, A., Zafiropulo, V., Van den Brink, O. F., Heeren, R. M. A., Teule, R. & Silva, A. 2003. Evaluation of the chemical and physical changes induced by KrF laser irradiation of tempera paints. Journal of Cultural Heritage 4: 257s–263s. Chappé, M., Hildenhagen, J., Dickmann, K. & Bredol, M. 2003. Laser irradiation of medieval pigments at IR, VIS and UV wavelengths. Journal of Cultural Heritage 4: 264s–270s. Dumitru, G., Romano, V., Weber, H. P., Sentis, M. & Marine, W. 2002. Femtosecond ablation of ultrahard materials. Applied Physics A 74: 729–739. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari V. (ed.) 2005. Lasers in the preservation of Cultural Heritage, Principles and Applications. In Series in Optics and Optoelectronics. NewYork: Taylor and Francis group. Fotakis, C., Zorba, V., Stratakis, E., Athanassiou, A., Tzanetakis, P., Zergioti, I, Papagoglou, D. G, Sambani, K., Filippidis, G., Farsari, M., Pouli, P., Bounos G. & Georgiou, S. 2007. Novel Aspects of Materials Processing by Ultrafast Lasers: From Electronic to Biological and Cultural Heritage Applications. Journal of Physics: Conference Series 59: 266–272. Gaspard, S., Oujja, M., Abrusci, C., Catalina, F., Lazare, S., Desvergne, J. P. & Castillejo, M. 2008a. Laser induced foaming and chemical modifications of gelatine films. Journal of Photochemistry and Photobiology A 193: 187–192. Gaspard, S., Oujja, M., Moreno, P., García,A., Domingo, C. & Castillejo, M. 2008b. Interaction of femtosecond laser pulses with tempera paints. Applied Physics A, in press. Georgiou, S., Zafiropulos, V., Anglos, D., Balas, C., Tornari, V. & Fotakis, C. 1998. Excimer laser restoration of painted artworks: procedures, mechanisms and effects. Applied Surface Science 127–129: 738–745. Gordon Sobott, R. J., Heinze, T., Neumeister, K. & Hildenhagen, J. 2003. Laser interaction with polychromy: laboratory investigations and on-site observations. Journal of Cultural Heritage 4: 276s–286s. Hildenhagen, J. & Dickmann, K. 2003. Nd:YAG laser with wavelengths from IR to UV (ω, 2ω, 3ω, 4ω) and corresponding applications in conservation of various artworks. Journal of Cultural Heritage 4: 174s–178s. Krüger, J. & Kautek, W. 2004. Ultrashort pulse laser interaction with dielectrics and polymers. Advance polymers Science 168: 247–289. Moreno, P., Méndez, C., García, A., Arias, I. & Roso, L. 2006. Femtosecond laser ablation of carbon reinforced polymers. Applied Surface Science 252: 4110–4119. Nevin, A., Cather, S., Anglos, D. & Fotakis, C. 2006. Analysis of protein-based binding media found in paintings

using laser induced fluorescence spectroscopy. Analytical Chimica Acta 573–574: 341–346. Nevin, A., Pouli, P., Georgiou, S. & Fotakis, C. 2007. Laser conservation of art. Nature Materials 6: 320–322. Pouli, P., Emmony, D.C., Madden, C. E. & Sutherland, I. 2003. Studies towards a thorough understanding of the laserinduced discoloration mechanisms of medieval pigments. Journal of Cultural Heritage 4: 271s–275s. Pouli, P., Bounos, G., Georgiou, S. & Fotakis, C. 2007. Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward? In J. Nimmrichter, W. Kautek & M. Schreiner (eds), Proceedings of Lasers in the Conservation of Artworks, LACONA 6, Vienna 21–25 September 2005: 287–293. Berlin: Springer. Teule, J. M., Ullenius, U., Larsson, I., Hesterman, W., van den Brink, O. F., Heeren, R. M. A. & Zafiropulos, V.

2002. Controlled laser cleaning of fire-damaged paintings. ICOM Committee for Conservation. Wisniewski, M., Sionkowskaa,A., Kaczmarek, H., Lazare, S., Tokarev, V. & Belin, C. 2007. Spectroscopic study of a KrF excimer laser treated surface of the thin collagen films. Journal of Photochemistry and Photobiology A 188: 192– 199. Zafiropulos, V., Balas, C., Manousaki, A., Marakis, Y., Maravelaki-Kalaitzaki, P., Melesanaki, K., Pouli, P., Stratoudaki, T., Klein, S., Hildenhagen, J., Dickmann, K., Luk’Yanchuk, B. S., Mujat, C. & Dogariu, A. 2003. Yellowing effect and discoloration of pigments: experimental and theoretical studies. Journal of Cultural Heritage 4: 249s–256s.

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Cleaning of paint with high repetition rate laser: Scanning the laser beam A.V. Rode, D. Freeman, N.R. Madsen & K.G.H. Baldwin Research School of Physical Sciences and Engineering, the Australian National University, Canberra, Australia

A. Wain Collection Services Section, the Australian War Memorial, Canberra, Australia

O. Uteza & P. Delaporte Lasers, Plasmas and Photonic Processes, CNRS – Mediterranean University, Marseille, France

ABSTRACT: Powerful ultrafast femtosecond laser pulses have the unique ability to ablate material with minimal collateral damage. This ability offers the potential for new applications of ultrafast lasers for removing surface contamination and unwanted surface layers in the conservation of artworks and heritage objects. In this paper, we concentrate on the problem of precise and fast scanning of the laser spot over the treated surface for cleaning relatively large surface areas. Preliminary results are presented for the removal of intrusive paint layers, using a 12 ps laser with 1.5 MHz repetition rate and 0.5 ps laser with 10 kHz repetition rate.

1

INTRODUCTION surgery, which are also very sensitive to collateral damage (Feit et al. 1998, Loesel et al. 1998, Juhasz et al 2000, Rode et al. 2002). Compared to long-pulse nanosecond lasers, femtosecond lasers offer higher etching resolution, minimal collateral damage and minimal photochemical modification of the surface. These qualities have opened up new possibilities in the cleaning of sensitive and technically demanding artworks and other objects of historical and cultural importance (Pouli et al. 2005). Currently, however, the application of femtosecond lasers to the cleaning of artifacts has been limited to small surface areas, and to a limited volume of the order of few mm3 at most. This is due to the fact that the low energy of the ultrashort laser pulse requires tight focusing to achieve the required ablation threshold intensity, which is of the order 1011 W/cm2 – 1012 W/cm2 . Moreover, the lower ablated mass per pulse, as compared with conventional long-pulse lasercleaning techniques, requires a significant increase in the laser repetition rate to achieve a reasonable rate of surface treatment. A technique is therefore required to provide fast and precise scanning of the laser beam over the surface area to be treated. In this paper, we analyse the potential advantages and challenges of using ultrafast laser pulses for precise removal of surface layers, and present preliminary

Laser ablation with ultrafast laser pulses possesses a unique ability to remove thin submicron surface layers without generating heat or shock waves in the bulk of the material. This is due to the very fast delivery of electromagnetic laser energy into the surface layer, which occurs too quickly for the absorbed laser energy to be transferred into the bulk material through heat conduction or shock wave propagation (Du et al. 1994, Stuart et al. 1995, Gamaly et al. 2002, Gamaly et al. 2007). As a result, a heated surface layer is ablated while the bulk of the material remains cold. The pulse duration required to remove a surface layer in this non-equilibrium, short-pulse regime of ablation depends on the properties of the material, such as heat diffusion, thermal capacity, ionisation energy and density, as well as electronic and optical properties (Gamaly et al. 2002, Gamaly et al. 2007). In general, the pulse should be shorter than the electron-lattice coupling time and the heat diffusion time from the surface layer. This usually lies in the picosecond time domain. The thickness of the layer removed at the laser fluence just above the ablation threshold is of the order of 100 nm per pulse. The advantages of using ultrafast lasers for micromachining (precise removal) of surfaces have already been proven in applications such as dentistry and eye

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results on using ps and sub-ps lasers to remove paint for the conservation of artworks and heritage objects. 2 2.1

LASER SCANNING SYSTEM Scanning requirements

A key requirement for precise removal of surface layers of sub-micron thickness is a high degree of control over the scanning of the focused laser beam. There is a number of general requirements for scanning systems for the short-pulse high-repetition-rate laser ablation technique: – very precise control of the ablated depth at the level of 10 nm/shot requires that each laser shot be located at a fresh point on the surface; – scanning speed along the surface should be faster than the propagation of the heat wave, to avoid heat accumulation; – the beam should move with a constant speed over the target surface; – the scanning pattern should cover the surface uniformly, so that every surface point is given the same exposure; – the spot size and shape should be the same across the entire scanning pattern.

Figure 1. Scheme for a constant scanning velocity alternating spiral pattern. This is the most suitable for fast scanning of the laser beam due to the absence of sharp turning points.

placing the scanning mirrors after the focusing lens so that the difference in optical path of the deflected beam is within the lens caustic. This is, however, at expense of the tight focusing required for low-energy short pulses. 2.2 Constant velocity alternating spiral

The first and second requirements lead to a relatively high scanning speed v = dfoc × Rrep , where dfoc is the focal spot diameter, and Rrep is the laser repetition rate. For organic materials with thermal diffusivity D of the order of 10−2 cm2 /s and with 1 MHz repetition rate the speed is typically

The scanning pattern must be chosen so as to respect the limitations of each scanner; namely, the maximum acceleration (given by the available torque and the moment of inertia of the motor and mirror) and the maximum power dissipation in the motor windings (governed by the average magnitude of acceleration throughout the pattern). One should therefore avoid patterns that involve sharp corners, such as raster scanning, unless a fast beam blanker is available to allow “dead time” for a safer acceleration without uneven illumination of the target. While it is feasible to use a “polygon” scanner for the fast axis of a raster pattern, achieving even higher scanning speeds, there is still a problem at the edges of the scan lines. The beam is split between the end of one line and the start of the next, destroying the quality of the focused spot and reducing the fluence below the ablation threshold, so that a fast blanker would still be required. Blanking an ultrafast laser at high average and peak powers is not a trivial proposition, so we have used an alternative pattern for our work. A smooth and continuous spiral pattern, alternating between increasing and decreasing radius, has no corners and provides continuous scanning at a constant linear speed with nearly uniform coverage and no wasted dead-time (Figs. 1, 2). It suffers, however, from the disadvantage of a hole in the centre, which is required to limit the acceleration at small radii. The scanners were computer-controlled as follows. The path was traced at a constant linear speed vscan by

where lth is the thermal wave propagation length between two laser pulses. This scanning speed can be obtained with relative ease with an orthogonal pair of galvanometer scanners located at a suitable distance from the target. The third and forth requirements lead to an even coverage of the surface by the scanning laser spot, so that the number of laser pulses per unit area is constant. Galvanometer scanners typically consist of a mirror mounted onto an electric motor, with positional feedback supplied to the driver circuit. Under closed-loop operation, a return spring is unnecessary, since the drive electronics can accurately apply the required torque to start and stop the mirror even for a sudden jump in position. Proper tuning of the driver is essential, particularly when different mirrors are fitted. The fifth requirement, for a beam of constant size and shape, can be achieved with a telecentric scanning lens. Such a lens is designed so that the laser beam strikes normal (perpendicular) to the working surface over the entire scanning field. This can also be achieved with conventional long-focus lenses by

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Figure 2. Laser scanning patterns: Lissajous figure (left), Lissajous figure superimposed on a circle (centre), and an expanding/collapsing spiral (right). Clearly, the spiral pattern provides the most uniform coverage, although it has a hole in the centre. The images are experimental results obtained by scanning onto a paper diffuser and acquiring an image using a CCD camera with a long integration time.

Figure 3. Racetrack scanning patterns at various laser fluences from 0.09 J/cm2 to 5.2 J/cm2 (the numbers under the patterns are fluences in J/cm2 ).

generating a pair of analogue waveforms, which were sampled at discrete time intervals (10 µs). The sample points were computed at the target, and converted to deflection angles for the scanners ±40◦ using the optical distance from each scanner to the target, and then output using 16-bit digital-to-analogue converters, resulting in angular steps of 80◦ /216 = 21.3 µrad. The outward spiral curve used, expressed in polar coordinates (r, θ), is:

uneven surface, the confocal parameter (the depth of focus) should be longer than the surface profile variations. The longer the focal length of the focusing optics and the smaller the numerical aperture, the longer the confocal parameter, which converts into a larger focal spot and lower laser fluence. To compensate the loss in fluence, which must be above the threshold value, one needs to increase the energy per pulse. This can only be achieved by increasing the average power of the laser or by reducing the repetition rate. 3 ABLATION EXPERIMENTS

where p is the pitch, the radial distance between successive turns; here, the pitch is larger than the focal spot. The value of θ at each sample time was computed using a discrete integration of the angular velocity determined at the preceding sample time. The angular velocity of this curve is given by:

Two laser systems were used to perform ultrafast laser ablation of paint. The aim of the tests was to analyse the effectiveness of ultrafast laser ablation in the removal of paint from firstly an underlying paint layer, and secondly a metal surface. One of the lasers used was a Nd:YVO4 laser, designed and built at the Australian National University for applications in micro-machining and the deposition of optical thin films (Luther-Davies et al. 2004). This laser produces an average power of 50 W in 12 ps pulses at a rate of 1.5 × 106 pulses per second and was converted to the second harmonic (λ2 = 532 nm). The second laser used was a commercialYb:YAG laser from ‘Amplitude Systems’ which generates 500 fs pulses at a rate 104 pulses per second; this laser was converted to the second harmonic (λ2 = 515 nm). The experiments were conducted with a light-grey ZnO based paint. The first series of experiments aimed to determine the required speed of scanning. Laser beams were directed to a painted metal target via a telescentric lens and focused to a spot size of 20 µm (532 nm) and 35 µm (515 nm). The laser was then scanned in a constant velocity ‘moving race track’ pattern with a size of 20 mm (532 nm) and 2 mm (515 nm) (Fig. 3a). The race-track pattern was chosen as a compromise to avoid the hole in the middle of the pattern but still preserve the constant scanning velocity over the linear tracks of the pattern.

When the radius reached the maximum allowed, the spiral changed to the inward type in which the radius decreased with further increase of angle, until the desired inner radius was reached and the next outward spiral began. Since the angular frequency (and therefore scanning mirror acceleration) is greatest at the centre of the pattern, the radius of the central hole must be large enough to respect the limitations of the scanners as described above, or else the pattern would become distorted, with non-uniform speed and area coverage at the centre. To ensure the individual spiral traces were not visible in the final result, a small arc θ was included at the outermost radius to produce a suitable angular offset between successive spirals, resulting in uniform coverage after a large number of spirals had been traced. This pattern has been the most successful to date in our laser deposition experiments (Luther-Davies et al. 2005, Gamaly et al. 2007). Keeping the laser parameters constant on the surface to be treated is another challenge. To treat an

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Figure 4. Ablation of paint using 515 nm, 500 fs laser pulses with scanning speeds 1 shot/spot, 10 shot/spot and 100 shot/spot. Redeposition of debris can be clearly seen at the edges of the treated areas at the scanning speeds 10 shots/spot and 100 shots/spot.

Figure 5. An example of ablation rate measurements with 532 nm 1.5 MHz laser scanning with a speed of 10 m/s or 3 shots per spot.

It was clear from the experiments that a slow scanning speed, above 3 shots per spot, led to re-deposition of debris and decomposed vapour near the ablated area (see the dark rims around the ablated area in Fig. 4). This undesired effect with slow scanning was due to absorption of the laser radiation in the ablated vapour and possibly, due to accumulation of heat, as the effect was not observed with a fast scanning speed above 0.35 m/s corresponding to 1 shot/spot exposure with 104 Hz repetition rate laser and ∼20 m/s with 1.5 × 106 Hz repetition rate. The second series of experiments aimed at measuring the ablation threshold Fth . The ablation threshold was determined in a standard way (Stuart et al. 1995) by fitting the dependence of the ablated depth per pulse in a semi-logarithmic plot and approximating the curve to a ‘zero’ ablation thickness, which was taken to be a single atomic layer thickness of the order of 0.3 nm (Fig. 5). The ablation threshold was found to be 0.25 ± 0.1 J/cm2 (the error is due to the different samples used) for 12 ps pulses; and ∼0.10 ± 0.05 J/cm2 , for 500 fs pulses. We noted that, with both 500 fs and 12 ps pulses, discoloration is observed at the fluence close to, but lower the ablation threshold (see Fig. 3), second pattern at 0.21 J/cm2 . There was no discoloration at laser fluences below 0.5 Fth (see Fig. 3), patterns at 0.05 J/cm2 and 0.09 J/cm2 . 4

However, the need for tightly focused, low-energy laser pulses introduces new challenges in precise manipulation of the laser beam. Fast scanning of the beam over the treated surface is one of these challenges to be addressed. We demonstrate here that scanning patterns and the scanning speed of the laser beam are important issues in laser cleaning of surfaces with high repetition rate, ultrashort laser pulses. The morphology of the ablated areas strongly depends on the homogeneity of the scanning pattern, while the short time between pulses leads to a need for a relatively high scanning speed. To avoid thermal accumulation, the scanning speed should also be faster than the heat diffusion rate, which for most organic materials is above 1 m/s. We show that a constant velocity alternating spiral and its modification, a moving racetrack pattern, provide the best practical options for homogeneous coverage of the treated area. In spite of the obvious advantages of applying ultrafast, high-repetition-rate lasers to the conservation of artworks and heritage objects, there are still a few important issues to be addressed. For instance: – using a top-hat profile for the laser beam will further improve the uniformity of the surface illumination and the morphology of the treated area; – fast scanning and fast removal of the ablated vapour is required to avoid redeposition of ablated material; – laser beam delivery and precise focusing for cleaning of three-dimensional objects; – ultrafast lasers must be optimised for use in conservation, as to date they have only been used in experimental facilities and are prohibitively expensive.

CONCLUSIONS

High precision treatment of surfaces and minimum invasiveness are the main advantages of using shortpulse, high-repetition rate lasers for laser cleaning. Contaminant material can be removed in individual layers of precise depth, with sub-micron precision.

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(ed), Pulsed Laser Deposition of Thin Films: Applications in Electronics, Sensors, and Biomaterials, 99–130. Hoboken, New Jersey: John Wiley & Sons. Juhasz, T., Djotyan, G., Loesel, F. H., Kurtz, R. M., Horvath, C., Bille, J. F. & Mourou, G. 2000. Laser Phys. 10: 495. Loesel, F. H., Fischer, J. P., Gotz, M. H., Horvath, C., Juhasz, T., Noack, F., Suhm, N. & Bille J. F. 1998. Appl. Phys. B 66: 121. Luther-Davies, B., Kolev, V. Z., Lederer, M. J., Madsen, N. R., Rode, A. V., Giesekus, J., Du, K.-M. & Duering M. 2004. Table-Top 50 W Laser System for Ultra-Fast Laser Ablation, Appl. Phys. A 79: 1051–1055. Luther-Davies, B., Rode, A. V., Madsen, N. R. & Gamaly, E. G. 2005, Picosecond high repetition rate pulsed laser ablation of dielectrics: The effect of energy accumulation between pulses, Optical Engineering 44: 051102. Pouli, P., Bounos, G., Georgiou, S. & Fotakis, C. 2005. Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward? LACONA VI Conference Proceedings, Vienna, Austria. Rode, A. V., Gamaly, E. G., Luther-Davies, B., Taylor, B. T., Dawes, J., Chan, A., Lowe R. M. & Hannaford, P. 2002. Subpicosecond laser ablation of dental enamel, Journ. Appl. Phys. 92: 2153–2158. Stuart, B. C., Feit, M. D., Rubenchik, A. M., Shore, B. W. & Perry M. D. 1995. Phys. Rev. Lett. 74: 2248–2251.

Nevertheless, with the fast development of powerful and compact femtosecond lasers, ultrafast laser ablation has the potential to become a standard tool in the conservation armoury and a key technique for conserving some previously untreatable artworks and heritage objects. ACKNOWLEDGEMENTS This work is supported by the Australian Research Council through the Linkage Project Scheme. REFERENCES Du, D., Liu X, Korn, G., Squier, J. & Mourou G. 1994. Appl. Phys. Lett. 64: 3071–3073. Feit, M. D., Rubenchik, A. M., Kim, B. M., Da Silva, L. B. & Perry, M. D. 1998. Appl Surf. Sc. 127–129: 869–874. Gamaly, E. G., Rode, A. V., Luther-Davies, B., & Tikhonchuk, V. T. 2002. Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics, Phys. Plasmas 9: 949–957. Gamaly, E. G., Rode, A. V., & Luther-Davies, B. 2007. Ultrafast laser ablation and film deposition. In R.W. Eason

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Removal of unwanted material from surfaces of artistic value by means of Nd:YAG laser in combination with Cold Atmospheric-Pressure Plasma C. Pflugfelder, N. Mainusch & W. Viöl HAWK, University of Applied Sciences and Arts, Göttingen, Germany

J. Ihlemann Laser-Laboratorium Göttingen, Göttingen, Germany

ABSTRACT: The removal of unwanted material such as spray lacquers (graffiti), organic residues or corrosion products is a very demanding task, especially in the case of sensitive surfaces and objects with historical value that conservators usually deal with. Since many years Nd:YAG lasers are applied to clear away specific substances. Recently the cleaning potentials of a commercially available plasma jet which emits a “cold” stream of exited gas were investigated at HAWK. The application of chemically reactive plasma species that decompose organic and inorganic compounds supplemented by laser pulses is a new approach to solve specific “cleaning problems”. Ongoing tests are being performed within the research project “PROKLAMO”, a collaboration of three scientific facilities and eight companies. The hybrid system permits the removal of organic binding media respectively varnishes and paint layers that are usually restricted to (expensive) UV laser ablation. Furthermore metallic corrosion products on archeological artifacts can be reduced and removed as well.

1

INTRODUCTION

of dissolved matter can complicate any thorough extraction. Furthermore whenever organic solvents, acids or alkali agents are to be used, danger to health and environment exists (Dignard et al. 2005). The use of an atmospheric-pressure plasma jet as a device for surface cleaning has been investigated in the course of a research project at HAWK between 2004–2006. Specific substances were removable with a plasma jet due to the thermal and especially the chemical impact of free radicals of the air-fed jet. On the one hand polymers like acrylic resins and polyester based sprays could be removed from smooth surfaces whereas other substances such as nitrocellulose were resistant (Dignard et al. 2005). Furthermore archaeological metal objects were treated by plasma with the goal to diminish oxidic crusts. As process gases, mixtures containing hydrogen were used. Especially in case of a strongly damaged silvered copper coin we have obtained good cleaning results: plasma reduces silver oxides very well. In contrast to this, copper and iron oxides proved to show weaker reaction to the plasma. In the case of completely corroded iron objects any plasma treatment proved to be dangerous because it destabilizes the structure and causes severe decomposition of the material (Pflugfelder 2005). As we sum up limitations and benefits of an atmospheric-pressure plasma jet and a Nd:YAG

A common goal of all cleaning procedures is the removal of unwanted material from a substrate. In conservation of archaeological metal artefacts, iron concretion or dark corrosion products have to be taken off in order to retain details from, for example, the coins original design. All such cleaning tasks have to be carried out selectively and without harming any substance of artistic value. Since the mid 1970’s lasers have been tested for a wide range of conservation activities (Asmus 1973). Many case studies prove short-pulse (5–10 ns) Nd:YAG-lasers to be a tool to overcome the aforementioned disadvantages (Cooper 1998). Essential for their successful employment is the appropriate absorption of laser energy from the unwanted material. Besides this, the underlying components with artistic value have to exhibit sufficient resistance to the laser impact so that selective ablation is possible and, in the most desirable case, a “self-limiting-cleaningprocess” can take place (Dickmann et al. 2005). Conventional cleaning methods include abrasive techniques and wet-chemical procedures. A disadvantage of mechanical intervention is minor accuracy and selectivity. Wet-chemical techniques bear the risk of the solvent’s uncontrolled permeation through a substrate due to capillarity. Unintended deposition

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laser, it becomes obvious that the coupling of these devices should be an smart approach that offers new possibilities in dealing with specific cleaning problems. By means of initial investigations aimed at the removal of polymers we already specified cases in which the laser-plasma-coupling increases working efficiency (Mainusch 2006). Objective of this contribution now is the presentation of new results from cleaning attempts of metal artefacts.

2 2.1

EXPERIMENTAL PART Experimental setup Figure 1. Experimental setup.

In the experimental set-up the laser source was a solidstate, flash lamp-pumped Nd:YAG laser (LUMONICS, model MiniQ) that operates at 1064 nm. The intensity of the Q-switched laser beam (with pulse duration of 4 ns) was adjusted by means of an external trigger generator that provided 30 V signals for the activation of the Pockels Cell. These pulses were synchronized with a signal from the laser’s flash lamps. The time gap in between flash lamp signal and trigger pulse (Q-switch delay) was to be modified (125 µs to 250 µs). By this, fluences in the range from 200 mJ/cm2 to 800 mJ/cm2 and a laser spot size of approximately 0.04 cm2 could be generated. As depicted in Figure 1 the plasma stream is directed upon the target with an angle of incidence of around 30◦ . The plasma source (model “Plasma Blaster” with generator “V06”, manufactured by TIGRES GmbH, Rellingen) provides a potential-free plasma jet resulting from a gas discharge that is ignited between a centered electrode and a grounded nozzle. The plasma generator produces a voltage at about 10 kV that is applied with a frequency of 20–40 kHz to continuously feed the gas discharge. The currents are limited to 10–20 mA by means of a specific electric control. Therefore only transient arcing between the electrodes occurs and subsequently plasma with moderate escape temperatures is being generated. The plasma jet can be fed with either air or various process gases such as argon, argon-hydrogenmixtures, nitrogen etc.The gas flow rate was controlled by means of a variable area flowmeter. Throughout our experiments, the plasma jet was constantly driven at output power 140 W. A nozzle with a diameter of 1 mm was implemented. By this, luminance and extension of the stream as function of the gas flow rate 30 l/min could be investigated.

2.2

Figure 2. Original state of the coin.

containing up to 5% hydrogen. Thus a significant amount of free hydrogen radicals with the capability to chemically reduce oxidised metal surfaces is be generated.

2.3 Specimen A variety of heavily spoiled and corroded metal coins without any historical value were subjected to the combined laser-plasma jet treatment. Figure 2 represents such a coin, a “Dinara” from former Yugoslavia. The coin apparently consists of a copper alloy yielding a greenish copper patina and a white veil on its surface. After a series of tests, an archeological finding that approximately dates back to the year 1000 was treated as well. As to be seen in Figure 3, there is a leftover of crust in the upper part of the coin. White and green efflorescence with the brown corrosion products disfigure the surface. The pattern of its coinage is hardly readable.

Process gas

As mentioned before, the plasma jet is designed for the use of various process gases. All through our tests, the plasma jet was supplied with argon based gas mixtures

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Figure 5. State of archaeological metal object after cleaning process; plasma-laser combination on the left and only laser on the right.

Figure 3. Original state of the archaeological metal object.

loose efflorescence followed by laser ablation (right half) and laser-plasma jet application (left). We chose a low laser fluence of 0.3 J/cm2 , because the finding pieces were fairly fragile. Once again, the application of the coupled device results in different cleaning effects in comparison to only laser ablation.The slightly brownish discoloration as well as corrosion remains that are visible in the right half of the coin cannot be detected in the left zone anymore. Moreover the homogenous appearance of the laser-plasma jet treated area is striking. Figure 4. State of coin after cleaning process; plasma laser combination on the left, only laser middle and only plasma on the right.

3

4

CONCLUSIONS

According to the documented cleaning effects, we believe that there is a significant contribution of plasma species in the cleaning procedure of superficially corroded metals. By means of former spectrographic investigation on the plasma stream, a certain amount of hydrogen atoms could be detected, whenever generating plasma with hydrogen gas mixtures (Pflugfelder 2006). It is well known that hydrogen possesses the capability to chemically reduce metal oxides and thus in many cases our approach should be promising. From the conservator’s point of view the phenomenon “homogeneity” of the cleaned area due to the coupling as mentioned above is interesting since any cleaning intervention in conservation has to result in a homogenous appearance of the piece of art. The fact that the plasma generator provides a jet that continuously blows onto the surface has to be regarded positively: the jet gradually disintegrates respectively reduces unwanted material and this favours a sensitive and controllable intervention.

RESULTS

Figure 4 depicts the “Dinara” after the treatment. Three areas can be distinguished: at the very right side only plasma jet treatment was performed. Compared to the original state only a little change is visible. The adjacent area documents the effect of stand alone laser ablation. Ablation has been carried out with a fluence of 0.5 J/cm2 to 0.6 J/cm2 . The repetition rate was 10 Hz running a stage speed of 3 mm/s. As a cleaning result we note that by single scanning the superficial alteration could be diminished quite well. A grey veil is preserved. On the left half of the object an intensified cleaning result was obtained by means of a coupled laser-plasma jet application even though the laser fluence amounted to only 0.4 J/cm2 . In Figure 5 there are two zones to be distinguished that resulted from an initial (mechanical) removal of

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REFERENCES

Mainusch, N. 2006. Plasma Jet Coupled with Nd:YAG Laser: A NewApproach to Surface Cleaning. Proceedings of 10th International Conference on Plasma Surface Engineering (PSE), S33-S38, WILEY-VCH, Weinheim. Pflugfelder, C. 2005. Einsatz von Atmosphärendruckplasma im Bereich der Restaurierung. Diplomarbeit, HAWKHHG. Pflugfelder, C. 2006. Cleaning Wall Paintings and Architectural Surfaces by Plasma, Proceedings of 10th International Conference on Plasma Surface Engineering (PSE), S516–S521, WILEY-VCH, Weinheim.

Asmus, J. F., Murphy, C. G. & Munk, W. H. 1973. Studies on the interaction of laser radiation with art artifacts. Proceedings of SPIE. Cooper, M. 1998. Laser Cleaning in Conservation: An Introduction. Butterworth-Heinemann. Dignard, C., Lai, W., Binnie, N., Young, G., Abraham, M. & Scheerer, S. 2005. Cleaning of Soiled white feathers using the Nd:YAG laser and traditional methods. Proceedings of Lasers in the Conservation of Artworks conference, LACONA V, Springer Verlag-Berlin, Heidelberg. Dickmann. K., Fotakis, C. & Asmus, J. F. eds. Proceedings of Lasers in the Conservation of Artworks conference, LACONA V, Springer Verlag-Berlin, Heidelberg.

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Analytical Techniques

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Optical coherence tomography for structural imaging of artworks P. Targowski, M. Góra, T. Bajraszewski, M. Szkulmowski, M. Wojtkowski & A. Kowalczyk Institute of Physics, Nicolaus Copernicus University, Torun, Poland

B. Rouba, L. Tymi´nska-Widmer & M. Iwanicka Institute for the Study, Restoration and Conservation of Cultural Heritage, Nicolaus Copernicus University, Torun, Poland

ABSTRACT: The Optical Coherence Tomography (OCT) technique is applied to image internal structure of an oil painting in non-destructive and non-invasive way. Different varnish layers, glaze and paint layers are clearly visible. This technique can also determine the deep location of an artist’s signature providing a method of confirmation of its authenticity.

1

INTRODUCTION

(Liang et al. 2005). In that year, OCT was presented for the first time at LACONA conference series, applied to imaging of varnish and paint layers (Gorczy´nska et al. 2007, Szkulmowska et al 2007, Arecchi et al. 2007). All applications of OCT to artwork diagnostics were recently reviewed by Targowski et al. (2006). Lately, a new use of the OCT technique to monitor laser ablation of varnish on easel paintings has been reported (Góra et al. 2006, Targowski et al. 2007).

Determination of internal structure of the artwork is essential not only for the purpose of its documentation or technical and historical research, but also for understanding causes of the decay and planning a proper conservation-restoration treatment. Routine method of stratigraphic examination is based on collection of samples from the artwork structure. The obvious limitation of this procedure is its invasive and destructive nature. Additionally, it can be performed in painting regions of lesser importance, which obviously disqualifies its use in inviolable regions of painting such as the areas of signature. Moreover, because of the large variability of the structure, the physical sample may not represent its neighbourhood. Optical Coherence Tomography (OCT) is an alternative method which gives the possibility of sampling in a fast and non-invasive way, for an unlimited number of locations in any region of the painting surface. Its utilization is, however, limited to transparent or semitransparent structures of weak absorption and scattering of infrared light. This relatively new, but already well-established technology, mostly used in biological research and diagnostics, especially in ophthalmology (Huang et al. 1991, Srinivasan et al. 2006) was quickly transferred to materials science (Stifter 2007, for review). OCT was first applied to obtain images of internal structures within archaic jades (Yang et al. 2004). In the same year, the first application of OCT to examination of paintings, namely to varnish layer imaging and to profilometry of picture surface, was reported (Targowski et al. 2004). In the next year, OCT was used to image underdrawings and model paint layers

2

INSTRUMENTATION

The tomograms shown in this paper have been obtained with a prototype SOCT instrument based on an optical fibre Michelson interferometer setup (Fig. 1), constructed in the Nicolaus Copernicus University. A broadband (λ = 50 nm, central wavelength 845 nm) superluminescent diode (LS) was employed as the light source. The light of high spatial but low temporal coherence is launched into a single mode 50 : 50 fibre coupler (FC) through an optical isolator (OI). The optical isolator protects the light source from the light back reflected from the elements of the interferometer. Light beam is then divided by the fibre coupler into two arms of the interferometer. The light propagating in a reference arm passes through a polarization controller (PC) to provide the optimal conditions for interference, the neutral density filter (NDF) for adjustment of the power of light to achieve the shot noise limited detection and a block of glass acting as a dispersion compensator (DC). The light is then back reflected from the stationary reference mirror (RM) to the reference arm fibre and coupler (FC). The sample arm

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Figure 2. A) “Portrait of Sir James Wylie”, oil on canvas, 19th century (fragment); B) UV induced luminescence of the same fragment. Restorer’s interventions are clearly visible as dark spots. Places where OCT tomograms were recorded are indicated by letters a–f.

Figure 1. A top view of the experimental setup.

in varnish and paint layer. The transverse resolution depends mostly on the optical properties of the system and is kept below 20 µm. The sensitivity of the system is 104 dB. The optical power of the incident beam at the surface of the object ranges between 500 and 2000 µW. In the tomograms presented, the intensity of scattered and/or reflected light from the internal structures within the sample is coded in a grey scale – the darker the shade, the higher level of reflectivity of the structure. Due to the fact that all originally measured axial distances are optical ones, a conversion to real lengths is required. To calibrate the in-depth axis, the group refractive index of the medium was assumed to be equal to 1.5.

comprises transversal scanners (X–Y) and lens which form the measuring head. The light beam is scanned across the object and backscatters and/or reflects from the elements of its structure and returns to the coupler FC. The light beams returning from the reference mirror and from the sample are brought to interference at the output of the interferometer and analyzed by a customized spectrometer. It consists of a volume phase holographic grating (DG) with 1200 lines/mm and achromatic lens (SL) which focuses the spectrum on a 12 bit line scan CCD camera (2048 pixels, 12 bit A/D conversion, Atmel). The spectral fringe pattern registered by this detector is then transferred to a personal computer (COMP). This signal, after Fourier transformation, yields one line of the crosssectional image (A-scan). The A-scan carries information about the location of structural interfaces in the object along the path of the penetrating beam. Scanning across the sample enables collecting 2D slice cross-sections (B-scan). Additional scanning in the perpendicular direction gives 3D information about a structure. The acquisition process and scanning protocols are controlled by a custom-designed compact electronic driving unit. To define precisely the examined area, an industrial camera registering the surface of the object is coupled to the OCT instrument. A high-density image (B-scan) is composed of at least 1000 A-scans per one millimetre and covers an area of 3.6 mm in depth. The exposure time (during which the object is illuminated) is 40 µs per single A-scan and it will increase to 200 ms for typical B-scan comprising 5000 lines. The axial resolution of the system (equals to one half of the coherence length of the light) is 6 µm

3

RESULTS

3.1 Stratigraphy of an oil painting A 19th century Portrait of Sir James Wylie (Fig. 2) was chosen for the OCT examination due to its plain and typical painting techniques. The original varnish layer was preserved during the last conservation-restoration procedures which took place about 30 years ago and included vast reconstruction of the composition. Finally, a thin layer of synthetic final varnish was applied onto the whole surface of the painting. The presence of original glazes with two layers of varnish – the original thick one in combination with a thin but already aged restorer’s varnish – brought an opportunity to investigate the possibility of imaging these layers as separate morphological structures by means of OCT.

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Figure 3. Tomograms of the inner structure of the painting “Portrait of Sir James Wylie”. Orientation of tomograms corresponds to direction of scanning. All images are scanned over 12 mm. Arrows point to following objects: 1: air-varnish interface; 2: re-storer’s final varnish layer; 3: restorer’s varnish–original varnish interface; 4: original varnish layers; 5: interface between layers of original varnish; 6: varnish–paint layer interface; 7: glaze layer; 8: opaque paint layer.

However, paint layers are in many cases transparent enough to enable deeper imaging. In Figures 3a, c–f one can observe the cross-sectional view of at least one layer of glaze underneath the varnish. The thickness of glaze layers in different parts of painting’s composition varies significantly, as may be noticed from the comparison of Figures 3c and 3e. This not only provides information about the artist’s original technique, but also may have great importance for planning conservation-restoration treatment. Detection and precise location of glaze under darkened or yellowed varnish will help decreasing risk of irreversible damage to the glaze layer during a process of varnish removal. In most of the tomograms, it proved to be possible to detect the presence of at least two films within the original varnish layer. Additionally, in Figures 3a and 3b, both taken across the boundary between the original and reconstructed paint layer, a thin superficial layer of restorer’s final varnish is revealed.

Figure 3 presents tomograms taken at locations indicated by letters a–f in Figure 2B. The light penetrates the object (Fig. 3) either from left (a–d) or from top (e, f ). As an example of diversity of the painting’s inner structure, the tomogram corresponding to Figure 3a may be considered. An interface between air and the surface of the painting is imaged as a thick black contour (1). The thin but recognizable light strip below is a layer of restorer’s final varnish (2). Then, two layers of original varnish (4) are visible as white strips due to its high transparency. The last visible layers are glaze (7) and opaque paint layer (8). Obviously, not all of the above described structures may be found in every tomogram, depending on the painting region examined. For instance, Figure 3b was taken from the area where the paint layer mostly consists of lead white, a pigment of great opacity and infrared impermeability. Therefore, there is no possibility of OCT imaging through this layer.

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Such counterfeit, as shown in Figure 4, can be discovered by means of the OCT technique. In the OCT cross-sectional image, the exact location of the signature is clearly visible. It proves not to be lying directly on the paint layer, but rather “hovering” above it. The white gaps under the edges of the signature give evidence that it is localized between two varnish layers and, hence, it is probably not authentic.

4

CONCLUSIONS

In this paper, we have shown the applicability of the OCT technique to unravelling the sequence of semitransparent strata of painting in a non-invasive way. Varnish layers imaging by means of OCT has proven to deliver quite detailed information: the possibility of imaging and distinguishing two chemically and historically different types of varnish and up to three consecutively applied films within the original varnish layer. Beyond varnish, semitransparent layers of glazes are also visible in OCT tomograms. The last structure available for OCT examination is the upper boundary of opaque paint layer. Especially interesting and promising application of structural imaging of paintings seems to be examination of the inviolable region of artist’s signature in order to confirm its authenticity.

Figure 4. Example of OCT examination of the signature region. In the OCT cross-sectional image, the exact location of the signature is clearly visible inside the varnish layer and thus the counterfeit becomes obvious.

Ancient varnishes, since they consist of natural resins and oils, are in most cases inclined to form layers of greater thickness than contemporary synthetic resin varnishes. Due to the natural ageing process, these layers present more inhomogeneous structure and thus scatter light more efficiently than the contemporary ones. These features of varnish coatings make the OCT technique a suitable tool for quick and non-destructive analysis of the range of restorers’ interventions, which not always may, due to various reasons, be revealed during routine UV-fluorescence examination.

REFERENCES Arecchi, T., Bellini, M., Corsi, C., Fontana, R., Materazzi, M., Pezzati, L. & Tortora, A. 2007. Optical coherence tomography for painting diagnostics. In J. Nimmrichter, W. Kautek and M. Schreiner (eds.). Lasers in the Conservation of Artworks, LACONA VI Proceedings, Vienna/Austria, Sept. 21–25, 2005, Berlin-HeidelbergNew York: Springer Verlag. Gorczyñska, I., Wojtkowski, M., Szkulmowski, M., Bajraszewski,T., Rouba, B., Kowalczyk,A. &Targowski, P. 2007. Varnish Thickness Determination by Spectral domain Optical Coherence Tomography. In J. Nimmrichter, W. Kautek & M. Schreiner (eds.). Lasers in the Conservation of Artworks, LACONA VI Proceedings, Vienna/Austria, Sept. 21–25, 2005, Berlin-HeidelbergNew York: Springer Verlag. Góra, M., Targowski, P., Rycyk, A. & Marczak, J. 2006. Varnish ablation control by Optical Coherence Tomography. Laser Chemistry doi:10.1155/2006/10647, http://www.hindawi.com/journals/lc/. Huang, D., Swanson, E. A., Lin, C. P., Schuman, J. S., Stinson, W. G., Chang, W., Hee, M. R., Flotte, T., Gregory, K., Puliafito, C. A. & Fujimoto J. G. 1991. Optical coherence tomography. Science 254: 1178–1181 Liang, H., Cid, M., Cucu, R., Dobre, G., Podoleanu, A., Pedro, J. & Saunders, D. 2005. En-face optical coherence tomography–a novel application of

3.2 Examination of artist’s signature Another interesting and not yet well explored application of OCT to the analysis of paintings is examination of artists’ signatures of uncertain authenticity. If a forger draws a signature on an original varnish layer and then intentionally covers it with another varnish of high fluorescence, the routine UV examination may not reveal the fake. A model sample of oil painting was prepared and covered with dammar varnish (Maimeri). Subsequently, to imitate a common forgery technique, the signature was drawn on the varnish surface and then the whole sample was covered with a few layers of the same varnish.

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non-invasive imaging to art conservation. Optics Express 13: 6133–6144. Srinivasan, V. J., Wojtkowski, M., Witkin, A. J., Duker, J. S., Ko, T. H., Carvalho, M., Schuman, J. S., Kowalczyk, A. & Fujimoto, J.G. 2006. High-definition and 3-dimensional imaging of macular pathologies with highspeed ultrahigh-resolution optical coherence tomography Ophthalmology 113: 2054 2065.e3. Stifter, D. 2007. Beyond biomedicine: a review of alternative applications and developments for optical coherence tomography. Appl. Phys. B DOI: 10.1007/s00340-0072743-2. Szkulmowska, A., Góra, M., Targowska, M., Rouba, B., Stifter, Breuer, E. & Targowski, P. 2007. The Applicability of Optical Coherence Tomography at 1.55 mm to the Examination of Oil Paintings. In J. Nimmrichter, W. Kautek & M. Schreiner (eds.).Lasers in the Conservation of Artworks, LACONA VI Proceedings, Vienna/Austria,

Sept. 21–25, 2005, Berlin-Heidelberg-NewYork: Springer Verlag. Targowski, P., Rouba, B., Wojtkowski, M & Kowalczyk, A. 2004. The application of optical coherence tomography to non-destructive examination of museum objects. Studies in conservation 49: 107–114. Targowski, P., Góra, M. & Wojtkowski, M. 2006. Optical Coherence Tomography for Artwork Diagnostics. Laser Chemistry doi:10.1155/2006/35373 http://www.hindawi. com/journals/lc/. Targowski, P., Marczak, J., Góra, M., Rycyk, A. & Kowalczyk,A. 2007. Optical CoherenceTomography forVarnish Ablation Monitoring.Proc. of SPIE 6618: 661803-1 – 661803-7. Yang, M. L., Lu, C. W., Hsu, I. J. & Yang, C. C. 2004. The use of optical coherence tomography for monitoring the subsurface morphologies of archaic jades. Archeometry 46: 171–182.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Atmospheric Pressure Laser Desorption Mass Spectrometry based methods for the study of traditional painting materials M.P. Licciardello, R. D’Agata & G. Grasso Dipartimento di Scienze Chimiche, Università di Catania, Catania, Italy

S. Simone Dipietro Automazione S.r.l., C. da Cava Sorciaro, Priolo G. (SR), Italy

G. Spoto Dipartimento di Scienze Chimiche, Università di Catania, Catania, Italy Istituto Biostrutture e Bioimmagini, CNR, Catania, Italy

ABSTRACT: The study of ancient works of art is a very challenging task mainly due to the impossibility of applying experimental approaches that could damage anyhow the object from which analytical information has to be obtained. Spatially resolved analytical methods have significantly enhanced our capacity to study ancient materials since they cause minimal and at times no damage to the studied object. Unfortunately, only few analytical techniques operating within the requested spatial resolution are applicable for the investigation of the organic components of artistic and archaeological objects. In this scenario, Atmospheric Pressure/Matrix Assisted Laser Desorption Ionization-Mass Spectrometry (AP/MALDI-MS) has already proven to be a very valuable technique for the study of ancient materials as it combines the good spatial resolution of the conventional MALDI to the possibility of working in air. In this work we present results from the AP/MALDI-MS investigation of the indigo dye and the carmine and brazilwood lakes. All the studied systems can be found in traditional paintings and the experimental conditions used are aimed to simulate real ancient materials.

1

INTRODUCTION

tools. The destructive approach thus remains as a last resort for the extraction of analytical information from artistic and/or archaeological samples. Tremendous improvements have been made as regards the scope and efficiency of today’s analytical instruments. This has led to the development of new analytical methodologies that satisfy specific requirements to a greater degree such as microdestructiveness or non-destructiveness of the sample to be analysed (Ciliberto & Spoto 2000, Spoto 2007). A wider range of information is now available and a greater sensitivity and reproducibility of analyses is thus ensured. In this context, the use of spatially resolved analytical techniques have provided new opportunities for micro-destructive and, at times, completely non-destructive analyses thus opening up new diagnostic approaches for the study of samples of artistic and/or archaeological importance (Spoto et al. 2000). They have also amplified the range of analytical information obtainable from ancient and valuable objects (Spoto 2002). Spatial resolution allows analysing tiny fragments of samples scraped from the object of interest with minimal damage to the artefact itself. Moreover, the

All scientists involved in studies concerning works of art or samples of archaeological interest will recognize the importance of making an appropriate selection of the analytical method to be used in their studies. The main questions usually raised concern how the proposed analytical procedure will affect the integrity of the object to be examined. From this point of view, only those techniques which do not alter the integrity and appearance of artistic and/or archaeological objects are eligible as “ideal” techniques. Techniques which operate in situ, making sample-taking unnecessary, come close to this ideal. In the attempt to find a balance between the requirements of scientific methods and the need to maintain the integrity of the object under study, the only alternatives to in situ analysis require the object itself to be placed in the analysing chambers of the analytical instrument or tiny fragments of samples to be scraped from its surface. The former approach cannot be applied in all cases, since only small objects such as coins, certain jewellery and statuettes are of a size and shape which will fit those of common analytical

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of artistic materials is the need of placing these samples under vacuum. The low-medium vacuum conditions required for the MALDI-MS analysis significantly limit its potential for in situ analysis of works of art and archaeological objects. The recently introduced Atmospheric Pressure (AP) MALDI-MS (Laiko et al. 2000) combines the advantages of the vacuum MALDI-MS analytical approach to the possibility of analysing samples in air. (Schneider et al. 2005) Two important advantages offered by AP/MALDI with respect to vacuum MALDI are the ability to produce lower internal energy molecular ions with minimal fragmentation (softness) (Gabelica et al. 2004) and the decoupling of the ion source from the mass analyser. In order to demonstrate the potential offered by AP/MALDI-MS in the study of ancient objects and materials, we recently presented an AP/MALDI study of the most important ink in western history: the irongall ink. (D’Agata et al. 2007) The study demonstrated that AP/MALDI can be used to identify the main gallotannic components of the ink by analysing in air strokes of ink directly from paper and parchment. To further expand our basic knowledge of the AP/MALDI potential in the identification of organic materials used in art, in this work we present results from the study of the indigo dye and the carmine and brazilwood lakes. All the studied systems were used in traditional painting. The study was conducted by using a commercially available AP/MALDI source previously modified in order to allow the direct analysis of real objects.

in situ analysis of microscopic areas of the artwork may also be accomplished by spatial resolution, thereby allowing the extraction of a wide range of valuable analytical information that can sometimes be imaged into 2D or 3D graphics. In spite of the powerful support provided by the today available analytical tools, the micro-destructive study of organic materials constituting the structure of works of art and archaeological objects is still a challenge. These organic materials are mainly natural products and, therefore, they are composed of complex mixtures of molecular and biomolecular components (Mills & White 1994). Detailed compositional information from such materials is obtained by making use of a wealth of instrumental methodologies. In this scenario, spatially resolved analytical methods offer valuable tools for the analysis of works of art with a micro-destructive approach. Unfortunately, only few analytical techniques operating within the requested spatial resolution are applicable for the investigation of the organic components of artistic and archaeological objects. Among them Raman microscopy (Vandenabeele et al. 2007, Clark 2007), secondary ion mass spectrometry (Darque-Ceretti & Aucouturier 2004) and Fourier Transform-IR micro-spectroscopy (Salvado et al. 2005) have been shown to be the most powerful analytical tools that are available today. However, the latter have limitations that prompt the quest for further, hopefully more versatile, spatially resolved analytical methods. In recent years, the potential showed in the above mentioned field by spatially resolved mass spectrometry (MS) techniques that make use of ionic sources based on direct laser desorption ionization (LDI) has been investigated (Boon & Learner 2002; Grim & Allison 2004). However, direct LDI is only effective in the study of a limited range of materials, while the use of matrices that assist the ionization process induced by the laser desorption (Matrix Assisted LDI, MALDI) has expanded the applicability of LDI-based MS methods to the field of spatially resolved studies of organic components from works of art. The matrix is a compound that absorbs light at a given laser wavelength and allows compounds that do not absorb laser light to be desorbed and ionized without much fragmentation. In other words, it protects and assists the analyte during the desorption and ionization processes and very often it is only by choosing the appropriate matrix for the particular system investigated that good quality mass spectra can be obtained (Williams et al. 2007). Examples of the applicability of MALDI-MS to analysis of pigments (Maier et al. 2004), siccative oils (Van Den Berg et al. 2004), proteinaceous binders (Tokarski et al. 2006, Kuckova et al. 2007), and varnishes (Zumbühl et al. 1998) are indeed reported in the literature. On the other hand, a significant disadvantage of using MALDI-MS for spatially resolved study

2

EXPERIMENTAL

Indigo, carmine naccarat, and brazilwood were purchased from Kremer (Germany). α-cyano-4hydroxycinnamic acid (CHCA), trifluoro acetic acid (TFA), acetonitrile (C2 H3 N), methanol (CH3 OH) were purchased from Sigma-Aldrich. All the AP/MALDI-MS measurements were carried out by using a Finnigan LCQ Deca XP PLUS (Thermo Electron Corporation, USA) ion trap spectrometer which was fitted with a MassTech Inc. (USA) AP/MALDI source. The source was modified in order to allow the direct analysis of real objects (Fig. 1). A home-built XYZ stage was used to place tridimensional objects on the focus of both the ion trap spectrometer and the laser source with a micrometric control. The sample positioning was controlled by home-built software operating in LabView framework that was also interfaced with two different digital cameras. A pulsed nitrogen laser (wavelength 337 nm, pulse width 4 ns, pulse energy 300 µJ, repetition rate up to 10 Hz, focus diameter ∼200 µm) was used as source. Laser power was attenuated to about 55%. Pulsed dynamic focusing (PDF) has been shown to improve S/N ratios in the AP-MALDI spectra,

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Mulholland 2006) were carried out by applying a multipole offset voltage of +100 V for negative ions and of −100 V for positive ions. The matrix solution was a CHCA solution (1 mg/ml) in 30% TFA (0.1%) and 70% C2 H3 N. Typically AP/MALDI-MS experiments were carried out by mixing 1 µl of the pigment in CH3 OH with 5 µl of matrix solution. 1 µl of the final solution was spotted on theAP/MALDI plate and irradiated with the laser source after solvent evaporation. In situ experiments were carried out by spotting 0.5 µl of matrix solution (spot diameter about 1000–500 µm) or by depositing CHCA crystals directly on the surface to be analysed. Egg tempera was obtained by separating out the yolk of one chicken egg and by draining it into a container. Pigment was added to obtain the final tempera paint. Once ready, the paint mixture was applied on a flat glass surface and allowed to dry in sunlight. The paint layers were analysed by AP/MALDI-MS as both freshly prepared (after visual solidification of the paint medium, typically after two days) and after two months. To acquire the necessary fundamental knowledge about the AP/MALDI capability in the characterization of organic materials, organic dyes and pigments traditionally used in painting and dyeing were analysed. The studied organic systems were the indigo (a traditional organic dye also used as a pigment) and two lakes: carmine and brazilwood lake. It is worth noting that all the analyses were carried out by operating in air and with no previous chromatographic separation of the samples components. Similar experimental conditions must be assured when in situ spatially resolved analyses of solid objects are going to be planned.

Figure 1. Home-built XYZ stage used to place tridimensional objects on the focus of both the ion trap spectrometer and the laser source with a micrometric control. The sample positioning was controlled by home-built software operating in LabView framework that was also interfaced with two different digital cameras (a). Aluminum foil in contact with the sample ensured the application of the target voltage to the sample surface (b).

(Berkout et al. 2007) therefore a PDF module that imposes a delay of 25 µs between the laser pulse and the application of the high voltage to the AP/MALDI target was also used. The target voltage was applied to the sample surface by connecting an aluminum foil carefully placed on the sample surface in close proximity to the area to be analysed. The applied target voltage was 1.8 kV. The ion trap inlet capillary temperature was 200◦ C. Capillary and tube lens offset voltages of 30 and 15 V, respectively, were applied. Automatic Gain Control (AGC) was turned off and instead the scan time was fixed by setting the injection time to 220 ms and 5 microscans per scan. MS/MS scans were acquired using an isolation width of 5 m/z, activation qz of 0.250, activation time of 30 ms, and normalized collision energy (NCE) in the range 30–40%, dependent on the ion. (NCE is the amplitude of the resonance excitation RF voltage scaled to the precursor mass based on the formula: RF amplitude = [NCE%/30%] (precursor ion mass x tick amp slope + tick amp intercept), where the tick amp slope and tick amp intercept are instrument specific values. For our LCQ Deca, 35% NCE for m/z 1000 = 1.8 V.). In source collision-induced dissociation (CID) experiments (Baranov & Bohme 1996; Peterman &

3

RESULTS AND DISCUSSION

3.1 Indigo Natural indigo was traditionally obtained from a variety of plants. The most common indigobearing plants belong to the indigofera species (indigofera tinctoria). Independent of the plant source, the chemistry of dye extraction requires a fermentation stage during which an enzymatic hydrolysis of the original compounds present in the plant is followed by an oxidation process caused by the exposure to air. At the end a blue pigment is collected. Evidence of the use of indigoid dyes dates back to 2000 B.C. in Egypt. Indigo has been widely used as a dye or as colouring matter for blue paper in the renaissance or later drawings and, to some extent, as a pigment in paintings (Chiavari et al. 2005). It is stable when used with tempera medium while unstable with siccative oils.

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O

H N

N H

O

Figure 2. Molecular structure of indigotin (MW = 262.2 Da). 50 40

Figure 4. Image of the indigo bearing paint layer. The brighter spots are due to the HCCA matrix crystals deposited on the different regions of the paint layer.

30

40

20

263.1

Relative Intensity (%)

Relative Intensity (%)

263.2

10 0 150

200

250

300

m/z Figure 3. Representative positive ion AP/MALDI spectrum obtained from the analysis of indigo.

30

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0 200

The main chemical component of natural indigo is the indigotin (Fig. 2) (Andreotti et al. 2004). A representative AP/MALDI positive ion spectrum obtained from the natural indigo is shown in Figure 3. A quite evident signal at m/z 263.2 is present in the spectrum. The signal is attributed to the pseudomolecular species obtained from the protonation of the indigotin (MW = 262.2 Da). The spectrum shows a number of other signals most of which are due to ionic species formed by the CHCA matrix. To further verify the ability of AP/MALDI to identify organic pigments when present in complex matrices the indigo was mixed to an egg tempera (see Experimental section) and the obtained paint layer was analysed after its aging (Fig. 4). The mass spectra obtained from replicate analyses carried out by operating in air confirmed the presence of the protonated indigotin ion as shown in Figure 5 (m/z 263.1). The egg tempera was previously analyzed to check for the presence of ionic species having the same m/z observed for the indigo. A signal, due to unidentified ionic species formed by the tempera products, was observed at m/z 265.8 and did not affect the pigment identification (the experimental condition assured a spectral mass resolution lower that 0.5 Da).

265.8

220

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280

300

m/z Figure 5. Representative positive ion AP/MALDI spectrum obtained from the analysis of indigo egg tempera paint layer. The signal at m/z 265.8 is attributed to ion species formed by the egg medium.

3.2 Carmine Plant parasites from the coccidea family have been traditionally used for dye extraction. The dried body of the egg-filled female scale insect was typically used in both Central Americas and Europe for dye extraction. Carmine used as a pigment in traditional painting was prepared by precipitating the aluminium complex of the cochineal extract dye (also known as crimson lake) (Schweppe & Roosen-Runge 1986). The chromophores in all scale insect dyes are derivatives of anthraquinone. The major constituent is carminic acid (MW = 492.2 Da, see Fig. 6) but the various species have characteristic fingerprints of anthraquinone minor components. Also in this case the AP/MALDI analysis provided a clear identification of the pigment, as ions at m/z 493.1 resulting from the protonation of the carminic acid were easily detected (Fig. 7).

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CH3

O

OH

OH

HO

HOOC

Relative Intensity (%)

OH O

HO

OH

OH O

OH

Figure 6. Molecular structure of carminic acid (MW = 492 Da).

80 60 40

285.1

20

493.1

100

267.3

100

0 200

250

300

350

80

Figure 9. Representative positive ion AP/MALDI spectrum obtained from the analysis of the brazilwood pigment. The signal at m/z 285.1 is attributed to ions formed by the protonation of the brazilin ([M+H]+ ). The signal at m/z 267.3 is formed by dehydrated fragment ions ([M−H2 O+H]+ ).

60 40

10

20

Relative Intensity (%)

Relative Intensity (%)

m/z

0 420

450

480

510

540

570

m/z Figure 7. Representative positive ion AP/MALDI spectrum obtained from the analysis of the carmine lake pigment.

267.3

8 6 4

285.1 2

OH 0 200

HO O OH

300

350

Figure 10. Representative positive ion AP/MALDI spectrum obtained from the analysis of brazilwood egg tempera paint layer.

Figure 8. Molecular structure of brazilein (MW = 284.3).

3.3

250

m/z

O

TheAP/MALDI positive ion spectrum of the Brazilwood lake shows two intense signals at m/z 285.1 and m/z 267.3 (Fig. 9). The former is attributed to the pseudo-molecular ions formed by the protonation of the brazilin ([M+H]+ ). The latter is caused by dehydrated fragment ions ([M−H2 O+H]+ ). Both the signals allow a clear identification of the brazilwood dye also from the tempera paint film (Fig. 10).

Brazilwood

Soluble redwood dyes are extracted from various species of the genus caesalpinia and are collectively known as brazilwood. Their colouring principles are readily soluble in water and they are normally used as mordant dyes. Soluble redwoods were considered to have less dying properties than carmine or madder because they have poor fastness properties and therefore were usually used in combination with other dyes. Nevertheless, brazilwood has been reported to be used in European Renaissance paintings (Roy et al. 2004, Kirby et al. 2006). The main chromophore in brazilwoods is the brazilein (Fig. 8) which is obtained from the oxidation of the brazilin.

CONCLUSIONS AP/MALDI-MS investigation of traditional pigments was carried out in order to evaluate the applicability of such experimental approach to the analysis of organic

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Gabelica, V. et al. 2004. Internal energy build-up in matrixassisted laser desorption/ionization. J. Mass Spectrom. 39: 579–593. Grim, D. M. & Allison, J. 2004. Laser Desorption Mass Spectrometry as a Tool for the Analysis of Colorants: The Identification of Pigments Used in Illuminated Manuscripts. Archaeometry 46: 283–299. Kirby, J. et al. 2006. Proscribed pigments in northern european renaissance paintings and the case of paris red. Preprints of the 21st IIC Congress: The object in context, crossing conservation boundaries. Munich. Kuckova, S. et al. 2007. Identification of proteinaceous binders used in artworks by MALDI-TOF mass spectrometry Anal. Bioanal. Chem. 388: 201. Laiko, V. V. et al. 2000. Atmospheric pressure matrixassisted laser desorption/ionization Mass Spectrometry. Anal. Chem. 72: 652. Maier, M. S. et al. 2004. Matrix-assisted laser desorption and electrospray ionization mass spectrometry of carminic acid isolated from cochineal. Int. J. Mass Spectr. 232: 225. Mills, J. S. & White, R. 1994. The Organic Chemistry of Museum Objects. Oxford: Butterworth-Heinemann, Ltd. Peterman, S. M. & Mulholland, J. J. 2006. A novel approach for identification and characterization of glycoproteins using a hybrid linear ion trap/FT-ICR mass spectrometer. J. Am. Soc. Mass Spectrom. 17: 168. Roy, A. et al. 2004. Raphael’s Early work in the National Gallery. National Gallery Technical Bulletin 25: 5–35. Salvado, N. et al. 2005. Advantages of the use of SR-FTIR microspectroscopy: applications to cultural heritage. Anal. Chem. 77: 3444–3451. Schneider, B. B. et al. 2005. AP and vacuum MALDI on a QqLIT instrument. J. Am. Soc. Mass. Spectrom. 16: 176. Schweppe, H. & Roosen-Runge, H. 1986. Carmine – Cochineal Carmine and Kermes Carmine. In R. Feller (ed.), Artists’ Pigments: A Handbook of their History and Characteristics: 255–283. Cambridge University Press. Spoto, G. et al. 2000. Probing archaeological and artistic solid materials by spatially resolved analytical techniques. Chem. Soc. Rev. 29: 429–439. Spoto, G. 2002. Detecting Past Attempts To Restore Two Important Works of Art. Acc. Chem. Res. 35: 652–659. Spoto, G. 2007. Chemical Methods in Archaeology. In KirkOthmer (ed.), Encyclopedia of Chemical Technology. 5th Edition. New York: John Wiley & Sons Inc. Tokarski, C. et al. 2006. Identification of proteins in renaissance paintings by proteomics. Anal. Chem. 78: 1494. Vandenabeele, P. et al. 2007. A Decade of Raman Spectroscopy in Art and Archaeology. Chem. Rev. 107: 675–686. Van Den Berg, J. D. J. et al. 2004. Effects of traditional processing methods of linseed oil on the composition of its triacylglycerols. J. Sep. Sci. 27: 181. Williams, T. I. et al. 2007. Effect of matrix crystal structure on ion abundance of carbohydrates by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid Communications in Mass Spectrometry 21: 807–811. Zumbühl, S. et al. 1998. A graphite-assisted laser desorption/ionization study of light-induced aging in triterpene dammar and mastic varnishes. Anal. Chem. 70: 707.

materials used in art. Indigo, carmine and brazilwood lakes were analysed either alone or from egg tempera paint films. MS results were interpreted so that the main chromophore species could be identified even in the case of tempera paint films, demonstrating the applicability of this approach for in situ analysis. A home-built apparatus was used to place tridimensional objects on the focus of both the ion trap spectrometer and the laser source with a micrometric control, opening the field to a wide range of in situ applications. We think that the obtained results contribute to the overall quest for the most applicable and least destructive experimental approachs for the examination of the organic components of ancient works of art. ACKNOWLEDGEMENTS We would like to thank Dr. Paolo Cremonesi for comments and helpful discussions. We also thank MIUR for financial support (S.I.D.ART. project, contract n. 12828/SSPAR/2001).

REFERENCES Andreotti, A. et al. 2004. Characterisation of natural indigo and shellfish purple by mass spectrometric techniques. Rapid Comm. Mass Spectrom. 18: 1213–1220. Baranov, V. & Bohme, D. K. 1996. In situ collisional dissociation in a selected-ion flow tube: A novel, inexpensive SIFT-CID operation. Int. J. Mass Spectrom. 154 (1–2): 71. Berkout, V. D. et al. 2007. Modeling of ion processes in atmospheric pressure matrix-assisted laser desorption/ ionization. Rapid Communications in Mass Spectrometry 21 (13): 2046–2050. Boon, J. J. & Learner, T. 2002. Analytical mass spectrometry of artists’ acrylic emulsion paints by direct temperature resolved mass spectrometry and laser desorption ionisation mass spectrometry. J. Anal. Appl. Pyrolysis 64 (2): 327–344. Chiavari, G. et al. 2005. Identification of Indigo Dyes in Painting Layers by Pyrolysis Methylation and Silylation. A Case Study: “The Dinner of Emmaus” by G. Preti. Chromatographia 61, April (No. 7/8): 403–408. Ciliberto, E. & Spoto, G. 2000. Modern Analytical Methods in Art and Archaeology. New York: John Wiley & Sons Inc. Clark, R. J. H. 2007. Raman microscopy as a structural and analytical tool in the fields of art and archaeology. J. Mol. Struct. 74: 834–836. D’Agata, R. et al. 2007. The use of atmospheric pressure laser desorption mass spectrometry for the study of iron-gall ink. Applied Physics A: Materials Science & Processing 89: 91–95. Darque-Ceretti, E. & Aucouturier, M. 2004. Secondary ion mass spectrometry. Application to archaeology and art objects. Compr. Anal. Chem. 42: 397–461.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Study of chromophores of Islamic glasses from Al-Andalus (Murcia, Spain) N. Carmona, M. García-Heras & M.A. Villegas Instituto de Historia, Centro de Ciencias Humanas y Sociales, CCHS-CSIC, and Centro Nacional de Investigaciones Metalúrgicas, CENIM-CSIC, Madrid, Spain

P. Jiménez & J. Navarro Escuela de Estudios Árabes, EEA-CSIC, Granada, Spain

ABSTRACT: The recent discovery and excavation of 12th century AD urban glass workshops in the city of Murcia (Spain) have provided good evidences of glass production in the ancient Islamic territory of Al-Andalus. Among other findings, an important amount of bulk coloured and colourless glass fragments were unearthed during the archaeological works undertaken. This research presents the results obtained in the characterization of the chromophores responsible of the different colours found in the glass ensemble, namely turquoise blue, green bluish, emerald green, purple, yellow and red. The main goal of the study was to get some insights into the technology developed to obtain different colours in glasses. The resulting data have allowed the assignment of the ions responsible for each colour studied and have provided outstanding information on the colouring techniques used by the Islamic glassmakers of Al-Andalus.

1

INTRODUCTION

Up to now, little was known on technological aspects of Islamic glasses manufactured in the ancient territory of Al-Andalus (Southern Spain, AD 711–1492). However, the recent discovery and systematic archaeological excavation of urban glass workshops in the city of Murcia has changed this situation since, for the first time, it is possible to have glasses related to production sites with a chronology of mostly the 12th century AD (Jiménez Castillo et al. 2000). These workshops are the first evidence of glass production in the region of Murcia and are currently the only well documented inAl-Andalus, except the workshop of Pechina (Castillo & Martínez 2000). Throughout the 12th century and, particularly, during the kingdom of Ibn Mardanish, Murcia reached a high splendour to such as extent that it was one of the most prominent west Mediterranean cities of that time. For this reason, some Arab written sources reported that Murcia was an important glass production centre (e.g., De Gayangos 1984, Jiménez Castillo 2000). Ibn Mardanish fought against the Almohads, who came from north of Africa and rapidly dominated the most part of Al-Andalus, during more than two decades until the city fell inAD 1172 (Jiménez Castillo 2003). The location of Murcia and the approximate boundary between the Christian and Islamic territories

Figure 1. Map of the Iberian Peninsula showing the location of Murcia and the approximate boundary between the Christian and the Islamic territories around the 12th century AD.

of Al-Andalus around the 12th century AD are shown in Figure 1. One of these Murcian workshops is located in the Puxmarina street. The archaeological excavation of this site revealed a total of five well preserved furnaces and some remains of three others. These furnaces have been dated by archeomagnetism between AD 1100 and

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of the glass ensemble. This selection encompassed the whole range of colours. In the second place, a sample of turquoise blue, green bluish, emerald green, purple, yellow and red glasses were taken to characterize their corresponding chromophores. In both cases, those fragments without a recognizable typological form were preferentially selected to undertake destructive analyses. 2.2 Analytical techniques Chemical analyses were carried out by X-ray Fluorescence (XRF) using a Philips PW-1404 wavelength dispersed X-ray spectrometer equipped with a tube of rhodium. Analytical determinations were obtained through the standard-less analytical software Uniquant 4.22 based on fundamental parameters. Once external deposits were removed by polishing to avoid contaminations, powder samples were prepared by grinding body glass fragments in an agate mortar. Then, pressed boric acid pellets, using a mixture of n-butylmethacrylate and acetone (10:90 wt %) as bonding medium, were made for the XRF analyses. The characterization of the glass chromophores was undertaken by UV/VIS absorption spectrophotometry using a Shimadzu 3100 spectrophotometer attached with an integrating sphere. Spectra were acquired in the 380–800 nm range on transparent glass samples of approximately 1 mm in thickness. The samples were obtained by polishing both sides of the glasses with a manual rotating polisher using an aqueous suspension of cerium oxide to remove external deposits. To the best of the authors’ knowledge UV/VIS absorption spectrophotometry has been little used in archaeological glasses despite its advantages to investigate their colours and chromophores (Sanderson & Hutchings 1987, García-Heras & Villegas 2004).

Figure 2. Some of the glass fragments from the Puxmarina workshop (Murcia, Spain) in the state as-received in the laboratory. Scales are in cm.

1200 (Gómez-Paccard et al. 2006). Contextual information suggested that at least three of the furnaces could have been used for glass melting. The excavation also provided a very fragmented ensemble of glasses, together with some glassworking waste evidences such as glass dribbles and threads, melts from batches and crucible remains (Jiménez Castillo et al. 2005). The main glass forming technique was blowing, even though some flat glass fragments were also present. The majority of glasses were colourless or slightly yellowish and, less frequently, turquoise blue, green bluish and purple. Only a few fragments of emerald green, yellow and red glasses were documented. All of them were transparent and bulk coloured in those cases in which they had colour. Due to the fragmentary state of the ensemble, a very few number of shapes could be reconstructed, including small vases or unguentaria and small necked bottles. Decoration is only present in a reduced number of fragments and is composed of black, white and red paints. Figure 2 shows some of the glass fragments recovered in the excavation. The relevance of such findings has been explored through a project focused on the archaeometric characterization of the glass productions, using different physical-chemical techniques (Carmona et al., in press). One of the key goals of the project was the characterization of the chromophores or chemical species responsible of the different colours exhibited by the glasses found in the Puxmarina workshop. Such a research is presented in this paper and was aimed at providing some insights into the technology developed by the glassmakers of Murcia, in order to shed new light on the general topic of the Islamic glass technology of Al-Andalus.

2 2.1

3

RESULTS AND DISCUSSION

3.1 Chemical analysis According to the chemical data obtained by XRF, the glasses studied can be classified into two distinct groups: 1) soda-lime-silicate glasses [Na2 O-CaOSiO2 ] and 2) soda-lime lead-silicate glasses [Na2 OCaO-PbO-SiO2 ], which are characterized by a high content of lead oxide. All the colourless and most of the bulk coloured glasses belong to the first group, whereas the second one is only represented by emerald green glasses. Mean and standard deviation of the 13 main components of both groups are displayed in Table 1. The major component of soda-lime-silicate glasses is the network-former SiO2 (58.83 wt %). The glass network-modifier Na2 O shows a relatively high concentration (19.28 wt %), while the content of the

EXPERIMENTAL Samples selected

In the first place, a total of 21 fragments, including glasses and remains of melt batches from furnaces, were selected to determine the chemical composition

74

Table 1. Results derived from the XRF chemical analysis of glasses (weight %). Soda-lime-silicate (n = 419)

Soda-lime lead-silicate (n = 2)

Mean

Mean

Na2 O MgO Al2 O3 SiO2 P2 O5 SO2 Cl− K2 O CaO TiO2 MnO Fe2 O3 PbO

19.28 4.89 3.77 58.83 0.27 0.15 1.06 2.16 7.27 0.19 0.34 0.96 0.83

Total

100.00

S.D. 1.88 0.89 1.47 2.64 0.06 0.04 0.17 0.48 0.98 0.08 0.26 0.22 1.35

12.46 2.61 3.00 49.77 0.12 0.16 0.84 1.52 4.92 0.14 0.10 1.39 22.97

S.D. 3.10 0.69 1.51 3.68 0.02 0.04 0.08 0.50 0.14 0.03 0.04 1.29 7.79 Figure 3. Visible absorption spectrum from a bulk emerald green soda-lime lead-silicate glass.

100.00

S.D. Standard deviation (±).

(2.61 wt %) than in the first group of glasses. The rest of components are otherwise very similar in both groups. Soda-lime lead-silicate glasses can be classified as Islamic high lead oxide glasses, following the terminology of Sayre & Smith (1961), and are documented in the same period of time.

network-stabilizer CaO is 7.27 wt %. The amounts of other network-modifiers such as MgO and K2 O are 4.89 and 2.16 wt %, respectively. The content ofAl2 O3 , which is also a network-former oxide, is 3.77 wt %. The percentages of P2 O5 and SO2 are not higher than 0.30 wt % and chloride ions range around 1 wt %. Minor components determined were transition metals such as TiO2 (0.19 wt %), MnO (0.34 wt %) and Fe2 O3 (0.96 wt %). Iron and titanium oxides can be considered as impurities of the raw materials. However, the manganese oxide was intentionally added as a chromophore to provide the purple colour as is discussed in the next section. This first group of glasses can be classified as high magnesia plant ash glasses (HMG) according to the terminology proposed by Sayre & Smith (1961). The use of plant ashes as a source of sodium oxide is documented throughout the Islamic world between the nine and fifteenth centuries AD and is strongly indicated by the high contents of Na2 O and MgO, as well as the noticeable concentration of K2 O (Tab. 1). These indicators suggest that natron was not used as alkali source because the concentrations of MgO and K2 O had to be then lower or around 1.00 wt %. In the second group, that of soda-lime leadsilicate glasses, the major component is also SiO2 (49.77 wt %). In this group the content of PbO is around 23.00 wt %, which at high concentrations can play the role of a network-former oxide (Götz et al. 1976, Fernández Navarro 2003). The percentage of Na2 O is 12.46 wt %, that of CaO is around 5.00 wt % and the content of K2 O is 1.52 wt %. On the other hand, the concentration of MgO is a little bit lower

3.2 Characterization of chromophores Figure 3 shows the absorption spectrum from a bulk emerald green glass. According to chemical analysis data, it seems that this colour was only produced in soda-lime lead-silicate glasses. The sample presents a unique wide absorption band of high intensity which can be assigned to Cu2+ ions. The band shifts towards lower wavelengths up to 740 nm. This can be attributed to the incorporation of high contents of lead oxide to the glass network since, as mentioned above, it can play the role of a network-former oxide at high concentrations. The high polarisability of the Pb2+ ions induces the glass to asymmetric structures able to be deformed (Fernández Navarro 2003: 453). This fact shifts the absorption band of Cu2+ ions from ∼800 to ∼740 nm, thereby changing the colour from blue to emerald green. The presence of copper oxide in this glass (4.22 wt %) was confirmed by chemical analysis data obtained by XRF. The UV absorption edge of this sample is around 425 nm and is probably due to the presence of Fe3+ ions which absorb at 380, 420 and 440 nm. Therefore, the intense emerald green colour was achieved by the synergic effect of the Cu2+ ions, which progressively shift their absorption band from blue to green due to the presence of a high lead oxide content in the glass, and the yellow colour

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Figure 5. Visible absorption spectrum from a bulk green bluish soda-lime-silicate glass.

Figure 4. Visible absorption spectrum from a bulk turquoise blue soda-lime-silicate glass.

provided by the Fe3+ ions. On the other hand, in modern glasses the emerald green colouring is obtained through Cr3+ ions, which have a triple absorption band at 630, 650 and 675 nm (Bamford 1977). These bands do not appear in the spectrum of the emerald green glass. Figure 4 displays the absorption spectra obtained from a bulk turquoise blue soda-lime-silicate glass. It presents a unique wide band of moderate intensity, in comparison with the emerald green glass, between 780 and 810 nm produced by Cu2+ ions, which provide the characteristic bright turquoise blue colour.The presence of copper oxide in this glass was confirmed by XRF (1.70 wt %). In this case the band does not experience a shift towards lower wavelength, since the content of lead oxide is very low (1.19 wt % according to XRF data). It is important to note the absence of the triple absorption band produced by the Co2+ ions at 540, 590 and 640 nm (Bamford 1977), which gives rise also to a more intense blue colour. Cobalt oxide, therefore, was not used as a chromophore in the Puxmarina turquoise blue glasses. Figure 5 shows the absorption spectrum from a bulk green bluish soda-lime-silicate glass. It also presents a unique wide band between 780 and 810 nm which can be likewise assigned to Cu2+ ions. This band exhibits, however, a lower intensity in comparison with the turquoise blue glass. In addition, the absorption band shows a slightly shift up to approximately 760 nm, which produces that the turquoise blue colour turns into a light green hue. Such as shift can be also attributed to the noticeable amount of lead oxide in the glass (4.47 wt %). The concentration, however, is considerably lower than that determined in the emerald green glass and, therefore, the green colour is much

Figure 6. Visible absorption spectrum from a bulk purple soda-lime-silicate glass.

weaker. The presence of copper oxide in this sample (1.65 wt %) was also confirmed in this sample by means of XRF. As in emerald green or turquoise blue glasses, the absorption bands of Cr3+ and Co2+ ions do not appear either. The visible absorption spectrum from a bulk purple soda-lime-silicate glass is shown in Figure 6. The purple colouring presents a unique absorption band at around 499 nm. This band is produced by the Mn3+ ions, which provides an intense purple colouring to the glass. The purple colour is intensified in silica glasses as far as the glass alkalinity increases (Fernández Navarro 2003: 448) and, in fact, this sample was made from a type of glass with a high degree of alkalinity as is the case of the soda-lime-silicate glasses (Tab. 1). Chemical determinations by XRF confirmed

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Figure 7. Visible absorption spectrum from a bulk silver yellow glass probably of the soda-lime-silicate type.

Figure 8. Visible absorption spectrum from a bulk ruby red soda-lime-silicate glass.

or yellow colouring. Therefore, from the technological point of view, obtaining such colours is difficult. It is important to point out that the solubility of both copper and silver nanoparticles rises as far as the alkalinity and the lead oxide concentration of the glass increase (Fernández Navarro 2003: 472). This fact could explain why a certain amount of lead oxide (2.84 wt % detected by XRF) was present in the bulk ruby red glass.

the optical absorption results, since the concentration of manganese oxide in this glass was 1.14 wt %. The bulk yellow colour is due to the formation of Ag silver colloidal nanoparticles, which are responsible of the absorption recorded at around 400 nm in the spectrum of Figure 7. The introduction of silver compounds in the glass gives rise to the formation of colloidal nanoparticles through the following three steps process: 1) dissolution of Ag+ ions and incorporation into the glass network, 2) thermal reduction of Ag+ ions to Ag atoms by means of a reducing atmosphere, and 3) precipitation and aggregation of Ag atoms which tend to form colloidal nanoparticles responsible of the yellow colouring (Fernández Navarro & La Iglesia 1994). Due to the reduced number of bulk yellow glass fragments found and the low weight available, it was not possible to analyze them chemically by XRF. Consequently, the content of silver could not be analytically confirmed. In any case, a very low concentration of silver (from 0.05 to 0.50 wt %) is enough to provide the characteristic silver yellow colour to the glass (Fernández Navarro 2003: 472). Finally, Figure 8 displays the absorption spectrum from a bulk red soda-lime-silicate glass. It shows an intense and well-defined absorption band at around 560 nm, which is characteristic of copper ruby red glasses. The red colour is due to the formation of Cu+ /Cu colloidal nanoparticles in a similar process to the silver ones but using copper compounds. The minimum concentration of copper oxide to produce the ruby red colour is estimated in ∼0.50 wt % (Fernández Navarro 2003: 468). The content of copper oxide determined in this sample by XRF was 0.59 wt %. Either in the case of copper or in the case of silver colloids it is necessary to produce critical reducing conditions during the glass melting to develop ruby red

4

CONCLUSIONS

The results of the present research have allowed the assignment by UV/VIS absorption spectrophotometry of the ions responsible for each glass colouring studied. The characterization of these chromophores indicated that Murcian glassmakers of the 12th century AD used copper oxide compounds to obtain emerald green, turquoise blue, green bluish and ruby red colouring in glasses. That is, by using the same copperbased chromophore, they were able to produce three different bulk colours varying the sensitive redox conditions of copper oxide during the melting process of glass. They also employed manganese oxide to obtain purple and some silver compounds to obtain the yellow colour. The presence of these ions was confirmed through chemical analysis by XRF, except in the case of the silver yellow in which there was not enough amount of sample available to be chemically analyzed. The resulting data suggest, therefore, a deep knowledge of glass colouring techniques. This implies a high degree of specialization in glass production in which control over different colours and glass compositions was achieved. In this sense it is important to emphasize the use of a soda-lime lead-silicate glass to specifically produce the emerald green colour, since both

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en al-Andalus: 83–101. Madrid: Coeditions de la Casa de Velázquez. De Gayangos, P. 1984. The History of the Mohammadan Dinasties in Spain. Delhi: 2 vols. Fernández Navarro, J.M. 2003. El vidrio. Constitución, fabricación, propiedades. Madrid: CSIC (3rd edition). Fernández Navarro, J.M. & La Iglesia, A. 1994. Study of the red and yellow colour of glasses from the Cathedral of Toledo. Boletín de la Sociedad Española de Cerámica y Vidrio 33: 333–336. García-Heras, M. & Villegas, M.A. 2004. Notas para el estudio científico del vidrio antiguo. Zephyrus 57: 377–390. Gómez-Paccard, M., Chauvin, A., Lanos, Ph., Thiriot, J. & Jiménez Castillo, P. 2006. Archeomagnetic study of seven contemporaneous kilns from Murcia (Spain). Physics of the Earth and Planetary Interiors 157: 16–32. Götz, J., Hoebbel, D. & Wieker, W. 1976. On the constitution of silicate groupings in binary lead-silicate glasses. Journal of Non-Crystalline Solids 22: 391–398. Jiménez Castillo, P. 2000. El vidrio andalusí en Murcia. In P. Cressier (ed.), El vidrio en al-Andalus: 117–148. Madrid: Coeditions de la Casa de Velázquez. Jiménez Castillo, P. 2003. Murcia islámica. Una visión a través de la arqueología. Murcia: Ayuntamiento de Murcia. Jiménez Castillo, P., Muñoz, F. & Thiriot, J. 2000. Les ateliers urbains de verriers de Murcia au XIIè siècle (c. Puxmarina et pl. Belluga). In P. Pétrequin, P. Fluzin, J. Thiriot & P. Benoit (eds.), Arts du feu et productions artisanales, XX Rencontres Internationales d’Archéologie et d’Histoire d’Antibes: 433–452. Antibes: Editions APDCA. Jiménez Castillo, P., Navarro, J. & Thiriot, J. 2005. Taller de vidrio y casas andalusíes en Murcia. La excavación arqueológica del casón de Puxmarina. Memorias de Arqueología 13: 419–458. Sanderson, D.C.W. & Hutchings, J.B. 1987. The origin and measurement of colour in archaeological glasses. Glass Technology 28: 99–105. Sayre, E.V. & Smith, R.W. 1961. Compositional categories of ancient glass. Science 133: 1824–1826.

high contents of lead oxide and glass alkalinity favour the incorporation and solubility of copper oxide to the glass. However, it is unlikely that Murcian glassmakers were aware of this point beyond the empirical level. Overall, this research sheds new light on the Islamic glass technology developed in the ancient territory of Al-Andalus in which, up to now, little scientific evidence was available. ACKNOWLEDGEMENTS This work has been financed by projects 200510M068 co-founded by the General Office of Universities and Research from the Regional Government of Madrid and the Spanish National Research Council (CSIC), and CSIC-PIE 200610I031. The authors acknowledge the General Office of Culture from the Regional Government of Murcia and the Archaeological Museum of this city for providing glass samples and for their useful collaboration. Dr. N. Carmona acknowledges an I3P (CSIC-ESF) postdoctoral contract. Finally, the authors are indebted to the CSIC Thematic Network on Cultural Heritage for its professional support. REFERENCES Bamford, C.R. 1977. Colour generation and control in glass. Amsterdam: Elsevier Science Publishers. Carmona, N., Villegas, M.A., Jiménez Castillo, P., Navarro, J. & García-Heras, M. in press. Caracterización arqueométrica de vidrios andalusíes procedentes de talleres murcianos. Actas de las Jornadas sobre Vidrio de la Alta Edad Media y Andalusí 2006. La Granja: Fundación Centro Nacional del Vidrio. Castillo, F. & Martínez, R. 2000. Un taller de vidrio en Bayyana-Pechina (Almería). In P. Cressier (ed.), El vidrio

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Polychromed sculptures of Mercadante and Millán analysed by XRF non-destructive technique A. Križnar & M.A. Respaldiza Centro Nacional de Aceleradores, Universidad de Sevilla, Seville, Spain

M.V. Muñoz, F. de la Paz & M. Vega Museo de Bellas Artes de Sevilla, Seville, Spain

ABSTRACT: Lorenzo Mercadante de Bretaña and his pupil Pedro Millán belong to the most important medieval artists that worked in Andalusia in the second half of the 15th century. The Museum of Fine Arts of Seville has an exceptional collection of their big polychromed terracotta sculptures, among which are Mercadante’s “Virgin and Child” and three Milláns’s works, “Entombment of Christ”, “Christ Man of Sorrows” and “Christ bound to the column”. Their support and pigments were analysed by the X-ray Fluorescence (XRF) non-destructive technique. A high presence of Pb is related to Pb based compounds applied as a preparation/ imprimation, as a pigment or as a dryer. Red pigments are cinnabar, red earth and probably carmin. Blue pigments found are azurite, some organic blue and vivianite. Green colour is a Cu based pigment; in one case green earth was found. Brown colour is umbra. In several areas gold and silver were detected.

1

INTRODUCTION

the polychromy. They all form part of an exceptional collection of big terracotta sculptures from the late Gothic and Renaissance times and are permanently exhibited in the first room of the museum. Among them are Mercadante’s “Virgin and Child” (second half of the 15th Century) and three Millán’s works: “Entombment of Christ” (around 1490), “Christ Man of Sorrows” (1485–1503) and “Christ bound to the column” (second half of the 15th century). The last one was found broken in many pieces and had to be fully reconstructed. It lost most of the colour decoration as well.

Lorenzo Mercadante de Bretaña and his pupil Pedro Millán belong to the most important medieval artists that worked in Andalusia in the second half of the 15th and beginning of the 16th century. Their elaborated style and exceptional quality had a remarkable influence on the artistic production in southern Spain of that time. Lorenzo Mercadante could have been born in Italy (Moreno Mendoza et al. 1991), but he learned the arts in France, in Brittany, as reveals his name (de Bretaña). From there he came to Seville, where he is documented between 1454 and 1467, working on several orders for the town cathedral. Among the most important are the porticos of Nativity and Baptism. His style reveals a mixture of French court forms and Nordic spiritualism, as well as a realistic and meticulous modelling, inspired by the Flemish painting. Pedro Millán was his pupil and in the sixties his collaborator on the cathedral’s porticos. Millán carried on the master’s style, which he enriched with the traditional andalusian expression. He is documented between 1485 and 1507 and probably died before 1526. Both artists worked mostly in terracotta, but used also other materials. The terracotta sculptures had been polychromed, although a great part of the colour decoration is lost today, especially in the sculptures located outside. The Museum of Fine Arts of Seville holds four artworks from both masters that still preserve some of

2

OBJECTIVES

Until today, only few examples of polychromed terracotta sculpture have been preserved. The bibliography on this topic is also quite scarce. In this area, Seville has a special place, regarding a rich patrimony of big terracotta sculptures from the Late Gothic and Renasaince. Several studies on the cathedral sculptures of Mercadante and Millán have been carried out. Most of them are dealing with the ceramic composition and the environmental pollution (Arquillo Torres & Arquillo Torres 1992, Pérez-Rodríguez et al. 1995), while the technique of polychromy was not sufficiently studied (Jiménez del Haro et al. 2001). In all these mentioned four sculptures, the colour palette and the technique of

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analysing some single elements reference materials. All the measurements were done with fixed instrumental conditions (29.5 kV of applied high voltage, 80 µA of cathode current and 300 s of preset lifetime). A semi-quantitative analysis was done by using the areas of XRF peaks obtained in the multi-channel analyser. These areas can give a semi-quantitative estimation of the element concentrations, because they are proportional to the weight concentrations and their square root can serve as a measure of experimental error. Therefore, comparisons between the content of one particular element in different samples of similar composition can be made directly through the respective peaks of that element. However, a comparison of the concentrations between different elements can be only possible if a complete calculation (that is, making the corrections for X-ray production cross-sections, self-absortion, etc.) of the concentrations are made. The pigments applied in the polychromed sculptures were recognized on the basis of characteristic chemical elements from the XRF spectra of analysed points. The elements are identified by the energies of their characteristic X-ray peaks. The comparison of the counts per second of the different elements in a particular point with regard to the background, gives the possibility to ascertain the presence or not of a particular element in that analysed point. The spectra were also compared with a pigment database that was elaborated at CNA, analysing commercial pure pigments from old traditional recipes. Because of the complex 3D modelling of the sculptures it was difficult or even impossible to reach with the XRF head to many points of interest, especially those inside the drapery folds, where pigments are usually better preserved. Besides, in many areas some later polychromies applied over the original ones were appreciated, as well as a thick hard layer of wax and resin over the surface of the sculptures. All these facts complicated the interpretation of the XRF spectra and made more difficult to distinguish original colour layers from the posterior restoration interventions. On the other hand, it is not always possible to identify the precise pigment applied, because the XRF analysis offers the elemental and not the compositional results. There exist several pigments with the same characteristic chemical elements. In this case, this technique alone can not distinguish between them. Such is the case, for example, of green pigments based on Cu. There are several types of Cu pigments; some of them are historic (have been used in the past), which narrows the choices in the case of studying old artworks. But it is still not possible to say with exactitude whether it is malachite, verdigris or maybe some copper resinate. Also the XRF technique does not serve to identify organic materials, because it does not detect elements with atomic number Z lower than 13 or 14. That is why

the polychromy are of interest, as well as the artistic relationship between the master and the pupil. Until now this relationship was based only on the style comparison, but not on the materials and techniques applied by both artists. This paper tries to cover this deficiency.

3

EXPERIMENTAL PROCEDURE

The sculptures are not in a restoration process, so it was not considered convenient to extract any microsamples. For this reason the non-destructive technique of X-Ray Fluorescence (XRF) was chosen for the analysis of support and pigments. The XRF equipment used has an X-ray tube of 30 kV with anode of W and one SDD detector with energy resolution of 140 eV. A 1 mm Al filter was coupled to the tube to suppress the characteristic peaks of the anode. This instrument was used directly in situ, in the exhibition room (Fig. 1), during the days when the museum is closed to the public. All four artworks were analysed on the front side and on both lateral sides, radiating many different points of interest. Furthermore, “Entombment of Christ” and “Christ bound to the column” were analysed also on the back side. Only these two sculptures could have been removed from the wall without any risk of damages. It should be pointed out that in all four sculptures, their back side was not meant to be presented to the public eye, so they did not use to be elaborated or polychromated. Before each new measurement session, the XRF equipment was calibrated in energy, radiating the air and showing characteristic peaks of Ar, the Zr peaks from an internal collimator of the detector and

Figure 1. In situ analysis of the sculpture by portable XRF.

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dryer. According to Gómez (Gómez 2000), the use of minium was common in the technique of polychromy. Pacheco (Bassegoda i Hugas 1990) also speaks in his treaty about the use of a red lead substance in the imprimation, which he called azarcón. In the case of Mercadante’s and Millán’s sculptures there must be a combination of two or three of these uses, demonstrated by the variable quantities of Pb compounds in the analysed areas. Nevertheless, although the count numbers for Pb peaks in the XRF spectra vary widely (from 30 counts per second or cps to 720 cps), they always remain high if compared with other elements in each spectra. The high concentration of Pb in all polychromed areas reveals the possible existence of a preparation or an imprimation layer covering the whole surface. On the other hand, in the parts without polychromy, as on the back side of the sculpture “Entombment of Christ”, the count numbers of Pb are very low, almost insignificant. This difference confirms that lead exists above all in the polychromed areas.

organic pigments can not be detected by XRF. This fact complicates also the analysis of clay or terracotta with many chemical elements with a low Z (Na, Mg, Al, Si). Information on surface alteration processes can not be obtained by XRF. Nevertheless, the XRF technique is very important in the study of materials, especially in art, as it offers the first exam of the artwork, without touching it or damaging it in any way. With portable equipments, the tests can be run in situ, without the need to move or transport the piece of art. It is one of the best ways to obtain information about the materials applied in the artwork and to have the first overlook in the materials and technique of the artist. It also serves to discover possible later interventions revealing modern materials where there should only be traditional ones. Nevertheless, it is a good practice, when possible, to combine XRF with other complementary techniques to obtain more specific results. In the case of the four sculptures presented in this paper, there was no possibility to do so, as explained above. 4

4.3

RESULTS AND DISCUSSION

Pigments

A lot of literature has been published about historical pigments used in painting and in polychromy (Calvo 1997, Gómez 2000, Knoepfli et al. 1990, Montagna 1993, Serchi 1999, Schram & Herling 1995, West Fitzhugh et al. 1987–1997). The colour pallette found on the examined sculptures is very similar: white, carnations, red, blue, green, brown. Some decorative parts are gilded. Also the presence of silver was found in some areas, that could belong to some drapery decoration. Next, more detailed results for each sculpture are exposed.

4.1 Terracotta The analysis of the terracotta bulk of the four selected sculptures was limited to a number of small areas, where the colour layers are already lost. In all four of them, the spectra show two predominant peaks for Ca, Fe, and other peaks of lower intensities as Mg, Si and Mn. High peaks of Sr appear in all the spectra where also Ca peaks are of high intensity. Sr is associated to Ca and it is common to find them both together in the preparation layers of painted surfaces as well as in ceramic materials (Seccaroni & Moioli 2002). Zr is present in all XRF spectra due to an internal collimator of the SDD detector made with this material.

4.3.1 Lorenzo Mercadante: Virgin and Child The sculpture (Fig. 2) follows the iconographical type of gentle Madonna that was developed by Mercadante himself. Virgin Mary is standing; she wears a long red dress with a blue coat over it. Her hair is covered by a white wimple. She carries the Child on her left hand, her right hand is lost. The Child wears a blue tunic and his hands are also missing. On the other hand, another blue pigment seems to be used for the Virgin’s coat. The lack of characteristic chemical elements in the spectra indicates the use of some organic blue pigment or inorganic ultramar blue, which is difficult to detect by XRF because of the low Z of characteristic chemical elements of this pigment (Fig. 4). The inner part of the coat was decorated with a Cu based green and in parts with red earth (Ca, Fe). Gold (Au) as well as some traces of silver (Ag) were confirmed in the Virgin’s vestments, belonging to the decoration elements, mostly lost today. Silver

4.2 Lead compounds A considerable quantity of lead was detected in all analysed sculptures. In all spectra obtained from the polychromed areas, the L peaks of Pb are the highest and the predominant ones as compared with those of bulk terracotta. Such an intense presence of lead shows an important role of some lead pigment(s) in the sculptures. Lead can be due to various lead compounds, not possible to distinguish by XRF: lead white, basic lead carbonate (2 Pb(CO3 )2 · Pb(OH)2 ), yellow litharge, lead oxide (PbO) or orange-red minium, also lead oxide (Pb3 O4 ). (Seccaroni & Moioli 2002, West Fitzhugh et al. 1987–1997, Dornheim & San Andrés Moya 2004). In painting and polychromed sculpture, they can be used as a preparation/imprimation, as a pigment (pure or added to another one), or as a

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MERCADANTE: Virgin and child blue coat Virgin Pb Pb

Counts

104 Pb

Pb 103

Pb

Pb

Pb Fe Fe

Ca

Zr Zn

Pb

Zr

102

5

10

15

20

25

Energy (keV)

Figure 4. XRF spectrum of an analysed point from the Virgin’s blue coat (“Virgin and Child”). An organic blue pigment or inorganic ultramar blue was probably used.

Figure 2. Polychromed terracotta sculpture “Virgin and Child” by Lorenzo Mercadante (second half of 15th century). Height 1.34 m. 5

10

MERCADANTE: Virgin and child blue dress child Pb Pb 4

Counts

10

3

Pb

Pb

Fe

Pb

Pb

Zn Zn Pb

10

Ca

Zr

Fe Pb

Cu

Zr

2

10

5

10

15

20

Figure 5. Polychromed terracotta sculpture “Entombment of Christ” by Pedro Millán (around 1490), a) front side and b) back side. Height 0,64 m.

25

Energy (keV)

4.3.2 Pedro Millán: Entombment of Christ A horizontal composition of seven figures standing around the dead Christ lying on the white sarcophagus is one of the most beautiful Millán’s works (Fig. 5). The artist’s signature appears in the centre of the tomb. The figures are dressed in blue, red and green vestments. The women and the two men figures behind the tomb have their head covered with wimples, while the both

Figure 3. XRF spectrum of one analysed point from the Child’s blue dress (“Virgin and Child”). The pigment used could be vivianite.

is detected on dark areas on the red dress and in some parts of the blue coat. High peaks of Zn in the Mary’s nose reveal some earlier intervention on the face, applying zinc white.

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MILLÁN: Entombmentof Christ blue dress Pb Pb

Pb 104

104 Pb

Pb 103

Fe Pb Ca Ca

Pb

Cu Zn

Counts

Counts

MILLÁN: Entombment ofChrist blue back side Pb

Pb Zr

102

10

15

Pb Zr

Hg Pb

Pb

Fe

Zn Cu

Ca Zr

5

10

Pb

Fe

Pb

Pb Fe

3

Pb

Zr

2

10

20

5

25

10

15

20

25

Energy (keV)

Energy (keV)

Figure 7. XRF spectrum of an analysed point from the blue colour on the back side of the sculpture (“Entombment of Christ”). An organic blue pigment was used.

Figure 6. XRF spectrum of an analysed point from the blue tunic, left male figure (“Entombment of Christ”). Azurite was found to be the applied pigment.

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men on the side with a cap. Christ lies on a sheet; his head is turned towards the spectator. This sculpture (0.64 m high) was examined in 45 points. These were chosen on both lateral figures, on the sarcophagus and on the Christ’s body. The figures behind the tomb could not be reached by XRF equipment from the front side, but they were analysed in some points on the back side, polychromed on the upper area of heads and shoulders. As said above, this sculpture was one of two that offered the possibility of analysis also on the back side. Lead white (Pb) was used for white colours. For carnations it was mixed with cinnabar (Hg). The Christ’s skin is paler than that of other figures in the presentation, that is why it contains less red pigment; the count numbers of Hg are lower and those of Pb much higher than in other analysed points of carnations. The red colour is a mixture of cinnabar (Hg) and red earth (Fe). Some interesting differences between the pigments on the front and on the back side were discovered. The blue pigment in the front is azurite, confirmed by the high peaks of Cu (Fig. 6), while in the back some cheaper, probably organic pigment was applied. There are no characteristic chemical elements for an inorganic blue pigment revealed by the spectra (Fig. 7). It is possible that the blue colour found on the back side belongs to some restoration work and not to the original palette. For the green colour, two different pigments were also used, identified by different characteristic chemical elements. In the front side a copper based green pigment was applied, revealed by high count numbers of Cu peaks (Fig. 8). It could belong to malachite, verdigris or some copper resinate. In the back side, the peaks of Si, Mn and Fe show the use of a green earth. The brown pigment on the hair is umbra (Mn, Fe). The edges of the sarcophagus are gilded (Au). On

MILLÁN: Entombment of Christ green dress Pb Pb

Cu 104 Counts

Cu Fe 103

Pb Pb

Ca Pb

Mn Ca

Fe

Pb Zr

Pb Hg

Pb Zr

2

10

5

10

15

20

25

Energy (keV)

Figure 8. XRF spectrum of an analysed point from the green tunic of the right male figure (“Entombment of Christ”). A copper based green pigment was applied.

the tools and in some azurite areas silver (Ag) was detected. Probably the tools were covered by a silver foil that oxidised. On the other hand, Ag peaks in the spectra of blue azurite areas could belong to the blue pigment itself (Seccaroni & Moioli 2002). 4.3.3 Pedro Millán: Christ Man of Sorrows This artistic work (Fig. 9) is organized in a sense of symbolic hierarchy with the dominant central figure of Christ, smaller figures of two angels at both sides and the smallest and iconographically less important kneeling donor. The sculpture presents a complex example of polychromy, although lost in parts. Christ wears a white shroud, a red coat over his back and shoulders and the spine crown on his head. He is defined by his five sores. Two angels are supporting the heavy coat of Christ with their inner hand, while with the outer one they carry

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MILLÁN: Christ Manof Sorrows carnation angel face angel

Pb

Pb

Counts

104 Fe

10

Pb

Pb Hg Hg Pb

3

Zr Pb

Fe Cu

Ca

Pb

Zr

Pb Ca

Ti

102

5

10

15

20

25

Energy (keV)

Figure 10. Comparison of two XRF spectra of the left angel’s face: area with polychromy (straight line) and area without it (dot line) (“Christ Man of Sorrow”). 105

MILLÁN: Christ Man of Sorrows grey coat Christ Pb

Pb

Counts

104 Hg Fe

3

10

Ca

2

10

golden attributes, related to the Passion of Christ. They both wear white tunics and a blue coat over it, golden strip is decorating their heads. The donor, kneeling at the left side of Christ, wears a blue dress and a red coat. The ground and the abundant vegetation seem of green-greyish colour. The sculpture (1.65 m high) was analysed in 84 points. They were chosen in diverse areas of carnations and vestments to compare results of different colours. A detailed study of this sculpture has been published elsewhere (Kriznar et al. 2008). The colour layers are very complex, with at least one re-touching belonging to some earlier restoration works. It was possible to establish the basic colour palette, but without stratigraphic sections it could not be determined whether they belong to the original or restored parts. The white pigment is lead white (Pb). The carnations are made with lead white (Pb) and cinnabar (Hg) (Fig. 10). Maybe also an organic red pigment as carmine was added, but it is not possible to identify it by XRF. In the Christ’s carnation also a low presence of copper based green pigment was detected. In the spectra of this area appear Cu peaks that are not present in the spectra of other carnations in this sculpture. The red colour is a mixture of cinnabar (Hg) and small amount of red earth (Fe). In some areas of the

Cu

Pb

Pb Pb Hg

Fe Hg

Pb

Figure 9. Polychromed terracotta sculpture “Christ Man of Sorrow” by Pedro Millán (1485–1503). Height 1,65 m.

Hg

Pb

Pb Zr

5

Ag Zr

Ca

10

15

20

25

Energy (keV)

Figure 11. XRF spectra of an analysed point of a grayish layer from the Christ’s coat (“Christ Man of Sorrow”). Detected silver probably belongs to already lost silver foil.

Christ’s coat, a dark red colour can be seen at a closer look in situ. The XRF spectra of selected analysed points do not show any characteristic chemical elements for any inorganic red pigment. In this case, the dark red layer is probably an organic colour. The blue pigment is azurite (Cu), maybe mixed with vivianite (Fe) in the angel’s coats. Fe could belong also to some red imprimation under the blue layer. On both coats blue over paint is clearly seen whose rests cover the gilded borders. The green colour of the plants is some Cu based pigment, malachite, some copper resinate or verdigris. With the XRF technique it is not possible to distinguish between them. The brownish hair colour is made with umbra (Mn, Fe). Gold (Au) was confirmed on gilded parts of the vestments, on Christ’s sores and on angels’s hair. Well defined peaks of silver (Ag) were detected in some areas of the Christ’s brooch, his coat and the

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MILLÁN: Christbound to the coloumn carnation leg

104 Counts

Fe Pb 103

Ca Ti K Ca Ti

102

Pb Zr

Fe

Pb Pb Zn

Mn

Zr

Pb

Cu 101 5

10

15

20

25

Energy (keV)

Figure 13. XRF spectrum of an analysed point in the restored area from the Christ’s left leg (“Christ bound to the column”).

polychromy is conserved, except on Christ’s hands and face. For this reason only 20 points were chosen in order to get information about the basic material used that is terracotta. Some rests of the polychromy were analysed, as well. Also this sculpture is the highest of all four (1.88 m), could be removed from the wall and analysed on the back side. On the bases of the column, a small probe was found. It was previously made by the restorers of the museum, and allowed the examination of the original material of the sculpture. This was also possible on the damaged right elbow. The cleaning test in layers revealed that the whole surface of the artwork was covered with a unifying layer to hide contacts between the broken pieces. The presence of this new layer was confirmed also by the XRF results, which show an important difference between the areas that still conserve original polychromy and the ones with the new unifying layer. The basic difference is the high presence of Ti, Zn and Fe in the areas with the mentioned layer (Fig. 13), while in the ones with polychromy the predominant element is Pb. This element reveals the presence of lead white, together with a small amount of cinnabar (Hg) for the carnation (Fig. 14), conserved in both hands, on the face and on the left knee. The Ti and Zn elements belong to Ti-Zn white, mixed with ochre (Fe). With the help of the XRF analysis it was possible to determine which parts are covered with this layer and which are not.

Figure 12. Polychromed terracotta sculpture “Christ bound to the column” by Pedro Millán (2/2 15th century). Height 1.88 m.

coats of both angels. In the case of the Christ’s coat (Fig. 11) it probably belongs to an already lost silver foil which was later decorated with the organic red colour to render it shinier. The blue azurite and red cinnabar, also discovered in this coat, belong to later re-polychromies. Silver found on the angel’s coats could be a part of the blue pigment itself (Seccaroni, Moioli 2002) or of some drapery decoration that is not preserved anymore.

5

4.3.4 Pedro Millán: Christ bound to the column This sculpture (Fig. 12) was considered lost (Gestoso 1984), but it was found during excavations in the church of Santa Ana in Seville in 1971 (Moreno Mendoza et al. 1991). It was broken into 168 pieces and it had to be totally reconstructed. Almost no

CONCLUSIONS

The polychromed terracotta sculptures of Lorenzo Mercadante and Pedro Millán, exposed in the Museum of Fine Arts of Seville were analysed by the nondestructive technique of XRF. The support and the

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pigments applied are of interest, as well as the technical connection between the master and the pupil. The results, summarised in Table 1, revealed that both of them used the same bulk terracotta base, not much elaborated at the back sides of the sculptures. The polychromy was applied only on the front side and not at the back. On all the sculptures the results showed the use of similar inorganic pigments: lead white (Pb), red cinnabar (Hg), red earth (Ca, Fe), copper based green pigments (Cu) and umbra (Mn, Fe). In the choice of blue pigments the two artists differed. Mercadante preferred to use an organic blue colour or an inorganic

105

Counts

10

ultramar, difficult to determine with XRF. Also vivianite was detected in his work. On the other hand, Millán preferred to apply azurite. In all four sculptures an organic red pigment, such as carmine (which can not be identified by XRF), could be present. In some cases, it is difficult to know for sure if the colour layers are the original ones or belong to some later retouching. In all analysed artworks, gold was confirmed for the decorative elements. The presence of silver, that is not seen with the naked eye but is present in the spectra, is difficult to understand. It can belong to some already lost silver foil or could form part of a blue pigment. REFERENCES

MILLÁN: Christ bound to the coloumn hand

4

Pb

Arquillo Torres F. & Arquillo Torres J. 1992. ¡¡Salvemos los “Mercadantes” de las portadas del Bautismo y del Nacimiento en la catedral de Sevilla!! IX Congreso de conservación y restauración de bienes culturales: 403–421. Bassegoda i Hugas B. (ed.) 1990. Francisco Pacheco: El Arte de la Pintura. Madrid: Cátedra. Calvo A. 1997. Conservación y Restauración, Materiales, técnicas y procedimientos, de la A a la Z. Edición del Serbal: Barcelona. Dornheim S.D. & San Andrés Moya M. 2004. Litargirio y masicote. Terminología, propiedades y usos. Reproducción a escala de laboratorio de algunos de sus procesos de obtención. XV. Congreso de conservación y restauración de bienes culturales 2004, Actas, Vol. I: 533–546. Gestoso J. 1984. Sevilla Monumental y Artística. Sevilla: Monte de Piedad y Caja de Ahorros Sevilla. Gómez M. L. 2000. Examen científico aplicado a la conservación de obras de arte. Madrid: Cátedra, Instituto del Patrimonio Histórico Español.

Pb

Fe 103 Fe

Ca

Pb Pb

Pb

K Ca Ar Ti

2

10

Zr

Zn Hg Mn

5

Pb Zr Pb Pb

Cu

10

15

20

25

Energy (keV)

Figure 14. XRF spectrum of an analysed point in an original polychromated area from Christ’s left hand (“Christ bound to the column”) showing the use of lead white (Pb) and cinnabar (Hg) pigments.

Table 1.

Comparison of the used pigments in different areas of the four analysed sculptures.

Virgin and Child Terracotta

Terracotta (Ca, Fe/Mg, Si, Mn) White pigment Lead white (Pb) Carnations Lead white (Pb)+ cinnabar (Hg)

Entombment of Christ

Christ Man of Sorrows

Terracotta (Ca, Fe/Mg, Si, Mn) Lead white (Pb) Lead white (Pb) + cinnabar (Hg)

Terracotta (Ca, Fe/Mg, Si, Mn) Lead white (Pb) Lead white (Pb) + cinnabar (Hg) (+Cu based green pigment − Christ) Cinnabar (Hg) + red earth (Fe) Carmine? Azurite (Cu) vivianite? (Fe) − angel’s coats Cu based pigment (Cu)

Cinnabar (Hg) + red Cinnabar (Hg) + red earth (Fe) Carmine? earth (Fe) Carmine? Blue pigment Vivianite? (Fe, Zn) − Azurite (Cu) − front side Virgin Organic/ ultramar Organic blue − back side blue − Child Green pigment Cu based pigment (Cu) Cu based pigment − front side Green earth (Si, Mn, Fe) − back Brown pigment Umbra (Mn, Fe) Umbra (Mn, Fe) Umbra (Mn, Fe)

Red pigment

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Christ bound to the column Terracotta (Ca, Fe/Mg, Si, Mn) Lead white (Pb) Lead white (Pb) + cinnabar (Hg) Not preserved Not preserved

Not preserved

Not preserved

Jiménez del Haro M.C., Pérez Rodríguez J.L. & Justo A. 2001. La técnica del brocado para decorar las cerámicas de las puertas de la catedral de Sevilla. III. Congreso Nacional de Arqueometría: 325–333. Knoepfli A., Emmenegger O., Koller M. & Meyer A. (eds) 1990. Reclams Handbuch der künstlerischen Techniken. Vol 1–3. Stuttgart: Philipp Reclam jun. Kriznar A., Muñoz M.V., De la Paz F., Respaldiza M.A. & Vega M. 2008. Pigment identification using X-ray fluorescence in a polychromated sculpture by Pedro Millán. X-ray Spectrometry. Vol. 37. Chichester: Wiley Publisher. (in press) Montanga G. 1993. I pigmenti. Prontuario per l’arte e il restauro. Firenze: Nardini editore. Moreno Mendoza A., Pareja López E., Sanz Serrano M. & Valdivieso Gonzáles E. 1991. Museo de Bellas Artes de Sevilla. Sevilla: Ediciones Gever, S.L.

Pérez-Rodríguez J.L., Jiménez de Haro M.C., Justo A., Maqueda C. & Ruiz Conde A. 1995. Study of the ceramic sculptures of the birth and baptism porticos of Seville Cathedral. The Ceramics Cultural Heritage: 635–642. Seccaroni C. & Moioli P. 2002. Fluorescenza X. Prontuario per l’analisi XRF portatile applicata a superfici policrome. Firenze: Nardini editore. Serchi M. (ed) 1999. Cennino Cennini: Il Libro dell’Arte. Firenze: Felice Le Monnier. Schram H.P. & Herling B. 1995 Historische Malmaterialen und ihre Identifizierung. Stuttgart: Ravensburg Buchverlag. West Fitzhugh E., Feller R. & Roy A. (eds.) 1987-1997. Artists’ pigments. A Handbook of their history and characterisation. New York, Oxford: National Gallery of Art: Washington, Oxford University Press.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Litharge and massicot: Thermal decomposition synthetic route for basic lead(II) carbonate and Raman spectroscopy analysis M. San Andrés, J.M. De la Roja & S.D. Dornheim Facultad de Bellas Artes, Universidad Complutense, Madrid, Spain

V.G. Baonza Facultad de Ciencias Químicas, Universidad Complutense, Madrid, Spain

ABSTRACT: Litharge (α-PbO) and massicot (β-PbO) are historical lead-based pigments which have been identified in paintings. In this work we reproduce in the laboratory the synthetic route of both by roasting lead white. The resulting products have been analyzed using X-ray Diffraction (XRD) and Raman microscopy. These analyses indicate that after 45 minutes of roasting at 873 K, lead white is transformed into massicot and this composition remains unchanged after 2 hours and 15 minutes. However, when the roasting is at 603 K, the transformation depends on heating time. In the first stages the roasting process gives rise to the formation of mixtures of (xPbCO3 · yPbO), with x and y = 1 or 2. Litharge appears after 2 hours and after 20 hours the compound formed is red lead (Pb3 O4 ) mixed with other lead oxides. The presence of different compounds influences the final colour of the pigment

1

INTRODUCTION

historical pigments, in which the properties of the products are described. For instance, litharge tonality is defined within a range varying from clear yellow to reddish yellow. Merrifield (1967) and Gettens (1996) define the colour range of litharge as yelloworange; Burgio et al. (2001) oscillating from yellow to red; Bearn (1923) describe three litharge forms: canary litharge (pale yellow), ordinary litharge (reddish yellow) and litharge in grudges (reddish). The wide chromatic variation found for this pigment very likely responds to changes in composition that could be related to the synthetic process. The aim of our study work is to reproduce, at laboratory scale, the synthetic process of these pigments and to study its evolution. In studies made by other authors, the effect of temperature was studied (Ciomartan et al. 1996); in our case, temperature was kept constant and the evolution of the process on terms of the roasting time is studied. Given the specificity of Raman microscopy, we use it to identify lead white and all the compounds formed at different stages of the transformation. Our results have been complemented with X-ray diffraction (XRD) experiments. In addition, we have carried out a colorimetric characterization of the products formed at each stage (De la Roja et al. 2007). Considering the previous studies (Burgio et al. 2001), our samples have been excited at low power with a He-Ne laser (632.8 nm) as excitation source.

There exist a large variety of lead pigments, all are of artificial origin and their synthetic procedures were known in the Antiquity. The less studied are litharge (α-PbO) and massicot (β-PbO) with the same elemental composition (like lead monoxide PbO) but differ in their crystalline structure. The old terminology used to refer to both is confusing and only a few recipes appear in the old texts (Dornheim et al. 2006). In general, the two varieties are rather reactive. However, in spite of these disadvantages, they have been identified in paintings. The characterization by Raman microscopy of litharge (α-PbO) and massicot (β-PbO) as pure chemical compounds is well defined in several available sources (Bell et al. 1997, Bouchard et al. 2003). Studies on the synthesis process of these pigments from roasting the lead white reveal that the evolution of the process is determined by different factors: amount of lead white, surface of sample directly exposed to heat and, of course, the temperature and duration of the roasting process. Although all these experimental variables can be precisely controlled nowadays, in the Antiquity it was not an easy task. Therefore the results obtained in the synthesis process could vary in relation to the composition of the product and its colorimetric characteristics. This fact is reflected in several literature sources on

89

2 2.1

EXPERIMENTAL Samples

We used lead white samples purchased to Panreac© to obtain the desired pigments. XRD analysis indicates that the pigment consists of lead (II) dihydro-biscarbonate [Pb3 (CO3 )2 (OH)2 ] and lead (II) carbonate (PbCO3 ). Litharge was obtained from lead white subjected to 603 K during 20 h, while the synthesis of massicot requires application of 873 K and this value was kept for 2 hours and 15 minutes. In both cases, we took samples at selected time intervals along the transformation process for analysis. When the processes were completed, the samples were removed, stirred and kept at ambient conditions. 2.2 Analytical techniques The resulting products have been analyzed by combined XRD and Raman spectroscopy, two techniques well-established in the literature (Ciomartan et al. 1996). The Raman spectrum was excited using the 632.8 nm line of a He-Ne laser. The equipment is composed of a 100X microscope, an ISA HR460 monochromator, and a CCD detector (1024 × 256 pixels). The spectral resolution is less than 4 cm−1 using a 600 grooves/mm holographic grating. The XRD study was performed with a Philips X‘PERT diffractometer using a voltage of 45 kV and intensity of 40 mA. This uses two slits, a 1◦ divergence slit for primary optics and a 1◦ anti-scatter slit (receiving slip 0.5 mm) for secondary optics. A curved Cu monochromator was used to eliminate the contribution of the Kβ line. 3

Figure 1. Pigments obtained by thermal decomposition of lead white at 873 K as a function of time. Table 1. Results of XRD analysis of thermal decomposition of lead white at 873 K as a function of roasting time. Sample (roasting time in hours) 0 1/4

3/4

RESULTS AND DISCUSION

Figure 1 shows the products obtained after roasting of lead white at 873 K. The analysis of the XRD patterns revealed that after 15 minutes treatment a mixture of litharge (α-PbO), massicot (β-PbO), and lead oxide carbonate [Pb3 O2 (CO3 )] is formed, together with unreacted lead white. However, if the treatment is extended up to 45 minutes, only the massicot phase is obtained as a primary product, together with a mixture of lead oxides (Pb2 O3 ) and (Pb5 O8 ). When the treatment is extended to 75 minutes, the final product is a mixture of massicot and Pb5 O8 . This composition remains unaltered after 135 minutes of treatment (see Table 1). The comparison of our Raman results with existing spectra (Ciomartan et al. 1996, Bell et al. 1997) confirms the results obtained by X-ray analysis. After 15 minutes the characteristic bands can be assigned to litharge (339 cm−1 ) and massicot (289 and 384 cm−1 ). Other bands can be attributed to the

1 1/4 1 3/4 2 1/4

Ref. Code (JCPDS)

Score

00-047-1734 00-013-0131 03-065-0398 03-065-0129 00-019-0681 00-001-0680 00-001-0687

60 42 51 45 38 20 18

03-065-0129 00-052-0772 00-036-0725 03-065-0129 00-052-0772 03-065-0129 00-052-0772 03-065-0129 00-052-0772

85 17 12 68 12 67 11 56 48

Crystalline species identified PbCO3 Pb3 (CO3 )2 (OH)2

α-PbO (litharge) β-PbO (massicot) Pb3 O2 CO3 Pb10 (CO3 )6 (OH)6 O 2PbCO3 · Pb(OH)2 β-PbO (massicot) Pb5 O8 Pb2 O3

β-PbO (massicot) Pb5 O8 β-PbO (massicot) Pb5 O8 β-PbO (massicot) Pb5 O8

lead (II) hydroxycarbonates [xPbCO3 · yPbO · (OH)z ] [1033, 1051 and 1056 cm−1 ]. After 45 minutes the bands assigned to massicot and a new band appears around 422 cm−1 . These spectral features remain unaltered after 135 minutes (288, 384, 422 cm−1 ) (see Table 2 and Fig. 2). The band appearing at 422 cm−1 might be related to the formation of the Pb5 O8 compound identified by X-ray analysis. Figure 3 shows the products obtained after a roasting treatment of lead white at 603 K. It is obvious that

90

Table 2. Raman bands (wavenumber/cm−1 ) of thermal decomposition of basic lead (II) carbonate at 873 K as a function of roasting time and reference compounds. Samples

0h – – – – – – – 414 w(br) – – – 678 vw – 694 vw – 835 vw

Reference Compounds

1/4 h

– 289 m – – 339 m – 384 vw – – – – – – – – – – – 1033 vw 1052 vs 1051 vw – 1056 vw(sh) 1369 w(br) – – – 1476 vw(br) – – – – – – –

3/4 h

2PbCO3 . Litharge Massicot Pb(OH)2 (Bell et al. (Bell et al. (Ciomartan 1 1/4 h 1 3/4 h 2 1/4 h 1997) 1997) et al. 1996)

Red Lead PbCO3 (Bell et al. (Ciomartan 1997) et al. 1996)

– 288 m – – – – 385 m – 422 w – – – – – – – – – 1051 vw – – – – – – –

– 289 s – – – – 384 m – 423 w – – – – – – – – – – – – – – – – –

– – 313 w – 340 vw – 390 w – – 480 vw 548 vs – – – – – – – – – – – – – – –

– 288 s – – – – 384 m – 424 w – – – – – – – – – – – – – – – – –

– 288 s – – – – 384 m – 422 w – – – – – – – – – – – – – – – – –

– 285 vw – – 336 w – – – – – – – – – – – – – – – – – – – – –

– 289 s – – – – 385 w – – – – – – – – – – – – – – – – – – –

267 vw – – 321 vw – – – 411 w – – – 679 vw – 693 vw 707 vw 837 vw 862 vw – 1050 vs – 1365 m – 1467 vw – – 1731 vw

– – – – – – – – – – – 673 w 682 w 694 vw 715 vw 837 w – – – 1054 vs 1364 m 1425 w 1476 m 1679 vw – 1735 vw

Very weak (vw); weak (w); medium (m); strong (s); very strong (vs); shoulder (sh); broad (br)

Figure 3. Pigments obtained by thermal decomposition of basic lead (II) carbonate at 603 K as a function of time.

Figure 2. Raman spectra of pigments obtained by thermal decomposition of lead white at 873 K as a function of time.

91

Table 3. Raman bands (wavenumber/cm−1 ) of thermal decomposition of basic lead (II) carbonate at 603 K as a function of time samples and reference compounds. Samples

Reference Compounds

0h

1/2 h

1h

2h

3h

4h

20 h

Litharge (Bell et al. 1997)

Massicot (Bell et al. 1997)

2PbCO3 . Pb(OH)2 (Ciomartan et al. 1996)

Red Lead (Bell et al. 1997)

PbCO3 (Ciomartan et al. 1996)

– – – – –

261 w 285 w – – –

262 vw 283 vw – – –

– 280 w – – 338 w

– 284 w – – 340 m

– 286 vw 310 vw – –

– 285 vw – – 336 w

– 289 s – – –

267 vw – – 321 vw –

– – 313 w – 340 vw

– – – – –

– – – 414 w (br) – – – – – 678 vw – 694 vw – 835 vw – – 1052 vs

360 w – – –

360 w – – –

261 vw 283 vw – – 338 vw (sh) 360 w – – –

359 m – 407 vw –

359 w – 406 vw –

– 389 vw – –

– – – –

– 385 w – –

– – – 411 w

– 390 w – –

– – – –

467 w – 494 vw – – 674 vw – 699 vw – – – 1034 vw 1052 s (sh) 1056 vs – – 1372 w (br) 1420 vw (br) –

467 w – – – – 675 vw – 697 vw – – – 1033 vw 1051 s (sh) 1055 vs – – 1373 w

465 w – – – 654 vw 674 vw – 698 vw – – – 1033 vw 1050 s (sh) 1057 vs – – 1371 w

463 m – – – 656 w 674 w – 696 vw – – – 1033 s 1049 vs

463 m – – – 655 w 675 w – 696 vw – – – 1035 s 1049 vs

– 475 vw – 545 vs – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – 679 vw – 693 vw 707 vw 837 vw 862 vw – 1050 vs

– 480 vw – 548 vs – – – – – – – – –

– – – – – 673 w 682 w 694 vw 715 vw 837 w – – –

– 1284 vw 1327 vw –

– – – –

– – – –

– – – –

– – – 1365 m

– – – –

1054 vs – – 1364 m

1425 vw (br) –

1428 w (br) –

– 1282 vw 1325 vw 1370 vw (br) 1431 m

1430 m

–

–

–

–

–

1425 w

–

–

–

–

–

1467 vw

–

1476 m

– – –

– – –

– – –

1679 vw 1693 vw –

1679 vw 1693 vw –

– – –

– – –

– – –

– – 1731 vw

– – –

1679 vw – 1735 vw

– – – 1369 w (br) – 1476 vw (br) – – –

Very weak (vw); weak (w); medium (m); strong (s); very strong (vs); shoulder (sh); broad (br)

the roasting time leads to important colour variations. Our analyses confirm that this phenomenon is intimately related to the composition of the transformed products (Tables 3 and 4, Fig. 4). The analysis of the XRD patterns (Table 4) indicate that the roasting process gives rise to the formation of mixtures of (xPbCO3 · yPbO), with x and y = 1 or 2. Litharge, mixed with lead oxides carbonates, is obtained after 3–4 hours of roasting time. After 20 hours the product obtained is a mixture of red lead (Pb3 O4 ) and Pb2 O3 , as confirmed

by XRD. Previous studies suggested that red lead already appears after 12 hours (De la Roja et al. 2007). The comparison of our Raman spectra and those reported by Ciomartan (1996) reveals that some characteristic bands can be attributed to lead oxide carbonates: 359, 463, 656, 676, 697 and 1431 cm−1 . The bands of litharge: 285 and 338 cm−1 appear after 2 h and after 20 h the compound formed is red lead (Pb3 O4 ): 310, 389, 475 and 545 cm−1 (Fig. 4 and Table 3).

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Table 4. Results of XRD analysis of thermal decomposition of lead white at 603 K as a function of roasting time. Sample (roasting time) (h)

Ref. Code (JCPDS)

Score

0

00-047-1734 00-013-0131

60 42

1/2

00-017-0730 00-048-1888 00-019-0681

46 28 23

1

00-017-0730 00-019-0681 00-048-1888

44 37 31

2

00-019-0681 00-017-0730 00-019-0682

46 45 22

Pb3 C2 O7 Pb3 O2 CO3 Pb2 OCO3 Pb3 O2 CO3 Pb3 C2 O7 Pb2 OCO3

3

00-019-0681 00-017-0730 00-048-1888 03-065-0401 00-019-0681 03-065-0400 00-008-0019 00-001-0654 01-076-1832

53 26 21 18 54 36 52 25 47

Pb3 O2 CO3 Pb3 C2 O7 Pb2 OCO3 α-PbO (litharge) Pb3 O2 CO3 α-PbO (litharge) Pb3 O4 (red lead) Pb3 O4 Pb2 O3

4 20

depends strongly on both the temperature and the roasting time.The results for the two temperatures used in this work (603 and 873 K) indicated that the early stages of the process leads to the formation of complex mixtures of lead oxide carbonates. After 45 minutes of roasting at 873 K massicot (β-PbO) is formed along with other lead oxides. At still higher roasting times the composition seems to stabilize and the transformed product is a mixture of massicot and Pb5 O8 . Longer roasting times (3–4 hours) are required at lower temperatures to obtain litharge (β-PbO). Finally, if the roasting time is extended up to 20 hours the product recovered is red lead (Pb3 O4 ) mixed with other lead oxides. Our study is directly related to the conservation scientific and art work documentation, as our results will help to interpret future analysis on pictorial samples. For example, those analyses in which litharge or massicot are mixed with other compounds (lead white, lead carbonate, etc) should lead to the conclusion that the presence of the secondary compounds might be associated to the synthetic process. Furthermore, in the case of litharge, we have verified the wide chromatic range derived from the roasting process (from yellowish to reddish brown). These observations based on our results points to the question that, in the Antiquity, other pigments could be obtained from lead white (for instance, the pigment known as “pardillo de albayalde” according to the Spanish terminology.

Crystalline species identified PbCO3 Pb3 (CO3 )2 (OH)2 Pb3 C2 O7 Pb2 OCO3 Pb3 O2 CO3

ACKNOWLEDGEMENTS This work was financed under Project BHA200202085, supported by the MCYT. We also express our gratitude to the Centre for X-ray Diffraction of the Universidad Complutense de Madrid.

REFERENCES Bearn, J.G. 1923. The Chemistry of Paints, Pigments and Varnishes, London, Ernest Benn Limited. Bell, I.M., Clark, R.J.H., Gibbs, P.J. 1997. Raman spectroscopy of natural and synthetic pigments (pre- ∼1850AD), Spectrochimica Acta Part A 53: 2159–2179. Bouchard, M. & Smith, D.C. 2003. Catalogue of 45 reference Raman spectra of minerals concerning research in art history or archaeology, especially on corroded metals and coloured glass, Spectrochimica Acta Part A, 59: 2247–2266. Burgio, L. Clark, R.J.H. & Firth, S. 2001. Raman spectroscopy as a means for the identification of plattnerite (PbO2 ), of lead pigments and their degradation products, Analyst, 126: 222–227. Ciomartan, D. A. Clark, R.J.H. McDonald, L.J. & Odlyha, M. 1996. Studies on the thermal decomposition of basic(II)

Figure 4. Raman spectra of pigments obtained by thermal decomposition of lead white at 603 K as a function of time.

Overall, our these results confirm the XRD analysis listed in Table 4, except for the presence of Pb2 O3 , which could be identified in our Raman experiments.

4

CONCLUSIONS

Our results demonstrate that the thermal decomposition of lead white is a quite complex process that

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carbonate by Fourier-transform Raman spectroscopy, Xray diffraction and thermal analysis. J. Chem. Soc., Dalton Trans., 3639–3645. De la Roja, Sancho, N, San Andrés, M., Baonza V.G. 2007. Obtención de litargirio a partir de la tostación de blanco de plomo. Caracterización cromática de los productos obtenidos. Proceedings VIII Congreso Nacional del Color, Madrid 19–21 September, 117–118. Dornheim, D. & San Andrés, M. 2006. Litargirio y masicote. Terminología, propiedades y usos. Reproducción

a escala de laboratorio de algunos de sus procesos de obtención. In Proceedings XV Congreso de Conservación y Restauración, Murcia, 21–24 Octubre, 2004, Vol. 1: 535–546. Gettens, R. J. & Stout, G. L. 1996. Paintings Materials, a Short Encyclopedia, New York: Dover. Merrifield, M.P. 1967. Medieval and Renaissance treatises on the arts of painting: original texts with English translations, New York, Dover Publications.

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Contamination identification on historical paper by means of the NIR spectroscopic technique M. Sawczak Polish Academy of Sciences – IFFM, Gdansk, Poland

A. Kaminska Agency for Integration of Conservation Activities, Gdansk, Poland

ABSTRACT: The soiling on historical documents and artworks on paper represents one of the most serious conservation problems. The selection of an effective and secure cleaning method preceded by a reliable examination of the contamination chemical composition is a key for recovering and preservation of the historical and aesthetical value of the paper object. The soiling and substrate analysis has therefore to be considered as an important part of the conservation project. In this work the potential usefulness of Near Infrared (NIR) Spectroscopy for analysis and classification of different types of stains on paper documents is studied.

1

INTRODUCTION

relatively deep, even few millimetres into the sample. As a consequence of low absorption, NIR is not a particularly sensitive technique but it can be very useful in probing bulk material with little or no sample preparation what makes this technique non-invasive. The molecular overtone and combination bands seen in the near IR are very broad and it can be difficult to assign absorption bands to specific chemical components. For analyzing the results obtained from NIR measurements the multivariate calibration techniques, e.g. Principal Components Analysis (PCA), Partial Least Squares (PLS), etc. are often employed to extract the desired chemical information. In this work, the absorption spectra in the near IR region were obtained for a variety of original and model stains and adhesives on historical and contemporary paper samples.

The application of analytical techniques is the essential part of evaluating the collection condition as well as an important part of conservation or restoration practice. These techniques can detect a wide range of issues including evidence of damage, vulnerability, etc. These evaluations lead to conservation decisions or maintenance guidance for the curators. The majority of the analytical techniques used for paper analysis are destructive and require a relatively great amount of samples that in the case of valuable pieces of art or historical documents is almost impossible. The necessity of implementation of non-destructive analytical techniques enabling examination of historical papers seems to be a crucial issue. Scientific interest is currently focused on the applicability of NIR for non-destructive and fast evaluation of paper condition (Lichtblau & Anders 2004, Trafela et al. 2007). The purpose of this research project is to establish whether it is possible to implement NIR for classification of contamination on historical papers. We expect a sufficient level of soiling recognition can be achieved with those techniques. Furthermore, we expect that soiling recognition by NIR techniques could enable objects assessment by providing reliable data and be an alternative for the chromatographic and spectroscopic techniques which demand sampling. NIR is based on molecular overtone and combination vibrations. Since the transitions are forbidden by the selection rules of quantum mechanics, the absorption in the near IR region is small and the radiation can penetrate

2

EXPERIMENTAL METHOD

NIR spectra were collected in the reflectance mode using a 0.5 m spectrometer equipped with a broadband radiation source (quartz tungsten bulb, 3000 K) and a PbS detector. Spectral data were acquired in a diffuse reflection by scanning between 1300 and 2500 nm with a step of 2 nm. Data of each sample were collected at five different locations. The absorbance spectra represented as log(1/R), where R is reflection coefficient, were obtained by averaging measurements and comparing them to the reference sample (Spectralon). The statistical analysis of the

95

experimental data was carried out using Principal Component Analysis (PCA) method. Spectral data were processed using own procedures written in Scilab software. 3

RESULTS AND DISCUSSION

Typical NIR spectra consist of broad overlapping bands which are very difficult to assign to specific chemical compounds. Figure 1 presents representative NIR spectra recorded for non-contaminated, contemporary and historical papers. The analysed papers are listed in Table 1. The curves have been offset for clarity. The spectra are visually very similar. Indicated selected bands are characteristic of C=O, C-C and C-H bonds of cellulose molecules. A global PCA performed on all data enable to separate them into groups. The main objective of PCA is to reduce the dimension of the matrix of data so that useful information can be extracted from the overlapped chemical information. Principal components (PC) can be taken as the projection of the original data in a new space. Results presented on Figure 2 show that spectra, represented by single points in the PC space, can be classified into separate groups. Historical papers having very similar composition are located in one group while contemporary papers characterized by different chemical composition are classified in separate groups. Applying PCA to historical papers enable to split them depending on individual chemical and physical properties. Building appropriate calibration model for PCA, the qualitative analysis of papers can be carried out. 3.1

Figure 1. Representative NIR spectra recorded for non-contaminated, contemporary and historical papers. Table 1.

Description of paper samples.

contemp_1 contemp_2 contemp_3 archiv_6, 8, 15, 20, 22

Whatman filter paper contemporary newspaper offset print paper (1986) historical rag papers 17th, 18th century

Model contamination

For the experiment, a set of samples using three contemporary papers (Table 1) covered with model contaminants was prepared. Substances often found on historical papers such as fats, waxes, tea, coffee, milk, were used as contaminants. Methylcellulose, starch, etc., as adhesives. In most cases, contaminants and residues responsible for the soiling and discoloration of paper cannot be determined by characteristic spectral bands. Typical examples of soiling resulting from inappropriate maintenance of historical paper indicate that the data can be grouped depending of chemical composition similarities. Global PCA analysis performed to differential spectra obtained by subtracting the spectra of contaminated and clean paper indicated that substances can be split into three groups: group A – fats (mainly: triglycerides, glycosides, alkane hydrocarbons); group B – adhesives: long chain carbohydrate, polymers; C – substances containing mainly carbohydrates (honey, fruit juice, etc). In

Figure 2. First two PC scores for spectra presented in Figure 1.

Figure 3, representative spectra of samples covered with different fats and paraffin are presented. There is a clear split in the group A (Fig. 4): margarine and oil gather together away from the principal components as an effect of their chemical composition similarities, mostly characterized by monosaturated and polysaturated fatty acids. The difference in the chemical nature of the other two substances: milk (4% fat: saturated, monosaturated and polysaturated fatty acids) and paraffin (paraffin wax, C25 H52 ) is

96

8

differential spectra

0,5

1725 1755

2295 2340

paraffin

Absorbance [a.u.]

6

Absorbance [a.u.]

margarine

oil

0,0 4

2

clean paper contaminated area

-0,5

0

-2 milk

1400

1600

1800

2000

2200

2400

λ [nm]

1400

1600

1800 λ [nm]

2000

2200

Figure 5. Spectra recorded for finger print near the pagination of one page of the book.

2400

Figure 3. Differential spectra of model samples (Whatman filter paper) contaminated with fats and paraffin. 8

1732 1766

differential spectra

Absorbance [a.u.]

2432

4 2310 2348

6

2 0

4

-2

clean paper contaminated area

2

-4 -6

0

-8 -2 1400

1600

1800

2000

2200

2400

λ [nm]

Figure 6. Wax stain from a page of a old print.

gelatine

methylcellulose

Absorbance [a.u.]

Figure 4. Result of PCA analysis of spectra recorded for model paper contaminated with fats and paraffin.

remarkable, although they are commonly considered as a fatty soiling on paper, too. Ages of candle lighting for reading left significant amounts of traces in the form of greenish, yellowish or brownish wax stains on book pages. Traces of fat and wax are likewise recognizable on the historical papers. Spectra of a trace of a finger print (usually containing traces of fat, lactic acid, particular matter, etc) found near the pagination of one page of the book and of a wax stain from a page of an old print are shown in Figures 5 and 6. The typical C-H bonds located near 1730, 1760, 2300 and 2340 nm can be observed distinctly. In Figure 7, representative differential spectra measured for samples covered with typical adhesives

starch

polyvinyl acetate

glue stick

1400

1600

1800

2000

2200

2400

λ [nm]

Figure 7. Differential spectra of model samples (Whatman filter paper) contaminated with adhesives.

97

Figure 8. First two PC scores for spectra presented in Figure 7.

Figure 9. First two PC scores for spectra recorded for samples contaminated with substances containing carbohydrates. Figure 10. Result of PCA analysis of spectra recorded for three contemporary papers covered with model contaminants.

applied on paper are shown. Each of the adhesives split into different space of PC1 and PC2 (Fig. 8). It should be expected according to their chemical nature: polysaccharides, protein, polymers as vinyl polyacetate and vinyl polypyrolidone (glue stick). This gives us the possibility to distinguish different kinds of adhesives used in paper conservation and book binding. In Figure 9, results of PCA analysis of group C, substances containing carbohydrates, are presented. The main compounds of these substances are saccharides and polysaccharides. The group shows the highest similarities for honey and sugar. The split into the other groups is probably caused by the difference in the fructose level in the fruits and additional components. Preparation of model samples sets for PCA procedure enables to classify unknown contaminants regarding its chemical composition. 3.2

the set of samples was prepared by covering three different contemporary papers (Table 1) with four model contaminants (milk, stick glue, methylcellulose and cream). In Figure 10, results of the PCA analysis performed on differential spectra of contaminated and clean paper are presented. Despite considerable differences in the composition of model papers used in the experiment, distinction between substances used as contaminants is still possible.

4

CONCLUSIONS AND FUTURE RESEARCH

The application of NIR spectroscopy for nondestructive, user-friendly recognition of contamination on paper documents promises to be a powerful tool for performing fast, in situ sample examination. The purpose of this paper was to demonstrate the potential usefulness of this technique for classification of contaminants, residues, adhesives, etc. Results of this work evidence that the technique can be used to classify groups of substances of similar chemical composition. The investigation is in the initial stage and will be continued for a large variety of materials and contaminants in order to obtain comprehensive and reliable characterization.

Influence of paper in PCA results

Changes in the spectra of paper due to contamination are very small. Moreover, in the case of historical papers, variability of paper parameters results in big differences of spectra. The question is whether it will be possible to classify different contaminants and distinguish compounds of very similar chemical composition using the NIR technique. For the experiment,

98

ACKNOWLEDGEMENTS

the International Conference, Durability of Paper and Writing, November 16–19, 2004, Ljubljana, Slovenia. Scilab scientific software homepage [www.scilab.org]. Trafela, T., Strlic, M., Kolar, J., Lichtblau, D. A., Anders, M., Pucko Mencigar, D. & Pihlar, B. 2007. Nondestructive Analysis and Dating of Historical Paper Based on IR Spectroscopy and Chemometric Data Evaluation. Anal. Chem. 79: 6319–6323.

Work is supported by the Ministry of Science and Higher Education under the project No 2059/B/T02/2007/33 REFERENCES Lichtblau, D. & Anders, M. 2004. Characterization of paper by near infrared spectroscopy. Proceedings of

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Three dimensional survey of paint layer profile measurements E. Pampaloni, R. Fontana, M.C. Gambino, M. Mastroianni & L. Pezzati Istituto Nazionale di Ottica Applicata (CNR – INOA), Firenze, Italy

P. Carcagnì, R. Piccolo & P. Pingi Istituto Nazionale di Ottica Applicata (CNR – INOA) – Sez. di Lecce, Arnesano (LE), Italy

R. Bellucci & A. Casaccia Opificio delle Pietre Dure (OPD), Firenze, Italy

ABSTRACT: The quantitative morphological analysis of a painting surface allows to evidence form defects and thus to study their influence on the stability of the paint and preparatory layers, as well as on the support. Therefore a three-dimensional survey can be very useful in planning the restoration intervention of a painting. In this work we present the results of the surface analysis carried out on the painting “Ultima Cena” by Giorgio Vasari. This panel painting is severely affected by paint film wrinkling produced as a consequence of the flood that occurred in Florence in 1966. Our analysis, accomplished to quantify the lengthening of the paint layer with respect to the one of the support, was made in order to plan the restoration intervention, which was performed on 25 profiles separated each by 10 cm to cover the whole painting surface. A data analysis, based on morphological filtering named “Rolling Ball” transformation, was used to evaluate the length difference between the paint layer and the panel support along each profile.

1

INTRODUCTION

Shape represents highly significant data for the historical and artistic evaluation, the diagnostic analysis and the conservation of an artwork. The possible scenarios involved in the use of 3D digital models range from the monitoring of deterioration due to pollutants, to the realization of digital archives, from reverseengineering to fast-prototyping, and from the analysis of the conservation state to the monitoring of restoration interventions. Moreover, it is possible to monitor the form of variations by computing the differences between measurements at different times. At present, a variety of instruments is available on the market for surface measurement. The commonly used techniques for in situ roughness measurements are contact techniques and they make use of stylus profilometers. The sample surface is investigated by means of a stylus or needle (often a diamond point) that is moved along the surface, its profile is then recorded. The system is usually calibrated with a known flat surface and the depth information is obtained by calculating the difference between sample and reference measurements. These profilometers have a very good axial resolution (up to some tens of nanometers), whereas lateral resolution depends on stylus diameter. The typical measured areas extend to

a few tens of square millimeters. This method is suited for measuring hard surfaces, but it is not applicable for surveying frail and precious objects like paintings, as stylus sharpness can damage the surface causing micro-scratches. In the diagnostics of paintings the non-contact characteristic is a mandatory step; this requirement makes the optical techniques particularly suitable for this purpose. That is why they are widely used and extremely well received in the field of conservation together with their effectiveness and safety (Bertani et al. 1990, Fabbri et al. 2000, Fontana et al. 2003, Carcagnı et al. 2007, Bellucci et al. 2007). Optical techniques for shape measurements are often derived from industrial metrology but the peculiarity of an artwork does not allow for a straightforward application. In fact, industrial manufacture is generally regular in shape, with uniformly coloured surfaces. On the contrary, artworks are unique in their shape, having polychrome and highly contrasted surfaces such as painting surfaces. Conoscopic micro-profilometry is particularly well suited for surveying the surface of paintings due its unsensitivity to color contrast, it enables measurements on surfaces with almost any reflectivity and it allows the survey of microscopic details working with an incident angle very close to grazing incidence.

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Figure 1. The working principle of the conoscopic system.

Figure 2. The laser scanner micro-profilometer during the measurements.

2

INSTRUMENTS

Our conoscopic micro-profilometer is composed of a rangefinder (Conoprobe 1000 by Optimet) mounted on a high-precision scanning system. The Conoprobe is substantially a video camera within which, between objective and CCD, is placed the conoscopic module, consisting in a uniaxial birefringent crystal sandwiched between two circular polarizers (Fig. 1). The probe working principle is as follows: a light beam, projected by a diode laser, is both reflected and back scattered by the sample surface and then is collected by the lens. The conical light beam, after impinging on the crystal is split into two beams, the ordinary and extraordinary one. These two beams running along slightly different optical paths produce an interference pattern that depends on the beam aperture angle and is related to the distance of the object. By measuring the fringe spacing, the distance of the investigated point from the conoscopic probe can be retrieved (Sirat et al.

1985, 1988, Charlot 1988). The probe is equipped with a 250 mm lens setting an axial resolution better than 15 µm and a measurement range of ±90 mm at a standoff distance of about 240 mm. The overall accuracy is better than 100 µm and the transversal resolution of about 100 µm. The system allows measurements on a maximum area of about 1.5 × 1 m2 . The device has a maximum acquisition rate of 800 points/s, but due to downtimes and scanning parameterization, the typical acquisition rate ranges from 100 to 500 points/s. The whole system is computer controlled (Fig. 2). 3

MEASUREMENTS AND DATA ANALYSIS

3.1 Painting surface measurement We performed a profile analysis on the surface of a panel painting severely affected by paint film wrinkling and colour raisings. This crumpling of the paint layer is the consequence of the flood caused

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Figure 3. The “Ultima Cena” by Giorgio Vasari (1546), 263 × 660 cm2 . Photo executed in 1956. The analyzed panel is enclosed by the dashed rectangle.

Figure 4. The measured panel: a) photograph in 1956, b) recent photograph with velinatura, c) raking light photograph, d) photograph of the back.

by the Arno river in Florence in 1966. The painting entitled “Ultima Cena” realized in 1546 by Giorgio Vasari for the monastery “Le Murate” in Florence, is currently under restoration at the laboratory of the Opificio delle Pietre Dure in Florence, where it arrived in 2004 after a period characterized by a long wandering in several restoration laboratories without success. The conservation aim is both the historical reconstruction of unlucky vicissitudes suffered by the panel painting and its restoration. Our analysis, accomplished to measure the amount of lengthening of the paint layer with respect to the support, allows the restorers to plan an appropriate restoration intervention.

Because of the painting huge dimensions, we performed our surface analysis only on the second panel constituting the artwork (Figs. 3, 4), that is representative of the conservation status of the whole painting. In order to achieve information useful for planning the restoration intervention of the painting surface and not excessively time-consuming, we acquired 25 profiles every 10 cm along the panel length (Fig. 5), with a sampling step of 0.25 mm (4 points/mm). Microprofilometry is a surface measurement that gives information on the painting layer and not on the morphology of the support. Measuring the rear part of the panel will not give the correct information because the two surfaces of the support are

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Figure 5. The measured panel (raking light photograph) and the 25 measured profiles.

not supposed to be identical. In order to evaluate the support length we need an appropriate data processing. The application of morphological transformations, like the “Rolling Ball” filtering, allows to discriminate the contribution of the wrinkled paint layer from that one of the support in the acquired profile curve. We implemented the “Rolling Ball” algorithm in the MatLab computing environment. 3.2 The Rolling Ball transformation The “Rolling Ball” transformation is a mathematical morphological algorithm first proposed by Sternberg (Sternberg 1983) to minimise image background noises. It consists in the application of morphological openings or closings (Sternberg 1986, Hashim 1996, Lee et al. 2005) to grayscale images by using a spheric structuring element. To best understand of this algorithm, we consider a grayscale image as a surface where bright areas are hills or peaks and dark areas are pits or valleys, and then we consider a large sphere rolling over the grayscale surface tracing a path as it rolls. This path represents the set of points where the ball fits the surface. This new surface is smooth relative to the original. By taking the grayscale difference image, we obtain an image of all the places where the ball could not fit into crevaces in the surface because it is too large. In this example, the “Rolling Ball” algorithm is a morfological transformation (closing, dotted line in Fig. 6a) followed by an image subtraction. Similarly, the morphological opening can be visualized as a ball rolling under the grayscale surface. In this case, any

Figure 6. The Rolling Ball algorithm representation: closing dotted line, opening dashed line, b) the top hat transformation.

protrusions of high curvature, such as sharp edges and ridges, are lost by the procedure (opening, dashed line in Fig. 6a). Therefore the “Rolling Ball” transformation describes the smoother features of a surface. The grayscale difference image, obtained by subtracting the opening (closing) “Rolling Ball” transformation from the original surface, is called a “Top Hat” (“Bottom Hat”) transformation (Fig. 6b). The main problem when using these morphological filters is to choose the right ball radius R to seize the desired surface features. In our case it was necessary to discriminate the crests of the wrinkled paint surface from the smooth surface of the wooden support and thus we used the “Top Hat” transformation. In order to choose the appropriate ball radius, we calculated the length of the curve L for each

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Figure 7. a) The length L as function of the ball radius R, b) the derivative dL/dR as function of R.

profile as a function of R, by means of the opening “Rolling Ball” transformation. The typical result for the profile is shown in Figure 7a. Obviously, for an increasing R, the length L decreases and for R → ∞, L(R) tends to the limit represented by the length of the projection of the profile on a horizontal line or equivalently by the sampling step length multiplied by the number of samples. But, for particular R values, L(R) decreases more slowly and presents a flex indicated by the arrow in Figure 7a. This, can be seen more clearly in the derivative dL/dR, where the curve presents a local maximum (the arrow in Fig. 7b). For this value of R the Opening “Rolling Ball” algorithm extracts all the profile features corresponding to the smooth surface of the wooden support. For instance, Figure 8 shows the 22nd profile, compared to the curve resulting by the Opening “Rolling Ball” algorithm for R = 1250 mm, equivalent to a points number N = 5000 (indicated by the arrows in Fig. 7). Figure 4d shows that the panel support is constituted by four planks linked by wooden crossbars, as is also visible in Figure 8. In fact, there are three local minima in the “Rolling Ball” filtered curve corresponding to the union points between two adjacent planks. These minima are located near the abscissas at 400, 650 and 880 mm.

4

RESULTS

For each profile the lengths of the paint layer and the wooden support were computed, and their differences L are shown in Figure 9. As the painting panel is composed by four planks, for restoration purpose, its important to evaluate not only the length difference L between paint layer and the wooden support over the whole panel, but we have also to consider how it is distributed on each plank. So all the profiles were

Figure 8. The 22nd profile (continuous line) compared to the “Rolling Ball” filtered curve(dashed line) for R = 1250 mm. The arrows indicate the union points between two adjacent planks. The ordinate scale is 10 times the abscissa scale.

subdivided into four parts, one for each plank, and the length differences between paint layer and support were calculated for each part. Figures 10a–d, show the results obtained by this calculation. The first plank variations range between 0.3 and 2.9 mm for the first ten acquired profiles, however in the other profiles it ranges between 4 and 8 mm. The maximum value for the deformation in this plank is 8.2 mm, for the profile number 21, which is the profile with the maximum dimensional variation between pictorial film and support (near 20.03 mm). In the second plank, dimensional variation is ranging between 1.5 and 4 mm (there is a maximum of 6 mm in the profile number 18 where detachments are more easily discernible). The third plank shows variations ranging between 0.4 and 3 mm which are nearly uniform along the whole plank. The fourth plank presents a more complex situation, the profiles from 1 to 10 have a dimensional variation ranging between 1 and 3 mm, however, the profiles from 11 to 16 have a variation ranging between 3 to 4 mm. Finally the profiles from 17 to 23 have a deformation ranging between 3 to 7 mm. Clearly we have more variations in the first (28.5 cm long) and in the fourth (38.5 cm long) plank; probably due to the different working process used for their preparation and to their spatial position which

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Figure 9. Length differences between the paint layer and the support along the painting.

Figure 10. Paint-support length differences for the first a), second b), third c) and fourth d) planks.

allows them to deform more easily. The two planks in the middle, being linked to the others, present less deformations (Figs. 10a–d). From the acquired profiles it is possible to obtain not only quantitative information about dimensional variations of the support with respect to the preparatory layer and to the painting layer, but also about the heights of the wrinkled paint layer. Considering the results shown in Figure 9, we estimate the difference between the paint layer length and the whole surface of the plank in contact with the preparatory layer. This difference is about 21.03 mm and its probably due to the planks shrinkage after the drying process. Wood dimensional variation is not constant along each plank due to the natural anisotropy of this material and to its different permeability. When the amplitude of wrinkling is greater, the shrinkage of the planks is also greater. Moreover the lower part of the panel is more affected by wrinkling (near 15 mm). These bigger deformations are surely due to the different water absorption of each plank

and consequently to the wrinkling of the paint layer. 5

CONCLUSIONS

In this work we have presented the results of a surface analysis performed on 25 profiles of the “Ultima Cena” by Giorgio Vasari, a panel painting severely affected by paint wrinkling, consequence of the flood occurred in Florence in 1966. These profiles were surveyed by means of an optical high resolution micro-profilometer. The results of the data analysis, based on the “Rolling Ball” morphological filtering, were used to evaluate the difference length between the paint layer and the panel support along each profile. This allowed also the evaluation of the height of each single wrinkle. This investigation produced a lot of useful informations to plan the restoration intervention in order to decide whether to substitute integrally the original panel support or to keep it with a few interventions.

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REFERENCES Bertani, D., Cetica, M., Poggi, P., Puccioni, G., Buzzegoli, E., Kunzelman, D. & Cecchi, S. 1990. A scanning device for infrared reflectography. Studies in Conservation 35: 113– 119. Bellucci, R., Carcagnı, P.L., Della Patria, A., Fontana, R., Frosinini, C., Gambino, M. C., Greco, M., Mastroianni, M., Materazzi, M., Pampaloni, E., Pezzati, L., Piccolo, R. & Poggi, P. 2007. Integration of image data from 2D and 3D optical techniques for painting conservation applications. The Imaging Science Journal 55: 80–89. Carcagnı, P., Della Patria, A., Fontana, R., Greco, M., Mastroianni, M., Materazzi, M., Pampaloni, E., Pezzati, L. 2007. Multispectral imaging of paintings by optical scanning. Opt. Lasers Eng. 45: 360–367. Charlot, D. 1988. Holographie conoscopique – reconstructions numeriques. Annales des Telecommunications 9: 23–26. Fabbri, F., Mazzinghi, P. & Aldrovandi, A. 2000. Tecnica di identificazione di materiali pittorici attraverso l’acquisizione di immagini digitali multispettrali in fluorescenza UV. Quaderni di Ottica e Fotonica 6: 94–104.

Fontana, R., Gambino, M.C., Greco, M., Pampaloni, E., Pezzati, L. & Scopigno, R. 2003. High-resolution 3D digital models of artworks. Proc. SPIE 5146: 34–43. Fontana, R., Gambino, M.C., Greco, M., Marras, L., Materazzi, M., Pampaloni, E. & Pezzati, L. 2003. Thediagnostics of statues: a high-precision surface analysis of roughness of Michelangelo’s David. Proc. SPIE 5146: 236–243. Hashim, M. 1996. New Texural Extraction Method Using Rolling Ball and Riping Membrane Transforms. Proceeding of the 17th Asian Conference on Remote Sensing ACRS. Lee, J.R.J., Smith, M.L., Smith, L.N. & Midha, P.S. 2005. A mathematical morphology approach to image based 3D particle shape analysis. Machine Vision and Applications 16: 282–288. Sirat, G. Y. & Psaltis, D. 1988. Conoscopic holograms. Opt. Comm. 9: 243–245. Sirat, G.Y. & Psaltis, D. 1985. Conoscopic holography. Optics Letters 10: 4–6. Sternberg, S. R. 1983. Biomedical image processing. IEEE Computer Society 16: 22–34. Sternberg, S. R. 1986. Grayscale morphology. Comp. Vision, Graphics, Image Process. 35: 333–355.

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Use of LA-ICP-MS technique with SEM-EDS analysis in the study of finishing layers L. Rampazzi & B. Rizzo Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, Como, Italy

C. Colombo, C. Conti & M. Realini Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, CNR, Milano, Italy

ABSTRACT: The investigation of the finishing layers in artworks is of major concern, as important information on the artistic technique can be suggested. The study is usually carried out with Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS), but in those cases where very similar compositions are detected, more sensitive techniques are called for. Laser Ablation-Mass Spectrometry equipped with Plasma source (LA-ICP-MS) allows very sensitive element analysis of solid surfaces sampled by means of a laser. It is an emerging methodology, applied only in a few cases in the field of cultural heritage. The integration of the two techniques has been studied and applied on the finishing layers of Baroque stucco decorations, in order to define the artistic technique in terms of raw materials, stratigraphy and working tools.

1

INTRODUCTION

The surfaces of artworks may answer important questions about the artistic technique such as: “Which materials have been used?. How?” Stratigraphy provides all the information on the raw materials, the technological process, and the finishing layers applied in the past. In some cases, shedding light on the technique could be very problematic: the materials detected in the decoration of mortars and stuccoes are, as an example, quite the same. In particular, the technique of stucco decorations is difficult to define, since: – the finishing layers are often complex, as many layers are present; – the layers may be very similar from the compositional point of view, being the binder mainly composed by lime; – the thickness of the layers can be very small, around tens of micron. The structure, the morphology and the element composition of stuccoes are usually determined by means of Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS) (Toniolo et al. 1998), characterised by a high visual resolution and a good sensitivity. Those techniques are usually resolutive and successful in defining

the artistic technique. When a complex stratigraphy or a very similar composition is observed, trace components become the qualifying and peculiar elements for the classification of the samples and conservation scientists call for sensitive analyses. Laser Ablation-Mass Spectrometry equipped with Plasma source (LA-ICP-MS) is an emerging analytical technique which allows a very sensitive element analysis of solid samples. The surface is observed with an optical microscope and the area of interest ablated by means of a laser beam and analysed. The technique has been often employed in environmental investigations of solid surfaces and only in a few cases in the field of cultural heritage, particularly for ceramic, metal and glass objects (Manson & Mank 2001, Robertson et al. 2002, Sanborn & Telmer 2003, Dussubieux & Van Zelst 2004, Wagner & Bulska 2004). The element analysis is much more sensitive than the one performed by SEM-EDS technique, but the resolution of the microscope is usually lower, thus preventing the precise mapping of elements. The authors have set up a methodology which has taken advantage of SEM-EDS high resolution and of LA-ICP-MS high sensitivity, in order to overcome the critical points of both techniques. The new methodology has been tested during a project called L’arte dello Stucco nel Parco dei Magistri Comacini – Valorizzazione, conservazione

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e promozione (The art of stucco in the Magistri Comacini Park – Enhancement, conservation and promotion), funded by EU Interreg IIIA Program and aimed to enhance the Baroque stucco decorations in the borderline area between Como (Italy) and Lugano (Switzerland). The aim of the integrated investigation was to corroborate decorating phases, ‘recipes’ or hypotheses about building sequences, previously suggested, on the basis of historical and artistic data, for the stucco decorations inside three churches: San Lorenzo in Laino, Santa Maria dei Ghirli in Campione d’Italia and Santa Maria del Restello in Castiglione Intelvi. The stucco cycles date back to 17th and 18th century and have been made by the Magistri Comacini, among the most important artists of Northern Italy.

2

Optical microscopy

Polished cross-sections of the samples were observed in reflected light using a Leitz Ortholux microscope with Ultropack illuminator equipped with a digitalisation image system. Fragments and cross sections were observed using Leitz Wild M420 stereomicroscope equipped with a digitalisation image system.

2.2

Laser Ablation Mass Spectrometry equipped with Plasma source (LA-ICP-MS)

Analyses were performed by ICP-MS (Thermo X-serie due, 10 ms dwell time, 1 channel of mass, sweep number 100, power 1300 W) equipped with a Laser Ablation system (UP266, New Wave Research). The ablation step was optimised as follows: laser output 1.56 mJ, surface power (55%), repetition rate 20 Hz, spot size 20 µm, scanning speed 2 µm/s in continuous mode. A pre-ablation step was performed to clean the sample surfaces using a weak energy laser spot. Element signals were corrected with an internal standard, as used in LA-ICP-MS methodology to normalize the data toward different local ablation efficiencies and general instrumental drift. In this study, Cu was selected due to the homogeneous distribution on the samples surface.

MATERIALS AND METHODS

Sampling was carried out in areas suggested by art historians, supplying a number of representative stratigraphies. Several micro samples were collected using a scalpel according to the indication of UNI-Normal rules 3/80 (Raccomandazione Normal 3/80 ‘Campionamento’ ICR-CNR 1980). Polished cross-sections of the samples were prepared, observed by an optical microscope and analysed by SEM-EDS, in order to determine the major and minor elements and to define in detail the stratigraphy, i.e. morphology, number and thickness of the layers. Then, LA-ICP-MS element analysis was carried out moving the laser beam from the outer layer to the inner one by means of a motorised stage. Thus, the distribution of elements could be precisely linked to the single layers of the stratigraphy. Fourier Transform Infrared (FTIR) analyses were performed to determine the presence of organic compounds.

2.1

2.3

Scanning Electron Microscope

SEM investigations were carried out by a JEOL 5910LV microscopy equipped with an X-ray spectrometer IXRF System/EDS 2000. Observations were carried out on polished cross sections.

2.4 Fourier Transform Infrared Spectroscopy FTIR spectra were recorded in transmission through a diamond cell by an FTIR spectrophotometer Nicolet Nexus, equipped with microscope Continuµm (400 to 4000 cm−1 , 4 cm−1 resolution). In order to avoid contamination by the substrate, samples were carefully collected under a stereomicroscope by means of a needle-sampler.

3

RESULTS AND DISCUSSION

The analyses distinguished three typologies of stucco decorations: simple and complex stratigraphies and gilding finishing layers. As far as the raw materials are concerned, the stucco and the finishing layers were mainly made of magnesiac lime mortar and only in a few cases of gypsum. For centuries, most of the conservation works affected most of the stuccoes and as a consequence, an overlapping of many white-finishing layers was observed in the samples. Finally, in some of the fragments gilding traces were detected among the white layers or on the top of the decorations. In case of simple stratigraphies, only one or two layers were observed onto the bulk. As an example, the samples coming from the church of Santa Maria del Restello showed the presence of a single layer on the gypsum bulk, constituted of magnesiac lime lacking in aggregate and many round clots of lime in the external layer (Fig. 1). The backscattered observation showed the different morphology of the external portion due to the re-crystallisation of rich in magnesium components (Fig. 2); EDS analysis detected only calcium and magnesium (Fig. 3). On the contrary LA-ICP-MS analyses pointed out also the presence of traces of lead, as showed in Figure 4.

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Figure 1. Optical microscopy image of polished cross section: only one white layer is perfectly adhered on the gypsum bulk (bar = 100 µm).

Figure 3. EDS spectrum of the external white layer.

Figure 4. LA-ICP-MS spectrum showing the presence of lead in the external surface.

Figure 2. Backscattered image of polished cross section: in the upper part of the external layer rich in Mg crystals are visible (bar = 100 µm).

The presence of lead can hint the use of metal working tool. First, lead is abundant only in the external portion of white layers. Second, lead was detected only in decorative elements, whose repeated shape could be achieved only with the use of moulds. As regards the gilding decorations, optical microscope investigations determined the presence of a gold leaf on a yellow preparation layer (Fig. 5), and SEM morphological observation pointed out the presence of a thin homogeneous and continuous layer between them (Fig. 6). The signal of lead detected by EDS analysis should be ascribed only to the preparation layer composition, i.e. chrome yellow (PbCrO4 , Fig. 7). The FTIR analysis carried out on this layer did not show the presence of lead white, easy detectable through the characteristic peaks at 1402, 836 and 683 cm−1 , while XRD analyses showed the presence of crocoite (PbCrO4 ). Because of the absence of lead sulphate, as showed

Figure 5. Optical microscopy image of polished cross section: gold leaf applied on a yellow preparation layer (bar = 100 µm).

by the X-ray diffraction (XRD) analysis, it is possible to suppose that the pigment corresponds to the middle (PbCrO4 ) or (PbCrO4 PbO) orange shade of a chrome compound according to the classification

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Figure 8. LA-ICP-MS spectrum showing the distribution of lead between gold leaf and mordant layer.

Figure 6. Backscattered image of polished cross section: a thin homogeneous and continuous layer is well distinguishable under a gold leaf.

Figure 9. FTIR spectrum showing the presence of calcite, gypsum and oil.

Figure 7. EDS spectrum acquired from the yellow layer.

of lead chromate pigments of Schiek (Kuhn & Curran 1986). The LA-ICP-MS analyses showed a wide abundance of lead in the yellow layer and between this one and the gold leaf (Fig. 8). The presence of lead just under the gold leaf is probably due to the employ of lead oxide as a dryer for lipidic adhesive (Figs. 6, 8). Literature corroborates this hypothesis, dealing with the use of lipidic compounds for the application of the gold leaf with the employ of a mordant (Gettens & Stout 1966). FTIR analyses of other gold finishing layers analysed in this work, revealed the presence of absorbance peaks at 1739, 2856, 2927 cm−1 , which can be ascribed to lipidic substances (Fig. 9). So, at the light of FTIR and SEM analyses, the presence of lead pointed out in the above discussed LA-ICP-MS results may indirectly suggest the presence of a lipidic adhesive. Coming to the study of stratigraphies characterized by many white layers (Fig. 10), SEM-EDS distinguished well the morphological differences among the layers and the different calcium to magnesium ratios (Figs. 11–13).

Figure 10. Optical microscopy image of polished cross section: three layers are applied onto the bulk (bar = 200 µm).

Once again LA-ICP-MS detected some trace elements, which discriminated the layers and suggested their application had been carried out at successive times.

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Figure 11. Backscattered image of polished cross section: the different morphological structure of layers is well distinguishable (bar = 100 µm).

Figure 13. EDS map of magnesium distribution in the area showed in Figure 11.

Figure 12. EDS map of calcium distribution in the area showed in Figure 11.

In particular, iron and lead distribution stressed the peculiarity of the external layer (Fig. 14). Iron and lead distribution are different: iron is randomly scattered in the whole layer while lead is mainly located close to the surface. Iron has to be considered as an impurity (such as iron oxide, hematite, hydroxide and limonite) of the raw material, carbonated stones, employed for the preparation of the lime. Differently, the presence of lead in the external portion of white finishing layer should be correlated to metal working tools, such as broad knives, trowels and scrapers employed by the artists to mould the lime. 4

CONCLUSIONS

The possibility of using SEM-EDS and LA-ICPMS integrated methodology for the investigation of stucco finishing layers emerged, in particular during

Figure 14. LA-ICP-MS spectrum showing the distribution of iron (top) and lead (bottom) in the stratigraphy.

the compositional, chronological and technological characterisation. The sensitive LA-ICP-MS analysis clarified the element composition in the stratigraphy previously defined by SEM-EDS.

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Here, we list the main critical points of the methodology: – a deep stratigraphic investigation is required; – LA-ICP-MS investigations have to be focused on well-defined areas, as layer’s thickness is very variable. The marking of cross-sections is under investigation in order to overcome these problems. REFERENCES Dussubieux, L. & Van Zelst, L. 2004. LA-ICP-MS analysis of platinum-group elements and other elements of interest in ancient gold. Applied Physics A 79: 353–356. Gettens, J. & Stout, G. L. 1966. Painting materials: 33 and 132. Kuhn, H. & Curran, M. 1986. Chrome yellow and other chromate pigments. Artist’s pigments, 1: 187–217.

Manson, P. R. D. & Mank, A. J. G. 2001. Depth-resolved analysis in multi-layered glass and metal materials using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Journal of Analytical Atomic Spectrometry 16: 1381–1388. Robertson, J. D., Neff, H. & Higgins, B. 2002. Microanalysis of ceramics with PIXE and LA-ICP-MS. Nuclear Instruments and Methods in Physics Research B 189: 378–381. Sanborn, M. & Telmer, K. 2003. The spatial resolution of LA-ICP-MS line scans across heterogeneous materials such as fish otoliths and zoned minerals. Journal of Analytical Atomic Spectrometry 18: 1231–1237. Toniolo, L., Colombo, C., Bruni, S., Fermo, P., Casoli, A & Palla, G. 1998. Gilded stuccoes of the Italian Baroque, Studies in Conservation 43: 201–208. Wagner, B. & Bulska, E. 2004. On the use of laser ablation inductively coupled plasma mass spectrometry for the investigation of the written heritage. Journal of Analytical Atomic Spectrometry 19: 1325–1329.

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Compositional depth profiles of gilded wood polychromes by means of LIBS A.J. López, A. Ramil, M.P. Mateo, C. Álvarez & A. Yáñez Departamento de Enxeñaría Industrial II, CIT, Universidade da Coruña, Ferrol (A Coruña), Spain

ABSTRACT: Gilded wood polychromes are important artistic expressions in Galicia (NW Spain) during the baroque period. The variety of techniques applied in gilded ornamentation and the possible existence of products of previous interventions make difficult the restoration process of these artworks which frequently include the removal of a brass-based paint (purpurin) added to cover the lack of gold leaf. The aim of this work is the characterization by means of LIBS of the different layers in gilded wood polychromes. For this purpose small pieces taken from two baroque altarpieces were analyzed using a Nd:YAG laser source operating at the wavelength of 355 nm to obtain characteristic LIBS spectra and compositional depth profiles.

1

INTRODUCTION

During the baroque period, polychromes on wood were the most important forms of sculpture in Spain. One of the most genuine Spanish baroque creation was the “retablo” which is a large, architectural panel divided into compartments which contains religious statues. These artistic workpieces were frequently polychromed and decorated with gold. In Galicia (NW of Spain) the baroque style appeared in the first half of the 17th century and it lasted during the whole 18th century in a great number of “retablos” located in rural churches and chapels. In polychromes the wood substrate is usually covered with different layers; ground, paint layers, gold leaf, varnish and so on. Characterization of the different layers becomes of great importance before any restoration work can be attempted. Several studies have shown the capability of laserinduced breakdown spectroscopy (LIBS) to identify the elemental composition of pigments and to characterize the different layers in painted artworks (Anglos et al. 1997, Burgio et al. 2000, Castillejo et al. 2000, Castillejo et al. 2001, Melessanaki et al. 2001, Clark 2002, Oujja et al. 2005b, Kaminska et al. 2006). LIBS technique is based on the spectral analysis of the emission from the plasma produced during laser ablation. In this sense, when a pulsed laser beam is focused onto the sample surface, it induces not only the ejection and vaporization of material from the sample surface, but also the formation of a plasma plume which emits light at wavelengths characteristic of the elemental composition of the removed layer.

Therefore, the analysis of the emission spectra can provide detailed information about surface composition at each pulse, that is, the in-depth compositional profile of the sample. On the other hand, in the last years, laser cleaning has been increasingly applied to artworks. The process must be controlled to avoid the damage of the substrate (gold leaf and paint layers in our case). In this context, LIBS appears as an adequate diagnostic tool (Gobernado-Mitre et al. 1997, Maravelaki et al. 1997, Tornari et al. 2000, Castillejo et al. 2002, Acquaviva et al. 2004, Colao et al. 2004, Oujja et al. 2005a, Acquaviva et al. 2005). The aim of this work is to test the capability of LIBS for the characterization of the different layers in gilded wood polychromes and to obtain compositional depth profiles which can help, not only in the knowledge of the structure and composition of these materials, but also in a controlled laser cleaning of these artworks. For these purposes LIBS has been applied to the identification of the different layers in real samples of gilded wood polychromes taken from two baroque altarpieces: Antiguo Retablo de Santa María la Mayor (17th century), nowadays at the Museo de Pontevedra, and Retablo de la Capilla del Pazo de San José de Vistalegre (18th century) Tui, Pontevedra; both involved in restoration processes. 2

EXPERIMENTAL

The results reported here were obtained with a Q-switched Nd:YAG laser source (Quantel, model

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Brilliant B) operating at the third harmonic, 355 nm. The samples were irradiated in air at room temperature and pressure. The laser beam was focused onto the sample surface by a plano-convex quartz lens with a focal length of 300 nm situated on a ruled rail that allowed to change the distance between the lens and the sample in a controlled way. X–Y translation stages and an alignment system consisting of two He-Ne lasers was used to help in sample positioning in terms of laser focal point situation and spectral collection. The emission of the plasma was collected and guided to the spectrograph (Oriel, model MS257) with a fiber optic. Light was dispersed by using the 600 grooves/mm grating of the spectro-graph. An intensified coupled charge device, ICCD (Andor, model DH5H7-18F03) detected the spectral resolved emission from the plasma. The depths reached by the laser ranged from 40 to 60 µm, depending on the sample, with a typical etch per pulse of 5 µm and a crater area of around 0.2 mm2 . This procedure allows to distinguish by LIBS the different layers which constitute the polychrome without causing an important damage to the piece. In addition to LIBS a stratigraphic analysis with the optical microscope was performed over a minuscule specimen taken with scalpel from the altarpiece and embedded in epoxy resin. 3

RESULTS AND DISCUSSION

As it has been previously pointed out, the carved wooden substrate is covered with a series of layers in polychromes; the number and characteristics of each one depend on the artistic technique used. In the case of gilded work, there are many different techniques (González-Alonso 1997); in brief, the carved wooden substrate is usually covered with a white ground layer or gesso (generally a mixture of hide glue and chalk), a layer of red bole (a mixture of clay and glue), gold leaf and in some cases a paint layer above. The quality and characteristics of the gold leaf can be different; the gold usually is alloyed with Ag and Cu and even Pt or other metals. 3.1 Antiguo Retablo de Santa María la Mayor The ancient altarpiece of Santa María la Mayor (Pontevedra) consists of five wooden panels showing important episodes of the life of The Virgin. They are bas-reliefs carved by Jácome de Prado (1623– 1626) and stylistically could be classified as an early and popular baroque piece (Filgueira-Valverde 1991). In Figure 1 one of the panels corresponding to the “Natividad de La Virgen” is shown. The vestments and other parts of the panel were ornamented using the method of sgraffito on gold which consists in covering

Figure 1. Natividad de la Virgen (153 × 76.5 cm2 ). One of the five panels which constitute the altarpiece Antiguo Retablo de Santa María la Mayor (17th century), nowadays at the Museo de Pontevedra (Spain). The square in the upper right side indicates the white zone where LIBS analysis were performed.

the gold leaf with a paint layer which is incised or scratched through, to reveal the gold underneath. Figure 2 depicts the optical micrography of a stratigraphic section (cross section) of a white-colored area of the polychrome. Different layers can be observed: on the top a white pigment layer (≈30µm depth), underneath a thin layer of gold (2–3 µm) and finally the layer of red bole (25–30 µm), a mixture of red clay and glue. LIBS depth profiles were carried out by subsequent ablation of the sample surface at the same irradiated spot. Figure 3 shows LIBS spectra obtained in

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Figure 2. Embedded cross section of a small fragment removed from the altarpiece Antiguo Retablo de Santa María la Mayor. On the top, a layer of white pigment followed by a thin layer of gold leaf and a layer of red bole underneath, can be appreciated.

Figure 4. Variation of intensity of () Au (I) 312.278, (
) Pb (I) 282.319, and (∗ ) Fe (I) 274.356 lines as a function of the number of laser shot; i.e. sample depth in Antiguo Retablo de Santa María la Mayor. Lead is the characteristic element of the external layer (lead white), gold indicates the intermediate layer and finally iron content is characteristic of the red bole.

The in-depth variation of the intensity of all the elements which appeared in successive LIBS spectra has been obtained after normalization of signals to take into account the decrease of the plasma signal with depth (López et al. 2005). Figure 4 shows the variation of intensity as a function of the number of laser shots, i.e. sample depth, for the peaks Pb (I) 282.319, Au (I) 312.278 and Fe (I) 274.356, characteristics elements of each layer (lead white pigment, gold leaf and bole). Lead content decreases in the first four laser pulses whereas gold content increases, taking its higher value around the third pulse. The iron content, characteristic of the red bole, remains practically constant after the fourth pulse. 3.2

Figure 3. LIBS spectra corresponding to: a) 1st pulse and b) 3rd pulse of a series of ten laser shots delivered at the same point of the sample taken from the Antiguo Retablo de Santa María la Mayor.

the 1st and 3rd pulses of a series of ten laser shots. The first two laser pulses produced a spectrum in which emission lines due to lead can be distinguished (Fig. 3a) confirming the use of lead white pigment (2PbCO3 · Pb(OH)2 ) in the external layer of the polychrome. The next two pulses result in an increase of the relative intensities of peaks attributed to Au, Cu and Ag (Fig. 3b) characteristics of a gold alloy.

Retablo del Pazo de San José de Vistalegre

The altarpiece of the chapel in Pazo de San José de Vistalegre (Tui-Pontevedra) is shown in Figure 5. It was built in 18th century by an unknown author and represents an example of baroque altarpieces located in rural churches or chapels in Galicia (NW Spain). It was made of gilded and polychromed wood. The altarpiece presents a central niche with the image of San José surrounded by six small statues of other saints. The process of restoration of this altarpiece included the removal of a brass-based paint (purpurin) probably added in previous interventions to cover the lack of gold leaf. Purpurin turned opaque due to the oxidation process of its constituents; furthermore, oxidation products increased the adhesion to the substrate making difficult the mechanical or chemical elimination of such substance. This problem is quite frequent for restorers because purpurin was used extensively throughout the last century to cover loss of gold leaf. In this case, the capability of LIBS as a control tool

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Figure 5. The altarpiece of the chapel in Pazo de San José de Vistalegre (Tui-Pontevedra), 18th century. Gilded and polychromed wood. The square in the bottom left side indicates the zone studied. Figure 7. LIBS spectra of a) gold leaf and b) purpurin layers in the spectral window from 260 to 340 nm. Gold used was alloyed withAg and Cu and purpurin is a brass based material.

Figure 6. Two optical microscopy images of cross sections obtained at different points of the Retablo del Pazo de San José de Vistalegre. In (a) no purpurin was added. In (b) a thick pile of purpurin can be appreciated.

in a laser cleaning process focused on the elimination of purpurin in gilded polychromes was tested. Specifically, LIBS was used to distinguish between original materials (gold leaf) and purpurin added in previous interventions. Figure 6 shows the cross sections obtained by means of optical microscopy of specimens taken from the altarpiece. Figure 6a corresponds to a sample free of purpurin showing a top layer of gold leaf ≈3 µm, a layer of bole ≈ 20 µm and the layer of white ground > 50 µm. Conversely, Figure 6b depicts a sample where purpurin was added. A thick and mat pile of purpurin over the layer of bole can be appreciated. LIBS spectra, in the range 260 nm to 340 nm, of gold leaf and purpurin layers, respectively, are shown in Figure 7. As depicted in the plots, gold was alloyed with silver and copper and purpurin consists basically of copper and zinc. Where purpurin was added (Fig. 8b), LIBS depth profiles show that copper (peak Cu (I) 327.396) characteristic of the purpurin and iron (peak Fe (I) 274.356) are present for the 8 first pulses, which, in addition to stratigraphy in Figure 6b, indicates that the purpurin is probably mixed with the bole. The intensity of Ca (II) 315.887 increases in the last two pulses indicating

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ACKNOWLEDGMENTS The authors are grateful to Museo de Pontevedra, specially Carlos Valle, director, and Sonia Briones, conservator; and Galicia Proarte S.L., for providing the samples to be analyzed. This work was partially supported by Xunta de Galicia, Project PGIDIT06CCP00901CT.

REFERENCES

Figure 8. LIBS depth profiles obtained in areas of the altarpiece (a) free of purpurin and (b) with purpurin added. The intensity of the peaks Au (I) 274.356, Cu (I) 327.396, Fe (I) 274.356 and Ca (II) 315.887, which characterize the different layers in polycromed samples from the altarpiece of the chapel in Pazo de San José de Vistalegre, are shown as a function of the number of laser shots.

that the ground layer has been reached. These results confirm the structure of the polychromes obtained by means of optical microscopy and demonstrate the capability of LIBS, not only to give information about the structure and composition of the different layers in polychromes, but also to monitor the laser cleaning of these artistic materials; specifically, the elimination of purpurin added to cover the losses of gold leaf.

4

CONCLUSIONS

Laser Induced Breakdown Spectroscopy, LIBS, has allowed to perform the elemental characterization of the different layers in gilded wood polychromes and to obtain compositional in-depth profiles of real samples from two baroque altarpieces. Specifically LIBS has allowed to characterize the different layers in a polychrome decorated with the technique of sgraffito on gold and to distinguish added layers of purpurin, a brass based pigment, extensively used during the last century to cover the losses of gold leaf. For these reasons LIBS appears as an adequate technique not only for the knowledge of the structure and composition of gilded wood polychromes, but also as a control tool in the laser cleaning of these materials.

Acquaviva, S., De Giorgi, M. L., Marini, C. & Poso, R. 2004. Elemental analyses by Laser Induced Breakdown Spectroscopy as restoration test on a piece of ordnance. Journal of Cultural Heritage 5: 365–369. Acquaviva, S., De Giorgi, M. L., Marini, C. & Poso, R. 2005. A support of restoration intervention of the bust of St. Gregory the Armenian: Compositional investigations by Laser Induced Breakdown Spectroscopy. Applied Surface Science 248: 218–223. Anglos, D., Couris, S. & Fotakis, C. 1997. Laser diagnostics of painted artworks: Laser-Induced Breakdown Spectroscopy in pigment identification. Applied Spectroscopy 51: 1025–1030. Burgio, L., Clark, R. J. H., Stratoudaki, T., Doulgeridis, M. & Anglos, D. 2000. Pigment identification in painted artworks: A dual analytical approach employing Laser-Induced Breakdown Spectroscopy and Raman microscopy. Applied Spectroscopy 54: 463–469. Castillejo, M., Martin, M., Oujja, M., Silva, D., Torres, R., Domingo, C., Garcia-Ramos, J. V. & Sanchez-Cortes, S. 2001. Spectroscopic analysis of pigments and binding media of polychromes by the combination of optical laserbased and vibrational techniques. Applied Spectroscopy 55: 992–998. Castillejo, M., Martin, M., Oujja, M., Silva, D., Torres, R., Manousaki, A., Zafiropulos, V., van den Brink, O. F., Heeren, R. M. A., Teule, R., Silva, A. & Gouveia, H. 2002. Analytical study of the chemical and physical changes induced by KrF laser cleaning of tempera paints. Analytical Chemistry 74: 4662–4671. Castillejo, M., Martin, M., Silva, D., Stratoudaki, T., Anglos, D., Burgio, L. & Clark, R. J. H. 2000. Analysis of pigments in polychromes by use of Laser Induced Breakdown Spectroscopy and Raman microscopy. Journal of Molecular Structure 550: 191–198. Clark, R. J. H. 2002. Pigment identification by spectroscopic means: an arts. Comptes Rendus Chimie 5: 7–20. Colao, F., Fantoni, R., Lazic, V., Caneve, L., Giardini, A. & Spizzichino, V. 2004. LIBS as a diagnostic tool during the laser cleaning of copper based alloys: experimental results. Journal of Analytical Atomic Spectrometry 19: 502–504. Filgueira-Valverde, J. 1991. La basílica de Santa María de Pontevedra. A Coruña: Fundación Pedro Barrié de la Maza. Gobernado-Mitre, I., Prieto, A. C., Zafiropulos, V., Spetsidou, Y. & Fotakis, C. 1997. On-line monitoring of laser cleaning of limestone by Laser-Induced Breakdown Spectroscopy and Laser-Induced Fluorescence. Applied Spectroscopy 51: 1125–1129.

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González-Alonso, E. 1997. Tratado del dorado, plateado y su policromía. Tecnología, conservación y restauración. Valencia: Servicio de Publicaciones de la Universidad Politécnica de Valencia. Kaminska, A., Sawczak, M., Oujja, M., Domingo, C., Castillejo, M. & Sliwinski, G. 2006. Pigment identification of a XIVwooden crucifix. Journal of Raman Spectroscopy 37: 1125–1130. López, A. J., Nicolás, G., Mateo, M. P., Piñon, V., Tobar, M. J. & Ramil, A. 2005. Compositional analysis of Hispanic Terra Sigillata by Laser Induced Breakdown Spectroscopy. Spectrochimica Acta Part B-Atomic Spectroscopy 60: 1149–1154. Maravelaki, P. V., Zafiropulos, V., Kilikoglou, V., Kalaitzaki, M. & Fotakis, C. 1997. Laser Induced Breakdown Spectroscopy as a diagnostic technique for the laser cleaning of marble. Spectrochimica Acta Part B-Atomic Spectroscopy 52: 41–53. Melessanaki, K., Papadakis, V., Balas, C. & Anglos, D. 2001. Laser Induced Breakdown Spectroscopy and

hyper-spectral imaging analysis of pigments on an illuminated manuscript. Spectrochimica Acta Part B-Atomic Spectroscopy 56: 2337–2346. Oujja, M., Rebollar, E., Castillejo, M., Domingo, C., Cirujano, C. & Guerra-Librero, F. 2005a. Laser cleaning of terracotta decorations of the portal of Palos of the Cathedral of Seville. Journal of Cultural Heritage 6: 321–327. Oujja, M., Vila, A., Rebollar, E., Garcia, J. F. & Castillejo, M. 2005b. Identification of inks and structural characterization of contemporary artistic prints by Laser Induced Breakdown Spectroscopy. Spectrochimica Acta Part B-Atomic Spectroscopy 60: 1140–1148. Tornari, V., Zafiropulos, V., Bonarou, A., Vainos, N. A. & Fotakis, C. 2000. Modern technology in artwork conservation: a laser-based approach for process control and evaluation. Optics and Lasers in Engineering 34: 309–326.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Classification of archaeological ceramics by means of Laser Induced Breakdown Spectroscopy (LIBS) and Artificial Neural Networks A. Ramil, A.J. López, M.P. Mateo & A. Yáñez Departamento de Enxeñaría Industrial II, CIT, Universidade da Coruña, Ferrol (A Coruña), Spain

ABSTRACT: The aim of this work is to analyze the feasibility of Artificial Neural Networks (ANN) for the classification, in function of the provenance, of archaeological ceramics Terra Sigillata analyzed by means of Laser Induced Breakdown Spectroscopy (LIBS). An ANN algorithm which is fed with the whole LIBS spectra of the ceramic samples was proposed and an analysis of the network architecture as a function of the number of hidden neurons and number of epochs of training was carried out in order to optimize the performance of the network. Finally, the correct classification of Terra Sigillata pieces from their LIBS spectra has been achieved in a systematic and objective way.

1

INTRODUCTION

This work deals with a provenance study of archaeological ceramics Terra Sigillata, a Roman fineware characterized by a red sintered slip. The production of Terra Sigillata began in Central Italy in the 1st century BC and from there it spread over many areas of the Roman Empire. Due to extensive trading in the ancient world, findings of Terra Sigillata from an excavated site may include products from different workshops and periods which frequently differ in technological features. The reconstruction of trade is important for archaeologists and the ability to determine the source of ancient ceramics becomes of great interest (Maggetti 2001, López-Pérez 2004). Different analytical techniques have been used for provenance studies of archaeological materials; among them, Laser-Induced Breakdown Spectroscopy (LIBS) provides some significant advantages: This technique is almost non-destructive and the study can be performed over the whole piece in air at room temperature without preparation or fragmentation requirements (Fotakis et al. 2007). In a previous work (López et al. 2006) we have shown that it is possible to obtain a reliable classification of Terra Sigillata shreds as a function of the provenance by considering the whole LIBS spectrum as a fingerprint of the sample and using linear correlation techniques for quantifying the degree of similarity among them. In this work, however, a different approach based in artificial neural networks is tried.

Artificial neural network (ANN) algorithms are now being used in a wide variety of data processing applications giving solutions to classification and prediction problems with high degree of accuracy. An advantage of this technique is the fast pattern recognition with an appropriate training (Bishop 1994, Peterson 2000). There is a number of papers in the field of archaeometry concerning the use of ANN (Remola et al. 1996, Bell & Croson 1998, López-Molinero et al. 2000, Maa et al. 2000, Fermo et al. 2004). However, despite of the fact that artificial neural networks have found great impact in spectral analysis, up to now there is little work concerning the application of ANN in LIBS spectroscopy (Sattmann et al. 1998, Samek et al. 2001, Inakollu 2003, Sirven et al. 2006). This paper is focused on the use of neural networks for the classification of Terra Sigillata samples as a function of the provenance. The advantage of ANN over traditional computing techniques like linear regression is the adaptability to unknown situations (generalization), the ability of learning, ease of use and strong tolerance to errors or noise (Haykin 1999, Ham & Kostanic 2001). The proposed ANN algorithm processes the whole LIBS spectrum of the analyzed samples. The optimum configuration of the neural network is analyzed and the performance for the correct assignation of provenances is evaluated and compared with results obtained from linear correlation techniques.

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2 2.1

EXPERIMENTAL Experimental setup

A standard LIBS configuration was used in this study consisting of a Q-switched Nd:YAG laser (Quantel, model Brilliant B, pulse length (FWHM) 6 ns) operating at the third harmonic, 355 nm. The laser beam was focused onto the sample surface at normal incidence with a quartz plano-convex lens (300 mm focal length) leading to an analyzed area of 0.5 mm2 . The emission of the plasma was collected and guided to the spectrograph (Oriel, model MS257) with an optic fibre. Light was dispersed by using the 600 grooves/mm grating of the spectrograph. An ICCD (Andor, model DH5H718F-03) which consisted of 512 × 512 pixels detected the spectral resolved emission from the plasma.

Figure 1. Characteristic LIBS spectra of Terra Sigillata samples from Hispanic, Gaulic and African workshops, in the spectral window between 260 and 340 nm.

2.2

subtle variations in the ratios of line intensities which are not obvious in a visual examination.

Samples

A total of 36 pieces of Terra Sigillata from different provenances (Hispanic, Gaulic and African workshops) and different periods (Higher Roman Empire and Lower Roman Empire) were analyzed by LIBS. On the basis of archaeological information, they were classified as follows: – 24 pottery samples (from S1 to S24) were assigned to Hispanic workshops in Tricio (La Rioja) and Andújar (Andalucía). Most of them dated from 1st – 2nd century AC. However, only four samples correspond to the Lower Roman Empire (4th–5th century AC). – 6 pieces (from S25 to S30) were attributed to the Gaulic center of La Graufesanque (1st – 2nd century AC). – 6 samples correspond to the North-African workshops (from S31 to S36). 2.3

LIBS data

Each ceramic shred was analyzed at different random points of a fresh fracture (10 positions in the case of samples used for training the network, and 3 for the rest) and ten laser shots were recorded over the same sample position. In all the cases, the emission signal reached higher values of intensity at the first shot than in the next nine, in which it remained stable. Due to this the first LIBS spectrum was always disregarded for the analysis. The spectral window between 260 and 340 nm was selected as the most adequate for the analysis of these materials (López et al. 2005). Characteristic LIBS spectra of samples from the Hispanic, Gaulic and African workshops are shown in Figure 1. All the samples analyzed present similar elemental composition in terms of major constituents (López et al. 2006). Consequently, as depicted in the plots, the differences between their LIBS spectra consist of

3

DATA PROCESSING

3.1 Designing the networks ANN are computational algorithms appropriate for complex classification and pattern recognition problems. Inspired by human mind, the neural networks are implemented using simple nonlinear or linear processing elements named neurons, which may interconnect with other ones by means of transfer functions, forming complex processing networks. These networks may be trained adjusting the interconnection branch loads (synapse weights) through training algorithms, being the back-propagation one of the most common. In this algorithm, a number of example data whose outputs are known are used as input data. The calculated output is then compared with the known output (target). The difference between the two outputs is then back-propagated to recalculate the weights. The mean square error mse given by Equation 1, where N is the number of outputs used to quantify such difference. Such an iterative procedure is continued until the difference becomes small enough (Haykin 1999).

After training, the ANN can be used to perform certain tasks depending on the particular application. Different types of neural networks are described in the literature; the most commonly used is the multilayer feed-forward ANN. This kind of network is composed of one input layer that receives the vector to be classified, one output layer that presents the classification results and a set of intermediate layers named hidden layers. The number of intermediate layers and the

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Table 2. Values of the mean squared error for training (mseT ) and optimization (mseO ) sets obtained as a function of the number of neurons in hidden layer and epochs of training.

Table 1. Network desired outputs targets in neurons n1 , n2 and n3 of the output layer for Hispanic (H), Gaulic (G) and African provenances.

Hispanic Gaulic African

n1

n2

n3

Neurons

Epochs

mseT

mseO

1 0 0

0 1 0

0 0 1

0-3 10-3 20-3 25-3 30-3

5000 4479 1399 1797 990

0.228782 0.005000 0.004997 0.004997 0.004983

0.154623 0.161673 0.071905 0.031622 0.095472

number of neurons in each intermediate layer depend on the complexity of the problem to be solved. One of the goals of the classification by means of ANN is the selection of a low complexity neural network having a good ability for generalization. Given that, in our case the aim is the classification of Terra Sigillata samples in three groups, Hispanic, Gaulic and African; a network with three neurons in the output layer, n1 , n2 and n3 , seems to be the most appropriate. In order to have a good discrimination among the group outputs and an easy classification rule, the logsigmoid function, which gives values between 0 and 1, was chosen as transfer function. The network desired outputs (targets) were selected according to Table 1. The input vector consisted of 512 intensity values of the normalized LIBS spectra in the spectral window between 260 and 340 nm. The set of LIBS spectra were divided into two groups; one for the calibration or training of the network and the other one for validation purposes. The training set T consisted of samples S5 (Hispanic), S28 (Gaulic) and S32 (African) which were selected because they presented the lower dispersion in five LIBS spectra (López et al. 2006). The networks were trained by means of a gradient decrease with momentum and adaptive learning rate algorithm (Haykin 1999, Cirovic 1997). The complete procedure was designed and performed using the Matlab Neural Network Toolbox (Demuth et al. 1992).

Figure 2. Mean squared error surface for the optimization set, mseO , as a function of the number of neurons in hidden layer and training epochs.

3.2 Optimizing the networks In order to optimize the design of the network to obtain a reliable classification of the Terra Sigillata shreds, different number of neurons in hidden layer were tried. For the optimization process one sample from each provenance group was taken from the validation set; this optimization subset O comprises the samples S10, S26 and S33. The performance of ANN as a function of the number of hidden neurons: 0, 10, 20, 25 and 30, was analyzed. The training was programmed to stop if the mse dropped below 0.005 or the number of the cycles of training (epochs), reached 5000. As it can be appreciated in Table 2 a network with no hidden layer reaches 5000 epochs without achieving the desired performance and may be rejected. Clearly, increasing

Figure 3. Mean squared error for training set (mseT ) and optimization set (mseO ) as a function of the number of training epochs.

the number of hidden neurons decreases the training epochs necessary to attain the same value of mse. However a network with a large number of weights could cause overfit. For this reason, mean-squared error of the optimization set, mseO , must be taken into account to select the optimum network design (Haykin 1999). From the data in Table 2, ANN (25:3) i.e. a network with 25 neurons in the hidden layer and 3 neurons in the output layer can be considered as the best choice in terms of mse.

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The variation of mean squared error, mseO , represented as a function of the number of neurons in hidden layer and training epochs (Hernández-Caraballo & Marcó-Parra 2003) is depicted in Figure 2. mseO reaches a minimum for a number of hidden neurons around 25 (Fig. 3). A decrease in mseO by increasing the number of epochs can be appreciated. However the network could become overtrained and would lose the ability for generalization. To avoid this problem and to improve the network output i.e. the classification results, determination of when the training should stop, or early stopping criterium, is necessary. The procedure was as follows: ANN (25:3) was retrained for 100.000 epochs and the variation of mse for training and optimization sets as a function of the number of epochs is shown in Figure 3. mseT curve decreases monotonically for an increasing number of epochs. In contrast, mseO decreases monotonically to a minimum and then it starts to increase as the training continues. This behavior indicates an overtraining of the network and the minimum point in optimization learning curve must be used as criterion for stopping the training session (Haykin 1999). In the case of ANN (25:3) the minimum occurs at 20.200 epochs (see Fig. 3). 3.3 Classification efficiency Once the network was optimized and the stopping criteria established, the capability of classification was tested. By using as inputs the average spectrum of each sample in the validation set, the output of the network was represented in Figure 4 as a function of the sample number. The output of ANN (25:3) which takes the highest value in neuron n1 for Hispanic samples, in n2 for Gaulic samples, and in n3 for African samples, attains a correct identification of all the samples (100% of success). It can be observed that the outputs of some Hispanic samples are less precise than Gaulic or African samples; especially for S11 and S20 the output of neuron n2 is close to the value of neuron n1 . The ability of the designed network for the classification of Terra Sigillata samples with a single-shot spectrum was also evaluated, and the percentages of correct identification for each sample are shown in Figure 5. Averaging over the 35 Terra Sigillata shreds studied, a mean value of 91% was obtained by ANN (25:3) and 86% by linear correlation method. Moreover, taking into account all the LIBS spectra used for the analysis of the 35 samples, the ANN failed in 9% of the total cases and linear correlation in 18%, which demonstrates that neural networks present higher tolerance than linear correlation to small variations in spectra from the same provenance. Artificial neural networks may appear as a sophisticated method in comparison with linear correlation. However, an important advantage of ANN method lies

Figure 4. Values of the neurons n1 , n2 and n3 in the output layer of the neural network ANN (25:3) for the samples in the validation set. Hispanic group ranges from S1 to S24, Gaulic group from S25 to S30 and African group from S31 to S36.

Figure 5. Percentage of single shot correct identification of samples for both linear correlation (CORR) and artificial neural network (ANN). A mean value of 91% of success is attained in the case of ANN, and 86% in the case of linear correlation.

on the capability of automatization; i.e. once ANN (25:3) has been designed as the suitable network for the classification of Terra Sigillata it can be applied as a black box by non specialized users. Moreover, the possibility of being retrained with new samples to refine or expand the classification to new provenances, enhance the advantages of ANN over other conventional techniques.

4

CONCLUSIONS

The results of the present study are indicating that the use of neural networks to automate the classification of Terra Sigillata in function of the provenance by means of LIBS spectra is feasible and efficient. A feed-forward back-propagation algorithm network which works with the whole LIBS spectrum as input data was selected as the most appropriate for the classification of Terra Sigillata shreds. Simple topological configuration consisting of one hidden layer and one output layer was analyzed as a function of the number of hidden neurons and number of epochs of training. The optimum configuration obtained for the classification of Terra Sigillata in 3

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provenances: Hipanic, Gaulic and African was ANN (25:3) consisting of 25 neurons in the hidden layer and 3 neurons in the output layer. By means of ANN (25:3) correct classification of Terra Sigillata shreds using the average spectrum of each sample amounted 100%. When a single shot spectrum was used for classification, ANN (25:3) failed in 9% of the total cases and linear correlation in 18% demonstrating that neural networks present higher tolerance than linear correlation to small variations in spectra from the same provenance. To conclude, these results demonstrate that a network as simple as ANN (25:3) allows to classify with high precision Terra Sigillata shreds in function of the provenance by means of their LIBS spectra, achieving even better results than those obtained by linear correlation. ACKNOWLEDGMENTS Special thanks to M.C. López Pérez (archaeologist) and Museo de Prehistoria e Arqueoloxía de Vilalba (Lugo, Spain) for providing Terra Sigillata samples. This work was partially supported by Xunta de Galicia through Project PGIDIT06CCP00901CT. REFERENCES Bell, S. & Croson, C. 1998. Artificial neural networks as a tool for archaeological data analysis. Archaeometry 40: 139–151. Bishop, C. M. 1994. Neural networks and their applications. Review of Scientific Instruments 65: 1803–1832. Cirovic, D. A. 1997. Feed-forward artificial neural networks: Applications to spectroscopy. Trac-Trends in Analytical Chemistry 16: 148–155. Demuth, H., Beale, M. & Hagan, M. 1992. Neural Network Toolbox User’s Guide (2007 ed.). The Mathworks Inc. Fermo, P., Cariati, F., Ballabio, D., Consonni, V. & Gianni, G. B. 2004. Classification of ancient Etruscan ceramics using statistical multivariate analysis of data. Appl. Phys. A 79: 299–307. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2007. Lasers in the Preservation of Cultural Heritage. Principles and Applications. Taylor & Francis. Ham, F. M. & Kostanic, I. 2001. Principles of neurocomputing for science and engineering. McGraw Hill. Haykin, S. (1999). Neural Networks: A comprehensive foundation. New Jersey: Prentice Hall. Hernández-Caraballo, E. A. & Marcó-Parra, L. M. 2003. Direct analysis of blood serum by total reflection

X-ray fluorescence spectrometry and application of an artificial neural network approach for cancer diagnosis. Spectrochimica Acta Part B-Atomic Spectroscopy 58: 2205–2213. Inakollu, P. 2003. A study of the effectiveness of neural networks for elemental concentration from LIBS spectra. Master’s thesis, Faculty of Mississippi State University, Mississippi. López, A. J., Nicolás, G., Mateo, M. P., Piñon, V., Tobar, M. J. & Ramil, A. 2005. Compositional analysis of Hispanic Terra Sigillata by Laser-Induced Breakdown Spectroscopy. Spectrochimica Acta Part B-Atomic Spectroscopy 60: 1149–1154. López, A. J., Nicolás, G., Mateo, M. P., Ramil, A., Piñon, V. & Yáñez, A. 2006. LIPS and linear correlation analysis applied to the classification of Roman pottery Terra Sigillata. Applied Physics A-Materials Science & Processing 83: 695–698. López-Molinero, A., Castro, A., Pino, J., Pérez-Arantegui, J. & Castillo, J.R. 2000. Classification of ancient Roman glazed ceramics using the neural network of selforganizing maps. Fresenius Journal of Analytical Chemistry 367: 586–589. López-Pérez, M. 2004. El comercio de Terra Sigillata en la provincia de A Coruña. Brigantium. Museo Arqueolóxico e Histórico Castelo de San Antón, A Coruña 16. Maa, Q., Yana, A., Hu, Z., Lib, Z. & Fanc, B. 2000. Principal component analysis and artificial neural networks applied to the classification of Chinese pottery of Neolithic age. Anal. Chim. Acta 406: 247–256. Maggetti, M. 2001. Chemical analyses of ancient ceramics: What for? Chimia 55: 923–930. Peterson, K. L. 2000. Artificial neural networks and their use in chemistry. Reviews in Computational Chemistry 16: 53–140. Remola, J. A., Lozano, J., Ruisánchez, I., Larrechi, M. S., Rius, F. X. & Zupan, J. 1996. New chemometric tools to study the origin of amphorae produced in the Roman Empire.Trac-Trends inAnalytical Chemistry 15: 137–151. Samek, O., Telle, H. H. & Beddows, D. C. 2001. LaserInduced Breakdown Spectroscopy: a tool for real-time, in vitro and in vivo identification of carious teeth. BMC Oral Health 1. Sattmann, R., Monch, I., Krause, H., Noll, R., Couris, S., Hatziapostolou, A., Mavromanolakis, A., Fotakis, C., Larrauri, E. & Miguel, R. 1998. Laser-Induced Breakdown Spectroscopy for polymer identification. Applied Spectroscopy 52: 456–461. Sirven, J. B., Bousquet, B., Canioni, L., Sarger, L., Tellier, S., Potin-Gautier, M. & Hecho, I. L. 2006. Qualitative and quantitative investigation of chromium-polluted soils by laser Induced Breakdown Spectroscopy combined with neural networks analysis. Analytical and Bioanalytical Chemistry 385: 256–262.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Laser ablation- and LIBS-ranging by webcam and image processing during laser cleaning M. Lentjes, J. Hildenhagen & K. Dickmann Laser Center (LFM), Münster University of Applied Sciences, Steinfurt, Germany

ABSTRACT: Pulsed lasers with manual articulated beam delivery systems are usually used for cleaning of various artworks. The advantage of spatial manoeuvrability can be a disadvantage as well, since the exact irradiation position is unknown. During laser cleaning the coordinates of each laser pulse impinging on the artwork can be obtained by a camera placed above the sample in combination with image processing. This method was applied to obtain the corresponding coordinates of Laser Induced Breakdown Spectroscopy (LIBS) measurements whilst laser cleaning. If the position of the measurements is known, the results can be displayed as a transparent overlay with discreet colour variations on top of the picture of the artwork. The colour value corresponds to the result obtained at that coordinate. In this way, any kind of single value result can be displayed in a picture, e.g. element line intensity, intensity ratio, linear correlation coefficient, etc. We calculated the linear correlation coefficient r between the current measured spectrum and a pre-stored reference spectrum, and displayed the value of r by the corresponding colour in the picture of the sample.

1

INTRODUCTION

Due to the extension of laser cleaning to various conservation fields, the diversity of laser-cleaned artworks is also increasing. Whereas the cleaning process of encrusted marble by Nd:YAG laser is self-limiting (when using the suitable parameters), this is not the case for all polluted artworks. This means that in these cases, on time stopping of the cleaning process is essential to avoid over-cleaning. Without intervention, the ablation process continue through the surface to be preserved. In general, the pollution layers are not uniform. Therefore, the number of laser pulses at a certain position has to be adapted to the degree of pollution in this position to ensure an appropriate cleaning of the object. The ablation process is manually controlled by the restorers in most of the practical laser cleaning projects. They observe the laser cleaning process through a laser safety goggle and manually stop the laser emission as soon as the pollution is removed. However, there are different methods applied to avoid over-cleaning during laser treatment of artworks. Some existing methods are: – spectroscopic analysis of the plasma emission induced during laser cleaning (LIBS) (GobernadoMitre et al. 1997, Klein et al. 1999, Scholten et al. 2000, Teule et al. 2003)

– plasma intensity measurement (Hildenhagen & Dickmann 2003, Lentjes et al. 2005) – spectroscopic analysis of laser induced fluorescence (LIF) (Anglos et al. 1995, Gobernado-Mitre et al. 1997) – measuring the transmitted laser radiation (Chaoui et al. 2003) – acoustic monitoring of the induced snapping sound (Jankowska & Sliwinski 2003, Lee & Watkins 2000) – chromatic monitoring (Lee & Watkins 2000, Lee & Steen 2001). In general, these methods are applied to control/ monitor laser cleaning processing with systems equipped with scanners or automatic translation stages. In this paper, a procedure is presented for the use of laser cleaning monitoring methods in combination with laser systems with articulated beam delivery. In the case of cleaning with manual articulated beam delivery, monitoring the process can be used to support a human decision. The developed method is based on the correlation of LIB spectra with pre-stored reference spectra. Identifying the layers during laser cleaning by means of LIB spectra and correlation analysis is described in more detail in a former paper (Lentjes et al. 2007a).

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2

MONITORING LASER CLEANING WITH ARTICULATED BEAM DELIVERY

Nd:YAG laser systems used for cleaning of artworks are often equipped with an optical fibre or articulated mirror arm for beam delivery. The advantage of the spatial manoeuvrability can also be a disadvantage, since the exact irradiation point is not known and the beam cannot be manipulated automatically. Therefore, cleaning systems with manual articulated beam delivery are generally not adapted for closed-loop cleaning. However, during laser cleaning the coordinates of each laser pulse on the artwork can be obtained by a camera above the sample in combination with image processing (Figs. 1, 2). In the first attempts this method was applied to obtain the corresponding coordinates of LIB spectra measurements whilst laser cleaning. The position of

the interaction area of each pulse was achieved shortly after the laser pulse and plasma emission by analysing the position of a coaxial He-Ne laser spot on the sample. The He-Ne laser spot was used since the Nd:YAG laser pulse and the plasma emission saturated the CCD of the webcam. Since the positions of the laser pulses are known, the results of the corresponding LIBS measurements can be displayed as a transparent overlay with discreet colour variations on top of the picture of the artwork. The colour value corresponds to the result obtained at that coordinate. In this way, any kind of single value result can be displayed in a picture, e.g. element line intensity, intensity ratio, linear correlation coefficient, etc. In this research, the linear correlation results, calculated by correlating the current measured spectrum with a pre-stored reference spectrum, were displayed by the corresponding colour in a picture of the sample. On-line visualisation of the artwork with a current measurement-overlay on a (computer) screen during laser cleaning can be used to monitor/control the process (Figs. 1, 2). By means of the colour values, the restorer can recognise the parts of the artworks that are sufficiently cleaned to avoid over-cleaning. 3

Figure 1. Photograph of the webcam-LIBS set-up.

EXPERIMENTAL SETUP

The laser system used (by Thales, see Table 1), is equipped with an articulated mirror arm enabling manual spatial manipulation of the laser beam (Fig. 1). The laser system is a flash lamp pumped Q-switched Nd:YAG laser based on oscillator/amplifier principle with the possibility of frequency doubling, tripling and quadrupling. In this research project, only the first harmonic was applied. The hand piece of the articulated arm embodies a telescope consisting of

Figure 2. Schematic of the webcam-LIBS setup used to measure LIB spectra and the interaction area coordinates. The right picture shows the sample with ablation spots while the left picture shows the same picture with the transparent overlay. The sample in the figure is iron covered with a thin rust layer.

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Table 1.

Characteristics of the Nd:YAG cleaning laser.

Type Wavelengths

Repetition rate Max. Pulse energy Pulse duration

Thales Saga 220/10 1064 nm (Nd:YAG) 2ω, 532 nm 3ω, 355 nm 4ω, 266 nm 1–10 Hz 1500 mJ @ 1064 nm 10 ns

Figure 3. Cross section of the mirror holder developed for in-process plasma emission measurement.

a positive lens ( f = 102.3 mm), followed by a negative lens ( f = −101.0 mm). The distance between both lenses can be varied in order to adjust the working spot diameter and consequently the energy density on the object surface is adjusted as well. Since the output beam was not parallel but converging, a distance holder was applied to secure a constant energy density on the object surface (Fig. 1). The implemented He-Ne laser was coaxial with the Nd:YAG beam. The HR2000 spectrometer system is a userconfigured miniature fibre optic spectrometer from Ocean Optics. For collecting the plasma radiation into the spectrometer, a 2 m long and diameter 600 µm optical fibre, in combination with a f = 10 mm collimator with diameter 5 mm was used. The groove density (300 grooves/mm) and the entrance aperture of 25 µm result in a spectral range of 200–1100 nm with a resolution of 2 nm. The exposure time of the electronic “shutter” is factory-set on 2 ms. The optical fibre with collimator was implemented in the last mirror holder of the articulated arm to allow in-process plasma emission measurement (onaxis plasma emission collection). Fig. 3 shows a cross section of this especially constructed mirror holder. The camera is a FireWire 1394 Webcam with 640 × 480 VGA resolution. The process was controlled by in LabView 7.1 software. The time between attaining the coordinate, firing the laser and acquiring a LIB spectrum was arranged in the LabView program in combination with a DAQ-card with onboard counters. The standard LabView 7.1 software was extended with the LabView

Figure 4. Flow diagram of the webcam and LIBS based laser cleaning monitoring.

Vision Development Module which enables image processing. The sequence of the software for monitored laser cleaning with manual articulated beam delivery is shown in Figure 4.

4

RESULTS AND SIDE EFFECTS

First trials of this method applied to 3 different samples (black on white paint, polluted marble gravestone and rusty steel) showed the basic workability of this method. It was possible to acquire LIB spectra and the position of the measurements after the webcam and image processing settings were optimized for each sample. The image processing settings that could be changed were detection threshold, sharpness, saturation, brightness and post processing image filter. The results were repeatable at different sample positions under the condition that the LIBS experimental parameters were kept constant. This was achieved by applying a distance holder and constant irradiation angle (Fig. 1). Figure 5 shows the trend of the correlation coefficients obtained with the polluted marble sample at constant experimental parameters (the results of the remaining two samples are not displayed as they featured the same trend). The LIB

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Figure 5. Variation of the correlation coefficient versus ablation pulse number per spot calculated during first trials with the webcam Nd:YAG cleaning laser monitoring setup (constant experimental parameters).

beam delivery. The standard deviation will increase which results in a spread of the single distributions. This means that the acquired spectra, corresponding to the different layers, have to provide a higher difference in comparison to cleaning with constant experimental parameters. This was the limiting effect during the experiments with the low resolution spectrometer and articulated beam delivery. The differences between the correlation coefficients corresponding to the second and third pulse at spot 1 to 4 in Figure 5 were < 0.005 at constant experimental condition. This was too low for in-process identifying during cleaning without distance holder. If the output beam is parallel, a perpendicular object hand piece distance variation will not change the energy density. With these pre-conditions in-process identifying during cleaning without distance holder with the applied setup should be possible.

5

Figure 6. LIB-spectra acquired at the first and third laser ablation pulse on spot 1 (sample polluted marble). The delay between the laser pulse and recording the plasma emission was 0 µs.

spectra of pulse one and three at spot 1 are shown in Figure 6. Since the resolution of the screen image with overlay is too low for printing it is not shown in this paper. In practice, experimental parameters like lens object distance, spot size and irradiation angle can vary from pulse to pulse when the beam is manually manipulated. This influences the form and intensity of the acquired spectra and therewith the distribution of correlation coefficients of the different layers (Lentjes et al. 2007b). If the correlation coefficients distributions of the different layers barely overlap when using a distance holder and a constant irradiation angle, this method cannot be applied to identify layers when cleaning with complete manual articulated

CONCLUSIONS

In a first trial, the LIBS-correlation method has been used to monitor laser cleaning with a manual articulated mirror arm. Therefore, a webcam was placed above the sample to ascertain the irradiation position. The obtained correlation coefficients were visualized in a current picture of the sample by colour marks representing the progress of the cleaning. The obtained results for the tested samples (black on white paint, polluted marble gravestone and rusty steel) were repeatable, provided that the experimental parameters remain constant. This was achieved by applying a distance holder and constant irradiation angle. It was feasible to acquire LIB-spectra and the position of the measurements after the webcam and image processing settings were optimized per sample. REFERENCES Anglos, D., Couris, S., Mavromanolakis, A., Zergioto, I., Solomidou, M., Liu, W. Q., Papazoglou, T. G., Fotakis, C., Doulgeridis, M. & Fostiridou, A. 1995. Artworks Diagnostics Laser Induced Breakdown Spectroscopy (LIBS) and Laser Induced Fluorescence (LIF) Spectroscopy. In E. König & W. Kautek (eds.), Lasers in the Conservation of Artworks, Restauratorenblätter, Sonderband – LACONA. 1: 113–118. Chaoui, N., Solis, J., Afonso, C. N., Fourrier, T., Muehlberge, T., Schrems, G., Mosbacher, M., Bäuerle, D., Bertsch, M. & Leiderer, P. 2003. A high-sensitivity in situ optical diagnostic technique for laser cleaning of transparent substrates. Applied Physics A 76(5): 767–771. Gobernado-Mitre, I., Prieto, A. C., Zafiropulos, V., Spetsidou, Y. & Fotakis, C. 1997. On-Line Monitoring of Laser Cleaning of Limestone by Laser-Induced Breakdown Spectroscopy and Laser-Induced Fluorescence. Applied Spectroscopy 51(8): 1125–1129.

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Hildenhagen, J. & Dickmann, K. 2003. Low-cost sensor system for online monitoring during laser cleaning. Journal of Cultural Heritage 4: 343–346. Jankowska, M. & Sliwinski, G. 2003. Acoustic monitoring for the laser cleaning of sandstone. Journal of Cultural Heritage 4: 65–71. Klein, S., Stratoudaki, T., Zafiropulos, V., Hildenhagen, J., Dickmann, K. & Lehmkuhl,T. 1999. Laser-induced breakdown spectroscopy for on-line control of laser cleaning of sandstone and stained glass. Applied Physics A 69: 441–444. Lee, J. M. & Watkins, K. G. 2000. In-process monitoring techniques for laser cleaning. Optics and Lasers in Engineering 34: 429–442. Lee, J. M. & Steen, W. M. 2001. In-Process Surface Monitoring for Laser Cleaning Processes using a Chromatic Modulation Technique. The International Journal of Advanced Manufacturing Technology 17: 281–287. Lentjes, M., Klomp, D. & Dickmann, K. 2005. Sensor Concept for Controlled Laser Cleaning via Photodiode. In K. Dickmann, C. Fotakis & J. F. Asmus (eds.), Laser in the Conservation of Artworks, Springer Proceedings in Physics. 100: 427–433.

Lentjes, M., Dickmann, K. & Meijer, J. 2007a. Influence of process parameters on the distribution of single shot correlation coefficients obtained by correlating LIB-spectra. Applied Physics A 88: 661–666. Lentjes, M., Dickmann, K. & Meijer, J. 2007b. Low Resolution LIBS for Online-Monitoring During Laser Cleaning Based on Correlation with Reference Spectra. In H. Nimmrichter, W. Kautek & M. Schreiner (eds.), Laser in the Conservation of Artworks, Springer Proceedings in Physics. 116: 437. Scholten, J. H.,Teule, J. M., Zafiropulos,V. & Heeren, R. M.A. 2000. Controlled laser cleaning of painted artworks using accurate beam manipulation and on-line LIBS-detection. Journal of Cultural Heritage 1: 215–220. Teule, R., Scholten, H., Brink, O. F. v., Heeren, R. M. A., Zafiropulus, V., Hesterman, R., Castillejo, M., Martin, M., Ullenius, U., Larsson, I., Guerra-Librero, F., Silva, A., Gouveia, H. & Albuquerque, M. B. 2003. Controlled UV laser cleaning of painted artworks: a systematic effect study on egg tempera paint samples. Journal of Cultural Heritage 4: 209–215.

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LIBS analysis of metal artefacts from Sucevita Monastery, Romania M. Oujja & M. Castillejo Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

W. Maracineanu, M. Simileanu & R. Radvan National Institute for Research and Development for Optoelectronics INOE 2000, Bucharest, Romania

V. Zafiropulos Technological Educational Institute of Crete, Sitia, Crete, Greece

D. Ferro Istituto per lo Studio dei Materiali Nanostructurati, CNR, Rome, Italy

ABSTRACT: This work presents the results obtained in June 2006 during a campaign within the frame of an EU-funded 2000 Culture Project. Laser induced breakdown spectroscopy (LIBS) has been used to determine the elemental composition of different parts of two metal objects kept in Sucevita Monastery in Suceava, Romania: a cache-pot from the 18th century and a ligneous cross from the 19th century. The results obtained by LIBS were validated by the energy dispersive spectroscopy/scanning electron microscopy technique (EDS/SEM). The integration of results on compositional microanalyses obtained by both techniques on small areas of the objects reveals interesting aspects of the working process and of ancient restoration phases. The comparison of the LIBS results with the EDS/SEM characterisation demonstrates the advantages of this spectroscopic technique for the in situ practice of metal artefact analysis. Finally, optical microscopy was used to observe the fingerprint of LIBS effect and to assess the microstructure of the studied metal objects.

1

INTRODUCTION

Systematic chemical and structural analysis of ancient artistic objects provide important insight into the techniques available and used for processing materials in the manufacture of an art object. Various analytical techniques have been applied extensively in the study of art objects providing important physical and chemical information about the materials and the structure of objects. Among these techniques, scanning electron microscopy (SEM), X-ray fluorescence (XRF), proton induced X-ray emission (PIXE), Xray diffraction (XRD), energy dispersive spectroscopy (EDS) and Raman microscopy (Clark et al. & Bell et al. 1997, Mantler et al. 2000, Burgio et al. 2001, Mandrino et al. 2004, Elfwing et al. 2005), can be cited. However, most of these laboratory techniques require special sample preparation and handling procedures. In addition, transportation of artworks to specialised laboratories is often subject to strict regulations and requires lengthy procedures. Laser Induced Breakdown Spectroscopy (LIBS) offers several analytical advantages and in some cases can be a potential alternative to the mentioned techniques. LIBS is a

practically non-destructive as well as rapid elemental analysis technique with the critical advantage of being applicable in situ, thereby avoiding sampling and sample preparation. LIBS is currently used to characterise the elemental composition of different materials (Anglos & Rinaldi et al. 2001, Bustamante et al. 2002). Qualitative analyses of the elemental composition of stones (Klein et al. 1999), metals (Melessanaki et al. 2002), inks (Oujja et al. 2005), and paintings (Scholten et al. & Castillejo et al. & Burgio et al. 2000) have been carried out. Recent approaches to quantitative analysis by LIBS have lead to the determination of absolute concentration values for each element of the analysed material (Colao et al. 2002, Kuzuya et al. 2003, Carmona et al. 2005 & 2007, Corsi et al. & Yaroshchyk et al. 2006). Some of us have used LIBS in combination with XRF and SEM/EDX in previous works to determine the qualitative and quantitative composition of glasses and inks (Oujja et al. 2005, Carmona et al. 2005 & 2007). In this work, we present the results of the studies performed on two different historical objects kept in Sucevita Monastery, in Suceava, Romania. Analytical information from LIBS analyses was used in order

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Figure 1. General view of the ligneous cross broken in two parts (a) and cache-pot (b) kept in Sucevita Monastery, Romania; c), d) and e) localisation of zones analysed by LIBS and where the material has been extracted for EDS analyses.

to characterise the metal objects, and to compare these results with those obtained with the conventional analytical technique EDS/SEM. 2

OBJECTS AND METHODS

The study was concentrated on two metal objects; a ligneous cross and a cache-pot. The ligneous cross, 15 cm high, 10 cm wide and 2 cm thick (Fig. 1a) was kept in Sucevita Monastery but it originates from Balinesti, Suceava County (tagged with inventory n◦ 492, dated 19th century). The cross is composed of three parts carved in wood which represent the cycle of Christ’s life in bas-relief and is encapsulated in a metal box made using a mould and assembling separate parts.The lateral decorations and the base were soldered to the cross. Hand refining traces are indicative of the style of the manufacture. To make the conical base of the metal container the metal was moulded with a particular shape without any further chisel work. Regarding the state of conservation, the ligneous material appears

in good condition presenting only some mechanical fractures. The left beam of the cross seems to be made of different wood and with different structure; furthermore figures appear to be carved using a different less accurate style. The right beam is inverted. The metallic box containing the cross is separated from its base and some of its components are missing. Apart from the slight oxidation strata, most of the damage is probably due to mechanical stress. Some points of interest for further study were identified.These include mechanical deformations and the presence of additive materials.These aspects were considered indicative for the understanding of the history of the object. The second metallic object studied was a cachepot (Fig. 1b, tagged with inventory n◦ 590, dated end of 18th century, Parochia Zvoristea, Suceava county). This object has an oval section with axes of 33 and 22 cm and is 13 cm high. It consists of a metallic strip decorated by stamping with amphorae and floral motives. Four legs are soldered to the bottom part of the body and these are decorated with motives of leaves made by casting. Inside the object, a metallic layer covers the whole internal surface. The object is in a discrete state of conservation; the internal layer of the cache-pot is completely corroded and broken in some parts near the edge. Areas with soldering material are evident, particularly in the zones where the four legs are attached to the main body of the object. The peculiarity of the object, composed of different parts, each of them manufactured in a different way, induced the choice of a series of zones for analyses, aiming at individualising the characteristics of each part of the manufacture. The areas selected for LIBS analyses at the base of the cross, the cross and in the cache-pot are indicated in Figures 1c, 1d and 1e. For each object, samples were taken to be analysed by EDS/SEM in order to provide evidence of the elemental quantitative composition and to obtain qualitative data for comparison with the results obtained by LIBS. The sampling consisted in extracting very small amounts of material by scraping with a quartz dust abrasive card. Sample taking was done with the consent of the project conservators. For LIBS measurements, laser irradiation was carried out with the fundamental harmonic of a Q-switched Nd:YAG laser (Quanta Systems, pulses of 6 ns, repetition rate of 10 Hz, 1064 nm). The samples were irradiated by the focused laser using an f = 100 cm lens allowing to achieve fluences up to 9 J cm−2 . The plume emission was collected with a quartz optical fibre. LIB spectra were recorded at the 300– 700 nm wavelength range with a Mechelle spectrograph (ME5000) coupled to a time gated ICCD camera (iStar, Andor Technologies). The temporal gate was operated at 500 ns time delay (in order to discriminate the atomic emission from the continuum background)

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qualitative/quantitative determination (0.2 wt resolution-limit) by observing the backscattered electrons to get atomic contrast and to distinguish metal grains from the abrasive card components. For each sample, the material extracted by scrapping was extended to a thin layer. Analyses were done on several positions of the extended material layer.

3

Figure 2. LIB spectra in positions BC1 and BC2 on the base of the cross (Figure 1c).

Figure 3. LIB spectrum of the area C2 of the cross as indicated in Figure 1d.

and width of 3 µs. The spectra were obtained by using commercial software by Andor Technologies for a single pulse. The assignment of lines is based on the information taken from the NIST database (NIST Electronic database, at http://physics.nist.gov). The analyses can be characterised as practically nondestructive because the laser beam is focused on a spot of typical diameter in the range of 400–500 µm. The optical microscopy measurements were carried out using an optical microscope which can provide us images at 5 different magnifications: 3x, 6x, 16x, 32x and 50x, these images are acquired with a CCD camera and transmitted to the PC software. The EDS/SEM measurements were performed using a microanalysis system INCA 300 for

RESULTS AND DISCUSSION

LIB spectra on different zones analysed at the base and on the cross are shown in Figs. 2 and 3 respectively and the main results are summarised inTable 1. The characteristic lines of the most representative elements could be assigned. At the base of the cross, the areas labelled BC1, BC2 and BC3 were analysed (Fig. 1c, Table 1). The LIB spectrum taken in the area BC1 (Figure 2) reveals the presence of emissions attributed to copper (Cu I at 324.75, 327.39, 465.11, 510.55, 515.32, 521.82 and 578.21 nm), zinc (Zn I at 328.23, 330.29, 334.50, 468.01, 472.21 and 481.05 nm), silver (Ag I at 328.07, 338.29, 520.90 and 546.55 nm), nickel (Ni I at 341.47, 345.84, 352.45 and 361.93 nm), calcium (Ca II at 393.36 and 396.84 nm, and Ca I at 422.67 nm) and sodium (Na I at 588.99 and 589.59 nm). The analysis done in BC2 (Fig. 2) and BC3 revealed the same emissions as in BC1, except for the absence of zinc and nickel in BC2 and silver in BC3. Figure 3 shows the spectrum taken in the area C2 of the cross (Fig. 1d); there are emissions characteristic to copper, silver, calcium and sodium. The area C1 gives rise to the same emissions as in C2. LIB spectra collected in different areas of the cachepot (Fig. 1e, Table 1) allow the characterisation of the different materials used in the manufacture of this object. LIBS analyses of the inner flat of the cachepot (area CP-IF1) indicate that it is mainly composed of zinc as evidenced by lines of Zn I. In addition, there are emissions attributed to calcium (lines of Ca II), lead (Pb I at 405.78 nm) and sodium (lines of Na I). The soldering material between the cachepot and the legs (area CP-SM6) gives rise to emissions attributed to lead (PbI at 357.27, 363.95, 367.14, 368.34, 373.99 and 405.78), tin (Sn I at 317.50, 326.23 and 380.10 nm), vanadium (V I at 437.92 nm), calcium (lines of Ca II) and sodium (lines of Na I). The qualitative analysis made in the area CP-L2 (the back part of legs), the area CP-SP3 (soldering material of the decorated strip) and the area CP-SP4 (the metal of the decorated strip) of the cache-pot indicate the presence of emissions attributed to zinc, copper, calcium and sodium. In addition to the emissions observed in these points, the analyses in point CP-BS5 (back side of the cache-pot) indicate the presence of iron (Fe I at 385.99, 407.17and 438.35 nm). Figure 4 shows the LIB spectrum obtained for the area CP-BS5.

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Table 1. Elements identified by LIBS and EDS/SEM performed in different areas of the metallic studied objects (Figure 1). The elements present in higher concentration, as determined by EDS, are marked in bold. Analysed areas Cross Metal box Lateral decorations Base of the cross Top (soldering material) Back side of the base Central part Cache-pot Inner flat part Metal legs Decorated strip Back side Soldering material legs-main body

LIBS

EDS

C1 C2

Cu, Ag, Ca, Na Cu, Ag, Ca, Na

Cu, Ag, Zn, Ti

BC1 BC2 BC3

Cu, Zn, Ag, Ni, Ca, Na Cu, Ag, Ca, Na Cu, Zn, Ni, Ca, Na

Cu, Zn, Ag, Ni, Si, Ti, Cl, Ca Cu, Ag, Br, Mg, K, Ti, Ca, Na

CP-IF1 CP-L2 CP-SP3 CP-SP4 CP-BS5 CP-SM6

Zn, Pb, Ca, Na Zn, Cu, Ca, Na Zn, Cu, Ca, Na Zn, Cu, Ca, Na Zn, Cu, Fe, Ca, Na Pb, Sn, V, Ca, Na

Zn, Cu, Al, Si, Cr, Fe, Ca, Na Zn, Cu, C, Al, Si, As, Ca, Na

Table 2. EDS/SEM qualitative and quantitative composition given in wt% (±0.20%) in different regions of the material extracted from the area BC1 of the base of the cross. Positions tested Si 1 2 3 4 5 6 7

Figure 4. LIB spectrum of the area CP-BS 5 of the cache-pot as indicated in Figure 1e.

EDS determination on the sample obtained from the top of the base of the cross (Fig. 1c, area BC1) as shown in Table 1 reveals the presence of different elements. Apart from the elements coming from the composition of the scrapping card (Si) and from impurities (Cl and Ca), the other elemental compo nents (Cu, Zn, Ni and Ag) are in correspondence with those detected by LIBS (Fig. 2 and Table 1). The elements O and C, observed by EDS, and not indicated in Table 2 are not interesting for our purposes. It appears that Cu is in constant ratio with Zn, indicative of a brass alloy with an average composition Zn (40%)/Cu(60%), while Ag also observed by LIBS, seems to be present in discrete amounts in some areas. The fabrication techniques of Zn, Cu and Ag alloys were not available at the time the object was dated (19th cc). Other alternative is considered in which a silvered brass lamina was prepared by pressing a fine layer of Ag on the brass substrate by the technique of

Cl

33.88 5.75 33.75 5.10 25.17 7.32 31.36 6.85 29.84 7.76 31.19 6.51 22.39 10.68

Ca

Ti

Ni

Cu

Zn

Ag

4.82 5.75 3.27 5.28 6.55 4.64 4.58

1.99 2.05 1.68 2.21 3.81 2.45 4.05

4.70 5.53 4.30 4.74 6.16 4.15 3.93

23.98 23.05 26.86 23.42 21.37 22.86 24.44

17.99 17.49 20.03 18.83 17.48 18.21 23.89

3.60 3.72 6.33 3.61 2.35 5.91 2.05

lamination. Subsequent polishing by abrasion would remove the silver layer from most parts of the surface. On the other hand the anachronistic presence of Ti/Ni not in a constant ratio suggests the probable use of a material constituted by these two elements during a modern restoration intervention to repair the damage caused by the separation of the cross from its base. This kind of welding is very fragile and not suitable to join two heavy parts. This is the reason why the cross was detached from its base. The elements identified by EDS in the material extracted from the back side of the base of the cross (Fig. 1c, area BC2) are indicated in Table 3. From this analysis, Cu andAg appear as the main metallic species in good agreement with the LIBS results. The absence of zinc is very strange while the presence of silver could be explained by the diffusion of this element through copper during the lamination process. The other elements detected (Ca, Na, K, Mg and Br) are attributed to impurities adhered to the surface of the base of the cross. Titanium is also present in this

136

Table 3. As in Table 2 from the area BC2 of the base of the cross.

Table 6. As in Table 2 from the area CP-SP3 of the strip of cache-pot.

Positions tested Na

Mg

K

Ca

Ti

Cu

Br

Ag

Positions tested C

Na

Al

Si

Ca

Cu

Zn

As

1 2 3 4 5 6 7

11.67 7.74 14.60 9.43 9.66 12.72 10.30

2.91 1.77 3.09 3.24 1.56 2.70 2.26

11.34 11.94 11.25 10.59 9.42 10.28 11.01

6.34 3.66 8.68 6.65 6.23 8.00 9.02

16.28 21.31 15.28 18.70 24.87 16.47 14.98

18.11 17.25 18.17 18.33 17.26 20.33 17.51

13.19 14.36 9.77 12.97 14.43 14.03 10.62

1 2 3 4 5

0.83 0.72 1.26 0.90 0.67

1.84 1.48 1.78 1.34 1.16

1.90 2.18 2.53 2.31 1.94

0.16 0.24 0.40 0.34 0.22

37.40 35.84 41.41 33.45 36.79

16.71 14.68 1.66 21.59 19.34

0.17 0.18 0.38 0.06 0.22

16.10 18.03 14.84 14.99 12.04 12.04 15.09

37.35 40.09 44.98 36.00 35.33

Table 4. As in Table 2 from the area C1 of the cross. Positions tested

Ti

Cu

Zn

Ag

1 2 3 4 5

1.6 2.0 6.9 5.7 7.4

59.3 62.7 58.3 62.6 54.5

1.6 2.5 1.4 0 3.5

37.5 32.8 33.4 31.7 34.6

Table 5. As in Table 2 from the area CP-L2 of cache-pot. Positions tested Na

Al

1 2 3 4 5 6

0.46 3.58 0.44 0.04 0.39 3.89 0.48 0.01 0.33 3.74 0.50 0.01 0.28 20.05 2.76 0.04 0.29 2.63 0.38 0.06 0.34 5.20 0.54 0.08

1.67 1.49 1.54 5.44 1.58 1.08

Si

Ca

Cr

Fe

Cu

Zn

0.22 0.23 0.19 0.47 0.25 0.18

33.33 32.31 31.23 1.15 29.69 33.63

10.84 11.21 10.87 0.46 10.21 11.23

Figure 5. Micrography of the laser spot corresponding to position C2 on the cross (Fig. 1d).

area of the object in a constant proportion without any Ni. Carbon, oxygen and silicon are also detected and not indicated in Table 3 due to their less importance. Table 4 presents the elements observed by EDS in the metal extracted from the metallic box of the cross (Fig. 1d, area C1). Copper and silver are the main elements in good agreement with the results obtained by LIBS (Table 1). A silvered lamination after the fabrication of the cross is again considered. Zinc and titanium were also observed in discrete amounts. Table 5 presents the elemental composition of the sample extracted from the back side of the legs of the cache-pot (Fig. 1e, area CP-L2). The presence of Zn and Cu (observed also by LIBS) with average composition 25–75% indicates that the nature of the material of the cache-pot legs is a brass alloy. The other elements observed are typically the contamination constituents.

Finally, the constituents of the strip of the cachepot obtained by EDS analyses of the material extracted from the area CP-SP3 reveal the same composition as the legs (Table 6), with different percentages of Zn and Cu (30–70%). The qualitative composition of the decorated strip mainly based in Cu and Zn indicates the use of a brass alloy to manufacture this material with a low Cu content. The use of a different brass alloy for the decorated strip than for the legs could have been intentional for the embossing working process. In the region corresponding to the union between the two strip ends (Table 1, area CP-SP4), no other elements different than those used in the composition of the strip itself, were found by LIBS analysis. This suggests that an autogenic welding has been employed for connecting the two strip sides. Differently, the soldering between the body of the object and the legs was based on the brazing technique, using a Pb-Sn alloy determined by LIBS (Table 1, area CP-SM6). The use of this alloy is justified by the necessity of providing a strong connection to support the weight of the object. Optical microscopy was used to observe the fingerprint of LIBS effect, i.e. the crater created by laser ablation needed for LIBS analysis. Figure 5 shows

137

European medieval cultural heritage). Discussions with Teresa Sinigalia (INMI, Romania), Oliviu Boldura (CERECS ART, Romania) and Octaviana Marincas (Universitatea de Arte “G. Enescu”, la¸si, Romania) are acknowledged. MO thanks CSIC-ESF I3P program for a postdoctoral contract. We also acknowledge the support of the Red Temática de Patrimonio Histórico y Cultural, CSIC. Figure 6. Optical micrographs of different areas of the cache pot showing the microstructure of the metal used: a) strip and b) leg.

the dimensions of the mark left in the area C2 of the metallic box of the cross after a single laser pulse. The diameter of the laser impact did not exceed 400 µm. Optical microscopy was also performed on cache- pot strip and legs to assess the microstructure of the metals that were used. Different morphologies were observed that lead to the conclusion that the strip was made by lamination (Fig. 6a) and the legs by casting (Fig. 6b). This choice was surely related to the different function of the two parts. The strip had to be decorated by moulding, while for the legs it was necessary to use a strong metal structure to stand mechanical stresses. 4

CONCLUSIONS

LIBS analysis has allowed the identification of the elemental composition of two metallic historical objects kept in the Sucevita Monastery in Suceava, Romania. The qualitative and quantitative analysis Romania. The qualitative and quantitative analysis of the samples, extracted from different zones of the metallic objects, by conventional techniques (EDS/SEM) allowed the comparison of the obtained results with those achieved by LIBS. The main components detected by LIBS are in correspondence with those observed by EDS/SEM. These results show the potential of LIBS for the in situ determination of the elemental composition of metallic objects without previous preparation and by consuming a micrometric amount of sample and all that is required is optical access to the material to produce the plume and collect the emitted light. LIBS measurements performed on the cross at its base and on the cache-pot allowed the characterisation of the objects from a technological point of view. Indications for the restoration and conservation of the items have been suggested to conservators and restorers. ACKNOWLEDGEMENTS Work funded by 2000 Culture Project (CLT 2005/A1/CHLAB/RO-488, Saving sacred relics of

REFERENCES Anglos, D. 2001. Laser-induced breakdown spectroscopy in art and archaeology, Applied Spectroscopy 55: 186–205. Bell, I. M., Clark, R. J. H. & Gibbs, P. J. 1997. Raman spectroscopic library of natural and synthetic pigments (P re- N 1850 AD), Spectrochimica Acta Part A 53: 2159–2179. Burgio, L., Corsi, M., Fantoni, R., Palleschi, V., Sialvetta, A., Squarcialuppi, M. C. & Tognoni, E. 2000. Self-calibrated quantitative elemental analysis by laser-induced plasma spectroscopy: application to pigments analysis, Journal of Cultural Heritage 1: 281–286. Burgio, L. & Clark, R. J. H. 2001. Library of FT-Raman spectra of pigments, minerals, pigment media and varnishes, and supplement to existing library of Raman spectra of pigments with visible excitation, SpectrochimicaActa Part A 57: 1491–1521. Bustamante, M. F., Rinaldi, C. A. & Ferrero, J. C. 2002. Laserinduced breakdown spectroscopy characterization of Ca in soil depth profile, Spectrochimica Acta B 57: 303–309. Carmona, N., Oujja, M., Rebollar, E., Römich, H. & Castillejo, M. 2005. Analysis of corroded glasses by laser induced breakdown spectroscopy, Spectrochimica Acta B 60: 1155–1162. Carmona, N., Oujja, M., Gaspard, S., García-Heras, M., Villegas, M. A. & Castillejo, M. 2007. Lead determination in glasses by laser-induced breakdown spectroscopy, Spectrochimica Acta B 62: 94–100. Castillejo, M., Martín, M., Silva, D., Stratoudaki, T., Anglos, D., Burgio, L. & Clark, R. J. H. 2000. Analysis of pigments in polychromes by use of laser-induced breakdown spectroscopy and Raman microscopy, Journal of Molecular Structure 550–551: 191–198. Clark, R. J. H., Curri, L., Henshaw, G. & Laganara, C. 1997. Characterization of Brown–Black and Blue Pigments in Glazed Pottery Fragments from by Castel Fiorentino (Foggia, Italy) Raman Microscopy, X-Ray Powder Difractometry and X-Ray Photoelectron Spectroscopy, Journal of Raman Spectroscopy 28: 105–109. Colao, F., Fantoni, R., Lazic, V. & Spizzichino, V. 2002. Laserinduced breakdown spectroscopy for semiquantitative and quantitative analyses of artworks-application on multilayered ceramics and copper based alloys, Spectrochimica Acta B 57: 1219–1234. Corsi, M., Cristoforetti, G., Hidalgo, M., Legnaioli, S., Palleschi, V., Salvetti, A., Tognoni, E. & Vallebona, C. 2006. Double pulse, calibration-free laser-induced breakdown spectroscopy: A new technique for in situ standardless analysis of polluted soils, Applied Geochemistry 21: 748–755.

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Elfwing, M. & Norgren, S. 2005. Study of solid-state sintered fine-grained cemented carbides, Int. J. Refract. Met. Hard Mater. 23: 242–248. Klein, S., Stratoudaki, T., Zafiropulos, V., Hildenhagen, J., Dickmann, K. & Lehmkuhl, Th. 1999. Laser-induced breakdown spectroscopy for on-line control of laser cleaning of sandstone and stained glass, Applied Physics A 69: 441–444. Kuzuya, M., Murakami, M. & Maruyama, N. 2003. Quantitative analysis of ceramics by laser-induced breakdown spectroscopy, Spectrochimica Acta B 58: 957–965. Mandrino, Dj., Godec, M., Skraba, P., Sustarsic, B. & Jenko, M. 2004. AES, XPS and EDS analyses of an ironbased magnetic powder and an SMC material, Surface Interface Analysis 36: 912–916. Mantler, M. & Schreiner, M. 2000. X-ray fluorescence spectrometry in art and archaeology, X-ray Spectrometry 29: 3–17. Melessanaki, K., Mateo, M., Ferrence, S.C., Betancourt, P.P. & Anglos, D. 2002. The application of LIBS for the analysis of archaeological ceramic and metal artefacts, Applied

Surface Science 197-198: 156-163. Rinaldi, C. A & Ferrero, J. C. 2001. Analysis of Ca in BaCl2 matrix using laser-induced breakdown spectroscopy, Spectrochimica Acta B 56: 1419–1429. NIST Electronic Database.Available at http://physics.nist.gov Oujja, M., Vila, A., Rebollar, E., García, J. F. & Castillejo, M. 2005. Identification of inks and structural characterization of contemporary artistic prints by laser-induced breakdown spectroscopy, Spectrochimica Acta B 60: 1140– 1148. Rinaldi, C. A. & Ferrero, J. C. 2001. Analysis of Ca in BaCl2 matrix using laser-induced breakdown spectroscopy, Spectrochimica Acta B 56: 1419–1429. Scholten, J.H., Teule, J.M., Zafiropulos, V. & Heeren, R.M.A. 2000. Controlled laser cleaning of painted artworks using accurate beam manipulation and on-line LIBS-detection, Journal of Cultural Heritage 1: 215–220. Yaroshchyk, P., Body, D., Morrison, R. J. S. & Chadwick, B. L. 2006. A semi-quantitative standard-less analysis method for laser-induced breakdown spectroscopy, Spectrochimica Acta B 61: 200–209.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Comparative study of historic stained glass by LIBS and SEM/EDX K. Szelagowska & M. Szymonski Institute of Physics, Jagiellonian University, Krakow, Poland

F. Krok Institute of Physics, Jagiellonian University, Krakow, Poland & Faculty of Conservation and Restoration of Fine Arts, Academy of Fine Arts, Krakow, Poland

M. Walczak∗, P. Karaszkiewicz & J.S. Prauzner-Bechcicki Faculty of Conservation and Restoration of Fine Arts, Academy of Fine Arts, Krakow, Poland

ABSTRACT: Medieval stained glass windows from St. Mary’s Church and Corpus Christi Basilica in Krakow (14th–16th century), as well as glass originating from several Polish historical buildings (18th–20th century) have been studied by means of Laser-Induced Breakdown Spectroscopy and Scanning Electron Microscopy with Energy Dispersive X-Ray spectrometer. The results are compared in order to obtain chemical composition of the samples and correlate them with the sample morphologies. Investigated glass samples can be divided into two groups: soda-lime-silicate glasses (modern glasses) and potash-lime-silicate glasses (historic glasses). Furthermore, the analysis of a stained glass sample of unknown dating acquired from a small Polish town, Grodziec, is presented. It is demonstrated that Grodziec stained glass has the characteristic potash–lime–silicate chemical composition, indicating that it belongs to the historic group of samples.

1

INTRODUCTION

Glass can be described as a solid substance of unordered structure composed of long chain molecules. Historic glasses, as being main constituents of some outstanding works of art, are a valuable part of cultural heritage. The inorganic glass, widely used since about 3000 BC, consists of several components (García-Heras et al. 2003): oxides that are network formers, oxides as network modifiers (flux and stabilizers) and colouring elements (chromophores). The main glass network former is silicon oxide (SiO2 ), however, in some glasses other oxides, such as aluminium oxide (Al2 O3 ), phosphorus oxide (P2 O5 ), etc, are used. Flux components decrease the silica melting temperature. Nowadays sodium oxide (Na2 O) is mostly used. However, in the medieval period potassium oxide (K2 O) was more common, since it was easier to obtain from wooden ashes. Stabilizers such as calcium oxide (CaO), magnesium oxide (MgO), lead oxide (PbO) and others are added to the glass batch to enhance their general chemical durability. Finally transition metal oxides provide colour to the glass

∗

Present address: Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

(Co- blue, Cu- red or green, Cr- yellow, Fe- brown colour, Mn- pink colour, etc.). Detailed analysis of glass composition (qualitative and quantitative characterization) can play an important role in dating and locating the glass origin; e.g. the proportions of concentration of three main types of oxides can suggest the period of glass production. For instance, in the medieval glasses, a high concentration of K2 O usually suggests that it originates from Western and Central Europe, where wood ash, rich in potassium compounds, was used for the glass production (Newton & Davison 1989). Therefore, it is clear that application of new analytical methods and instruments give enhanced opportunities for glass characterization, so important for art and glass historians as well as for conservators. Moreover, it must be stressed that historic glasses, as a valuable part of cultural heritage, should be investigated in a non destructive way. In the field of glass research Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray spectrometer (EDX) has been widely used for many years (Schreiner 1988). Although it is a very useful technique to obtain information about the major glass components, the sensitivity of SEM/EDX is often insufficient to measure the concentration of trace impurities and light elements. Alternatively, LaserInduced Breakdown Spectroscopy (LIBS) has been

141

developed as a very promising diagnostic technique that permits real-time, qualitative and under specific circumstances also semi-quantitative measurements of elemental composition of solids, liquids and gases (Burgio et al. 2000, Anglos 2001, Melessanaki et al. 2002, Müller & Sterge 2003). The main advantage of this method is related to its micro destructive character (a laser spot diameter that is used for analysis is smaller than 0.2 mm), sensitivity and possibility of carrying the in situ analysis, which is very important for glass objects. Recently, semi-qualitative analysis of the elemental composition of stones, metals, inks, and wall paintings have been carried out (Anglos 2001, Melessanaki et al. 2002, Oujja et al. 2005). Also chromophores and trace elements have been identified by this technique (Burgio et al. 2000, Castillejo et al. 2000, Carmona et al. 2005). Recent approaches to quantitative LIBS measurements in minerals, ceramics and soils have been reported (Colao et al. 2002, Kuzuya et al. 2003, Carmona et al. 2007, Müller & Sterge 2003). In this paper it is reported on the successful application of two complementary methods mentioned above (SEM/EDX and LIBS) for comparative analysis of medieval stained glass windows originating from several Polish historical buildings and churches (18th–20th century). The obtained results allow to determine the manufacturing period of a glass sample of unknown origin.

2

Table 1. List of the investigated glass samples. Period

Origin

1 2 3

Modern Modern Modern

4

19th c (?)

5

18th c. (?)

6

16th c.

7

14th c.

8

Unknown

Model glass Hand blown, Jaslo, Poland Machine made window glass, Krakow, Poland Hand blown, Brzesko Nowe, Poland Hand blown, Brzesko Nowe, Poland Corpus Christi Basilica, Krakow, Poland Stained glass, St. Mary’s Church, Krakow, Poland Stained glass, Grodziec, Poland

Table 2. Chemical composition of the selected glass samples as determined by SEM/EDX. Component (wt. %)

EXPERIMENTAL

In this study 5 samples of historic glass from Polish stained glass windows from different periods and for comparison 3 samples of modern (20th century) glass have been investigated. The samples are listed in Table 1. Glasses 2 and 6 had a blue colour, glasses 7 and 8 were green, and the rest of the samples were colourless. To analyse properly the bulk composition, each sample was cut and all the measurements were carried out on the fresh made cross-section area. For SEM investigation samples of 1 cm2 size were introduced to the microscope chamber (JOEL 5550). The imaging was performed with 20 keV electron beam (the size of the spot was in the range of 20 nm). Prior to the SEM imaging, the glass samples were covered with a thin carbon layer to avoid the charging of the sample surface. Simultaneously with the SEM imaging, the EDX technique was used allowing to study the elemental composition of the samples. For these measurements the silicon detector cooled down to liquid nitrogen temperature was used (IXRF Systems). The X- ray photons of energy up to 20 keV were acquired. LIBS analysis was carried out by means of a LIBS2000+ system (Ocean Optics) with a Q-switched

Sample no.

Sample no.

Si

Na

K

Ca

Al

Mg

Mn

1 2 3 4 5 6 7 8

28.8 21.2 22.5 16.6 23.0 22.8 19.0 20.5

7.5 8.7 7.5 0.1 0.5 0.2 0.3 0.1

0.5 0.6 0.1 6.7 15.5 14.6 26.8 13.6

3.2 4.1 4.5 8.2 17.7 8.2 14.9 11.8

0.7 0.6 0.4 0.5 0.8 0.3 0.4 0.8

1.9 – – 0.6 1.1 1.3 1.0 1.9

– – – 0.5 1.3 0.3 1.1 0.5

Nd:YAG laser (Big Sky Laser Technologies). Samples were irradiated at the fundamental wavelength of 1064 nm (pulse duration of 6 ns, 1 KHz) with a fluence of 2.6 J/cm2 . The plasma emission from the neutral and ionized atoms was collected by a bundle of glass fibres placed near to the analysed surface and transmitted to the optical analyser consisting of a set of 7 spectrometers. The final spectrum includes the 200–900 nm range.

3

RESULTS AND DISCUSSION

Table 2 shows the results of SEM/EDX analysis of the glass samples listed in Table 1. It is seen that modern glass samples belong to the soda-lime-silicate class, while the historic glass belongs to the potassium-lime silicate class. To further differentiate samples within the groups, SEM/EDX data were compared with the LIBS results. A typical example of the LIBS spectrum obtained for sample no. 8 is shown in Figure 1. Characteristic

142

Ca I

Ca I, II 300

400

500

600

Ca II 700

KI

Ca I Ca I

KI

Si I Si I Si I

Na I Na I Si I Si I

Mg I

Si I

Mg I, Si II Ca I

Ca I

Ca II

Ca II

Si I

Si I

Mg I

Si I Ca II

Intensity [a. u.] 200

800

900

Wavelength [nm]

I (Na 589.59 nm) / I (Si 612.40nm)

Figure 1. LIBS spectrum obtained for sample no. 8.

14

2

1

12 Modern

10 8 3

6

Historic

4 5 2

7

4 6

0 0

1

2

8 3

4

5

6

7

I (K 766.49 nm) / I (Si 634.71 nm) Figure 2. Cluster analysis of the intensity ratios: I (Na 589.59 nm)/I (Si 612.40 nm) (LIBS) versus I (K 766.49 nm)/I (Si 634.71 nm) (LIBS).

lines of main elements, such as Si, Ca, K, Mg and Na, were undoubtedly identified and denoted in the figure. The corresponding wavelengths of those lines are: Si (I) at 243.52, 250.69 251.60, 288.18, 390.60, 393.39, 612.40 and 614.35 nm; Si (II) at 412.80, 413.08, 504.10, 505.59, 634.71 and 637,14 nm; Ca (I) at 422.78, 429.98, 430.32, 431.95, 443.60, 445.51, 458.59 and 558.87 nm; Ca (II) at 315.88 317.93, 393.36, 396.84, 528.53 and 866.2 nm; K (I) 766.49

and 770.11 nm; Mg (I) 278.14, 517.27 and 552.84 nm; Na (I) 588.99 and 589.59 nm (NIST, Müller & Sterge 2003). For comparative analysis of the investigated samples, the following scatter plots have been prepared (Figs. 2–4) displaying dependences for selected pairs of data. In Figure 2 the intensity of sodium line 589.59 nm versus the intensity of potassium line 766.49 nm is shown. Both intensities are normalized with respect to intensities of relevant silicon lines in order to get semi-quantitative values of sodium and potassium concentration. Sodium line is normalised with respect to silicon line 612.40 nm and potassium line with respect to silicon line 634.71 nm. As it can be seen, modern glass samples (no. 1–3) and historic glass samples (no. 4–8) form two distinct groups, although some differences among each of the two groups are present. First of all, modern glasses have higher sodium content, as expected (Garcia-Heras et al. 2003). However, 3rd sample seems to be well separated from the other two modern samples towards lower sodium content. Furthermore, it is clear that samples no. 4, 5 and 6 create their own group among the historic glass group, while samples 7 and 8 differ from that group and from each other. It is known that historic glasses, on their own, are divided into several groups and in order to investigate this aspect with LIBS technique further studies are required and undertaken.

143

I (Na 589.59 nm) / I (Si 612.40 nm)

14

samples 5 and 6 are fairly similar. Those discrepancies are due to the existence of different glass groups among the historical glasses, as mentioned above.

2

1

12 Modern

10

4

8 3

6 4

Historic 4

2

5

7 8 6

0 0

2

4

6

8

10

Na [wt %]

Figure 3. Cluster analysis of intensity ratios: I (Na 589.59 nm)/ I (Si 612.40 nm) (LIBS) versus concentration of Na in wt. % (SEM/EDX).

I (K 766.49 nm) / I (Si 634.71 nm)

7

8

6 5

Modern 7

4 3

2

4

6 5

1

2 1

Historic

3 0 0

5

10

15 K [wt%]

20

25

30

Figure 4. Cluster analysis of intensity ratios I(K 766.49 nm)/ I(Si 634.71 nm) (LIBS) versus concentration of potassium in wt. % (SEM/EDX).

In Figure 3 the intensity of the sodium Na line at 589.59 nm normalized with respect to the intensity of the silicon Si line at 612.40 nm is compared with the sodium content (in wt. %) obtained with SEM/EDX. As can be seen, points gather into two distinct groups corresponding to two classes of glass samples. Once again, sample no. 3 differs from other modern samples, whereas historic glasses are fairly close to each other. In Figure 4 results of measurements of potassium content are shown. Intensity of the potassium line 766.49 nm is normalized with respect to the intensity of silicon line 634.71 nm and plotted versus potassium content obtained with SEM/DEX. Contrary to sodium, potassium concentrations show high similarity for the modern glass samples (although sample 3 still differs from other samples). Rather large differences between historic glass samples are seen. Only

CONCLUSIONS

Summarizing, a comparative analysis of different glass samples (historic and modern) has been performed by means of LIBS and SEM/EDX techniques. From results obtained some conclusion may be drawn. Namely, in the modern glass group, being sodalime-silicate glasses, different composition of the 3rd sample may suggest that this particular sample is of different origin than the others (for instance imported from outside of the particular region). Furthermore, for the historic glass group, being potash-lime-silicate glasses, samples 4, 5 and 6 show clear similarities. This similarity is particularly interesting result, if one notes that the sample no. 6 is from 16th century, while samples no. 4 and 5 are dated to supposedly be from 18th/19th centuries. This may suggest that samples 4 and 5 could be older than expected before. This hypothesis, however, needs to be confirmed by further investigations. Furthermore, it is clear that glass sample no. 8, of unknown origin, belongs to the historic glass group (potash-lime-silicate). It means that this glass (and at least parts of the stained glass panel from which the sample was collected) has characteristic properties of the medieval stained glass. Finally, the study demonstrates that parallel application of both analytical methods, LIBS and SEM/EDX, can provide complementary material for comparative investigations of historic stained glasses allowing the identification of the sample origin in time and space.

ACKNOWLEDGEMENTS Authors thank the Polish Ministry of Science and Higher Education for the financial support. REFERENCES Anglos, D. 2001. Laser-induced breakdown spectroscopy in art and archaeology. Appl. Spectrosc. 55: 186A–205A. Burgio, L., Corsi, M., Fantoni, R., Palleschi, V., Sialvetta, A., Squarcialuppi, M.C., Tognoni, E. 2000. Self-calibrated quantitative elemental analysis by laser-induced plasma spectroscopy: application to pigments analysis. J. Cultural Heritage 1: 281–286. Carmona, N., Oujja, M., Rebollar, E., Römich, H., Castillejo, M. 2005. Analysis of corroded glasses by laser induced breakdown spectroscopy. Spectrochimica Acta B 60: 1155–1162. Carmona, N., Oujja, M., Gaspard, S., García-Heras, M., Villegas, M.A., Castillejo, M. 2007. Lead determination in glasses by LIBS. Spectrochimica Acta B 62: 94–100.

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Castillejo, M., Martín, M., Silva, D., Stratoudaki, T., Anglos, D., Burgio, L., Clark, R.J.H. 2000.Analysis of pigments in polychromes by use of laser-induced breakdown spectroscopy and Raman microscopy. J. Molec. Struct. 550–551: 191–198. Colao, F., Fantoni, R., Lazic, V., Spizzichino, V. 2002. Laserinduced breakdown spectroscopy for semiquantitative and quantitative analyses of artworks-application on multilayered ceramics and copper based alloys. Spectrochimica Acta B 57: 1219–1234. García-Heras, M., Gil, C., Carmona, N., Villegas, M.A. 2003. Weathering effects on materials from historical stained glass windows. Mater. Construct. 270: 21–34. Kuzuya, M., Murakami, M., Maruyama, N. 2003. Quantitative analysis of ceramics by laser-induced breakdown spectroscopy. Spectrochimica Acta B 58: 957–965. Melessanaki, K., Mateo, M., Ferrence, S.C., Betancourt, P.P., Anglos, D. 2002. The application of LIBS for the analysis of archaeological ceramic and metal artifacts. Appl. Surf. Sci. 197–198: 156–163.

Müller, K., Sterge, H. 2003. Evaluation of the analytical potential of laser-induced breakdown spectroscopy (LIBS) for the analysis of historical glasses. Archaeometry 45: 421–433. Newton, R., Davison, S. 1989. Conservation of Glass. London: Butterworths. NIST Electronic Database.Available at http://physics.nist.gov/ cgi-bin/AtData/lines-form. Oujja, M., Vila, A., Rebollar, E., García, J.F., Castillejo, M. 2005. Identification of inks and structural characterization of contemporary artistic prints by laser-induced breakdown spectroscopy. Spectrochimica Acta B 60: 1140–1148. Schreiner, M. 1988. Deterioration of stained medieval glass by atmospheric attack. Part 1. Scanning electron microscopic investigations of the weathering phenomena. Glastech. Ber. 61: 197–204.

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Portable Laser Systems for Remote and On-Site Applications

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Scanning hyperspectral lidar fluorosensor for fresco diagnostics in laboratory and field campaigns F. Colao, L. Caneve, R. Fantoni, L. Fiorani & A. Palucci ENEA, FIM-FISLAS, Frascati RM, Italy

ABSTRACT: A scanning hyperspectral system based on Laser Induced Fluorescence (LIF) has recently been developed for optical characterization of surfaces relevant to cultural heritage. This paper describes its field application for frescoes diagnostics in some monasteries in Bucovina, a Romanian region. The LIF system provides information on the present conservation status, identifies areas with biological attack and gives details on the restoration methods of the frescoes under study.

1

INTRODUCTION

The recent technological development of optical devices, able to measure spectral components in fast and accurate, way represents one of the major breakthroughs in the field of work art investigation. At present, a number of imaging multispectral and hyperspectral sensors as well as specialized software have been conceived (Anglos 1999, Carcagni 2007), making accessible the tools to take advantage of imaging technologies to a continuously increasing number of experts. Algorithms for digital image analysis, mainly matured in the field of remote sensing of land and vegetation, have been developed and their application to artworks investigation represents one of the most powerful, promising and fast growing technology. This paper describes the results obtained by the application of Laser Induced Fluorescence (LIF) technique for diagnostics of frescoes during the 2006 campaign in Bucovina. In particular, the sites under study were the Resurrection Church in the Suceviþa Monastery, the Saint Nicholas Church in the Popˇau¸ti Monastery near Boto¸sani and the Saint Nicholas Church in Bˇaline¸sti, all currently under restoration. Among a lot of techniques offering the possibility of pigments and biodeteriogens identification in frescoes, the laser based methods were applied successfully for in situ or remote characterization of artwork surface, both for diagnosis (Asmus 2003) and for maintenance (Klein 2001). In particular, the LIF technique has been recently applied to the remote sensing of fresco (Comelli 2004) to detect characteristics invisible with the naked eye without moving samples from their original location. Large images can be collected once a fluorescence lidar system equipped with a scanning device is utilized (Lognoli 2003).

However the unmixing of pixel spectral information still remains as one of the most challenging tasks to carry out in order to complete data pre processing thus allowing for an appropriate data interpretation (Coma 2000). Indeed the area pertaining to a single pixel contains different materials all of them contemporarily excited by the incident laser beam: the measured signal is then a combination of the fluorescence induced on all the different material layers. While some pigments as lime white have distinct LIF emission, other pigments, as for example red ochre, have a much less distinct signature (Anglos 1996, Nevin 2006). Nonetheless they can be distinguished when the signal emitted by plaster and substrates laying underneath is modulated both spectrally and in intensity by the absorption and re-emission of pigments located close to the superficial layers. By this rationale we try to present in this paper how to interpret the LIF images, although they are characterized by a very complex and extremely rich spectral signatures which at a first glance might appear as just undesired details. A compact scanning lidar fluorosensor apparatus has been designed and built in the ENEA laboratory and it has been used to perform the field measurements of this campaign. Several multispectral images were obtained and their combination was attempted in order to reveal the occurrence of surface biodegraded area.

2

EXPERIMENTAL

The hyperspectral scanning LIF system here used is based on a previous instrument’s version developed during former projects (Aristipini 2004). All the mechanical and optical elements have been renewed

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and installed in an aluminum box of 58 × 43 × 36 cm3 , weighting less than 15 kg. Its small size and light weight allow an easy transport of the system and its operation from scaffoldings, in the case of surfaces out of the current maximum range for remote operation (10 m). The layout of the system is given in Figure 1. The Thomsom DIVA is a diode pulsed Nd:YAG laser used as light source to generate radiation at 266 or 355 nm, producing laser pulses of 10 ns duration at a fluence of 1 to 50 mJ/cm2 . The spectrometer entrance is protected from the intense backscattered radiation by means of an appropriate dichroic filter. Proper selection of the energy per pulse content is a critical issue, especially in the case of precious frescoes which might be damaged by bleaching processes induced by UV light excitation (Athanassiou 2000, Sansonetti 2000). A systematic study on this topic has not yet been completed and the laser energy was kept as low as possible to ensure a signal to noise ratio of the order of few units. Since no optical elements are used to collimate the laser beam, the overall spatial resolution is a function of the operation distance and in the present case we may infer a resolution of approximately 1 to 2 mm from the spot size on the target. The digitized spectrum is transferred to a portable computer where a LabView program allows the user to set experimental parameters, to control data acquisition and to perform a preliminary data analysis.

Figure 1. Optical layout of hyperspectral scanning LIF system.

burning deposits etc.) and biological agents as well. Previous study on biodeterioration agents mainly identified the presence of fungi, while chlorophyll, easily detectable by the LIF system (Colao 2005), has not been found, probably because the low level of natural light inside the churches under study prevented the development of photosynthetically active microorganisms.

2.1 Data analysis The hyperspectral LIF scanner here described is able to measure the fluorescence radiation induced on a surface at a series of narrow and contiguous spectral bands. In every scanned image the light emitted by each pixel is decomposed in its components, providing the maximum extent of spectral information today conceivable in the UV to VIS wavelength range (200–800 nm). After the completion of an exhaustive reference spectral library for all the substances and pigments present in the sample under study, the analysis of hyperspectral images allows for distinguishing among different spectrally similar materials. However the image analysis and subsequent data interpretation still remain one the most difficult and challenging tasks, since for a variety of reasons the reference database is far from being completed. To work out this problem we resort to a less accurate data analysis, based on the attempt to isolate and identify spectral characteristics of selected area directly on the LIF images and validating a posteriori with complementary analyses. As it will be detailed in the following, we direct our efforts to the identification of deteriorated areas on fresco and mural paintings by environmental factors (wall humidity, salt efflorescence, beeswax

2.2 Data processing Various factors affect the signal measured by the LIF scanner; just to cite a few, it is worth mentioning (1) the thermal drift caused by the long times needed to complete an acquisition scan, (2) the radiometric distortion introduced by optical elements between the first collection mirror and the spectrometer entrance, (3) the spectral transfer function of the spectrometer itself. Also (4) the geometrical effects due to the change in incidence angle should be considered. Accurate data processing requires the correction for all of these effects. Actually the overall radiometric response of the apparatus has been experimentally measured by using traceable reference sources: a deuterium lamp for the 200 to 380 nm spectral region and a tungsten lamp for the region from 400 to 800 nm. As the cross talk between adjacent spectral band and the effects of finite spectral resolution of the spectrometer is concerned, we estimated them to affect the LIF signal for less than 10% and consequently they were ignored. The geometrical effect correction has been carefully considered, and it can be introduced in the data pre

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Figure 2. LIF spectra of selected fungi’s strain excited at λex = 355 nm. Strains starting from upper left are SCH = Schizophyllum commune, BJC = Paecylomices varioti, TRICH = Trichoderma viride, CHAET = Chaetomium, ALT = Alternaria, HORM = Hormodendron.

processing chain once a LRF scan of the same fresco’s portion is available. Details on this correction may be found in (Colao 2005). Finally the Spectral Angle Mapper (SAM) algorithm has been used to analyse images (Colao 2007). According to SAM, the spectrally resolved intensities can be treated as vectors’ components. Then it is possible to compute the angle between a given pixel spectrum and a reference spectrum: the smaller is the angle, the higher will be the similarity between pixel and reference spectra. To improve the diagnostic readability of images and to give relevance to selected areas, a threshold has been introduced for the spectral angle, thus giving a black and white version of the original scanned image. Whenever the reference spectra pertains to a bio contaminated sample, this processing marks well the areas relevant for the bio deterioration diagnostic. 3

RESULTS AND DISCUSSION

LIF measurements taken with the scanning hyperspectral lidar fluorosensor are grouped in two sections, the first concerning with excitation at third harmonic of the Nd:YAG laser (355 nm) and the second with excitation at fourth harmonic of the Nd:YAG laser (266 nm). The first wavelength is optimal for excitation of pigments and has a strong excitation efficiency in case of chlorophyll, thus allowing for identification of algae or autotrophic micro organisms; the second is better suited for identification of heterotrophic micro organisms like fungi and also for identification of organic binders lying over the pigments.

3.1

Laboratory data bases

Reference fluorescence spectra of biological samples and of acrylic resins used as consolidants were measured in order to support the assignment of features emerging in LIF analysis of frescoes. 3.1.1 Microorganisms identification Pure cultures of several fungi strains were excited at 355 nm. LIF acquired spectra (Fig. 2) sometimes show common broadband peaks, while in other cases show specific and distinctive features. Moreover some samples are more intensively fluorescent than others, while few samples with marginal fluorescence intensity were found once excited at this wavelength. As an example of common feature we might consider the UV/blue fluorescence peaked at 400–450 nm, which is present in almost all of the considered samples. Similarly the green fluorescence, with a maximum at 500 nm, is observed in almost all of the samples. Quite different is the case of red fluorescence with maximum at 600–650 nm, which has a significant intensity only in few samples. A preliminary evidence from the experimental findings is that the fluorescence intensity and bands ratios do depend on a number of factors like fungi strains, physiological state and environmental conditions as well (for example the temperature of the specimen under study). Data analysis made also evident that the classification of organisms could be possible only by using full spectral information, thus requiring for a hyperspectral detection system. However it might be possible that a double or multiple excitation wavelengths

151

LIF Intensity (a.u.)

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength [nm]

Figure 4. Processed images of a scan in Sucevita church (90 × 80 cm2 ). Left image (a) was taken with a conventional photo camera; in the right pane (b) it is shown a black and white LIF intensity on selected spectral bands.

Figure 3. LIF spectra of two consolidants on plaster: Calaton (dash dot), Mowilith (solid line).

could reduce the requirements on spectral information needed for unambiguous fungi strain identification. Further laboratory experiments are currently carried out to investigate this point. 3.1.2 Consolidants identification The identification of organic materials as protective varnishes and or consolidants in works of art is of particular interest, since they strongly influence the deterioration processes (Domenech 2001). Here we report some preliminary results obtained by applying the fluorescence induced by UV laser excitation at 266 nm to distinguish different consolidants on plaster laboratory samples. For the purpose of the present experiment three different consolidants were taken into account: Calaton, soluble nylon, Mowilith, vinyl acetate and Paraloid, ethyl acrylate (Cappitelli 2004, Abdel-Kareem 2000). In Figure 3, as example, we report the UV fluorescence spectra for Calaton (dashdot line), and for Mowilith (solid line). The spectra of Figure 3 were then used as references to apply SAM algorithm to the analysis of images acquired with the hyperspectral LIF scanner. The identification of areas treated with different consolidants types was excellent (not shown here) being independent from several factors including (1) the plaster used, (2) the pigment in the underlying layer and (3) the amount of consolidant sprayed on the surface (at least in the concentration range used for the present experiment). In all the tested cases we obtained an unambiguous and successful identification. Measurements of fluorescence emission from the same consolidant samples excited at 355 nm are in progress in our laboratory, in order to check the possibility to distinguish them meanwhile acquiring information on pigments and biodeterioration. No many data, in fact, are available in literature with respect to the fluorescence of consolidants, but the possibility of their identification by applying PCA to LIF spectra obtained upon excitation at 355 nm was demonstrated (Ballerini 2001).

3.2 Frescoes at 355 nm excitation The 355 nm laser excitation wavelength was used to excite fluorescence in Sucevita Monastery on four different portions of a painted dome. All the scanned areas (70 × 80 cm2 ) are partially restored and their preservation status is generally poor showing a blackish superficial deposit encompassing almost all of the frescoes. Restoration started from the gilded areas and actually it is not yet completed, thus it does not extend on other portion of the frescoes. Figure 4 shows details on the first scanned image. In the left panel (Fig. 4a) we can see a black and white image of the area taken with a standard photocamera (Canon PhotoShot 600), and in the right pane (Fig. 4b) the image of the same fresco region acquired by LIF. The acquired fluorescence spectral data have been combined to generate a false RGB color image using the spectral channel intensities at 340 nm, 480 nm and 560 nm respectively for Blue Green and Red (Colao 2005); this image is characterized by a high spectral contrast reproducing icon details with an exceptional accuracy. Spectral changes appear as a weak modulation over a mean LIF signal dominated by the plaster contribution. The superficial pigments and deposits act as quencher of the plaster fluorescence leaving their signatures as an entangled combination of (1) laser excitation absorption (2) plaster fluorescence absorption and (3) possibly pigments fluorescence emission. Gilded details appear in several different parts of the frescoes: in the centre the aureole of the main character at left side (Jesus), as well as in stars in the upper central part. From a qualitative point of view the gilding fluorescence represents a case quite different from the rest of the scanned area: despite that metal fluorescence is generally very low and frequently completely absent, we observe a strong spectral intensity and unique spectral features. On one hand this is explained by the fact that to prevent superficial layer detachment, during restoration gilded areas have been partially treated with organic consolidants like Paraloid.

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Figure 5. Processed images of a scan in Sucevita church. Left panel (a) is a SAM image with bio-attack obtained with a reference spectrum; in the right panel (b) a SAM image obtained with a reference spectrum from a portion of a cleaned fresco surface.

Areas with bio attack by fungi occur on upper left regions; the similarity map with SAM and a reference spectrum taken in a partially contaminated region is shown in Figure 5(a). To enhance the bio contaminated areas, this image has been processed with a threshold filter set at a spectral angle of approximately 5◦ , thus eliminating intermediate intensity gray levels. As result we obtain an easy recognition of contaminated and cleaned portions. In this case the heavily attacked areas appear in black, while non or slightly contaminated portions are in white. As a further check, Figure 5b reports areas with small biological attack; in this case the SAM similarity map has been computed using a reference spectrum taken in a cleaned region. 3.3

Frescoes at 266 nm excitation

The 266 nm laser excitation wavelength was used to excite fluorescence from fresco contained in Ballinesti Church, where a strong biodeterioration by fungi is visible on the Church walls sometimes also by the naked eye. To face with biological attack, the restorers working in this site were planning to use biocides and prudently were testing their effects on a small area of a church wall. While they were using standard methods to assess potential unwanted deterioration caused by the biocide, it was also of great interest to characterize the used chemical agents by means of LIF. Figure 6a shows a black and white image of a detail of a fresco on the church wall taken with a standard photo camera, the same area was then scanned with the hyperspectral LIF system. Three regions can be recognized in Figure 6a: the first one marked as i) is typical of a strong biological attack, a second one marked as ii) only treated with biocide and a third area marked as iii) treated with biocide and subsequently cleaned by restorers. SAM algorithm was used to identify selected regions in the scanned area.The three specified regions with different spectral characteristics were identified respectively for biological attack (from Penicillium

Figure 6. Processed images of a scan in Ballinesti church (35 × 35 cm2 ). Upper left (a) image by a conventional photo camera. SAM maps obtained with reference spectrum of (b) a bio-attack; (c) a biocide, (d) a cleaned area.

crysogenum, independently identified), treated with biocide, and areas which were treated with biocide and subsequently cleaned by restorers. SAM similarity map obtained using biological attack reference spectrum is shown in the upper right (Fig. 6b), Figures 6c and 6d show SAM mapping obtained respectively with biocide and cleaned reference spectra. SAM algorithm is able to correctly identify the treated and untreated regions, while Figure 6d shows that cleaned areas are identified less precisely as is made evident from the non negligible number of misidentified pixels in treated but not cleaned portion. Several considerations must be made to explain the partial failure of identification on cleaned areas. First of all we must observe that the spectral differences in treated only and treated plus cleaned areas are quite small. To quantify this observation we computed the average spectral correlation of these areas. It resulted that regions i) and ii) have a correlation coefficient of 0.96, meaning that a part for floral decoration appearing in the central part of the regions, they have been covered with the same kind of pigment. The spectral differences are then due only to the presence of the biocide chemicals, which evidently do not have any special and distinctive feature. A second point deserving some attention is related to the fact that the treatment and cleaning in region iii) was made at least six months before the LIF analysis. Since in the meantime the appropriate actions to avoid fungi proliferation were still to be completed, it happened that fungi started again to colonize the wall, thus contaminating with hyphae and mycelium also regions ii)

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ACKNOWLEDGEMENTS

Figure 7. Photo picture of a Sucevita fresco detail (12 × 12 cm2 ) (a) and corresponding SAM image for identification of consolidant (b).

and iii). Nonetheless SAM proves to be an effective and powerful tool for identification. As found in the laboratory database construction, the excitation at 266 nm is especially suitable to reveal the presence of specific consolidants utilized during the restoration. This could be verified in field measurements on frescoes with consolidated gilded details. In Sucevita monastery, a particular of the same image already examined at 355 nm and shown in Figure 4 (Jesus’ head picture) has been scanned at 266 nm in order to trace the extent of the restorer’s action (see Fig. 7). The presence of Paraloid at specific locations on the aureole has been revealed by its characteristic fluorescence emission band peaked at 310 nm. The SAM mapping image reconstruction of the scanned area, shown in Figure 7b, highlights the not uniform distribution of the consolidant due to the restorer’s intention to cover only the detaching gold leaf.

4

CONCLUSIONS

The new compact laser scanning LIF apparatus has successfully applied to the painted walls investigation of relevant cultural heritage. The advantages of the LIF technique as diagnostic tool for cultural heritage are mainly related to the capability of space resolved remote measurements with minimal invasiveness and of a rapid acquisition of data. The presented set-up is compact and solid, the optical system is in 58 × 43 × 36 cm3 , light enough (about 20 kg) and relatively cheap. Moreover, the technique gives additional, valuable and complementary information with respect to conventional visible or infrared imaging and the results presented demonstrate that hyperspectral LIF gives useful information to identify biological attacks areas on painted surfaces. Fluorescence emissions can be also related to the different materials and methods applied during the fresco realization. It is indeed an effective tool for specific diagnostics of cultural heritage.

The contribution of I. Gomoiu for the selection, handling and growing the fungi strains is gratefully acknowledged. Also we thank the contribution of I. Nemec for the preparation of plaster samples with pigments and consolidants. For the field campaign the invaluable support of R. Radvan, coordinator of the CULTURE project “Saving Sacred Relics of European Medieval Cultural Heritage” is gratefully acknowledged. Work partially supported by the European Union in the framework of the “CULTURE 2000” program (project CLT 2005/A1/ CHLAB/RO-488).

REFERENCES Abdel-Kareem 2000. Microbiological testing of polymers and resins used in conservation of linen textiles. 15th World Conference on Nondestructive Testing, Rome. Anglos D., Solomidou, Zergioti, Zafiropulos V., Papazoglou, Fotakis C., 1996. Laser-Induced Fluorescence in Artwork Diagnostics: An Application in Pigment Analysis, Appl. Spectroscopy, 50:1221–1337. Anglos D., Balas C., Fotakis, C., 1999. Laser spectroscopic and optical imaging techniques in chemical and structural diagnostic of painted artwork. American Laboratories 31: 60–67. Aristipini, P., Colao, F., Fantoni, R., Fiorani, L. & Palucci, A. 2004. Compact scanning lidar fluorosensor for cultural heritage diagnostics, Proceedings of SPIE 5880: 196–203. Asmus, J.F. 2003. Non-divestment laser applications in art conservation. Journal of Cultural Heritage 4: 289–293. Athanassiou, A., Hill, A.E., Fourrier, T., Burgio, L. & Clark, R.J.H. 2000. The effects of UV laser light radiation on artists pigments. Journal of Cultural Heritage 1: S209–S213. Ballerini, G., Bracci, S., Pantani, L. & Tiano, P. 2001. Lidar remote sensing of stone cultural heritage: detection of protective treatments. Optical Engineering 40: 1579–1583. Cappitelli F., Zanardini E., Sorlini C., 2004. The biodeterioration of synthetic resins used in conservation. Macromol. Biosci. 4:399–406. Carcagni, P. 2007. Multispectral imaging of paintings by optical scanning. Optics and Lasers in Engineering, 45: 360–367. Colao, F., Fantoni, R., Fiorani, L., Palucci, A. & Gomoiu, I. 2005. Compact scannino lidar fluorosensor for investigations of biodegradation on ancient painted surfaces. Journal of Optoelectronics and Advanced Materials 7: 3197–3208. Colao, F., Fantoni, R., Fiorani, L. & Palucci, A. 2007. In press. Scanning hyperspectral lidar fluorosensor: a new tool for fresco diagnostics. Proceedings of Conference Conservation Science 2007. Coma, L., Breitman, M., Ruiz-Moreno, S. 2000. Soft and hard modelling methods for deconvolution of mixtures of Raman spectra for pigment analysis. A qualitative and quantitative approach. Journal of Cultural Heritage 1: S273–S276.

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Comelli D., D’Andrea C., Valentini G., Cubeddu R., Colombo, C., Toniolo L., 2004. Fluorescence lifetime imaging and spectroscopy as tools for nondestructive analysis of works of art. Applied Optics 43: 2175–2183. Domenech-Carbo M.T., Gimeno-Adelantado J.V., Bosh-Reig F., 2001. Identification of synthetic resins in works of art by Fourier transform infrared spectroscopy. Applied Spectroscopy 55: 1590–1602. Klein, S., Fekrsanati, F., Hildenhagen, J., Dickmann, K., Uphoff, H., Marakis, Y.& Zafiropulos, V. 2001. Discoloration of marble during laser cleaning by Nd:YAG laser wavelengths. Appl. Surf. Sci. 171 : 242.

Lognoli, D., Cecchi, G., Mochi, I., Pantani, L., Raimondi, V., Chiari, R., Johansson, T., Weibring, P., Edner, H. & Svanberg, S. 2003. Fluorescence lidar imaging of the cathedral and baptistery of Parma, Applied Physics B 76: 457–465. Nevin A., Cather S., Anglos D., Fotakis C., 2006. Analysis of protein-based binding media found in paintings using laser induced fluorescence spectroscopy, Anal. Chim. Acta., 573-574C: 341–346. Sansonetti, A. & Realini, M. 2000. Nd:YAG laser effects on inorganic pigments. Journal of Cultural Heritage 1: S189–S198.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

A lidar experiment for the characterization of photoautotrophic and heterotrophic biodeteriogens by means of remote sensed autofluorescence spectra V. Raimondi, L. Palombi, D. Lognoli & G. Cecchi Institute for Applied Physics ‘Nello Carrara’ – National Research Council, Sesto Fiorentino, Florence, Italy

I. Gomoiu Institute of Biology, National Arts University of Bucharest, Splaiul Independentei, Bucharest, Romania

ABSTRACT: Remote laser-induced autofluorescence measurements have been performed on a set of biodeteriogens samples selected in an archaeological site, Tropaeum Traiani, near Constanta, Romania. Both photoautotrophic (lichens) and heterotrophic biodeteriogens (pure cultures of fungi and bacteria) were examined with a high resolution fluorescence lidar system featuring a UV laser (XeCl at 308 nm) as excitation source. The measurements were carried out on the in vivo samples placed at a distance of about 25 m in the outdoor, in full sunlight. The results confirm the fluorescence lidar technique as a powerful method for the remote detection and characterisation of photoautotrophic biodeteriogens and also open good prospects for remote, non-invasive monitoring of heterotrophic biodeteriogens on non-movable objects, also outdoors.

1

INTRODUCTION

Biodeterioration is a typical process that affects stone cultural heritage, especially outdoors. Its monitoring, treatment and timely prevention is thus essential for the conservation of cultural heritage. This, however, preliminarily requires the identification of the microorganisms affecting the surface of monuments and an assessment of the extension of the contaminated areas. Extensive diagnostics of the stone outdoor cultural heritage can become quite demanding in terms of time and costs when traditional techniques are applied. In this context, remote sensing can offer several advantages to assess the overall distribution of biodeteriogens over extended surfaces and to routinely monitor its modifications for a timely prevention. In particular, a preliminary assessment of the extension and type of biological growth on monuments’ surfaces can be exploited for the identification of the most suitable sampling areas for possible applications of specific analytical techniques. In vivo fluorescence spectral signatures of algae and cyanobacteria have been studied for a long time (Yentsch et al. 1979, Bazzani et al. 1992) and the fluorescence properties of these photoautotrophic biodeteriogens have been also exploited as a diagnostic tool for the monitoring of biodeterioration on stone cultural

heritage with remote sensing techniques (Raimondi et al. 1998, Weibring et al. 2001, Lognoli et al. 2002). However, although visual observation of fluorescence emitted from lichens under UV light has been carried out for a long time (e.g. observation with epifluorescence microscopy techniques or confocal microscopy; see e.g. Mathey et al. 2001), spectral signatures of lichens fluorescence have been not studied to a great extent up to now and only few works on this subject are available (Mathey et al. 2001, Hidalgo et al. 2002). As far as the autofluorescence of heterotrophic biodeteriogens is concerned, only few papers are available in the literature about their spectral signatures (Arcangeli et al. 1997, Bengtsson et al. 2005, Colao et al. 2005, Raimondi et al. 2007). This work illustrates the results of a set of measurements aimed at a fluorescence-based characterisation of different types of biodeteriogens and carried out during a joint experiment within a EU-funded Culture 2000 project at a Roman archaeological site, Tropaeum Traiani, near Constanta, Romania. Laser induced autofluorescence spectra acquired with a mobile lidar system featuring a UV laser as an excitation source were the measurements collected. The system was deployed at the site and both photoautotrophic and heterotrophic biodeteriogens were examined during the experiment in uncontrolled environmental conditions.

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Figure 2. Picture of the pure culture of the fungal strains of Verticillium sp. (left) and of Aureobasidium pullulans (right).

Figure 1. Picture of the stone samples affected by biological growth. Most stones showed the presence of lichens as listed in Table 1.

Table 2. Pure cultures of fungal and bacterial strains, and relevant nutrient media, examined during the experiment (second session).

Table 1. Lichens on stone substrate examined during the experiment (first session). Label

Description

Sample A Sample B Sample C Sample D

Parmelia sp. Caloplaca sp., Parmelia sp. Caloplaca sp., Physcia sp. White crust, not identified

2 2.1

MATERIALS AND METHODS Samples description

The samples consisted of two different sets: – a selection of stones affected by the growth of photoautotrophic biodeteriogens, particularly lichens; – a set of pure cultures of different fungal and bacterial strains, previously isolated from the samples collected in the archaeological site. All the samples were selected in the Tropaeum Traiani archaeological site. Figure 1 shows the set of stones affected by biological growth, mainly lichens, that were selected for the first session of measurements. A description of the biodeteriogen-affected stone samples presented in this paper can be found in Table 1. Figure 2 shows a picture of pure cultures of the two fungal strains – Verticillium sp. (left) and Aureobasidium pullulans (right) – among those collected at the archaeological site and isolated in the laboratory. A list of the bacterial and fungal strains examined during the measurement session is reported in Table 2. 2.2 Instrumentation Fluorescence lidar remote sensing allows to transfer the Laser Induced Fluorescence (LIF) technique to the

Label

Description

F-blank F1 F2 B-blank B1-a B1-b B2-a B2-b

Nutrient medium Aureobasidium pullulans Verticillium sp. Nutrient medium Bacillus sp.1 Bacillus sp.2 Pseudomonas sp.1 Pseudomonas sp.2

outdoor environment where uncontrolled environmental conditions are met. If the lidar is provided with a scanning system to scan the laser beam over the target, a fluorescence map can be acquired. A further description of the technique can be found in e.g. Weibring et al. (2001), Lognoli et al. (2003) and Svanberg (2005). The fluorescence measurements were carried out with a high spectral resolution fluorescence lidar that has been in-house developed at CNR-IFAC and has been operative aboard a mobile laboratory since 1991. Although originally intended for the monitoring of marine environment and afterwards of vegetation, the CNR-IFAC lidar system has already shown great potential for the remote non-invasive monitoring of stone cultural heritage since first experiments carried out in the mid 1990’s (Raimondi et al. 1998, Lognoli et al. 2003). The CNR-IFAC lidar system features a XeCl excimer laser emitting at 308 nm as an excitation source. The signal is collected with a 25 cm-diameter telescope and the detection system consists of a 275 mm focal length spectrometer coupled to a 512 channel photodiode array detector. The acquired fluorescence spectrum covers the 300–800 nm spectral window and is achieved by combining a measurement of the 300–600 nm range with a measurement of the 500–800 nm range to avoid second order superposition. The detailed description of the system can be found in e.g. Cecchi et al. (1992, 1994).

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2.3

Experimental setup

The measurements were performed during two sessions carried out in the frame of a EU-funded Culture 2000 project at the Roman archaeological site of Tropaeum Traiani (Romania). The first session was devoted to the acquisition of fluorescence spectra on stones affected by several biodeteriogens; the second was devoted to the measurements of autofluorescence from the pure cultures of fungal and bacterial strains. Both measurement sessions were carried out with the CNR-IFAC mobile lidar system. The mobile lidar system was deployed at a distance of about 25 m from the sample holder where samples were placed before each measurement. The sample holder was covered with non-fluorescent material to avoid spurious contributions to the collected fluorescence signal. The measurements were performed on the in vivo samples outdoors, in full sunlight, to reproduce as much as possible the experimental conditions during a lidar field campaign. In these experimental conditions the area of the target actually measured at each laser shot was a spot of about 2 cm diameter. Full-sunlight operation was possible by applying an electronic temporal gating (of the order of µsec) to the detector, in order to increase the signal-to-background ratio. In addition, a background spectrum was automatically acquired between each laser pulse and the following one and subtracted from the laser-excited spectrum to remove the possible residual background.

Figure 3. Autofluorescence spectra on stones affected by biodeteriogens. Fluorescence spectrum from the front and rear part of the stone (sample A): the rear part shows the typical fluorescence peak of phycocyanin at 660 nm.

Figure 4. Autofluorescence spectra on three different samples of stones affected by lichen growth.

3 3.1

RESULTS Lichens on stone substrates

Laser induced autofluorescence spectra of the examined biodeteriogens on stone substrates are shown in Figures 3–5. Figure 3 shows the fluorescence spectra of the front and rear section of the same stone: the fluorescence spectrum from the rear part of the stone shows a spectral profile typical of several cyanobacteria (see, e.g. Lognoli et al. 2002) characterised by the fluorescence peak of phycocyanin at about 660 nm. This leads to infer the presence of cyanobacteria on the rear surface of the stone, as it was also supported afterwards by visual and optical microscopy inspection of the rear surface of the stone by the biologists. Figure 4 shows the fluorescence spectra of different samples of stone with lichens on their surface (samples B, C and D; refer to Table 1 for their description). In particular, the areas effectively examined on the samples refer to: – sample B: area affected by Parmelia sp. (greyish lichen); – sample C: area affected by Caloplaca sp. (orange lichen);

Figure 5. Autofluorescence spectra on a stone affected by lichen growth (sample B) with the typical fluorescence peak due to Chl a at 680 nm after scratching the lichen surface.

– sample D: area affected by a white encrustation, probably due to by-products of the lichen acids. All the three samples show a typical fluorescence shape which is remarkably different from each other and could be used for their detection and mapping of the surface of a monument. Sample B shows the typical fluorescence peak of Chlorophyll a (Chl a) at about

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Figure 7. Autofluorescence spectra of pure cultures of four different bacterial strains (B1-a, B1-b, B2-a, B2-b) and of the relevant nutrient medium (B-blank).

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Figure 6. Autofluorescence spectra of pure cultures of two different fungal strains (F1, F2) and of the relevant nutrient medium (F-blank).

3.2

Pure cultures of fungal and bacterial strains

The second session of measurements was devoted to the acquisition of autofluorescence spectra on pure cultures of fungal and bacterial strains, as listed in Table 2. Several measurements were peformed for each culture. Figures 6–7 show the autofluorescence spectra for the examined fungal and bacterial strains, respectively. The spectra shown in the figures are obtained by operating a mean over the measurements performed on the

F1/F-blank F1/F-blank F1/F-blank F-blank'/F-blank F-blank/F-blank

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680 nm, although not very intense. Sample C features a very intense fluorescence band at 650 nm, while sample D has a sharp-rising fluorescence band peaked at 450 nm which could be due to calcium oxalates. Similar fluorescence spectra due to calcium oxalates in lichens, excited at 250 nm, are reported for example in Clark et al. (2001). Figure 5 shows the fluorescence spectrum obtained on the same sample B (whose spectrum is already shown in Fig. 4) after scratching the surface of the lichen: after the operation, the Chl a fluorescence at 680 nm is definitely more apparent, both because of the partial removal of the crust responsible for the fluorescence between 500–600 nm and because of a more efficient excitation of the algal layer partially covered by the metabolites crust and the hyphal network on the surface of the colonies. Some by-products have in fact the effect to protect the algae from UV radiation and effectively decrease the amount of radiation reaching the algal layer (Fernandez et al. 1996, Clark et al. 2001).

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Figure 8. The ratio between three spectra relative to the F1 sample and the F-blank spectrum. The ratio between another blank measurement (F-blank’) and the F-blank spectrum is reported as well.

same sample. The fluorescence spectra of the relevant nutrient media are also shown. Apart from some strain (e.g. B1-b bacterial sample) which features a noticeably different spectral shape, most fluorescence spectra are quite similar to the fluorescence spectrum of the nutrient medium. However, a preliminary analysis based on a very simple data processing, such as the ratio between the fluorescence spectra obtained on the strain and the relevant nutrient medium, reveals differences in the fluorescence contributions due to the presence of the bacterial or fungal strains. As an example, Figures 8–9 show the ratios between the fluorescence spectra obtained on the fungal strains

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Figure 9. The ratio between three spectra relative to the F2 sample and the F-blank spectrum. The ratio between another blank measurement (F-blank’) and the F-blank spectrum is reported as well.

and the fluorescence spectrum of the relevant nutrient medium. Data in both graphs are shown with the same X–Y axis scale to make the comparison easier. Noise at higher wavelengths (>650 nm) is due to very low intensities in the fluorescence spectra. While the sample F1 does not show significant different contributions to the fluorescence spectral shape with respect to the fluorescence of the nutrient medium (Fig. 8), the ratios between the fluorescence spectra from the F2 sample and the nutrient medium point out differences in the fluorescence contributions around 400 nm and 600 nm (Fig. 9). Similarly, Figure 10 shows the ratios between the fluorescence spectra obtained on the bacterial strains and the fluorescence spectrum of the relevant nutrient medium. For comparison, the graph also shows the ratio between the fluorescence spectrum of the only nutrient medium (B-blank) and another measurement (B-blank’) taken on the same nutrient medium at the end of the set of measurements on the bacterial strains. From Figure 10 it can be inferred that all the examined bacterial strains show peculiar contributions to the fluorescence spectral shape in the 400–600 nm spectral range with respect to the nutrient medium (B-blank’/B-blank). In addition, the graph shows how the ratios referring to B2-a and B2-b strains have a very similar spectral shape, making them not distinguishable from each other. These different contributions to the fluorescence spectral shape can be exploited, by using more refined processing methods like multivariate analysis, to characterise different strains. Fluorescence data were also analysed with multivariate statistical techniques, specifically Principal

Figure 10. The ratio between the fluorescence spectra of the bacterial strains and that of the nutrient medium (B-blank). The ratio between another blank measurement (B-blank’) and the B-blank spectrum is reported as well.

Component Analysis and Cluster Analysis (see, e.g. Rencher 2002), to investigate the possibility to distinguish the fluorescence of the fungal and bacterial strains from that of the relevant nutrient medium as well as to distinguish one strain from the other one. The results, presented in a separate publication (Raimondi et al. 2007), allowed to characterise all the examined strains on the basis of their fluorescence features, except for the Aureobasidium pullulans that did not allow its discrimination from the nutrient medium. The proposed data processing techniques allow to differentiate among genera and even strains opening good prospects for the differentiation of heterotrophic organisms with remote sensing techniques in the field, at least when the fluorescence background, due to e.g. the stone substrate, has a known, relatively homogeneous spectral shape as in the case of the nutrient medium.

4

CONCLUSIONS

Remote laser-induced autofluorescence measurements have been peformed on both photoautotrophic and heterotrophic biodeteriogen samples selected in an archaeological site with a high spectral resolution fluorescence lidar from a 25 m distance in full sunlight, under uncontrolled environmental conditions. Most examined photoautotrophic and heterotrophic biodeteriogen samples showed peculiar fluorescence features that can be exploited for their characterisation during on-site remote non-invasive monitoring of monuments outdoors. In particular, the examined pure cultures of fungal and bacteria strains showed fluorescence features that allow their characterisation. This opens good prospects for the remote fluorescence

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mapping of heterotrophic organisms on outdoor monuments. Remote fluorescence imaging-based documentation and diagnostics could actually be exploited to provide a preliminary, global assessment of the extension and typology of biodeterioration on the vast stone cultural heritage with the aim to plan at its best cleaning, maintenance and preventive conservation interventions of large-scale objects. ACKNOWLEDGEMENTS The authors want to thank all people that made it possible the realisation of this experiment. In particular, they are grateful to Roxana Radvan and Roxana Savastru for their helpful support all through the measurement campaign. They acknowledge the EU-funded Culture 2000 Project (Contract No. CLT 2003 A1 RO 515) for funding this experiment. REFERENCES Arcangeli, C. et al. 1997. Fluorescence study on whole Antarctic fungal spores under enhanced UV irradiation. Journal of Photochemistry and Photobiology B: Biology 39: 258–264. Bazzani, M. et al. 1992. Phytoplankton Monitoring by Laser Induced Fluorescence. EARSeLAdvances in Remote Sensing 1: 106–110. Bengtsson, M. et al. 2005. Fungus covered insulator materials studied with laser-induced fluorescence and principal component analysis. Applied Spectroscopy 39: 1037–1041. Cecchi, G. et al. 1992. FLIDAR: a multipurpose fluorosensorspectrometer. EARSeL Advances in Remote Sensing 1: 72–78. Cecchi, G. et al., 1994. Remote sensing of chlorophyll a fluorescence of vegetation canopies: I. near and far field techniquesRemote Sensing of Environment 47: 18–28.

Clark, B.M. et al. 2001. Characterization of mycobiont adaptations in the foliose lichen Xanthoparmelia chlorochroa (Parmeliaceae). American Journal of Botany 88: 1742– 1749. Colao, F. et al. 2005. Compact scanning lidar fluorosensor for investigations of biodegradation on ancient paited surfaces. Journal of optoelectronics and advanced materials 7: 3197–3208. Fernandez, E. et al. 1996. Photoprotector capacity of lichen metabolites against UV-A and UV-B radiation. Cosmetics Toiletries 111: 69–74. Hidalgo, M.E. et al. 2002. Photophysical, photochemical, and thermodynamic properties of shikimic acid derivatives: calycin and rhizocarpic acid (lichens). Journal of Photochemistry and Photobiology B: Biology 66: 213–217. Lognoli, D. et al. 2002. Detection and characterization of biodeteriogens on stone cultural heritage by fluorescence lidar. Applied Optics 41: 1780–1787. Lognoli, D. et al. 2003. Fluorescence lidar imaging of the cathedral and baptistery of Parma. Applied Physics B. 76: 457–465. Mathey,A. et al. 2001. Spatial distribution of perylenequinones in lichens and extended quinones in quincyte using confocal fluorescence microscopy. Micron 32: 107–113. Raimondi, V. et al. 1998. Fluorescence lidar monitoring of historic buildings. Applied Optics 37: 1089–1098. Raimondi, V. et al. 2007. Remote detection of laser-induced autofluorescence on pure cultures of fungal and bacterial strains and their analysis with multivariate techniques. Optics Communications 273: 219–225. Rencher, A. C. 2002. Methods of Multivariate Analysis. New York: Wiley Interscience. Svanberg, S. 2005. Fluorescence imaging of lidar targets. InT. Fujii & T. Fukuchi (eds), Laser Remote Sensing: 433–467. Boca Raton: CRC Press. Weibring, P. et al. 2001. Fluorescence lidar imaging of historical monuments. Applied Optics 40: 6111–6120. Yentsch, C.S. et al. 1979. Fluorescence spectral signature: the characterization of phyotoplankton populations by the use of excitation and emission spectra. Journal of Marine Research 37: 471–483.

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Design and development of a new high speed performance fluorescence imaging lidar for the diagnostics of indoor and outdoor cultural heritage V. Raimondi, L. Palombi, D. Lognoli & G. Cecchi Institute for Applied Physics ‘Nello Carrara’ – National Research Council, Sesto Fiorentino, Florence, Italy

L. Masotti Electronics and Communications Department, University of Florence, Italy

ABSTRACT: Fluorescence lidar imaging is a remote sensing technique that can be used to obtain hyperspectral fluorescence images on a monument from a distance by using a low-energy laser beam. The technique represents a particular economic advantage (in terms of time, money and specialised personnel) for an extensive monitoring of the outdoor stone cultural heritage. In addition, fluorescence lidar imaging data can also be presented in an easy-to-read output format, like false-colour thematic maps, which can become an effective support for restorers, historians and decision-makers. This paper presents the main technical features of a new fluorescence imaging lidar system specifically developed for the remote, non-invasive diagnostics on the cultural heritage by CNR-IFAC in collaboration with a private company, the El.En. Group S.p.A., in the frame of the SIDART National-Funded Project.

1

INTRODUCTION

The fluorescence lidar technique has been applied to the investigation of the cultural heritage since the mid 1990s (Raimondi et al. 1995). Since then, several experiments have proved it as a useful tool for the remote non invasive diagnostics of monuments, providing helpful information for the assessment of the state of conservation of monuments and for the characterisation of masonry materials. Up to now, main investigated issues include the detection and characterisation of different stones, mortars and other masonry materials (Raimondi et al. 1995, Cecchi et al. 1996b, Raimondi et al. 1998, Cecchi et al. 2000), of protective treatments (Ballerini et al. 2001), of frescoes (Colao et al. 2005), of biodeteriogens (Cecchi et al. 1996a, Lognoli et al. 2002, Colao et al. 2005 & Raimondi et al. 2007) and the analysis of the effects of biocide treatments (Lognoli et al. 2002). A step forward in the application of the fluorescence lidar technique to the monitoring of the cultural heritage has been achieved in 1997 with the use of lidar hyperspectral imaging during an experiment carried out on the Cathedral of Lund, Sweden (Weibring

et al. 2001). The results showed good potential for achieving thematic maps aimed at the characterisation of the lithotypes and the detection and characterisation of biodeteriogens. In the following years hyperspectral fluorescence lidar imaging has been applied on several other monuments, such as: the Cathedral and Baptistery of Parma (Lognoli et al. 2003), the roman archaeological site of Adamclisi in Romania (Cecchi et al. 2004) and the Coliseum and the Baptistery of San Giovanni in Laterano in Rome (Hällstrom et al. 2007). All these experiments were conducted by using multipurpose fluorescence lidar sensors, specifically the one developed at CNR-IFAC (Cecchi et al. 1992), the other developed at the Lund Institute of Technology, Sweden (Weibring et al. 2003). Both lidars were initially developed for different applications, such as atmospheric studies and sea, natural waters and vegetation monitoring, and were then applied to the diagnostics on the cultural heritage. A compact scanning lidar fluorosensor has been also developed for indoor operation on artworks and especially frescoes inside tombs where only a limited space is available (Colao et al. 2005). This prototype has been utilized to investigate painted walls of a Byzantine crypt in Constanta (Romania).

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This paper introduces a new fluorescence lidar sensor, specifically developed for cultural heritage applications, which has been built at the CNR-IFAC in collaboration with a private company, the El.En. Group S.p.A.. Design criteria have taken into account the following issues:

Figure 1. Block diagram of a fluorescence lidar system.

n-columns

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These characteristics make the sensor able to operate at different sites, e.g. outdoor as well as indoor, and also aboard a small-size van. The sensor is provided with a computer-controlled target scanning system and with a pointing laser system in the visible to reference the acquired hyper-spectral images on the target. The following sections briefly describe the hyperspectral fluorescence lidar imaging technique and the main features of the new prototype, specifically designed for cultural heritage applications, together with the results of a demonstrative test in the laboratory.

Intensity

– High scanning speed of the target for a quick image acquisition also on large areas; – Relatively high spatial resolution capabilities for the investigation of monument details; – High spectral resolution for a thorough spectral shape analysis by refined data processing techniques, such as multivariate statistical analysis; – Wide field of view and reduced minimum operational distance to enhance the sensor’s operational capabilities; – Compactness and on-site transportability.

Laser spot

(a) Scanning of the target to obtain an image with (mxn) pixels

Wavelength

(b) Detection of a high resolution fluorescence spectrum for each image pixel

(c) Realisation of thematic maps by analysing the fluorescence spectra with specific processing techniques

Figure 2. Operating principles of hyperspectral fluorescence lidar imaging.

2

HYPER-SPECTRAL FLUORESCENCE LIDAR IMAGING

A fluorescence lidar is essentially composed of a laser, a telescope, a dispersion system and a detector (Fig. 1). The laser beam, which can also be collimated by using a beam expander, is sent to the target and interacts with its constituents. The backscattered signal, including the fluorescence emitted by the target and containing information about its chemical-physical characteristics, is collected by the telescope and then fed to the dispersion and detection system, usually featuring high spectral resolution. The data are finally stored in a PC for the analysis of the signal. Hyper-spectral fluorescence lidar imaging technique essentially exploits a computer-controller scanning system to operate a scan of the target and thus acquire a complete spectrum for each scan position. In this way, at the end of the scan, a hyper-spectral fluorescence image of the investigated area can be retrieved. The measurement process is synthetically sketched in Figure 2; the investigated area on the surface of the monument can be ideally divided into m × n ‘squares’ (Fig. 2a) and then scanned with the lidar system to obtain a fluorescence image of the target. For

each ‘square’ of the target image, or ‘image pixel’, a full high spectral resolution fluorescence spectrum is acquired (Fig. 2b). The m × n set of fluorescence spectra is finally processed to obtain thematic maps that outline specific fluorescence features of the target (Fig. 3c). Fluorescence-based thematic maps are particularly attractive for the monitoring of monuments: firstly, they provide a comprehensive assessment on the status of the whole monument and a spatial definition that cannot be obtained by means of mere sampling. Moreover, the opportunity of recording time-dependent, repetitive fluorescence images opens new prospects for reliable monitoring, repeated in time, of the status changes of the monument. Another important aspect of thematic maps is that they make it easier to transfer information gained with sophisticated data processing to the conservation specialist to sustain the action of the decision maker. The hyper-spectral image analysis and then the thematic maps can be achieved in different ways, such as calculating a ratio between two selected spectral bands of the fluorescence spectra, applying Principal

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Figure 3. Layout of the lidar arrangement. Figure 4. The fluorescence lidar prototype during operation.

Component Analysis (PCA) or Cluster Analysis (CA) methods to the fluorescence data set (Rencher 2002). The results can be plotted as a function of the corresponding (x, y) position in a false-colour coded map.

Table 1.

3 THE NEW PROTOTYPE OF FLUORESCENCE IMAGING LIDAR 3.1

New lidar sensor main features.

Features

Description

Excitation laser source Optical reference system laser source Detector

3ω Nd:YAG, Q-Switched (@ 355 nm) 8 mJ @ 16 Hz or 40 mJ @ 50 Hz CW Laser @ 532 nm, 5 mW (IIIA class)

General description

A general layout of the new prototype is showed in Figure 3. The sensor has a UV laser for target fluorescence excitation and a laser emitting in the visible to reference the fluorescence image to the target. The two laser beams are coaxial to the optical axis of a telescope. A movable folding mirror is used as a pointing and scanning system to send the laser beam on the target and to scan the area. The telescope provides the collection of the backscattered radiation from the target. The collected radiation is focused on the entrance of an optical fibres bundle. The exit of the fibre bundle is mechanically coupled to the entrance slit of a spectrometer coupled to a 512 × 512 pixel matrix detector. The whole system is managed by a personal computer. The lidar sensor is arranged within an aluminium frame. The laser sources and relevant conditioning optics are placed on an aluminium plate, while the laser folding mirrors, the telescope and the optical fibre bundle support are fixed under the plate. The pointing and scanning system is also fixed on the aluminium frame. The dimension of the lidar sensor are 250 cm × 85 cm × 75 cm (l × h × w) with a weight of about 150 kg. The sensor has been designed so as to allow the transport aboard a small size van and its operation from the side door. Figure 4 shows a picture of the new lidar sensor during operation. Table 1 summarises the main features of the new lidar sensor.

Spectrometer Spectrometric linear resolution Telescope Telescope distance range Telescope Far field of view Pointing system Lidar Sensor Pointing field of view Pointing accuracy (both axes) Imaging single pixel acquisition time Weight and dimension

Intensified CCD 512 pixel × 512 pixel QE >10% (in 150 nm–870 nm range), Minimum gate width < 5 ns 300 mm focal length Three gratings (150 gg/mm, 600 gg/mm, 2400 gg/mm) 0.51 nm/pixel, 0.12 nm/pixel, 0.02 nm/pixel Newtonian Layout, 1 m focal length, 250 mm diameter 4 m up to inf 1 mrad Two axis motorized folding mirror Primary Axis: −30◦ up to +300◦ from the reference direction Secondary Axis: ±45◦ from the orthogonal direction to telescope axis 0.2 mrad <2 s Size: 250 cm × 85 cm × 75 cm (l × h × w), Weight: about 150 kg

3.2 Excitation sources, collecting optics and dispersion and detection system The system can be operated with two different commercial laser units as a fluorescence excitation source.

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The first one is a Q-switched, frequency tripled Nd:YAG (Quanta System, Handy YAG H-750 model) emitting 40 mJ at 355 nm with a pulse width of 5 ns and a maximum repetition rate of 50 Hz. The second unit is another Q-switched, frequency tripled Nd:YAG (Continuum, MiniLite II model) emitting 8 mJ at 355 nm with a pulse width of 5 ns and a maximum repetition rate of 16 Hz. This unit, although that has lower pulse energy and repetition rate than the first one, is smaller and lighter and its use is preferable for short range (typically < 30 m) or indoor applications. The excitation laser beam is made coaxial with the telescope by using high energy dielectric folding mirrors. The telescope is a 1000 mm focal length, 250 mm diameter Newtonian telescope. The optical design is optimised to allow operation starting from a minimum distance of 4 m. This characteristic can be useful to operate in indoor scenarios. The spectral dispersion system is a flat field Czerny-Turner f/4 spectrometer (Acton Research, SpectraPro-2300i) with 300 mm focal length. The spectrometer is equipped with three dispersion gratings (150 grooves/mm, 600 grooves/mm and 2400 grooves/mm) and a motorised filter-wheel mounting four long pass filters (Omega Optical) with cut-off wavelength at 330 nm, 500 nm, 370 nm and 550 nm, respectively. The detector is a gated CCD (Princeton Instruments/Acton, PIMAX:512 model) equipped with an intensifier (Unigen III Generation). 3.3 Pointing and scanning system The pointing and scanning system is constituted by a laser source in the visible to point the target and reference the fluorescence image and a computercontrolled stepping-motorised conditioning mirror to operate pointing and scanning of the target. The VIS laser used to reference the image to the target is a 3 mW cw laser emitting at 532 nm (class IIIa) with a divergence of 1.2 mrad and a starting beam diameter of 1.1 mm. The laser beam is made coaxial to the telescope by a high reflectance dielectric folding mirror. The VIS laser spot on the target surface corresponds to the position and size of the observed area. The VIS laser wavelength is in the spectral region close to the maximum sensitivity of the human eye and permits a high visibility of the spot on the monument surface from a distance. This allows the acquisition of an image with the reference spot on the examined area. Target pointing and scanning capabilities of the system rely on a movable folding mirror placed between the telescope and the target. The mirror is mounted on a motorised frame that permits the rotation on two orthogonal axes with a rotation accuracy better than 0.5 mrad. The field of view of the system is depicted in Figure 5.

Figure 5. Lidar pointing field of view in the upper hemisphere.

4

HYPERSPECTRAL FLUORESCENCE IMAGES ON STONES

A preliminary test of the new sensor was performed in the laboratory on an artificial target constituted by a set of different stones. The target was arranged juxtaposing a set of lithotypes: Botticino stone, Trani stone, Dolomitic and Carrara marbles. The target’s dimensions were about 16 cm × 20 cm. The distance of the sensor from the target was 10 m and the spot size diameter on the target was about 1 cm. The spatial resolution (i.e. the distance between the centres of two following pixels in the acquired image) was about 1.1 cm horizontally and about 2.7 cm vertically. The acquired hyperspectral image was 6 pixel × 20 pixel. Figure 6 shows some fluorescence spectra representative of the examined lithotypes; the spectra are normalized to their standard deviation. The spectra were acquired with the 8 mJ Nd:YAG laser (@355 nm) and have a spectral resolution better than 2 nm. Each fluorescence spectrum (944 channels distributed over the 350–830 nm spectral range) was obtained by merging two separate fluorescence measurements; the first centred at 480 nm and with the 370 nm cut-off filter; the second centred at 700 nm and with the 550 nm cutoff filter.The residual backscattered laser line is visible in the spectra. Such fluorescence features of the examined lithotypes, which are due to fluorescence activators such as impurities and lattice defects contained in the crystal lattice (see, e.g., Raimondi et al. 1998 and references therein), can be exploited as spectral signatures to distinguish the different lithotypes. As an example, Figure 7 shows, together with the photos of the four lithotypes used to arrange the target, a demonstrative thematic map obtained applying CA method to the non-normalised fluorescence data set in the 375–805 nm spectral range. CA was carried out using a ‘statistical’ distance and a ‘weighted’ linkage method. Such CA-based map clearly points out

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field of view and minimum operational distance from the target. The sensor features as well a relatively high spatial resolution to improve the operational capabilities of the sensor as for the investigation of the details of the target. These characteristics, together with its compactness and transportability, make the sensor able to operate in several scenarios both outdoor and indoor, including also ceilings and vaults. Last but not least, the prototype features a pointing laser system in the visible to reference the fluorescence image on the target. This input, in fact, plays a key role for the interpretation of the thematic maps obtained after processing the fluorescence data set. Figure 6. High spectral resolution fluorescence spectra obtained on a set of different lithotypes with the new prototype. Spectra are acquired from 10 m distance and 8 mJ Nd:YAG laser energy.

Figure 7. Photos of four types of stone samples and a fluorescence-based thematic map (bottom left) acquired from a 10 m distance on a composite target made with the four types of stone. The map was obtained by applying CA method to the fluorescence data set acquired with 8 mJ energy Nd:YAG laser.

the position of the different lithotypes in the artificial target arranged juxtaposing the stones, distinguishing them from each other. A1, A2 and A3 are three Dolomitic marbles, B1 is a Carrara marble; C1 and C2 are two Botticino stones and D1 is a Trani stone.

5

CONCLUSIONS

A new prototype of fluorescence lidar sensor has been designed and constructed for specific applications towards the monitoring of the cultural heritage. Main features consists of improved speed of acquisition of fluorescence hyperspectral images on extended areas and enhanced operational capabilities especially as for

REFERENCES Ballerini, G. et al. 2001. Lidar remote sensing of stone cultural heritage: detection of protective treatments, Optical Engineering 40: 1579–1583. Cecchi, G. et al. 1992. FLIDAR: a multipurpose fluorosensorspectrometer. EARSeL Advances in Remote Sensing 1: 72–78. Cecchi, G. et al. 1996a. Fluorescence lidar technique for the monitoring of biodeteriogens on the cultural heritage. In D. Arroyo-Bishop et al. (eds.), Remote Sensing for Geography, Geology, Land Planning, and Cultural Heritage: 137–147. SPIE 2960. Cecchi, G. et al. 1996b. The fluorescence lidar technique for the remote sensing of stony materials in ancient buildings. In D. Arroyo-Bishop et al. (eds.), Remote Sensing for Geography, Geology, Land Planning, and Cultural Heritage: 163–171. SPIE 2960. Cecchi, G. et al. 2000. Fluorescence lidar technique for the remote sensing of stone monuments, Journal of Cultural Heritage 1: 29–36. Cecchi, G. et al. 2004. A fluorescence lidar experiment at the archaeological site of Adamclisi, Dobruja (Romania). Report Culture 2000 project Ref. CLT 2003/A1/RO-515. Colao, F. et al. 2005. Compact scanning lidar fluorosensor for investigations of biodegradation on ancient painted surfaces, Journal of Optoelectronics and Advanced Materials 7(6): 3197–3208. Grönlund, R. 2007. Fluorescence lidar imaging of historical monuments – Övedskloster, a Swedish case study. In J. Nimmrichter et al. (eds.), Lasers in the Conservation of Artworks: LACONAVI Proceedings,Vienna, Austria, Sept. 21–25, 2005: 583–588. Berlin: Springer. Hällström, J. et al. 2007. Remote fluorescence lidar imaging of monuments: the Coliseum and the Lateran baptistery in Rome. In this volume. Lognoli, D. et al. 2002. Detection and characterisation of biodeteriogens on stone cultural heritage by fluorescence lidar. Applied Optics 41: 1780–1787. Lognoli, D. et al. 2003. Fluorescence lidar imaging of the cathedral and baptistery of Parma. Applied Physics B. 76: 457–465. Raimondi, V. et al. 1995. Remote sensing of cultural heritage: a new field for lidar fluorosensors. In Proceedings of 1st International Congress on Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin, Catania: 935–938.

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Raimondi, V. et al. 1998. Fluorescence lidar monitoring of historic buildings. Applied Optics 37: 1089–1098. Raimondi, V. et al. 2007. A lidar experiment for the characterization of photoautotrophic and heterotrophic biodeteriogens by means of remote sensed autofluorescence spectra. In this volume.

Rencher A.C., 2002. Methods of Multivariate Analysis. New York: Interscience. Weibring, P. et al. 2001. Fluorescence lidar imaging of historical monuments. Applied Optics 40: 6111–6120. Weibring, P. et al. 2003. Versatile mobile lidar system for environmental monitoring. Applied Optics 42: 3583–3594.

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Remote fluorescence lidar imaging of monuments: The Coliseum and the Lateran baptistery in Rome J. Hällström & K. Barup Architectural Conservation and Restoration, Lund University, Lund, Sweden

V. Raimondi, L. Palombi, D. Lognoli & G. Cecchi Institute for Applied Physics ‘Nello Carrara’ – National Research Council, Sesto Fiorentino, Florence, Italy

R. Grönlund, A. Johansson & S. Svanberg Atomic Physics Division, Lund University, Lund, Sweden

C. Conti Soprintendenza Archeologica di Roma, Rome, Italy

ABSTRACT: On site remote laser induced fluorescence measurements have been performed with application towards non-movable objects. This paper gives an overview of the results obtained from the application of non-invasive hyperspectral fluorescence imaging to two masonry monuments, the Coliseum and the Lateran Baptistery in Rome, during a joint Italian-Swedish experiment. The mobile systems of Lund Institute of Technology, Sweden, and CNR-IFAC, Italy, were placed at the distance of 18–65 m from the façades for the documentation and characterization of surfaces, including materials, protective treatments, biodeteriogens and historic layers.

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INTRODUCTION

The non-movable cultural heritage objects challenge the traditional analysis situation, due to the complexity of documentation methods and need for new noninvasive techniques. Fluorescence lidar imaging has already been applied successfully to create thematic maps for documentation of monuments and specifically outlining the presence of biodeteriogens on the surface, protective treatments or the distribution of different lithotypes in a stone monument, e.g. (Weibring et al. 2001, Lognoli et al. 2003). However, only very few measurements have been carried out on large complex monuments until now. This paper gives an overview from the results obtained from the extensive application of remote fluorescence lidar imaging to two monuments, the Flavian Amphitheatre Coliseum and the Lateran Baptistery in Rome. The field studies was carried out employing two fluorescence lidars: the first one, developed at the Lund Institute of Technology (LTH), Sweden, uses a multi-wavelength laser system as an excitation source (Weibring et al. 2003); the second one, developed at the Institute of Applied Physics of the Italian National Research Council (CNR) uses an excimer laser in the UV for target excitation and a laser pointer in the visible to reference the images on the target (Cecchi et al.

1992). Both lidars are equipped with a scanning system to acquire fluorescence hyperspectral images on the monument surface. The field study was carried out in January–February 2005 when several areas were examined on the external walls. Hyperspectral fluorescence data were exploited to create thematic maps which gave helpful information for the characterization of the investigated materials on the monument’s surface. The aim of the study was to use the fluorescencebased documentation of the facades, together with traditional surveying techniques, in order to understand the historical layers and interpret previous conservation interventions. The use of remote sensing as a non-invasive technique can provide informative images of the surfaces and give the cultural heritage sector a mobile tool for the analysis before and after conservation. 2

CAMPAIGNS

Fluorescence lidar studies were performed on two important cultural heritage sites, the Coliseum and the Lateran Baptistery in Rome. The sections to be examined of these vast monuments were chosen in collaboration with the authorities, also on the basis of

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Figure 1. The two mobile lidar systems deployed at the Coliseum site during the experiment. Measurements were carried out at night. Lund mobile system was placed about 65 m far from the façade; the CNR one about 18 m.

what was suitable for the extent of the study and for the progress of the work. These fluorescence lidar imaging studies, however, can be easily performed also on areas or parts situated at the top of the façade. The Flavian Amphitheatre in Rome, the Coliseum, inaugurated in 80 A.D. is a well known historical monument. The external wall is constructed of local travertine blocks, which today deals with numerous conservation concerns. The surveyed area in this study consisted of the sections L-LIV of the façade, (a width of about 33 m) situated northeast towards Via dei Fori Imperiali. In this section a set of smaller areas was selected and then scanned with the lidar systems. Previous conservation projects have been performed as well in this area, whose documentation gave valuable information in the interpretation of the data (Conti 2001). The selected areas were also chosen so as to involve both cleaned and heavily soiled travertine. The Lateran baptistery is situated in the south-east parts of Rome. Constructed with bricks and plastered, it was built in an octagonal shape during the fourth century A.D. During an intervention in the 1960s the façades were stripped of its plaster so that now we can see the masonry construction and the traces of reconstructions. Although the building has been in use since its inauguration few studies of the standing walls have been performed in modern times (Brandt 2002). The present external walls mainly consist of a complex texture of different types of historic bricks, restoration bricks and mortars with some insertions of marble spoils. Both campaigns were performed during night-time in order not to disturb the ongoing daily activities at the sites. Figure 1 shows the deployment of the two

Figure 2. The two mobile lidar systems during the experiment at the Lateran Baptistery. Measurements were carried out at night. Lund mobile system was placed about 40 m far from the façade; the CNR one about 15 m.

lidar systems at the Coliseum site, Figure 2 at the Baptistery site. The LTH lidar van was parked at the same location each night at a distance from the target of about 65 meters at the Coliseum site and 40 meters at the Baptistery site, and then it was driven away each morning. Similarly the CNR van was parked each night at a distance from the target of about 18 meters at the Coliseum site and 15 meters at the Baptistery site. The campaign was performed during a two-week period in late January and early February 2005 (between 9.00 PM and 5.00 AM). No samples were taken for this study; the analysis consisted of the fluorescence data, visual recordings and previous studies of the façades with written and image-based documentation.

3

METHODS

3.1 Fluorescence lidar Fluorescence lidar operating principles can be summarised as follows: a short ultraviolet laser pulse is directed to the target and the backscattered signal, including induced fluorescence, is collected by a telescope. The collected light is sent through a long-pass filter, used to reject the laser line and spectrometer higher orders, and then is fed to a spectrometer system with a linear array detector. By scanning the laser beam over the target, a fluorescence map can be acquired. A further description of the technique can be found in, e.g. Weibring et al. 2001, Weibring et al. 2003, Cecchi et al. 1992, Svanberg 2005.

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Experimental set-up

The measurements were performed using two different fluorescence lidar systems. Although none of the systems were originally intended for these kinds of measurements, they have both shown great potential in the field (Weibring et al. 2001, Lognoli et al. 2003, Raimondi et al. 1998, Grönlund et al. 2007). The Swedish LTH system is fully described in Weibring et al. 2003, and the Italian CNR system is introduced in Cecchi et al. 1992. The LTH system used a frequency-tripled Nd:YAG laser at 355 nm to induce fluorescence in the target. The signal was collected with a 40-cm-diameter Newtonian telescope and the detector was a spectrometer system with a 1024 channel CCD detector. The spectrum in the wavelength range 280–810 nm was collected in each point. The spot size on the target was about 2–4 cm and the system-target distance was ranging from 40–65 m, depending on the measured spot and the experiment site. Spectra were averaged over 200 shots corresponding to a measurement time of 10 seconds in each spot. The CNR system used a XeCl excimer laser at 308 nm to induce fluorescence in the target. The signal was collected using a 25 cm diameter telescope and the detector was a spectrometer with a 512 channel photodiode array detector. A complete fluorescence spectrum from 300 nm to 800 nm was achieved by combining a measurement in the 300–600 nm range with a measurement in the 500–800 nm range and by using proper long pass optical filters to reject the backscattered laser line and spectrometer higher order. The effectively measured spot size on the target was about 1.5–2 cm and the system-target distance varied between 15 and 18 meters, depending on the site. The measurement of each spot took about 1 minute. By a co-axial visible laser, each spot measured is referenced and stored together with a photograph. 3.3 Analysis methods A complete fluorescence spectrum in the VIS range is collected for each point of the scanned area and then processed to obtain a false colour image that can be compared with photographic and archive documentation or analysed together with the drawings and matrix from the stratigraphic studies. In situ survey for the investigation of the walls can also be useful sources of information to be combined with the outcomes of a fluorescence-based false colour map. Fluorescence-based false colour image of the examined areas can be obtained by using several processing methods: one of the simplest is to operate a ratio between selected spectral bands of the fluorescence spectra. More refined processing methods can instead rely on the use of multivariate statistical techniques, such as the Principal Component Analysis (PCA) and

the Cluster Analysis (CA), (see e.g. Esbensen 2002), that can be used to reduce the dimensionality of the data. The type of processing method, of course, is selected on the basis of the feature/parameter to be retrieved from the fluorescence spectra. For example, a preliminary assessment of the distribution of biodeteriogens on a surface can usually be obtained by plotting the ratio between two spectral bands, the first positioned inside the Chlorophyll fluorescence band (680 nm) and the other positioned outside. 4

RESULTS

4.1 The Coliseum The outcomes of the scanning of the monument showed good potential for the technique as a support for revealing possibilities for assessment of material diagnostics and information not visible to the naked eye, such as the distribution of protective treatments. Figure 3 shows the set of areas selected on the external travertine wall, comprising section L-LIV, where the scans were performed with the two lidar systems and a series of typical fluorescence spectra as detected on the surface of the monument. The fluorescence spectra refer to: (A) travertine stone in a heavily weathered area; (B) travertine affected by biological growth: biodeteriogens can be detected by means of the typical Chlorophyll a (Chl a) band centred at 680 nm, that is clearly visible in this spectrum; (C) old iron clamp dating back to the XIX century (conservation intervention by architect Valadier): the metal reinforcement structures are characterized by a very low fluorescence intensity; the elastically backscattered laser line at 308 nm is also visible. (D) grain mortar joint. (E) metal clamp treated with a corrosion-inhibiting coating as it can be inferred by the narrow fluorescence peak at about 380 nm. (F) old cement joint. Figure 4 shows the investigation of the presence of biological growth on a portion of the cleaned façade (Area 1 in Figure 3): in Figure 4a a detail of the investigated area, together with the grid indicating where the fluorescence measurements were effectively acquired, is shown; in Figure 4b the fluorescence-based map indicates the spots affected by biological growth as light colour pixels. In addition, the study included the assessment of travertine blocks dating back to the original structure and of blocks inserted during restoration interventions carried out in the 1950s. Protective treatments, specifically corrosive-inhibiting coatings, were found on metal reinforcement structures (see Fig. 3, spectrum E) and provided knowledge to estimate the extension of previous conservation interventions. A larger section of the studied parts of the façade had been previously cleaned with low-pressure fine water mist. Fluorescence analysis of these surfaces, compared to

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Figure 3. Areas selected on the Coliseum’s external wall, comprising gates L-LV, to be scanned with the two fluorescence lidar systems. Areas examined with the 308 nm excitation wavelength are black-squared (Area 1 refers to the area affected by biological growth, whose analysis is reported in Figure 4); areas examined with the 355 nm excitation wavelength are white-squared. Typical fluorescence spectra detected on the surface of the monument are also shown: (A) Fluorescence spectrum of travertine in a recently cleaned area. (B) Fluorescence spectrum showing the presence of biological growth, as it can be inferred by the typical Chlorophyll a (Chl a) peak at 680 nm. (C) Fluorescence spectrum of an old iron clamp dating back to the XIX century. (D) Fluorescence spectrum of grain mortar joint. (E) Fluorescence spectrum showing a narrow peak at about 380 nm due to the presence of a corrosion-inhibiting coating applied to metal clamps. (F) Fluorescence spectrum of an old cement joint. In some fluorescence spectra (C, D and E) residual elastic backscattering of the laser line (308 nm) is still visible.

heavily soiled areas, was performed in order to make a first assessment of the cleaned and weathered areas. Fluorescence data gave information which could be exploited, together with further diagnostic analytical techniques, to provide a full characterization of the patinas covering the surface. A thorough description of the results referring to the fluorescence scans of these areas is given in two separate works (Hällström et al. in prep a, Palombi et al. in prep.).

4.2 The baptistery This experiment was focused on the comparison of fluorescence lidar data, acquired on key sections of the brick masonry, and information obtained from visual inspection, previous documentation and stratigraphic studies of the historic layers concerning the external walls. The brick walls, in fact, constituted a real challenge for fluorescence lidar measurements due to their

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Figure 4. Analysis of an area partly affected by biological growth (Area 1 in Figure 3). The excitation wavelength was 308 nm. The distance of the sensor from the target was about 18 m and the spot size diameter on the target was about 2 cm. The spatial resolution (i.e. the distance between the centres of two following spots) was about 6 cm, both in horizontal and vertical directions. (a) shows the scanned area and reference grid automatically generated by the FLIDAR software to reference the acquired fluorescence image on the target. (b) shows a thematic map pointing out the distribution of biodeteriogens on the surface (light colours). The thematic map is obtained exploiting the typical fluorescence peak of Chlorophyll a contained in all photoautotrophic biodeteriogens (see a typical fluorescence spectrum of an area affected by biodeteriogens in Figure 3, spectrum B).

Figure 5. a) The set of areas selected on one of the brick walls of the Baptistery of S. Giovanni in Laterano to be scanned with the two fluorescence lidar systems. The area marked in black is that one shown in detail in the picture on the right. The analysis of a section of the scan, below the open window, also marked in black, is reported in Figure 6. b) shows a detailed picture of the examined area. This area comprehended a portion of an enclosed arch (partially visible on the right). c) shows a fluorescence-based analysis of the area: the map points out the presence of bricks (lighter colour) with peculiar fluorescence features (fluorescence peak at about 360 nm). The map was obtained by using the ratio between the integral of the spectral intensities in the 340–370 nm spectral range and the integral of the spectral intensities in the 400–430 nm spectral range. The excitation wavelength was 308 nm. The distance of the sensor from the target was about 15 m and the spot size diameter on the target was about 1.5 cm. The spatial resolution (i.e. the distance between the centres of two following spots) was about 4 cm in horizontal direction and about 3 cm in the vertical direction.

morphology and also to the numerous rebuilding and restoration phases. The areas for the lidar measurements have been chosen so as to include previous enclosures and openings in the façade and possibly more than one historic period.

The PCA-based analysis of the fluorescence data acquired on the selected areas allowed the characterization of some smaller areas featuring homogeneous fluorescence features whose location and extension in many cases were successfully compared with

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Figure 6. a) Part of the scanned area below the window opening of the brick façade (also indicated in Figure 5). The inserted graph shows a PCA-based analysis of the fluorescence spectra that separates the modern restoration bricks from the ancient rebuilding construction phase. The excitation wavelength was 355 nm. The distance of the sensor to the target was 40 m and the spot size diameter on the target was about 2 cm. B) shows a set of typical spectra acquired along the area marked in A): grey-coloured spectra refer to modern bricks, black-coloured spectra refer to the arch bricks.

stratigraphic studies. The architecture of the façade, the distribution of the bricks on the facade could be identified. A detailed description of the results is provided in Hällström et al. in prep. b. As an example, Figure 5 shows a high-spatial resolution scan (about 3 cm) carried out with the 308 nm excitation source: such fluorescence map provided very detailed data, required by this type of target, and the possibility to identify some bricks with peculiar fluorescence features (Fig. 5c). These data can supply a background to the interpretation of the stratigraphic layers, although the very micro level scale of the surface fluorescence data needs further analysis in comparison to the larger scale of traditional visual inspections. Figure 6 shows an area scanned with the 355-nm excitation: results indicate the restoration bricks inserted in the façade in modern times, below and under the arch of the enclosed opening. The window arch does not, however, have part of the original building construction but is part of a previous rebuilding phase in ancient times. 5

CONCLUSIONS

Remote fluorescence imaging has been successfully applied during joint Swedish-Italian campaigns on the Coliseum and on the Lateran Baptistery in Rome using mobile lidar systems. The Coliseum case study allowed to detect and to evaluate the growth extension of chlorophyllcontaining biodeteriogens, to create maps for the assessment of cleaned and weathered parts of the

façade and to evaluate the distribution of previouslyapplied conservation treatments on metal reinforcement structures. The Lateran Baptistery case study allowed to carry out a comparison between fluorescence based maps and stratigraphic studies regarding the location of historical bricks and rebuilt areas. Remote fluorescence imaging-based documentation and non-invasive diagnostics can provide conservation professionals with a first hand indication to plan maintenance and future preventive conservation of large-scale objects. Documentation over time can also be obtained without mounting scaffolding and taking samples, as changes and the speed of the deterioration processes can be monitored through repeated whisk-broom scanning at regular intervals over time.

ACKNOWLEDGEMENTS The authors are really indebted to all people that made it possible the realization of this experiment and supported them during the campaign; particularly to: Olof Brandt and Federico Guidobaldi; Pontificio Istituto di Archaeologica Christiana, Paolo Liverani; the Vatican Museums, Gunhild Eriksdotter and Barbro Santillo Frizell; the Swedish Institute in Rome. REFERENCES Brandt, O. 2002. Ipotesi sulla struttura del battistero lateranese tra Constantino e Sisto III. In F. Guidobaldi & A. Guidobaldi (eds), Ecclesiae Urbis. Atti del congresso

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internazionale di studi sulle chiese di Roma (IV-X secolo), Roma, 4–10 September 2000, Studi di antichità christiana pubblicati a cura del Pontificio Istituto di Archaeologia Christiana 59. Rome: Città del Vaticano. Cecchi, G. et al. 1992. FLIDAR: a multipurpose fluorosensorspectrometer. EARSeL Advances in Remote Sensing 1: 72–78. Conti, C. 2001. Anfiteatro Flavio: Il restauro delle superfici in travertino. Arkos: Scienza e Restauro 2: 22–27. Esbensen, K.H. 2002. Multivariate Data Analysis – in practice. Oslo: Camo AS. Grönlund, R. 2007. Fluorescence lidar imaging of historical monuments – Övedskloster, a Swedish case study. In J. Nimmrichter et al. (eds), Lasers in the Conservation of Artworks: LACONAVI Proceedings,Vienna, Austria, Sept. 21–25, 2005: 583–588. Berlin: Springer. Hällström, J. et al. in prep.a. Documentation of soiled and biodeteriorated façades: A case study on the Coliseum, Rome, using hyperspectral imaging fluorescence lidars.

Hällström, J. et al. in prep.b. A methodological merge of the stratigraphic and laser-induced fluorescence analysis of the Lateran Baptistery in Rome. Lognoli, D. et al. 2003. Fluorescence lidar imaging of the cathedral and baptistery of Parma. Applied Physics B. 76: 457–465. Palombi, L. et al. in prep. Hyperspectral fluorescence lidar imaging at the Colosseum, Rome: Elucidating past conservation interventions. Raimondi, V. et al. 1998. Fluorescence lidar monitoring of historic buildings. Applied Optics 37: 1089–1098. Svanberg, S. 2005. Fluorescence imaging of lidar targets. InT. Fujii & T. Fukuchi (eds), Laser Remote Sensing: 433–467. Boca Raton: CRC Press. Weibring, P., et al. 2001. Fluorescence lidar imaging of historical monuments. Applied Optics 40: 6111–6120. Weibring, P. et al, 2003. Versatile mobile lidar system for environmental monitoring. Applied Optics 42: 3583–3594.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Portable spectroscopic analysis of nitrates affecting to cultural heritage materials M. Maguregui, I. Martinez-Arkarazo, M. Angulo, K. Castro, L.A Fernández & J.M. Madariaga Department of Analytical Chemistry, University of Basque Country, Bilbao, Spain

ABSTRACT: Nitrates are highly damaging compounds affecting the conservation of Cultural Heritage (CH) materials. Depending on the nature of the CH materials and the nitrate compound impacting them, several nitrate salts can be formed. Sodium nitrate (nitratine), magnesium nitrate (nitromagnesite), calcium nitrate (nitrocalcite) and barium nitrate (nitrobarite) are claimed to be the most common decaying products of such materials. Such nitrate salts are identifiable by combining two totally non-destructive analytical techniques: Raman spectroscopy and portable micro X-ray Fluorescence spectroscopy (XRF). Although most of the times only the strongest band of each nitrate is detectable by Raman spectroscopy, in the 1060–1035 cm−1 spectral region and despite their closeness, it is often possible to distinguish among the different nitrate compounds. However, the help of XRF analysis is sometimes essential when (a) low concentration of those salts present in the sample, (b) there are two nitrate compounds with the main band at the same wavelength and (c) a mixture of several metal nitrates is present in the sample, giving wide bands in the Raman spectra.

1

INTRODUCTION

One of the main reasons for weathering of building stones is crystallization of soluble salts (Doehne 2002, Selwitz et al. 2002, Kamh 2005). These salts are originated by different pathways, even depending on the environment (Klemm et al. 2002), but they might also be part of the stone itself. Salts are transported in solution through a complex stone capillary system towards the inner parts of the stone. Salt decay processes in buildings, and in stone artworks in general, are caused when salts, instead of remaining in the outer parts of the stone or coming out to the surface, remain in the interior of the stone because they can exert internal pressures through crystallization-hydration cycles and thermal expansion. When these stresses exceed the compressive strength of the stone, disruption and decay occur irreversibly. Occasionally, these salts crystallize as efflorescences on the surface of the stone or as subefflorescences close to the surface. The higher is the porosity of the stone material the higher is the susceptibility to suffering from salt weathering. However, low porosity materials such as marbles are not free from undergoing such problems (Chabas et al. 2001). Nitrate salts are the most damaging compounds affecting to the conservation of Cultural Heritage

(CH) materials among those salts. The main nitrate sources in the environment are (a) the production of nitric acid aerosols through the NOx atmospheric gases formed by combustion of fossil fuels in power generation plant, urban traffic, maritime activities, etc. or by industrial activities like nitric acid production plants, production of nitrate based fertilizers (Massey 1999, Martinez-Arkarazo et al. 2007a) and (b) the production of ammonium nitrate (NH4 NO3 ) from agricultural runoff, decaying of organic matter, animal wastes, decomposition of dead animals (including human burials) or bacterial activity (Shrimali et al. 2001, Deutsch et al. 2006, Komorowska-Kaufman et al. 2006). Depending on the CH materials and the nitrate compound impacting them, several nitrate salts such as sodium nitrate (nitratine, NaNO3 ), magnesium nitrate (nitromagnesite, Mg(NO3 )2 .6H2 O), calcium nitrate (nitrocalcite, Ca(NO3 )2 .4H2 O) and barium nitrate (nitrobarite, Ba(NO3 )2 ) can be formed. The high solubility is one of the main characteristic of the nitrate salts, higher than the original materials of stone made artworks, in particular the carbonate based ones. For instance, decaying problems come when carbonate compounds become nitrates and these are progressively dissolved by the rain, giving as a result serious material losses similar to the one shown in the carbonate based sandstone of Figure 1,

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nitrates in Table 1) while the second one gives the elemental analysis of the material. Moreover, the chemical reactions producing the decaying products can be ascertained by combining such spectroscopic information (chemical composition of the original CH materials and new nitrate salts formed on it) with thermodynamic speciation. As an example of the usefulness of this new methodological approach we summarize the results of the analyses undertaken on stone materials affected by nitrates of different source, that is, stone materials exposed to aggressive environments and suffering from wet deposition of atmospheric NOx , and some examples of stone materials located indoor and affected by NH4 NO3 produced from wastes and organic matter decomposition processes.

Figure 1. Loss of material in carbonated sandstone in the façade of a palace house in the metropolitan Bilbao. Table 1. Raman characteristic bands of common nitrate compounds affecting stone materials. Compound

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belonging to a Palace House located in the main channel of Bilbao’s estuary and affected by maritime traffic (Martinez-Arkarazo et al. 2007a). Due to their high solubility, nitrate salts are easily removed from the stone surface, or they go into the inner parts of the stone material, by the action of water (rain, infiltrations, floods, etc.). That is why the zones protected from rain washing are the best candidates to find nitrate compounds in such concentrations as to be detected and identified. Nowadays, the identification of pollutants affecting to CH materials could be performed by in situ analyses and using totally non-destructive analytical techniques. In this work, we propose the combination of (a) portable fiber optic Raman microprobe spectroscopy and (b) portable micro X-ray Fluorescence spectroscopy (XRF) for the characterization of the cited specimens. The first one shows a high selectivity for nitrates over other anions, giving the most intense Raman signatures of the different nitrate compounds in the 1060–1035 cm−1 spectral region (see Raman characteristic bands of the most common

EXPERIMENTAL

The analysis of all samples was performed using a portable Raman microprobe, Renishaw RA 100 spectrometer with an excitation wavelength of 785 nm (diode laser) and a CCD detector (Peltier cooled). The nominal laser power was 150 mW at the source. Neutral filters allow us to work at 1%, 10% or 100% of that power in order to prevent the thermal decomposition of the samples. A microscope lens built-in the microprobe (long range type x4, x20 or x50) allowed the laser beam to be focussed on approximately 10–200 µm spots, depending on the lens used. The system is provided with a video camera controlled by a joystick that helps the user in the focussing process. The spectra were collected with a resolution of 4 cm−1 in the range 200–2200 cm−1 (wide enough for the detection of nitrates characteristic Raman bands) with integration times in the range of 1-200 seconds and accumulating several scans from each spectrum to improve the signal-to-noise ratio. Data acquisition was carried out with the Spectracalc software package GRAMS (Galactic Industries, Salem, NH, USA) and the analysis of the results undertaken with the Omnic software by Nicolet (Madison, Wis., U.S.A). Ultramobile equipments operable by an only person are available nowadays. These systems allow in situ analysis (without the need of sampling) even in hardly accessible areas. However, the miniaturization results in the loss of some characteristics of the Raman spectrometer which influences in the detection of low scattering compounds such as nitrates (Martinez-Arkarazo et al. 2007b). A Roentec (currently Bruker AG) ArtTax µ-XRF portable equipment, consisting on a X-ray tube with molybdenum anode working at a maximum voltage/current of 50 kV/0.6 mA and a special Xflash detector (5 mm2 ) was used for elemental analysis. The X-rays are collimated by a Tantalum collimator

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with a diameter of 0.65 mm. The measuring head of the equipment implements a CCD video camera that allows us to focus on the sample by a motorised XYZ positioning unit controlled by the computer. Acquisition parameters were fixed on 800 seconds with 600 W power and 50 mA current for a 200 µm2 analysis area and using a helium flow in order to allow the detection of light elements. Among samples located outdoor directly exposed to the environment, structural elements of civil buildings and decorative elements of funerary monuments were studied, while some wall paintings (in several churches) were selected as samples protected from the direct action of environmental stress. All of them were located in the northern area of the Iberian Peninsula. The Raman microprobe and micro XRF analysis were carried out in situ or directly on the samples (without any previous treatment) when sampling was performed.

3 3.1

RESULTS AND DISCUSSION Environmentally exposed materials

As already mentioned, protected zones of decayed stones are the best candidates for nitrates to be found. However, in most cases they do not appear as the only degradation product, but together with gypsum, they form the typical black crusts. Most authors report only gypsum as the main inorganic degradation product composing the black crusts, but in a recent work we have determined high concentration of nitrates in such black crusts, covering the façade in a Palace House located at the end of the main channel of Bilbao (Martinez-Arkarazo et al. 2007a). Figure 2 shows two examples of the different nitrate compounds found in the stones materials, limestone and carbonate sandstone, of that Palace House. The spectrum labeled (a) shows characteristic bands of amorphous carbon (deposited as soot), calcite of the original composition as well as a band at 1048 cm−1 measured in the black crusts of a limestone protected from the rainfall. The cited band was not possible to assign unambiguously to a given nitrate compound, since it could belong to both calcium and potassium nitrates according to the wavenumbers of their characteristic Raman bands summarized in Table 1. Unfortunately, the XRF analysis of this sample showed the presence of both calcium (which is obvious, since the original stone was composed of calcium carbonate) together with minor quantities of potassium. Hence, in this particular case it was not possible to identify the cation associated to the nitrate. In order to clarify this point complementary experimental information would be necessary, but the presence of nitrocalcite is more probable than that of

Figure 2. Raman spectra of stone materials of the facade of a palace house showing (a) calcite (1085, 712 cm−1 ), soot (1640, 1300 cm−1 ) and CaNO3 4H2 O or KNO3 in a limestone; (b) calcite, gypsum (1008 cm−1 ), nitratine and quartz (464 cm−1 ) in a protected zone of a sandstone.

nitre because there is a higher in the limestone amount of calcium than potassium. Spectrum (b) in Figure 2 shows bands of α-quartz (characteristic Raman band at 464 cm−1 ) and nitratine, apart from the main band of calcite in the black crusts of a carbonate sandstone sample of the Palace House. The main band of gypsum at 1008 cm−1 and the two bands of soot are shown in the spectrum as well. Nitratine is supposed to be formed from acid attack of nitrogen oxides to the calcium carbonate of the stone in the presence of marine aerosols, acting probably as the sodium source. Such formation can be exemplified through reactions (1) and (2).

The formation of nitrocalcite would start with the formation of the nitric aerosol from NOx gases (reaction (1)) followed by reaction (3).

The nitrate compounds formed following reactions (1) to (3) are the most common in samples exposed to atmospheric stress. However, nitromagnesite (in dolomitic or magnesium-rich stones, mortars, plasters, etc.) and nitrobarite (in stones surfaces affected by strong road traffic) have been also determined in the analyses of other samples. An example of nitromagnesite presence in a magnesium rich calcium carbonate based mortar is shown in Figure 3 (spectrum a). These nitrate species would be formed following a reaction analogous to (3). Notice that nitratine (1065 cm−1 ) appeared together with nitromagnesite (1058 cm−1 ) in this measuring point.

179

Figure 3. Raman spectrum of (a) nitromagnesite and nitratine appearing jointly in a mortar sample of the cemetery of Getxo, Biscay; (b) one of the measuring points of the wall painting appearing NH4 NO3 and iron oxide yellow (550, 395, 302 and 241 cm−1 ).

3.2

Figure 4. Raman spectrum of a mortar sample from the interior of a church. Calcite (1085, 712 cm−1 ), gypsum (1008 cm−1 ), non identified mixture (wide band centered at 1032 cm−1 ) and Ca(NO3 )2 4H2 O, assigned thanks to the information on the elemental composition given by the XRF analysis in Figure 5.

Indoor materials

Regarding indoor stone materials affected by nitrates, similar cases can be found. The formation of nitrate salts in those samples (out of the direct impact of atmospheric pollutants) could not be explained by reactions (1) to (3), but by other mechanisms. The presence of acid aerosols in indoor environments is not the cause of the degradation of the original materials. In these environments other precursors are the responsible for the decaying. Hence, nitrates do not appear associated to soot and gypsum formations. As mentioned in the introduction, the formation of nitrate salts could be the result of reactions between original materials and ammonium nitrate coming from different sources. For instance, ammonium nitrate was also recognized in several measuring points of the same wall painting. Figure 3 (spectrum b) shows the strongest characteristic Raman band of ammonium nitrate, which was supposed to be incorporated to the wall painting by infiltration of waters coming from the cemetery located behind the north wall of the church. Figure 4 shows the Raman spectrum of the bulk of a wall painting, where gypsum and calcite bands are visible. An additional band in the nitrate region is also visible. In this case, such band at 1048 cm−1 was assigned to nitrocalcite since the XRF analysis, which is able to detect trace concentration levels of any element, revealed the absence of significant amounts of potassium (see XRF spectrum in Figure 5). The Raman and XRF spectroscopy results shown in Figures 6 and 7 respectively are another example of characterization by combination of both techniques. The low intensity band at 1045 cm−1 in the Raman spectrum of Figure 6 was assigned without any doubt to nitrobarite thanks to the information given by the XRF spectrum, where the presence of barium was clearly recognized.

Figure 5. XRF spectrum of the mortar sample corresponding to the Raman spectrum shown in Figure 3. There is an absence of potassium and high amounts of calcium.

Figure 6. Raman spectrum of a mortar sample from the interior of a church in northern Iberian Peninsula. Calcite (1085, 712, 281 cm−1 ), iron oxide red (684, 408, 233 cm−1 ) and Ba(NO3 )2 (1045 cm−1 ) in agreement with XRF results shown in Figure 7.

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only one recorded in this kind of samples) at the same wavelength. This is the case of nitrocalcite and nitre with the main Raman band at 1048 cm−1 . (c) In samples with nitrate mixtures, which appear as wide bands in the Raman spectrum (Maguregui 2007). This methodology was successfully used in the characterization of stone and mortar samples, where several nitrate salts such as sodium nitrate, magnesium nitrate, calcium nitrate and barium nitrate were recognized. Figure 7. XRF spectrum appearing Ba bands, corresponding to sample with the Raman spectrum shown in figure 6.

5 ACKNOWLEDGEMENTS

Moreover, it is remarkable that the elements shown in the XRF spectrum agreed with the assignation of the Raman bands; that is, calcium (Ca) belongs to the bulk composition, while iron (Fe) is due to the presence of iron oxide, widely used as a red pigment for ages. Reactions (4) and (5) show the formation hypothesis for the calcium and barium nitrate salts starting from the infiltrated ammonium nitrate salt. Both carbonates are part of the mortars where the wall painting is located. Barium is thought to be an impurity of the original calcium material.

4

CONCLUSIONS

Nitrate salts formed from the attack of NOx gases in carbonated stones exposed to polluted environments (but protected from rain washing) and those formed from the reaction with NH4 NO3 are similar and identifiables by combining two totally non-destructive analytical techniques: Raman spectroscopy and portable micro X-ray Fluorescence spectroscopy (XRF). Although most of times only the strongest band of nitrates is visible in Raman in the 1060–1035 cm−1 spectral region, despite their closeness, it is often possible to distinguish among the different nitrate compounds. However, XRF analysis could be essential in the following situations: (a) When low concentrations of those salts have to be determined. Due to the low Raman scattering of nitrate compounds, significant amounts are necessary in order to generate a measurable spectrum. (b) In order to distinguish between two nitrate compounds with the main Raman band (sometimes the

This work has been financially supported by project CTQ2005-09267-C02-01/PPQ of the Spanish National R+D Programme (MEC). M. Maguregui acknowledges her fellowship from the University of the Basque Country. Dr. K. Castro is grateful to the Ministry of Education and Science (MEC) for his contract (PTA 2003-02-00050). REFERENCES Chabas A. & Jeannette D. 2001. Weathering of marbles and granites in marine environment: petrophysical properties and special role of atmospheric salts. Environmental Geology 40: 359–368. Deutsch, B., Kahle, P. & Voss, M. 2006. Assessing the source of nitrate pollution in water using stable N and O isotopes. Agronomy for Sustainable Development 26 (4): 263–267 Doehne, E. 2002. Salt weathering: a selective review. Geological Society Special Publication 20 5(Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies): 51–64. Kamh, G.M.E. 2005. The impact of landslides and SALT weathering on Roman structures at high latitudes- Conway Castle, Great Britain: a case study. Environ. Geol. 48: 238– 254. Klemm, W. & Siedel, H. 2002. Evaluation of the origin of sulphate compounds in building stone by sulphur isotope ratio. Geological Society Special Publication, 205 (Natural Stone,Weathering Phenomena, Conservation Strategies and Case Studies): 419–429 Komorowska-Kaufman M., Majcherek H. & Klaczyn’ski E. 2006. Factors affecting the biological nitrogen removal from wastewater. Process Biochemistry 41: 1015–1021. Maguregui M. 2007. Estudio del impacto medioambiental sobre ladrillos históricos mediante una nueva metodología analítica. Master Thesis (Environmental Contamination and Toxicology). University of the Basque Country (EHU/UPV), Leioa, Spain. Martinez-Arkarazo, I., Angulo, M., Bartolomé, L., Etxebarria, N. & Madariaga, J. M. 2007a. An integrated analytical approach to diagnose the conservation state of building materials of a palace house in the metropolitan Bilbao (Basque Country, North of Spain). Anal. Chim Acta 584: 350–359.

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Martinez-Arkarazo, I., Smith D.C., Zuloaga O., Olazábal M.A. & Madariaga, J. M. 2007b. Evaluation of ultramobile Raman instrumentation efficiency for the characterisation of degradation products on carbonate-based stones of Cultural Heritage buildings located in a coastal area of Biscay, North of Spain. IV International Congress on the Application of Raman Spectroscopy in Art and Archaeology. Modena. Massey, S.W. 1999. The effets of ozone and NOx on the deterioration of calcareous stone. The Science of the Total Environment 227: 109–121.

Selwitz C. & Doehne E. 2002. The evaluation of crystallization modifiers for controlling SALT damage to limestone. Journal of Cultural Heritage 3: 205–216. Shrimali M.& Singh K.P. 2001. New methods of nitrate removal from water. Environmental Pollution 112: 351–359.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

A study of laser cleaning parameters using a portable system on a gargoyle of the Torres de Serranos in Valencia, Spain B. Sáiz Department of Graphic Expression in Architecture, Institute for Heritage Restoration, Polytechnic University of Valencia, Spain

M. Iglesias Materias Primas Abrasivas (MPA), Barcelona, Spain

ABSTRACT: A laser technique was applied with the aim of determining the viability of the technique for cleaning an original gargoyle from the Torres de Serranos in Valencia. The laser system used was the Maestro, a portable Q-switched Nd:YAG pumped by diode and operating at a wavelength of 1064 nm. The study was carried out with optical microscopy (OM), scanning electron microscopy-energy dispersive X-ray (SEM/EDX), X-ray diffraction (XRD) and colorimetric analysis using a contact spectrophotometer. Thorough petrographic characterization and a deep study of the gargoyle alterography before and after cleaning were performed. The research carried out allowed the optimal laser parameters to be determined for the elimination of the contamination layers.

1

INTRODUCTION

This paper describes the research carried out to determine the feasibility of the laser cleaning of an original gargoyle in the Torres de Serranos, Valencia, a civilmilitary masterpiece of the Valencian Gothic period, built between 1392 and 1398. The methodology applied involved a historical and analytical study with the multiple aim of studying the building’s history, characterizing the original and non original materials, evaluating its state of conservation and choosing the most successful cleaning technique. Both the original construction materials and the numerous alterations and restorations were dated to ascertain whether the gargoyle was an original piece. The historical research provided information on the type of ornamental stones used for the gargoyle. Analytical studies confirmed not only the historical results but also the state of conservation and the patinas formed from the time of construction until the present day. The piece had been partially conserved since all the gargoyles of the monument were replaced in 1985. The state of conservation of the stone in the gargoyle was decisive in the choice of laser cleaning as the best way to ensure the conservation of the original material after the cleaning operation.

Figure 1. Gargoyle and laser machine.

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Figure 2. Thin section stone gargoyle with bioclastic elements, X40, NX+C.

Figure 3. Cross section of a gargoyle sample. A: stone, B: reddish layer and C: black crust.

The cleaning was under control at all times due to the low energy applied at each pulse of the laser diode used for the experimental procedure. The aim of the research was to test the effectiveness of laser irradiation on stone seriously damaged by exposure to the atmosphere and to design a cleaning protocol to ensure non-harmful cleaning of the original gargoyle (Sáiz 2003). 2

PRELIMINARY STUDIES

A preliminary investigation was carried out with the aim of characterizing the stone in the gargoyle and detecting any changes produced as a consequence of being exposed to atmospheric contaminants. Characterization of the mineralogical composition of the stone and deposition layers is decisive in selecting laser cleaning parameters (Rodriguez-Navarro et al. 2001). X-ray diffraction analysis (XRD) (Philips 1830 DMP 2000 model using the US International Center for Difraction Data norms) showed the stone to be of calcarenite origin. A thin section observed through a petrographic microscope (Nikon with low position and polarized system illumination with a Microflex UFX-Nikon photographic system) revealed the presence of oolites and bioclastic elements (Fig. 2). The petrographic study revealed the stone to be biocalcarenite. Cross section and thin samples analysis by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM-EDX) (JEOL, JSM 6300 model with LinkOxford-Isis microanalysis system) showed the presence of two layers with variable thickness, density and particle sizes. Figure 3 shows the stone (A), the reddish layer (B) characterized by EDX microanalysis as a sulfate layer with an amalgam of particles (Si, Fe, Na, K, Ti and S) of morphological diversity and varied sizes, estimated as being between 0.1–25 µm. Thickness of

Figure 4. Accessory and secondary minerals. A: glauconite, B: ferrous quarz, C: ferrous silicate and D: iron oxides over calcite crystal.

the layer varied between 0.2 and 0.9 mm. Layer B was considered a non original patina formed by the deposition of atmospheric and ground particles. The historical study also revealed that it could have been formed in the period before the ground surrounding the Serranos Tower was covered with asphalt. The third layer (C) presented a thickness between 0.1 mm and 0.3 mm in the samples analyzed, although this reached 5 mm in the most severely damaged areas of the gargoyle. EDX analysis identified the compound of layer C as a mixture of atmospheric particles consisting of coal dust from urban pollution and ground minerals, including calcite, quartz iron oxides and other ambient particles. All these compounds were intermixed in a sulfate mass with small quantities of chlorides and nitrites. A black crust formed the third superficial layer (C). Accessories and secondary minerals were characterized in order to identify chromatic alteration due to irradiation (Fig. 4).

184

Table 1. Energy per pulse at the output beam laser for the repetition rate tested. E per pulse (mJ) Repetition rate (Hz)

1

2

3

4

Max.

10 100 150 200

6.0 7.0 6.7 5.7

8.0 9.0 7.8 7.0

9.0 10.5 8.7 8.0

11.0 11.0 9.3 8.5

12.0 11.5 10.0 9.0

Figure 5. One of the four matrix quarry stones with carbon powder prepared for evaluation of the effects of laser irradiation.

The stratigraphic and petrographic studies provided data on the stone and deposition layers and thus a stone very similar to that in the gargoyle could be selected from the quarry for irradiation prior to testing the gargoyle samples. Colorimetric measurements were performed using a contact spectrophotometer (Minolta CM-508 i). The studies allowed us to determine the effects of laser irradiation on the original stone, once the black crust and the patina had been eliminated. 3 3.1

EXPERIMENTAL METHODS Laser system

The laser system used for the cleaning was the Maestro from MPA, a portable diode-pumped Q-switched Nd-YAG, pulse laser operating at a wavelength of 1064 nm; pulse duration of 4 ns, repetition rate range 10–200 Hz; pulse energy range 5–30 mJ/pulse; maximum average power 6W. The laser beam is delivered onto the surface by means of an optical fiber. The average working distance was calculated to be 2.4 ∼ = cm. This means a focus beam diameter of 1.5 mm, and working spot size of 1.8 mm2 . 3.2

Laser cleaning procedure

In order to evaluate the effects of laser cleaning on the surface of the gargoyle, a similar type of stone was laser irradiated, chromatically measured and systematically evaluated (Labouré et al. 1999). Four test matrices of 5 cm × 7 cm in size were made and an innocuous carbon powder layer was formed as shown in Figure 5, producing ten 1 cm2 squares for wet (top squares) and dry irradiation (lower squares). The carbon powder layer provided a guide to avoid irradiating the same area twice. This simulates the cleaning procedure once the black crust has been eliminated. The four

Figure 6. Cleaning matrix on the gargoyle to the test effect of irradiation at 10, 100, 150 and 200 Hz.

matrices were irradiated with all energies per pulse values for the four repetition rates of the laser (Table 1). All the irradiation procedures were performed on wet and dry surfaces. The average of the total energy per spot was calculated for each square. Colorimetric measurements and a deep optical microscope study of the surface topography were carried out before, during and after laser treatment. The irradiation of the quarry stone provided the necessary information for the selection of the best irradiation parameters to use in the test on six samples taken from the gargoyle before actually testing the piece itself (Fig. 6). The size of the samples meant the cleaning could be controlled by OM, which was also

185

Table 3. Instrumental and working conditions for gargoyle sample G1 to eliminate the black crust conserving the reddish layer.

Table 2. Instrumental and working conditions for irradiation at different repetitions rates on quarry stone. Instrumental conditions

Working conditions

EA spot EA spot f E Fluence t (dry) (dry) t (wet) (wet) (J) (s) (J) (Hz) Position (mJ) (J/cm2 ) (s) 10 10 100 100 150 ∗

1 Max. 2 4 Max

6.0 12.0 9.0 11.0 10.0

0.34 0.68 0.51 0.62 0.56

150 95 40 37 15

0.15 0.20 0.63 0.71 0.39

113 89 36 25 14

0.11 0.18 0.57 0.48 0.31

used to evaluate the ablation threshold for the black crust and reddish layer. Laser irradiation was evaluated directly on the surface through a cross section of each sample, as can be appreciated in Figures 8 and 9. The gargoyle stone presented areas with very different characteristics and different thicknesses for both the black crust and reddish layer. It was therefore decided to prepare a cleaning matrix of the gargoyle (Fig. 6) for irradiation with all the possible laser parameters (Table 1) calculating fluences, times of irradiation on dry and wet surfaces and averaged total energy per pulse, with the aim of designing the laser cleaning protocol. The results obtained by laser irradiation on the quarry stone and gargoyle samples proved useful in choosing the different affected areas on the gargoyle for irradiation with the different parameters in order to choose the best cleaning method without damaging the original stone (Cooper et al. 1995).

4.1

Working conditions

f (Hz)

Position

E (mJ)

Fluence (J/cm2 )

t (dry) (s)

EA spot (dry) (J)

10 10

1 3

6.0 9.0

0.34 0.51

11 32

0.04 0.16

∗

EA is the averaged total energy per spot.

4

Instrumental conditions

RESULTS AND DISCUSSION Laser irradiation on quarry stone

The preliminary results on the quarry stone coated with carbon powder, testing the energy per pulse at five different positions for the four repetition rates (Table 1) showed that none of the fluence values with different irradiation times revealed chromatic variation or topographic alteration with loss of mineralogical particles on the surface. The averaged total energy per spot values were achieved at a repetition rate of 100 Hz (Table 2). Laser test irradiation at a fluence of 0.34 J/cm2 or lower was not sufficient to eliminate the carbon powder, even on the wet surface, on which traces of carbon remained. Fluences of 0.45 J/cm2 , 0.51 J/cm2 , 0.62 J/cm2 and 0.68 J/cm2 not only eliminated the carbon layer without any problem but also no morphological damage was caused to the surface and no chromatic

EA is the averaged total energy per spot.

alteration was detected. All the squares of the four matrices were examined and photographed by OM and chromatically measured before and after cleaning. The highest values of total energy per spot needed to eliminate the carbon layer were quantified as 0.71 J at a repetition rate of 100 Hz on a dry surface and the minimal variances in the chromatic analysis showed that the parameters used to test laser cleaning were below the damage threshold of the quarry stone. The best results in the cleaning tests (considering time and total averaged energy) were achieved by previously moistening the surface, at a fluence of 0.56 J/cm2 , at a repetition rate of 150 Hz, in the shortest time (14 s) and with a total averaged energy per spot of 0.31 J. The selective removal of the fine carbon layer with no secondary irradiation effects shows that the threshold damage for this type of stone is above the laser irradiation parameters used. 4.2 Laser cleaning of gargoyle sample The results obtained from the irradiation of the quarry stone were used for the selection of laser parameters. Laser cleaning was tested on six gargoyle samples, showing the feasibility of eliminating the black crust while conserving the reddish patina (this was an interesting result, but, in fact, was not necessary because the layer was not original) and the possibility of eliminating both layers. Table 3 shows the parameters used for the sample test. Figure 7 shows the result of the irradiation on a dry surface with a repetition rate of 10 Hz. During the operation, the surface was observed under OM to get the best approximation to the reddish layer. The black crust presented depths that varied between 0.1 and 0.2 mm, and the red patina between 0.2 and 0.4 mm. The black crust threshold ablation was estimated to be irradiating at a fluence of 0.34 J/cm2 . Irradiation at a fluence of 0.62 J/cm2 on a wet surface showed the laser ablation of both the black and reddish layer. Figure 8 shows the cross section of the wet irradiation result.

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Figure 7. Cross section of gargoyle sample G1. Dry laser cleaning of the black crust conserving the red layer.

Figure 9. Laser cleaning at a repetition rate of 100 Hz. Upper side: dry surface and lower side: wet surface.

Table 5. Instrumental and working conditions for irradiation at a repetition rate of 100 Hz on wet and dry areas of the gargoyle. Instrumental conditions

Working conditions

EA spot EA spot f E Fluence t (dry) (dry) t (wet) (wet) (J) (s) (J) (Hz) Position (mJ) (J/cm2 ) (s) 100 100 100 100 100 ∗

1 2 3 4 Max

7.0 9.0 10.5 11.0 11.5

0.40 0.51 0.59 0.62 0.65

60 58 58 55 43

0.74 0.92 1.07 1.06 0.87

56 52 76 47 39

0.69 0.82 0.92 0.91 0.79

EA is the averaged total energy per spot.

Figure 8. Cross section of gargoyle sample G6. Right side shows laser cleaning of black crust and reddish layer.

4.3 Laser cleaning of gargoyle stone Table 4. Instrumental and working conditions for gargoyle sample G6 to eliminate the black crust and the reddish layer. Instrumental conditions

Working conditions

f (Hz)

Position

E (mJ)

Fluence (J/cm2 )

t (wet) (s)

EA spot (wet) (J)

100

1

11.0

0.62

63

0.99

∗

EA is the averaged total energy per spot.

In this sample, the thickness varied between 0.2 mm and 1.3 mm for the black crust and 0.1 mm and 1 mm for the reddish layer. Table 4 shows the irradiation parameters used to eliminate both layers without damaging the original stone surface. No loss of mineralogical particles was observed under OM during the laser irradiation.

Four areas were prepared, as shown in Figure 6, for irradiation on dry and wet surfaces with the parameters given in Table 1. Figure 9 shows the different results obtained from the irradiation at a repetition rate of 100 Hz. Irradiation parameters are shown in Table 5. Best cleaning results were obtained at fluences achieved at a repetition rate of 100 Hz. The three last upper and lower squares of Figure 9 showed the most intensive cleaning with no trace of dust particles. From the restoration point of view, the results obtained on the first two squares offered the best chromatic cleaning result to the naked eye. Both cases achieved complete elimination of the contamination layer, but it was not considered advisable to continue to the point of achieving the colour of the quarry stone. The local population sometimes reject old building cleaning, so in general, cleaning criteria should be carefully selected.

187

5

Figure 10. Laser cleaning at a repetition rate of 150 Hz. Upper side dry and lower side wet.

Table 6. Instrumental and working conditions for irradiation at a repetition rate of 100 Hz on wet and dry areas of the gargoyle. Instrumental conditions

∗

1 2 3 4 Max

6.7 7.8 8.7 9.3 10.0

0.38 0.44 0.49 0.53 0.56

56 52 36 37 47

0.99 1.07 0.83 0.91 1.24

50 49 31 31 41

Successful ablation of the black crust and reddish layer was carried out using the minimum energy per spot to reduce the possibility of causing laser-induced damage. The laser cleaning of this particular architectural masterpiece was designed after having analyzed the effects of laser irradiation on quarry stone and samples of the gargoyle stone in a preliminary test. The laser parameters tested in the gargoyle matrix provided data for the laser cleaning protocol, taking into account the optical laser parameters, threshold ablation of black crust and the reddish layer and the properties of the stone, in accordance with its state of conservation. The laser parameters achieved for this portable Q-switched diode-pumped Nd:YAG demonstrated its selectivity in offering the possibility of eliminating the black crust while conserving the reddish layer. Its portability and weight of only 16 Kg make this particular laser a very useful cleaning tool. ACKNOWLEDGEMENTS

Working conditions

EA spot EA spot f E Fluence t (dry) (dry) t (wet) (wet) (J) (s) (J) (Hz) Position (mJ) (J/cm2 ) (s) 150 150 150 150 150

CONCLUSIONS

0.88 1.01 0.71 0.76 1.08

The restoration work was supported by the Heritage Conservation Institute of the Polytechnic University of Valencia and the City Council of Valencia. We would like to thank the R&D&I Linguistic Assistance Office at the Polytechnic University of Valencia for their help in revising and correcting this paper. REFERENCES

EA is the averaged total energy per spot.

The irradiation results at a repetition rate of 150 Hz (Fig. 10) offer an interesting effect. In the first three squares, in both dry and wet cleaning, it can be observed that the black color of the contamination layer has been completely eliminated and the crust partially eliminated. This assessment can be carried out from close up with the naked eye. A distance of 50 cm is enough to appreciate a cleaned surface without variations in the surface topography. Keeping the discolored sulfate crust means leaving a protective layer against the attack of new contaminants. Irradiation parameters are given in Table 6.

Sáiz, B. 2003. Limpieza de materiales pétreos con la técnica láser. Determinación de los parámetros de limpieza láser para una Gárgola de las Torres de Serranos de Valencia. (Tesis doctoral: Universidad Politécnica de Valencia). Rodriguez-Navarro, C., Elert, K., Sebastián, E., Esbert, R.M., Grossi, C.M., Rojo, A., Montoto, M., Ordaz, J., Alonso, F.J., Escudero, C. & Pérez, M.C. 2001. Q-switched Nd-YAG laser cleaning of white marbre: induced damage evaluation trough combined use of XRD and SEM. Lasers in the Conservation of Artworks, LACONA IV; Book of Abstracts, Paris, 49–52. Labouré, M., Bromblet, P., Orial, G., Wiedemann, G. & Simon-Boisson, C. 1999. Assessment of laser cleaning rate on limestones and sandstones. Lasers in the Conservation of Artworks, LACONA III; Book of Abstracts, Florence, 21–27. Cooper, M.I., Emmony, D.C. & Larson, J. 1995. Characterization of laser cleaning of limestone. Optics & Laser Technology, 27: 69–73.

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Laser Cleaning of Monuments and Sculptures

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Castle of Quart, Aosta Valley: Laser uncovering of medieval wall paintings S. Siano Istituto di Fisica Applicata “Nello Carrara” – CNR, Sesto Fiorentino, Italy

L. Appolonia & A. Piccirillo Laboratorio Analisi Scientifica, Soprintendenza per i Beni e Attività Culturali, Regione Autonoma Valle d’Aosta, Aosta, Italy

A. Brunetto Restoration firm, Vicenza, Italy

ABSTRACT: We report a cleaning optimisation study aimed at solving the conservation problem of the medieval wall painting cycles discovered in the Castle of Quart, Aosta Valley. The paint layers were almost completely covered by whitewash and organic layers applied in the past. Cleaning tests and associated material characterisations provided evidences that the combination of Nd:YAG lasers emitting pulses with suitable durations can successfully uncover the painted surfaces with minimised risk of damage to the paint layers as compared with standalone mechanical and chemical approaches.

1

INTRODUCTION

The growing number of published papers concerning the laser cleaning of painted surfaces indicates an increase of interest on the topic from both the scientific and professional standpoints. In the present LACONA congress, for the first time, the number of contributions concerning the conservation of paint layers exceeds those dedicated to stone cleaning. This is a clear evidence of a massive effort of the scientific community to address the variety of complex problems encountered in the conservation of painted surfaces, which in general represents the most difficult challenge because of the relatively high sensitivity of pigments and binders to the laser irradiation. The removal of scialbaturas, such as lime and/or gypsum whitewashes, as well as of pigmented thin plasters including the possible presence of organic binders, represents a very difficult aim in the laser cleaning of wall paintings. The layers to be removed usually exhibit mechanical and thermal resistances comparable or higher than the ones of the paint layers underneath. Furthermore, in the case of whitewashes there is not a favourable reflectance jump for any of the possible laser wavelengths, i.e. the reflectance of the whitewash is higher or similar to that of the painted surface.

As is well known, the potential of excimer and YAG lasers to approach the cleaning of paintings has been investigated since the nineties (Zergiotti et al. 1997, Salimbeni et al. 1998, Gaetani et al. 2000, De Cruz et al. 2000). The former systems appear too complex, cumbersome, and expensive to foresee any optimistic perspective in the near future for the cleaning of wall paintings, even to remove organic layers where, in specific conditions, they could provide some advantages with respect to solid state lasers (Teule et al. 2003). This experimentation is hence strongly focused on the potential use of Nd:YAG (1064, 532, 355, 266 nm) and Er:YAG (2.94 µm) lasers. Several experimental works were carried out, which investigated the potential of Q-switched (QS) Nd:YAG lasers fundamental (1064 nm) and Free Running (FR) Er:YAG laser. Beside some not encouraging studies, which focused on discoloration effects (Pouli et al. 2000, 2003, Sansonetti et al. 2000), the tests showed that the most interesting feature of 1064 nm wavelength was the relatively high alteration fluence of various pigments and paint layers, which allowed broader operative irradiation ranges with respect to shorter wavelengths (Chappe et al. 2003, Hildenhagen et al. 2005, Schnell et al. 2005). Despite not available, also the damage thresholds of the main pigments at 2.94 µm, which are dominated by the strong OH

191

bond optical absorption, are expected to be lower than the corresponding ones at 1064 (by assuming the same pulse duration). Actually, the main feature of the Er:YAG laser irradiation is the very short optical penetration, which is in the order of 1 µm in water (De Cruz et al. 2000), as well as in most of the materials encountered in stratification and paint layers. Thus, it was mostly used to produce the photothermal decomposition of aged varnishes in order to either simplify their mechanical and chemical lighting or complete removal (Bracco et al. 2003). Following their successful application in the restoration of metal and stone masterpieces (Siano et al. 2001, 2003) “intermediate pulse duration lasers”, namely Long Q-switched (LQS) and Short Free Running (SFR) Nd:YAG lasers (Margheri et al. 2000, Salimbeni et al. 2003) were recently proposed to approach wall painting conservation problems. The available commercial versions emit pulses with duration of 120 ns (LQS) and 40–120 µs (SFR). These systems were successfully used for removing whitewashes and aged Paraloid from wall paintings (Siano et al. 2006, 2007). Here, we present insights on this specific topic (i.e. removal of scialbaturas) through the discussion of a new case study. We investigated the laser cleaning optimisation of the medieval wall painting cycles found in the donjon (keep) of the Castle of Quart, Aosta Valley. After compositional and micro-structural characterisation of pigments, deposits and applied layers, comparative tests were carried out using different pulse duration Nd:YAG lasers along with FR Er:YAG laser. Furthermore, the analytical comparison was extended to the cleaning tests previously carried out by another group using chemical and mechanical methods, which are briefly discussed here. 2

MATERIALS AND METHODS

2.1 The artwork The Castle of Quart is documented in ancient manuscripts since the first half of the 13th century (Rivolin 2003). It was built by the lords of Quart that were the owners up to 1375, when the Castle was formally annexed to the properties of the powerful Savoia family. They sold it in 1536 during a serious economic crisis. From then until 1612 the Castle had four more owners up to last ones (Perrone di San Martino), the current titular owners. All these vicissitudes involved major changes to the architecture and internal decorations of the Castle, which can be only vaguely argued from historical documents. The Soprintendenza per i Beni e Attività Culturali of the Autonomous Region Aosta Valley started a complex recovery intervention some years ago. It includes the structural restoration, the conservation of

Figure 1. Great hall of the donjon: north-east corner. The delimiting frieze of the fresco cycles and the tree of the sun are scarcely recognisable. The letters indicates the sites where XRF measurements were performed (see below).

wall paintings decorating the chapel, the Aula Magna and the donjon of the Castle, which is object of the present investigation. The wall paintings by unknown artist found in the great hall of the donjon, likely dated around the end of the 13th century, are almost completely covered by a whitewash (lime and gypsum scialabatura) applied in the past. Some scenes that can be indistinctly seen through the scialbatura reveal a linear style characterised by very marked contours and a poverty of colour. A relatively wide frieze (two pairs of red and white bands enclosing a black band with white decorations) running along the four walls of the room outlines the upper limits of the painting cycles. On the north side there is a representation of Alexander the great while interrogating the anthropomorphic tree of the sun, which is inspired by an episode of the romance of Alexander. The theme was very popular during middle ages and was reproduced in many miniatures. Whereas, the present one is the only case of wall painting, which presents a set of peculiar figurative features making it very important from the art history standpoint (Lupo et al. 2003). On the east wall of the room a representation of the twelve months of the calendar is expected, since January and February are already recognisable. The iconographic themes of the other two walls are still completely unknown. The cleaning tests reported in the present study were carried out on the north, east, and south walls. 2.2 Experimental The pigments used to execute the wall paintings of the north and east walls of the donjon’s great hall

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were preliminarily identified using a portable X-ray Fluorescence (XRF) instrument (Lithos 3000, ASSING SpA) providing elemental composition on a variable area between about 2–11 mm. The measurements were carried out in zones where different colour hues and some pictorial details were recognisable under the scialbatura. Then several samples were taken to assess the state of conservation and plan suitable cleaning tests. They were analysed using Infrared Spectroscopy (FT-IR) and Microscopy. Laboratory irradiation tests of prepared samples and cleaning trials of the artwork were performed using fibre-coupled QS, LQS, SFR Nd:YAG (1064 nm) lasers providing pulse durations of 8 ns, 120 ns, and 40–120 µs (FWHM), respectively. Furthermore, we also carried out some trials using FR (230 µs) Er:YAG in order to achieve a first evaluation of its potential in the removal of scialbaturas. The complete presentation of the laboratory investigation performed on prepared sample painted in fresco will be reported elsewhere. Here, we preliminarily discuss the measurements of the optical properties and the irradiation effects on iron and copper pigments, of interest for the present case study. The laboratory experimentation also provided information on the ablation processes associated with the different laser parameters, which allowed qualitative interpretations of the irradiation phenomenology observed on the wall paintings of the castle of Quart. Laser cleaning tests were carried out on the north (scene of Alexander and frieze), east (calendar) and south (frieze and vegetable motifs) walls. The irradiation effects and cleaning results were then characterised through the techniques mentioned above along with video-microscopy examination. 3 3.1

RESULTS State of conservation

XRF elemental characterisation of areas with emerging paint layers and of very small sites that were partially uncovered by a scalpel, provided a first overall picture of the pigments used by the artist. Some of the semi-quantitative evaluations are summarised in Table 1. They allowed distinguishing iron, copper and lead pigments that were afterwards identified through cross-section analysis as ochre (E1), malachite (N3), azurite (E6), and mixtures of malachite, ochre, and lead white (E5), lead white and ochre (N1, N2, E4). X-ray fluorescence spectra also evidenced a significant content of arsenic into the scialbatura, attributed to a specific feature of the quarry where the raw material was taken. FT-IR analysis of several powder samples allowed identifying calcite and gypsum as the main components of the whitewash with relevant quantities of calcium

Table 1. Semi-quantitative elemental analysis achieved using portable XRF. The measurement sites are reported in Figure 1. Site/element

Ca Fe

Cu Zn Pb

Sr

As

E1) East frieze. Red E2) East frieze. Black E3) East. Scialbaturas E4) East. Ab. fr. Yellow E5) East. Ab. fr. Green E6) East. February. Blue N1) North. Face. White N2) North. Branch. Pink N3) North. Plant. Green N4) North. Scialbatura S1) Scialbatura

xxx xxx xxx xxx xx xx xxx xxx xx xxx xx

x x x x xxx xxx x x xxx x x

xx xx xx xxx xx xx xxx xxx xx xx xx

– – xx – – – – – – – xxx

xxx xx xx xx xx x x xx x xx xx

x x x x x x x x x x x

x x – xx xx xx xxx xxx x xx –

Figure 2. Cross-section of a sample taken from a green vegetable motif in proximity of E5 in Figure 1. a) Lime plaster with sandy aggregate. b) Lime layer (preparation). c) Malachite spread through the underlying preparation on which it was applied in fresco. d) Coarse black carbon deposits. e) Whitewash (up to 35 µm). f) Thin layer with pale orange zones (5 µm).

oxalates. The analyses also revealed the presence of an organic component, which could be associated mainly to the outermost layer found in several cross-sections. Three representative stratigraphies are reported in Figures 2–4. They show the presence of two layers applied intentionally on the underlying paint (Figs. 1, 2). The outermost one at times exhibits a film like shape suggesting its possible organic nature. It was applied on a properly called whitewash of very variable thickness (up to some hundreds microns). Such a scialbatura is often separated from the underlying paint by carbon deposits (“ancient black crust”), which represents a very favourable situation for achieving a selective laser cleaning action, as discussed in the following. Even thought this absorption layer was not identified in all the stratigraphies (see for example Fig. 4),

193

Figure 3. Fluorescence microscopy detail of a cross-section of a sample taken from the black band of the frieze above N3 (Fig. 1). c) Carbon black paint is usually applied in secco on the underlying paint layer (b, missing). e) Whitewash (150 µm) with carbon inclusions. f ) Thin layer, likely of organic nature, with rare carbon and ochre (∼30 µm).

Figure 4. Cross-section of a sample taken from the red band of the frieze, in proximity of E3 (see Fig. 1). a) Lime plaster with sandy aggregate. b) Lime layer (preparation). c) Red ochre layer applied in fresco on the underlying preparation. e) White-wash with carbon inclusions (up to 240 µm).

a diffuse presence of carbon inclusions throughout the scialbatura was found (Figs. 3, 4). It suggests that in some areas the carbon deposits were likely partially spread by the application of the whitewash. 3.2

Cleaning tests

Comparative laser cleaning tests were carried out on the red bands of the frieze of the north and south walls. Laser irradiation was always water assisted unless differently specified. As displayed in Figure 5, no discrimination of the paint layer was possible using QS laser. It was aggressive already at the cleaning threshold (i.e. the

Figure 5. Cleaning tests performed on the frieze of the north wall using different laser pulse durations. QS: 8 ns. LQS: 120 ns. SFR: 50 µs. Darker grey corresponds to deeper red, i.e. better safeguard of the paint layer.

minimum fluence allowing a satisfactory removal of the scialbatura in the zones under test), which was about 0.7 J/cm2 . This and the following cleaning thresholds are referred to a typical stratification thickness in the order of few hundreds of microns. Obviously, the threshold could be significantly higher in zones with thicker and/or harder stratifications and the laser treatment at a constant fluence could leave some residues on the treated area. A more gradual action was provided by LQS laser working at about 0.9 J/cm2 . However, the ablation process did not present a practicable self-termination, since at the operative fluence an evident undesired lightening of the paint layer was sometimes induced (Fig. 5). Only the SFR laser allowed the complete safeguard of the red ochre paint layer. The associated cleaning and damage thresholds were 3.9 and 6.2 J/cm2 , respectively. Although this operative range is narrower than the typical ones for stone and metals, it is still enough practicable with minimal risk of damages to the pigment layer, as found through stratigraphic analyses. Figures 5 and 6 show a very satisfactory cleaning result and the absence of any relevant damage. This comprehensive evaluation holds even more when considering the results of alternative approaches. We will not enter into the details of the many systematic mechanical and chemical cleaning tests carried out by another group before the present study. Here, it is useful to summarise the main features. The traditional approaches did not provide practicable selective removal and the cleaning results were not satisfactory, as compared to the one achieved using laser ablation. In particular, the cleaning tests on the red ochre paint layer using scalpel and ammonium carbonate provided a variable degree of cleaning over the treated areas, from insufficient to invasive. In the best

194

Figure 6. Red band of frieze of the south wall: cross-section at the cleaning transition: (SFR laser, 50 µs, 3.9 J/cm2 ).

Figure 8. Extended SFR Nd:YAG laser cleaning treatment of an area painted with red ochre and black carbon: representation of December on the east wall close to the south-east corner. The cleaned area is about 60 × 90 cm.

Figure 7. Cleaning tests carried out on the red band of frieze of the south wall using ammonium carbonate poultice and scalpel: cross section at the cleaning transition. The cleaned zone (right) presents residues of scialbatura of variable thickness (up to 50 µm in this stratigraphic detail).

conditions significant residues of scialbatura along with calcite produced by precipitation were left over the surfaces (Fig. 7). Actually, laser irradiation after optimised chemical and mechanical treatments significantly improved the cleaning by removing such residues. Particular attention was needed to clean the black contours, which are a peculiar characteristic of the present pictorial style, using SFR laser. The high absorption reduces the cleaning threshold around 2 J/cm2 and there is not longer a self-termination of the ablation process. Nevertheless, the low ablation efficiency of the SFR laser on the pure black carbon layer allows stopping the irradiation before producing serious damage (occurring after some tens of shots). The result of the early preliminary trials and optimised treatment are displayed in Figure 5 (see the black line at the bottom of the testing area).

After the preliminary tests described above, SFR Nd:YAG laser was used to uncover an extended area on the east wall in proximity of the south-east corner with the representation of December, which is displayed in Figure 8. Very surprisingly the red and orange ochre paints of the robes exhibited a significant high resistance to the laser irradiation. Due likely to the coherence of the paint layers and good degree of carbonization, in the tested areas, visible damage occurred only above 8 J/cm2 . QS and LQS lasers were both effective to uncover the scene of Alexander at operative fluences around 0.9 and 1.1 J/cm2 , respectively. LQS allowed a slightly deeper cleaning degree and provided relatively larger operative margins for safeguarding black lines and inscriptions. The successful cleaning of such highly absorbing pigment is strictly determined by restorers skill. Actually, the fluence must be reduced by suitably defocusing the laser beam and the number of laser shots must be limited to the minimum providing a satisfactory readability. The risk of damage to the black paint along with the presence of lead white led us to operate on whitishpink adjacent areas without overcoming the cleaning threshold preliminarily determined. Even thought the removal of scialbatura was sometimes incomplete, the

195

Figure 9. Cleaning tests and extended treatments of the scene representing Alexander the great. The dark rectangles at the top are those of Figure 5. The area at the right side cleaned using QS laser is a green leaf of a plant.

laser treatment allowed a better recover of the readability with respect to mechanical ablation (scalpel), as shown in Figure 10. We found that an acceptable recovery of the readability was also possible by operating in dry condition (without spraying water on the irradiated surface) and at lower fluences (around 0.7 and 0.9 J/cm2 for QS and LQS, respectively). In this way, only the dark outermost layer (see Figs. 2, 3) of the stratification was removed whereas the scialbatura was only partially removed. Finally, cleaning tests were performed on areas painted with malachite and azurite, applied in fresco and with organic binder, respectively. As well-known these pigments are characterised by a pronounced thermal instability, which makes the use of of SFR Nd:YAG lasers harmful because of the relatively high operative fluences and low ablation rates. This is even truer when dealing with whitewashes such as the present one. For this reason, as in previous studies (Siano et al. 2007), the tests were carried out using QS and LQS Nd:YAG lasers. The former was employed in dry condition to clean the greenish vegetable motifs of the north and south walls (see for example the tests displayed in Fig. 9), while on a blue area in the scene representing February the best result was achieved by a combined irradiation with the two lasers. According to the stratigraphic characterisation, the malachite paint, which is spread in the underlying preparation (Fig. 2), did not show any chromatic or structural alteration after laser irradiation at 0.7 J/cm2 . Although this fluence is not enough for the overall removal of the scialbatura, a significant recover of the readability was achieved.

Figure 10. The anthropomorphic tree of the sun: cleaning result after laser uncovering using LQS Nd:YAG laser (about 1.1 J/cm2 ).

Likely due to the organic binder and coherence, the azurite paint layer exhibited a relatively higher sensitivity to the laser treatment. The zone cleaned using first LQS then finished with QS laser was apparently acceptable but the stratigraphies showed a slight ablation of the paint layer. No comparisons are possible since traditional techniques have not been experimented yet on azurite. The FR Er:YAG laser was tested in three areas on the north and south walls in water-assisted and dry conditions. The irradiation was ineffective on coherent scialbatura layers. Several hours were needed to partially uncover an area of about 10 cm2 continuously humidified with water. Only a minimal surface ablative effect was observed after several tens of pulses at operative fluences up to about 2.5 J/cm2 . The irradiation of uncovered painted surface was very harmful, especially in dry conditions, due to the high optical absorption of the paint layer, though the very low ablation rate could in principle allow stopping the irradiation before deep ablation occurs. 4

DISCUSSION AND CONCLUSIONS

The results of the cleaning tests described above confirm the significant potential of Nd:YAG lasers in the cleaning of wall paintings and the crucial importance of the pulse duration. The best selection depends on the optical and microstructural properties of the stratification to be removed and paint layer. According to the results of laboratory measurements carried out on prepared samples, the typical reflectance of a thick lime scialbaturas at 1064 nm is up to about 90%. The worst condition for laser cleaning is encountered when such a white layer is directly applied on the paint, without any absorbing component throughout the stratification. The tests allowed

196

demonstrating that even in this case the scialbatura can be removed from iron pigment paint layers using Nd:YAG lasers. Although the prepared samples could not represent any real condition, this result indicates that the pigment layer can take part in the photothermal transfer process. In other words, in the worst condition, the ablation dynamics is driven by surface pigment heating, which generates a very localised water vaporisation, and then a secondary spallation removal. Most of the pigments are actually opaque and exhibit a relatively high absorbance, between about 30–60 for iron and copper pigments. Fortunately, in practical cases, a “pure” whitewash is rarely encountered. Thus for example, as in the present case, absorbing deposits are typically observed between paint layer and scialbatura. More in general, in complex stratigraphies various absorbing components could be found throughout the stratification (dark organic compounds, spread deposits, etc.) The presence of these components drastically reduces the cleaning threshold and then the heating of the painted surface. The possibility to safeguard the paint layer depends essentially on the possibility to control the generated photomechanical effect (i.e. the laser pulse duration). The laboratory trials actually evidenced the primary damage in water-assisted conditions is the partial or complete ablation of the paint layer, whereas discoloration is a second order effect for iron and copper pigments since it occurs in dry conditions and at very high fluences in wet condition. This description can qualitatively explain the good cleaning results achieved in the present study. Concerning the general features of the pulse duration selection, the present results confirm that long pulses (SFR laser) can allow safeguarding paint layer with a weak adhesion, which do not allow using LQS laser because of the relatively intense pressure transient induced. Here, it was the case of iron pigments applied in fresco, whereas azurite, malachite, and pigment mixtures including lead white are better treatable using LQS laser and, in some specific situation, QS laser. In other words, there are cases where the damage threshold exhibits a main dependence on laser fluence and other cases where it depends essentially on laser intensity. Finally, after having provided a new concrete evidence of the significant performance of the laser ablation technique to approach the cleaning of wall paintings, we would also outline that this result was achieved using mostly the same laser systems employed for precise cleaning of stones and metal artworks, i.e. intermediate pulse duration Nd:YAG lasers. This is a good premise for the dissemination of the laser approach, even more whether considering that SFR and LQS lasers could be compacted in a single laser system in the near future.

ACKNOWLEDGEMENTS The authors wish to thank Alberto Felici, Mariarosa Lanfranchi, and Fabrizio Bandini of the Opificio delle Pietre Dure for having provided the data of their cleaning tests using traditional techniques.

REFERENCES Bracco, P., Lanterna, G., Matteini, M., Nakahara, K., Sartiani, O., De Cruz, A., Wolbarsht, M., Adamkiewicz, E., Colombini, M.P. 2003. Er:YAG laser: an innovative tool for controlled cleaning of old paintings: testing and evaluation. Journal of Cultural Heritage 4 Supplement 1: 202–208. Chappé, M., Hildenhagen, J., Dickmann, K., Bredol, M. 2003. Laser irradiation of medieval pigments at IR, VIS and UV wavelengths. Journal of Cultural Heritage 4, Supplement 1: 264–270. De Cruz, A., Wolbarsht M..L., Hauger, S.A. 2000. Laser removal of contaminants from painted surfaces. Journal of Cultural Heritage 1: S173–S180. Gaetani, M.C., Santamaria, U. 2000. The laser cleaning of wall paintings. Journal of Cultural Heritage 1, Supplement 1: S199–S207. Hildenhagen, J., Chappé, M., Dickmann, K. 2005. Reaction of historical colours and their components irradiated at different Nd:YAG laser wavelengths (w, 2w, 3w, 4w). In: K. Dickmann et al. (ed.s), LACONA V Proceedings: 297– 301. Berlin: Springer-Verlag. Lupo, M., Zidda, G. 2003. Alexander a gli alberi del sole e della luna: fonti iconografiche. In: E. Rossetti Brezzi (ed.), Frammenta picta, Testimonianze pittoriche dal castello di Quart, Secoli XIII-XVI: 10-11. Aosta: Soprintendenza per i Beni e Attività Culturali, Regione Autonoma Valle d’Aosta. Margheri F., Modi S., Masotti L., Mazzinghi P., Pini R., Siano S., Salimbeni R. 2000. SMART CLEAN: a new laser system with improved emission characteristics and transmission through long optical fibres. Journal of Cultural Heritage 1, Supplement 1: S119–S123. Pouli, P., Emmony, D.C. 2000. The effect of Nd:YAG laser radiation on medieval pigments. Journal of Cultural Heritage 1, supplement 1: S181–S188. Pouli, P., Emmony D.C., Madden, C.E., Sutherland, I. 2003. Studies towards a thorough understanding of the laserinduced discoloration mechanisms of medieval pigments. Journal of Cultural Heritage 4: 271–275. Rivolin, J.G. 2003. Momenti di storia: i signori di Quart e i Sarriod del La Tour. In E. Rossetti Brezzi (ed.), Frammenta picta, Testimonianze pittoriche dal castello di Quant, Secoli XIII-XVI: 22–23. Aosta: Soprintendenza per i Beni e Attività Culturali, Regione Autonoma Valle d’Aosta. Salimbeni, R., Mazzinghi, P., Pini, R., Siano, S., Vannini, M., Matteini, M., Aldrovandi, A. 1998. In: A. Guarino (ed.), Proceedings of the 1st International Congress on: Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin (Catania, Italy 1995): 811–815. Roma: Consiglio Nazionale delle Ricerche.

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Salimbeni R., Pini R., Siano S. 2001. Achievement of optimum laser cleaning in the restoration of artworks: expected improvements by on-line optical diagnostics. Spectrochimica Acta, Part B 56: 877–885. Salimbeni R., Pini R., Siano S. 2003, A variable pulsewidth Nd:YAG lase system for conservation Journal of Cultural Heritage 4: 72S–76S. Sansonetti, A., Realini, M. 2000. Nd:YAG laser effects on inorganic pigments. Journal of Cultural Heritage 1, Supplement 1: S189–S198 Schnell, A., Goretzki, L., Caps, Ch. 2005. IR-laser effects on pigments and paint layers. In: K. Dickmann et al. (eds) LACONA V Proceedings: 291–296. Berlin: Springer– Verlag. Siano, S., Casciani A., Giusti A., Matteini M., Pini R., Porcinai S., Salimbeni R. 2003. The Santi Quattro Coronati: cleaning of the gilded decorations. Journal of Cultural Heritage 4, Supplement 1: S123–S128. Siano, S., Salimbeni, R., 2001. The Gate of Paradise: physical optimization of the laser cleaning approach. Studies in Conservation 46: 169–281. Siano, S., Brunetto, A., Droghini, F., Guasparri, G., Scala, A. 2006. Cappella del Manto e Sagrestia Vecchia in Santa

Maria della Scala, Siena: rimozione laser di scialbature su dipinti murali. In: V. Dell’Aquila (ed.), Atti del IV Congresso Nazionale IGIIC–Lo Stato dell’Arte: 295–302. Firenze: Nardini Editore. Siano, S., Brunetto, A., Mencaglia, A., Guasparri, G., Scala, A., Droghini, F., Bagnoli, A. 2007. Integration of laser ablation techniques for cleaning the wall paintings of the Sagrestia Vecchia and Cappella del Manto in Santa Maria della Scala, Siena. In: LACONA VI Proceedings: Berlin: Springer-Verlag. Teule, R., Scholten, H., van den Brink, O. F., Heeren, R. M.A., Zafiropulos, V., Hesterman, R., Castillejo, M., Martin, M., Ullenius, U., Larsson, I., Guerra-Librero, F., Silva, A., Gouveia, H., Albuquerque, M.B. 2003. Controlled UV laser cleaning of painted artworks: a systematic effect study on egg tempera paint samples. Journal of Cultural Heritage 4, Supplement 1: 209–215. Zergiotti, I., Petralis, A., Zafiropulos, V., Fotakis, C., Fostiridou, A., Doulgeridis, M. 1997. Laser applications in painting conservation. In: W. Kautek et al. (eds.), Laser in the Conservation of Artworks, LACONA I: 57–60. Restauratorenblatter, Verlag Mayer & Comp., Wien.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Colour changes in Galician granitic stones induced by UV Nd:YAG laser irradiation A. Ramil, A.J. López, M.P. Mateo, C. Álvarez & A. Yáñez Departamento de Enxeñaría Industrial II, CIT, Universidade da Coruña, Ferrol (A Coruña), Spain

ABSTRACT: Laser cleaning has become a high profile and accepted specialized cleaning technique for most stone types. Given that local natural building stone in our region, Galicia (North West of Spain) is granite, our interest was focused on the response of this stone type to laser cleaning. The aim of this work is to determine the change in colour as index of the response of galician granites to UV-Nd:YAG laser exposure. For this purpose samples of commercial granites from different quarries were irradiated at increasing fluences. Colour changes were determined through the analysis of digital images and expressed in the L∗ a∗ b∗ and L∗ h∗ C ∗ colour models. The results enable the establishment of application limits of laser cleaning to these materials.

1

INTRODUCTION

Laser cleaning of stones has become a high profile and accepted specialized technique in restoration. The majority of the reported laser stone cleaning has been for limestone and marble and to a lesser extent for silicate rocks such as sandstone; experience with granite and other crystalline stones is rare at present (Fotakis et al. 2007). Given that Galicia (North West of Spain) has a rich heritage of buildings and monuments constructed from locally obtained granite, our interest was focused on the response of this stone type to laser cleaning. Besides, the current development of transport and construction techniques has led to the widespread use of granite in facing tiles, even in areas located far away from the product source, which enlarge the interest on this topic. Because of the wet climate in Galicia, the granite is almost permanently damp, which made it highly bioreceptive. Biological colonization and blackening of exterior surfaces can be observed in many buildings (Prieto et al. 1999, Aira et al. 2007); the main objective of the cleaning process is the removal of biological contamination. The effects of laser cleaning on stone depend on the laser parameters, the type of the stone and the characteristics of application. Between the undesirable effects, changes in colour have been observed for different stone types at different working parameters (Klein et al. 2001, Lee et al. 2001). In the case of granites, changes in coloration of “Rosa Porriño” under 1064 nm Nd:YAG laser irradiation, and different Scottish granites under 1064, 532 and 355 nm were

reported (Wakefield et al. 1997, Esbert et al. 2003, McStay et al. 2005, Grossi et al. 2007). The aim of this work is to determine the change in colour of Galician granites under UV laser exposure. For this purpose samples of commercial granites showing different colours were irradiated with a Nd:YAG laser source at the wavelength of 355 nm and different fluences. The colour changes were determined through the analysis of digital images of the irradiated and non irradiated samples. The results can help to establish the application limits of the laser cleaning technique for different types of Galician building granites. 2 2.1

EXPERIMENTAL Materials

The stone types used in this study are specifically those of Galician origin and comprised samples of pink (Rosa Porriño), pinkish-gray (Mondariz), and gray (Albero, Gris Morrazo) granites. All of them are commercial stones being Rosa Porriño, one of the most marketed ornamental stones in Spain and outside Spain. Fresh samples from stores were cut in tablets of around 10 × 10 cm2 . Surface finishes were polished for all the samples except for Albero which was sew. Table 1 summarizes some relevant characteristics of these stones (Quiroga-Calviño et al. 1998). 2.2 Laser irradiation Experiments were carried out using a Q-switched Nd:YAG laser source; operating at the third harmonic,

199

Table 1.

Table 2. Values of colour coordinates of granites.

Granites analyzed in this work.

Granite

Classification

Macroscopic description

Granite

Albero

Morrazo

Mondariz

Porriño

Porriño

Biotite granite

Mondariz

Biotite granite

L∗ a∗ b∗ C∗ h∗

44 ± 2 1.0 ± 0.9 4.7 ± 1.0 4.8 ± 1.0 79 ± 11

47 ± 3 0.1 ± 0.6 1.0 ± 0.8 1.2 ± 0.8 68 ± 59

45 ± 6 0.6 ± 1.0 3.4 ± 1.4 3.5 ± 1.4 80 ± 19

46 ± 4 5.7 ± 3 8.7 ± 3 10.5 ± 4 58 ± 8

Albero

Muscovite granite

Morrazo

Biotite granite

Pink colour due to the coloration of the feldspars. Medium to coarse grained. Pinkish-gray with mega-crystals of feldspar. Light gray, with a yellow to brown shade, mediumgrained, rich in muscovite. Gray, fine-grained.

355 nm, 10 Hz repetition rate, spot diameter around 8 mm; pulse duration 6 ns and maximum pulse energy varying between 10 and 60 mJ. The samples were irradiated at different fluences between 0.18 and 1.03 J/cm2 . The energy density was calculated from measured energy data and the spot size of the laser beam on heat sensitive paper. Samples were mounted onto a computer-controlled X–Y translation stage. Each sample was submitted to 3 scans at a scan speed of v = 2 mm/s. Under these conditions the average degree of overlapping given by the relation (d f/v) was 40 laser pulses in each scan, being f the frequency, d the diameter of the laser beam and v the speed of scan. Then, an average of 120 laser pulses was delivered over each point of the sample surface. 2.3

Colour measurements

Although the measure of colour used to be performed by conventional spectrophotometric techniques; at present available commercial colorimeters measure colour only over a very few square centimeters, and thus their measurements could not be very representative in heterogeneous materials such as granite items. For this reason, in order to quantify the colour changes induced by laser irradiation, a simple method based on computer vision techniques was applied (Yam et al. 2004, Thornbush et al. 2004, Leon et al. 2006). This method uses a digital camera to capture the colour image of the granite tablet under proper lighting. The captured image is a bitmap image consisting of many pixels; each pixel assign a specific location and colour (RGB) value which can be transformed into different colour coordinates by means of the adequate software (Westland and Ripamonti 2004). The digital camera was a Nikon D100 with exposure times ranging from 30 s to 1/4000 s and an objective Nikon micro 60/2.8. The CCD of the camera consists of 6.1 × 106 pixels and presents an active area of 23.7 × 15.6 mm2 . The images were stored in non-compressed file (TIF format) to avoid

loss of image quality. The standard light source D65 (http://www.cie.co.at/) which mimic variations of daylight and with a colour temperature of 6500 K was used. The position of the camera, sample, and light sources was arranged in order to capture the diffuse reflection responsible for the colour, which occurs at 45◦ from the incident light, and to ensure a uniform distribution of light intensity over the sample surface. A standard colored chart GretagMacbeth ColourChecker was used to calibrate and verify the experimental settings prior to actual measurements. The digital images were taken out of focus to obtain a fuzzy image which simulates the effect of the standard CIE viewing angle geometry (http://www.cie.co.at/).

3

RESULTS AND DISCUSSION

In order to quantify the colour changes, digital images of all the samples irradiated were analyzed and colour measurements were expressed using the CIE L∗ a∗ b∗ system. Here L∗ is the variable lightness or luminosity, which varies from 0 (black) to 100 (white); a∗ and b∗ are the chromatic coordinates. The attributes ∗ of chroma (Cab : saturation or colour purity) and hue (h∗ab : referring to the colour wheel) can be cal∗ culated by the equations: Cab = (a∗2 + b∗2 )1/2 and h∗ab = tan−1 (b∗ /a∗ )(180/π). Colour differences (L∗ , ∗ a∗ , b∗ , Cab , h∗ab ) were calculated and the total ∗ colour change (Eab ) was estimated by the expression: ∗ Eab = [(L∗ )2 + (a∗ )2 + (b∗ )2 ]1/2 . The colour of the granite stones results from a combination of colours of the granite rock-forming minerals; thus, the selected stones present different colour coordinates, which are summarized in Table 2. Different responses to UV-Nd:YAG laser irradiation of these stones are appreciable under naked eye examination showing changes in colour especially in the case of Rosa Porriño in which a clear loss of pink colouration was observed. Gris Mondariz, as well as Albero, present more subtle variations, however in the case of Gris Morrazo no changes were noticed. On the whole, colour changes seem to be higher at higher laser fluences.

200

60

10

b)

50

20

20

150

0 0.5 1 1.5

fluence /(J/cm2)

*

*

L 0 -0.5 0 0.5 1 1.5 fluence /(J/cm2)

45

C ab

6

a)

40 -0.5 0 0.5 1 1.5

∗ . Figure 1. Rosa Porriño L∗ , a∗ , b∗ , C ∗ , h∗ and Eab

4

80 60 -0.5 0 0.5 1 1.5

Rosa Porriño

fluence /(J/cm2)

d)

2

*

hab

Data of color variations, in terms of colour coordinates, as a function of the laser fluence are discussed below for each granite type:

b) 4 2 -0.5 0 0.5 1 1.5

100

c)

3.1

0 -5

50

5

60 -0.5 0 0.5 1 1.5 fluence /(J/cm2)

d)

5

∗ Figure 2. Mondariz L∗ , C ∗ , h∗ and Eab .

f) ∆E*ab

e)

50

fluence /(J/cm 2)

10

80

100

0 -0.5 0 0.5 1 1.5

0 -0.5 0 0.5 1 1.5

100

10

c)

∆E ab

*

0 -0.5 0 0.5 1 1.5

10

0 -0.5 0 0.5 1 1.5

*

d) C ab

*

b

10

30 -0.5 0 0.5 1 1.5

2

∆E ab

-10 -0.5 0 0.5 1 1.5

c)

hab

40

20 -0.5 0 0.5 1 1.5

b)

4

*

0

hab

40

L

a*

L

*

*

a)

6

a) C ab

60

0 -2

0 0.5 1 1.5

fluence /(J/cm2)

As can be appreciated in Figure 1a and Figure 1b, there are strong changes in chromatic coordinates a∗ and b∗ even at the lowest value of laser fluence. As a consequence, the chroma C ∗ (Fig. 1c) decreases showing a higher rate of change at the lower values of fluence (between 0 and 0.5 J/cm2 ). This decrease in C ∗ indicates that the colour of the surface was approaching the values corresponding to gray colouration. At the same time, the increase in hue h∗ (Fig. 1d) indicates a separation from the red-green axis; i.e. a loss of red colouration. Finally, from the measurements of L∗ (Fig. 1e), no discernible trend can be appreciated confirming that the loss in red colouration was not accompanied by a corresponding lightening or bleaching of the stone surface which was measurable using the L∗ component.

This granite presents a light gray colour with a shadow of yellow-brown. Changes in colour were discernible by eye at the higher fluences but they are very subtle. Figure 3b shows appreciable changes in C ∗ for fluence values of 0.5 J/cm2 onwards. Coordinate h∗ shows a subtle decrease (Fig. 3c). Values of L∗ do not evidence lightening or bleaching caused by the UV laser irradiation.

3.2

3.4

Gris Mondariz

As in the case of Rosa Porriño, a loss of pink colouration is appreciable at all the fluences analyzed giving a

∗ Figure 3. Albero L∗ , C ∗ , h∗ and Eab .

decrease in C ∗ with increasing fluence (Fig. 2b). Values of L∗ and h∗ do not show an appreciable trend (Fig. 2a and 2c). 3.3 Albero

Morrazo

This gray granite does not show changes in colouration caused by the laser irradiation.

201

Then, UV-Nd:YAG laser irradiation has caused colour changes in pinkish granites (Rosa Porriño and Gris Mondariz) even at the lowest fluences applied, probably associated to changes in the pink feldspars which contain iron oxides. In the case of Albero, gray granite with a yellow-brown shadow, changes were appreciated when the laser fluence reaches around 0.2 and 0.5 J/cm2 . In all cases chroma, i.e. C ∗ coordinate, is sensitive to the laser fluence, showing a clear decrease when the fluence increases. This decrease indicates a progressive approach to the gray axis in L∗ C ∗ h∗ space. The lightness, L∗ , does not show any trend indicating that no bleaching is produced by the laser. The behavior of hue, h∗ coordinate, under laser irradiation depends on the granite type but does not exhibit a great variation with laser fluence. As a result, ∗ colour difference Eab shows a clear increase with increasing fluences for all the granites analyzed, giving a maximum value of 5 for Rosa Porriño and around 2 for Mondariz and Alberto. 4

CONCLUSIONS

Different coloured granite stones were exposed to irradiation of a Q-switched Nd:YAG laser in the UV range (355 nm) at different energy densities to determine the effect of the laser on the colour of fresh stone surfaces. Except for the case of gray granite Gris Morrazo, a loss of colour with increasing fluence is visible with the naked eye. In order to obtain quantitative values of changes in coloration, a method based on the analysis of digital images was used, and coordinates of the L∗ C ∗ h∗ space were selected for quantification. The more sensitive coordinate to the laser energy density was the chroma C ∗ which decreases with the increase of laser fluence. For granites containing red colored minerals such as Rosa Porriño and Gris Mondariz, colour loss occurred at the lowest fluence applied, 0.18 J/cm2 . Exposure of these stone types to UV causes a loss in pink coloration from the feldspar grains which turn gray. In the case of gray granites, Albero, a light gray stone with a yellow-brown shadow, changes occurred at values around 0.5 J/cm2 , probably associated to changes in the minerals responsible of the yellow-brown shadow. ACKNOWLEDGEMENTS Special thanks are given to Dr. Francisco Fernández Martínez from Dept. Química Industrial y Polímeros,

Universidad Politécnica de Madrid. This work was partially supported by Xunta de Galicia through Project PGIDIT06CCP00901CT. REFERENCES Aira, N., V. Jurado, V., Silva, B. & Prieto, B. 2007. Gas chromatography applied to cultural heritage: Analysis of dark patinas on granite surfaces. Journal of Chromatography 1147: 79–84. Esbert, R. M., Grossi, C. M., Rojo, A., Alonso, F. J., Montoto, M., Ordaza, J., Pérez de Andrés, M. C., Sebastián, E., Rodríguez-Navarro, C. & Elert, K. 2003. Application limits of Q-switched Nd:YAG laser irradiation for stone cleaning based on colour measurements. Journal of Cultural Heritage 4 (Supplement 1): 50–55. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2007. Lasers in the Preservation of Cultural Heritage. Principles and Applications. Taylor & Francis. Grossi, C. M., Alonso, F. J., Esbert, R. M. & Rojo, A. 2007. Effect of laser cleaning on granite colour. Colour Research and Application 32: 152–159. Klein, S., Fekrsanati, F., Hildenhagen, J., Dickmann, K., Uphoff, H., Marakis, Y. & Zafiropulos, V. 2001. Discoloration of marble during laser cleaning by Nd:YAG laser wavelengths. Applied Surface Science 171: 242–251. Lee, J. M., Watkins, K. G. & Steen, W. M. 2001. In-process chromatic monitoring in the laser cleaning of marble. Journal of Laser Applications 13: 19–25. Leon, K., Mery, D., Pedreschi, F. & Leon, J. 2006. Colour measurement in L∗ a∗ b∗ units from RGB digital images. Food Research International 39: 1084–1091. McStay, D., Wakefield, R., Murray, M & Houston, J. 2005. Laser Stone Cleaning in Scotland. Research Reports. Edinburgh: Technical Conservation, Research and Education Group. Historic Scotland Conservation Bureau. Prieto, B., Rivas, T. & Silva, B. 1999. Environmental factors affecting the distribution of lichens on granitic monuments in the Iberian Peninsula. Lichenologist 31: 291–305. Quiroga-Calviño, J. R., Casares Gallego, A., Míguez Suárez, V. L. & Vidal Romaní, J. R. 1998. A pedra de Galicia. Santiago de Compostela: Xunta de Galicia. Thornbush, M. & H. Viles, H. 2004. Integrated digital photography and image processing for the quantification of colouration on soiled limestone surfaces in Oxford, England. Journal of Cultural Heritage 5: 285–290. Wakefield, R. D., Brechet, E. & McStay, D. 1997. The effect of laser cleaning on Scottish granite. Lasers as Tools for Manufacturing 2 2993, 246–251. Westland, S. W. & Ripamonti, C. 2004. Computational colour science using Matlab. England: John Wiley & Sons. Yam, K. L. & Papadakis, S. E. 2004. A simple digital imaging method for measuring and analyzing colour of food surfaces. Journal of Food Engineering 61: 137–142.

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Arch-collegiate church in Tum: Laser renovation of priceless architectural decorations A. Koss Inter-Academy Institute for Conservation and Restoration of Works of Art, Academy of Fine Arts in Warsaw, Poland

J. Marczak & M. Strzelec Institute of Optoelectronics, Military University of Technology, Warsaw, Poland

ABSTRACT: The aim of the present paper lies in the presentation of the results of laser renovation technique included in a greater project of conservation of arch-collegiate church in Tum, a leading Romanesque monument in Poland, which is still in progress after almost six years of realisation. The microscopic investigations of samples after laser cleaning have shown the possibility of preservation of natural patinas on stone objects. The cleaning process was realised using the lowest possible laser fluence that enabled removing the encrustation during a defined, reasonable time. The level of cleaning has been determined through the results of measurements of white light scattering coefficients for different small areas of objects before and after laser cleaning using various energy fluences.

1

INTRODUCTION

Among the leading monuments of Romanesque art in Poland, the largest one is the former arch-collegiate church in Tum near Łeczyca, which fulfilled impor˛ tant religious and political functions during the feudal disintegration period (12th and 13th centuries). In the course of its more than 800 years-long history, the church experienced turbulent and dramatic events, including serious damage during the campaign of 1939 (Stawicki 1999). Its particular, a priceless and famous decoration fragment is a Pantokrator limestone sculpture, made in the 12th–13th centuries, characterized by fine, millimeters depth of drawings. Limestone is the main substrate material of most important architectural parts of the temple. Its characteristic composition, susceptible to acid rains, as well as its physico-chemical features, cause high limestone dependence on weathering and degradation under the influence of frost, water and soluble salts. Acknowledge of those phenomena lead to frequent renovation procedures based for example on applying compact painting layers, which in principle was a wrong solution. The closure of pores and decrease of stone surface “breathing” contribute to the accumulation of water and acceleration of the internal degradation processes. Another problem is the frequent appearance of complex leaching damages and stone binder accumulation. In many cases, conventionally removed external

encrustation unveiled internal polluted and painted layers, that changed, in some parts the colour of the original material. Laser tests of removal of these bonding, painting layers and dirt showed that laser cleaning is an universal method. As a result, in the frame of a large conservation project in the collegiate church in Tum, the following, most important objects have been cleaned using a Q-switched, Nd:YAG ReNOVA Laser 1 and 2 systems (Marczak 2001): – Main Portal, – Sacristy Portal in Chancel, with preserved original remains of polychrome, – Romanesque sculpture of Christ Pantokrator (Koss 2005), – Epitaph of Knight with Sword, a bas-relief from 14th century, – Springers of vault, portal over the entrance of the northern aisle, – Substrates the painting layers of restored polychrome in church chancel. To preserve the original objects patinas, the cleaning process was realised using the lowest possible laser fluence, that enabled removal of encrustation during a defined, reasonable time. The level of cleaning has been determined through the results of measurements of white light scattering coefficients for different square areas of object before and after laser cleaning using various energy fluences in comparison with light

203

Figure 2. Tests of laser cleaning at the portal basement, documented six year after the first trials.

Figure 1. Front view of main portal in the Tum Collegiate.

backscattered from black carbon. Experiments were performed using an optical fibre spectrograph (Koss et al. 2007) and evaluated by a team of experienced conservators.

2 2.1

Figure 3. View of portal finial before (left side) and after (right side) laser cleaning.

CLEANED OBJECTS Main portal

The main (north) portal is sculpted and dates back to the first half of the 12th century (Fig. 1). First tests of laser cleaning, illustrated in Figure 2, were made in 2001 and remained still “fresh” till recent full portal renovation. Results of laser cleaning in different places of walls and reliefs are shown in Figures 2–6.

2.2

Sacristy Portal in Chancel

Figure 4. Sculptured finial cantilever during laser cleaning.

Sacristy Portal in Chancel was one of the first Church monuments restored with the use of laser (2001). Figure 7 shows its front view before and after conservation. Following pictures illustrate cleaning process in different areas of the portal ornaments (Figs 8, 9). 2.2.1 Remains of ancient polychrome Some specificity of ReNOVALaser2 system use in the case of discovered fragments of old polychrome consisted in selective removal of secondary lime layers and encrustation without use of liquids as well as in

preservation of fragile remains of older overpaintings (Fig. 10). 2.3 Epitaph of Knight with Sword Bas-relief called Epitaph of Knight with Sword belongs to collection of sculptures from 14th century. Figure 11 shows its view before and during laser treatment. Final result of conservation can be seen in Figure 12.

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Figure 8. Upper part of limestone portal decoration during laser cleaning. The arrow shows a restored area. Figure 5. Different sculptured fragments of the portal after laser cleaning.

Figure 6. Laser cleaning test at fragment of memorial plate (arrow). Figure 9. Detailed view of sculptured limestone wall during laser cleaning. The arrow indicates area after laser treatment.

Figure 10. Discovered remains of ancient polychrome. The arrows indicate places of laser cleaning. Figure 7. Front view of Sacristy Portal in Chancel. Left side photograph documents original view; right side: portal after full renovation, including laser cleaning of stone framing.

2.4

Sculpture of Christ Pantokrator

A particular fragment of the church decoration is a Pantokrator limestone sculpture, characterized by fine, millimeter depth drawing. As a result of the long interaction with atmospheric gases there was a layer of gypsum and upper black encrustation. The

use of precise ReNOVALaser1 system with optical fibre beam delivery and output energy below predetermined threshold allowed to remove all encrustation preserving the original substrate with a thin patina layer. A general view of the Pantokrator before and after restoration is shown in Figure 13. The cleaning progress at the element of the coat of the figure is illustrated in Figure 14. The evaluation of the level of cleaning of the limestone was based on spectrometric measurements of

205

Figure 14. Illustration of laser cleaning results at the fragment of Pantokrator’s coat. The square marks an enlarged area at the rightside photograph.

Figure 11. Bas-relief Epitaph of Knight with Sword before (left side) and during (right side) laser removal of dirt crust.

Figure 12. Sculpture after laser cleaning.

Figure 15. Illustration of “square method” for evaluation of cleaning level. Laser fluence is minimal at square 1 area and maximal at square 10. Numbers correspond to results presented in Figure 16.

Figure 13. Photograph of Pantokrator at the church wall before conservation (left side) and after laser cleaning (right side).

backscattered white light (Koss et al. 2007) after illumination of several squares, results of treatment with different, gradually selected fluences of the laser beam. Using ReNOVALaser 2 system with a pantograph and a focusing lens of a focal length of 1000 mm, the minimum beam diameter can be of 3.0 mm and the maximum fluence can reach 9 J/cm2 . Such high energy densities were not used because they could

Figure 16. Results of optical fibre spectrometer measurements of white light backscatter amplitude for different, gradually adjusted laser fluences.

damage the limestone substrate and could cause vitrification, oxidation, melting, destruction, and formation of micro-cracks at the surface. The so called “square method” is illustrated in Figure 15. The results of measurements are shown in Figure 16.

206

water are not only ineffective but dangerous for personnel and even damaging for objects, because they tend to increase the volume of the white cavities and have little impact on the crusts. The use of laser cleaning, in contrast, acts without physical contact and has no effect on the white parts, enabling a gradual and selective cleaning confined to the parts affected by black crusts. Furthermore, careful and experienced operators can also ensure good conservation of the patinas since the laser technique offers a greater degree of control over the cleaning process compared with chemical and mechanical methods and is less aggressive than mechanical techniques. Conducted experiments allowed to verify applicability of relatively simple measurement techniques for ensuring the safety of artworks and personnel: Figure 17. Results of the quantitative evaluation of concentration of pollutants concentrations generated during laser cleaning of limestone (the Pantokrator bas-relief).

Although laser cleaning is generally recognized as the safest techniques of renovation of artworks, ablation of secondary encrustation generates particles and molecular species comparable but not similar to the emission of air contaminants during the laser processing of various materials in industry. Model 1312 system, produced by Innova Air Tech, with the detection limit typically below 1 ppm was used for determination of pollution during laser cleaning inside the church. Innova system principle of operation is based on PhotoAcoustic Spectroscopy (PAS) in which the gas to be measured is irradiated by intermittent light of filtered pre-selected wavelength. The gas molecules absorb some of the light energy proportional to gas concentration and convert it into an acoustic signal which is detected by a microphone. Measured values of gas concentrations, emitted during laser cleaning of the Pantokrator (limestone), showed significant increase of CO emission and large changes from point to point, depending on the local properties of soiling, its quantity and type (Fig. 17).

3

CONCLUSIONS

As a conclusion from the first Polish laser renovation project presented here, the laser treatment provided the best results in the majority of situations, enabling the stone surfaces and previous protective treatments to be fully preserved. In the large but closed areas, cleaning methods involving the application of chemical poultices or spraying with nebulized or atomized deionized

– white light spectrometry for determination of the cleaning level (preservation of original substrate and patina), – IR PhotoAcoustic Spectroscopy method for the evaluation of pollution generated during laser ablation. However, in the last case, particular attention should be paid to long-term stone cleaning in small, closed spaces. Experience acquired during laser the works in Tum Collegiate, which started more than six years ago, allowed to implement the laser procedure as a proven and accepted restoration tool in the largest Polish project: the laser renovation of Sigismund Chapel, Wawel Castle, Krakow (Marczak et al. 2007). ACKNOWLEDGEMENTS Work has been supported by the Ministry of Science and Higher Education, Poland, project 217/E284/SPUB-M/EUREKA/T-11/DZ 203/2001-2003. REFERENCES Koss, A. & Marczak, J. 2005. Application of lasers in conservation of monuments and works of art. Scientific Reports IAICR 1, 49 pages, ISBN 83-922954-0-4. Koss, A. et al. 2007. Laser cleaning of set of 18th century ivory statues of Twelve Apostles. In this Volume. Marczak, J. 2001. Surface Cleaning of Art Work by UV, VIS and IR Pulse Laser Radiation. Proc. SPIE 4402. 202–209. Marczak, J. et al. 2007. Batory’s Chapel at Wawel Castle, Cracow: laser cleaning and hue measurements of epitaph and stalls. Proc. SPIE 6618. 66181C. Stawicki, S. 1999. Reflections about Tum. Cenne, Bezcenne, Utracone (in Polish). 178–188.

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Laser cleaning of the Nickerson Mansion: The first building in the US entirely cleaned using laser ablation A. Dajnowski CSOS Inc. Forest Park, Illinois, USA

ABSTRACT: In November of 2005 CSOS Inc. completed the first restoration of an entire architectural structure in the USA with lasers. The exterior walls of the Nickerson Mansion were almost entirely black prior to our treatment. All chemical methods of treatment failed to provide adequate results. Blasting with any media would have damaged the sandstone walls. After performing numerous chemical tests and some tests with lasers, it was decided that lasers would be used to restore the 25 000 square foot surface of the stone walls. This cleaning technique can be used for numerous applications in the treatment of architectural elements and objects. This was a very challenging project due to the size of the building and the many intricate carvings in the stone. This paper will discuss the practical aspects of using lasers on a large-scale architectural project.

1

INTRODUCTION

The Nickerson mansion (Figs. 1 and 2) was designed by Edward Burling and was completed in 1883 (Folson 1963). Since then its’ exterior was never cleaned. This was the first mansion that was built in Chicago after the great fire of 1871 that was constructed to be fire resistant. Berea sandstone from Cleveland, Ohio was selected for the exterior walls (Love et al. 1998). A number of exotic marbles and wood were used to finish the interior and the Nickerson Mansion came to be called “the Marble Palace” (Love et al. 1998). Based on historical pictures, the building was covered with a layer of black crust since 1923. The burning of coal, leaded fuel used by cars, and environmental pollution slowly darkened the light beige color of the building. The crust kept darkening with time due to accumulation of pollutants on the surface. When I was asked to evaluate this structure in 2003, it was almost entirely and evenly black. At this time, the Nickerson Mansion had another nick name: “the black building”.

2

Figure 1. Nickerson Mansion before treatment in 2004.

BEREA SAND STONE AND SELECTION OF CLEANING PROCESS

In order to understand the structure of the stone, the composition of the crust and nature of the surface problems, the following analytical techniques were used: Raman spectroscopy, SEM/EDS, petrographical

Figure 2. Nickerson Mansion after treatment in 2005.

209

Figure 3. SEM image showing a magnified cross-section of the stone taken from the building. Please note the large voids between the grains of the stone.

Figure 4. Same area as in Figure 3; the white dots represent iron distribution in the cross-section of the stone mapped using SEM/EDS.

microscopic analysis, diffraction qualitative analysis and wet chemical analysis. The sandstone is composed of fine grains of 0.1 to 0.2 mm in diameter (Fig. 3). Berea sandstone is medium, grained, Mississippian age graywacke. It is composed of quartz (∼80%), feldspar (∼5%), clay, predominantly kaolinite (∼8%), and calcite (∼6%). It has porosity of 19% (Hart et al. 1995) (Fig. 3). Minor amounts of quartzite, shale, muscovite and gypsum were also found during testing. Raman, SEM, EDS, XRD analysis indicated that the following iron oxide phases were present: Hematite (Fe2 O3 ), Magnetite (Fe3 04 ) both relatively stable, but hematite when exposed to NaOH can be converted to Lepidocrocite (FeO·OH), a less stable form that can react with water. Figure 4 illustrates high iron content in the structure of the stone. The Iron X-ray mapping was performed on a JEOL 5900 low vacuum scanning electron microscope equipped with an Oxford Instruments EDS Detector

Figure 5. Cross-section of the surface of the stone in plain light magnified 40× showing the uneven thickness of the black crust.

Model 7573. The instrument was operating in the high vacuum mode using an accelerating voltage of 15 kV. The black crust formed a layer, (Fig. 5) preventing rainwater from easily evaporating from the stone. The trapped water caused deterioration that was most visible in the carved areas. The removal of the black crust was as important aesthetically as it was crucial for structural stability of the stone. Numerous chemical tests were performed, but the cleaning results were inconsistent. Some test areas were acceptable while others were significantly stained and rusty in appearance. Mechanical cleaning tests such as sand blasting were performed on a small area in the past (by others), but this technique is very damaging. Using blasting methods in order to remove the surface contamination meant that it was necessary to remove more than 1 mm of the contaminated stone. The black crust encapsulated the surface grains almost entirely (Fig. 5). Therefore, chemical and mechanical cleaning methods were discarded. The black crust contained large amounts of C, Si, S and Fe (Table 1). Hydrocarbon oils similar to asphalt and gypsum were also detected. Raman analysis (Fig. 6) supplements EDS results. Diffraction analysis of dark gray samples indicated presence of: Cerussite (PbCO3 ), Hydrocerussite (Pb[CO3 ]2 [HO]), Anglesite (PbSO4 ). After extensive testing of various laser systems, the Clean Optical Laser System CL 120 Q.,1064 nm Nd:YAG was selected to be used in this project. The specifications of this system are:

210

– – – – – –

Scan width (blue gun): 5–50 mm Scan width (stylus): 1–25 mm Scan frequency: 50–150 Hz, Variable laser pulse frequency: 8–35 kHz Pulse energy: 3–15 mJ Pulse duration: 5 nanoseconds

Table 1. EDS.

Chemical analysis of the black crust obtained by

Element

% of weight

% Error

C Mg Al Si S K Ca Fe Na P Total

38.75 1.21 3.58 16.43 14.45 4.28 1.39 18.25 0.35 1.31 100

± 2.24 ± 0.13 ± 0.15 ± 0.32 ± 0.40 ± 0.30 ± 0.15 ± 0.89 ± 0.17 ± 0.16

Figure 7. “Blue Gun” of cleaning laser during works on the Nickerson Mansion.

Figure 6. Raman data comparing fireplace soot, candle soot, furnace soot, and our samples a, b, c taken from the Nickerson Mansion. The Raman data were collected with a Renishaw 1000 Raman microscope system.The samples were excited using a laser of 514 nm wavelength.

– Spot size: 0.5 mm – Focal diameter: 400 µm. This system uses a fibre optic to transfer light energy from the laser unit to the hand piece (blue gun or stylus). The length of the fibre is of extreme importance because it allows a conservator to work freely without moving the laser unit frequently. Our system had a 30 m fibre optic cable. The length of the cable allowed us to work a few days without moving the unit. The use of the fibre optic system allowed to keep the laser unit itself in a temperature-controlled environment while the fibre extended to the outside of the building. 3 ADVANTAGES OF THE USE OF LASERS Using a laser with an extended fibre optic cable to clean a building gives a conservator the opportunity to work on a project regardless of weather conditions.

We worked continuously from June of 2004 until the project was completed in November of 2005. During winter, the lasers were kept inside of the building and only the cleaning heads were taken outside onto the scaffolding where the cleaning was performed (Fig. 7). The work area was shielded with plastic and heated by blowing warm air into the workspace. During the hot Chicago summer the laser unit was kept inside as well, but this time in an air conditioned room in order to prevent overheating of the system. The laser is cooled by water, therefore it is very important to keep the unit away from extreme cold and heat. In order to achieve a consistent surface appearance some areas were cleaned dry while other more contaminated areas were ablated after spraying with water.The addition of a minimal amount of water improved the laser cleaning process and more pollution was removed due to the refractive and reflective properties of water. The wetting process was also used during wintertime because it takes only seconds to spray the water onto the surface and then clean the same area. Therefore, there was no risk of freezing of the sprayed water. We used one laser throughout the majority of the project but towards the end, in order to meet the deadline of the project, we had to use a total of three 120 watt Adapt (Clean) lasers. The level of pollutant accumulation varied among the stones and different parts of the building. The south elevation was the most difficult to clean. During the testing phase of the project we were allowed to work on the east side of the building, which was the easiest to clean.The cleaning rate for this part of the building was misleading compared to the other parts of the building which took much longer to clean. It is strongly suggested to perform tests on different parts of a building prior to giving a cost and time estimate for a project. The laser successfully cleaned all of the stones to the same degree of desired uniform appearance

211

Figure 8. Photomicrograph taken in plain light 36× magnification. The left side represents the condition of the stone before treatment and the right side is after laser cleaning.

Figure 9. This actively delaminating area has been laser cleaned. The flaking stone, if touched, would have fallen off of the surface. Laser ablation did not affect this unstable condition. Only laser ablation allows us to clean severely deteriorated stone so that it can be consolidated following the cleaning and further repairs can be done.

(Figs. 8–12). This cleaning technique can have numerous applications for the treatment of architectural elements and objects. This was a very challenging project due to the size of the building and the many intricate carvings in the stone.

4

DISCUSSION OF PRACTICAL ASPECTS OF LASER TREATMENT

Lasers can be used to clean stones in various conditions. Traditional cleaning methods do not allow to clean safely deteriorated stones. Flaking or powdery areas are impossible to clean using chemical or mechanical methods. Only the use of lasers allows a

Figure 10. Severely deteriorated freeze area during cleaning. Middle section was laser cleaned.

Figure 11. Laser cleaned flat and carved stone. When examined closely, tool marks can be easily identified.

conservator to safely remove black crust from a deteriorated stone (Figs. 9, 10). The use of laser technology allowed also to work all year round. Work was done above pedestrians walking under our scaffolding. This was possible, because it was possible to vacuum all of the ablated material. There was no hazardous byproducts (such as acidic runoff ) of the cleaning process that could contaminate the surrounding area. On request of the owner, the flat stones was cleaned to a pre-selected level and the stones framing windows were left slightly darker to achieve a desired surface effect. This level of quality control is possible only with lasers. At the present time, laser cleaning is slower than traditional methods. It was possible to clean from 1 square foot to 5 square feet of stone per hour. However, the slow cleaning rate is offset by the quality, precision, and inert character of this cleaning method. The laser used provided with a much faster cleaning rate than other laser systems tested. Based on these experiences,

212

Figure 12. By using laser ablation we were able to clean flat surfaces as well as complicated architectural details, revealing tool marks and surface details. Please note the very clear marks left on the stone by a pointing chisel. The marks look as if they were recently made although this wall is over 100 years old.

Figure 13. An example of incorrect use of a laser during a test performed by another company. This picture illustrates user-created lines as a result of a mistake during surface cleaning. The user divided this area into small squares and then worked in a straight up and down fashion.

the Clean laser provides one of the fastest cleaning rates of a laser available today for conservation treatments. The lasers were used for approximately 15 hours per day, six days per week. Technical problems arose with the equipment, but considering the fact this equipment was used almost nonstop, it was concluded that Adapt (Clean) lasers are durable and appropriate for use on large-scale architectural projects. It is important to note that quick service, when needed, from the manufacturer is of great importance. Using two or more lasers is strongly recommended when working on a large project. In addition to a faster cleaning rate, having two or more units is important for progress if

Figure 14. Prior to our treatment, vines were covering the west facade of the building.

one of the lasers has a technical problem or requires maintenance. Various laser systems have different properties of what is called the “cleaning spot”. Clean lasers use an oscillating mirror to move the cleaning spot from left to right. The spot oscillates from 50 to 150 times per second to form a line. The beam spends more time at both endpoints of the “cleaning line,” before changing direction, and as a result, the ends of the cleaning zone are exposed to more ablation impacts (hot spots). This phenomenon can be used to the advantage of the conservator, but it can also create lines, as in the picture above (Fig. 13), when the user thinks of the equipment as a systematic tool doing repetitive cleaning job for him/her. Moving the laser head repetitively up and down will create lines as in the above picture. In order to eliminate this effect the “hot spots” of the beam can be blocked with aluminum foil (blue gun) or the cleaning head has to be moved in an irregular fashion: left, right, and up and down, or in circular motions. Constantly moving the laser head while cleaning is the easiest way to avoid this problem. Other laser systems using a non-oscillating cleaning spot create a “dotted” effect. In order to avoid this undesired surface appearance it is necessary to overlap the cleaned spots / areas. The need for overlapping reduces the obtainable cleaning rate. Based on practical observations, it is much more difficult to create an even surface appearance using a laser providing a small cleaning spot as opposed to a larger one or a line beam as in the case of cleaning lasers. The quality of the cleaning is always in the hands of a conservator. It is important to remember that although laser ablation can be considered a self-limiting process, it is not an intelligent self-controlling solution to conservation problems. The skill and care of the operator makes all the difference. Vines were covering large portion of the west facade of the Nickerson Mansion (Fig. 14). At the beginning stages of the project their roots were cut and

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the vines were left to dry for about a year. The large and small branches were removed prior to laser cleaning. In order to provide the fastest cleaning result of this area, razor blades had to be used to cut off most of the organic material. Laser ablation of the organic matter was quick and successful once most of it was removed mechanically. It was discovered that the laser did not remove thicker organic material. If the beam was allowed to dwell on a spot with a residue of a vine in place it would burn the vine and create a dark stain on the stone. Therefore, as much of the organic material as possible was mechanically removed and then proceeded with laser cleaning of the stone.

ACKNOWLEDGEMENTS

5

REFERENCES

CONCLUSIONS

Laser ablation is a revolutionary way to clean buildings and architectural structures. It is extremely important to test the effect of the laser beam on several samples of the stone to be cleaned prior to starting the treatment. The use of lasers will likely become more common when environmental control requirements become more stringent. Lasers, regardless of their cleaning rates, will be used on important historical structures due to the quality of the final product.

It was a great pleasure to work with Mr. Joe Antunovich and Bill McMillan of Antunovich Associates Architects during the cleaning of the exterior of the Nickerson Mansion. The first laser cleaning of an architectural structure in the USA was possible due to the trust and visionary thinking of Mr. Richard Driehaus, who provided funding for this project. I would like to thank my staff (CSOS Inc) for enduring long work hours in often unpleasant temperatures, and my son Bartosz A. Dajnowski for his help with editing this paper.

Folson, M. 1963. Grate American Mansions and Their Stories, Hastings House, Publishers, New York. Love, R. H & Worley, M. P. 1998. The Samuel M. Nickerson House of Chicago Neo-renaissance Palazzo and Private Art Gallery of the Gilded Age, R.H. Love Galleries, Chicago. Hart, D. & Wang, H. F. 1995. Laboratory measurements of a complete set of poroelastic moduli for Berea sandstone and Indiana Limestone, Journal of Geophysical research, 100: 17741–17751.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Laser cleaning of a set of 18th century ivory statues of Twelve Apostles A. Koss Inter-Academy Institute for Conservation and Restoration of Works of Art, Academy of Fine Arts in Warsaw, Poland

D. Dre´scik Academy of Fine Arts, Kraków, Poland

J. Marczak, R. Ostrowski, A. Rycyk & M. Strzelec Institute of Optoelectronics, Military University of Technology, Warsaw, Poland

ABSTRACT: During the changes of temperature and humidity, ivory expands and contracts which can cause its warping, torsional deflection and fracturing. Therefore, particularly difficult in artworks restoration is the removal of thick clusters of greyish-yellow encrustation, well bounded to the ivory substrate in cracks and at deeper reliefs or engravings. This paper shows that ivory objects, sensitive to conventional restoration processes, could be effectively and safely cleaned using laser techniques. This method has been utilised for the cleaning of the Twelve Apostles collection (18th century), property of Wawel Cathedral Treasury in Cracow. The laser used was a Q-switched Nd:YAG system ReNOVALaser1, with a pulse width of about 8 ns, output energy up to 120 mJ, repetition rate 1–20 Hz, wavelength 1064 nm and beam delivery through optical fibre. We also present ivory damage threshold for different Nd:YAG laser harmonics (1064, 532 and 355 nm), determined by means of tunable ReNOVALaser5 system.

1

INTRODUCTION

The main aim of laser cleaning is removal of encrustation and recovery of the primary appearance without violation of the original material of the artwork. Conservation procedures often concern objects made of fragile and susceptible to damage materials. In such cases, laser cleaning is an irreplaceable technique. The above mentioned problems concern, among others, figures and sculptures carved out on different kinds of bones. The most common material is ivory (Strzelec et al. 2005). Authors categorize “ivory” as a material descendent from elephant tusks (African and Indian), considering dentine, i.e. mammoth, walrus, hippopotamus and boar tusks and mammals skeleton bones – as natural substitutes (Mann & Espinoza 1992). The main problem connected with bone objects is their hygroscopicity. During the changes of temperature and humidity ivory expands or contracts which can cause its warping, torsional deflection and fracturing. Moreover, ivory yellows with the lack of light and air, and loses its natural elasticity as the content of collagen decreases. Almost in every case, bone objects are covered with a thin layer of inorganic patina and organic compounds – mainly fats. Thicker clusters of

such encrustation, well bounded to the substrate occur inside deeper reliefs or cracks. Ivory structure and its physical-chemical properties generate problems to determine the optimal cleaning procedures. The anisotropy and hygroscopicity of those materials excludes the utilisation of aqueous solutions in their conservation. The acid, basic solutions and organic solvents as well as abrasive techniques are not recommended due to the possibility of artifact scratch or crumbling. Utilisation of lasers in treatment of living bone tissues has been described in the literature in detail. However, in the case of ancient objects, most of the reports were related to fossils (Landucci et al. 2000, 2003). Only one publication, to the authors’ knowledge, describes values of safe doses of laser energy in ivory cleaning process (Madden et al. 2003). On the other hand, application of safe laser cleaning techniques needs determination of ivory substrate damage threshold, i.e. laser fluence threshold values (Ostrowski et al. 2007). 2

OBJECT CHARACTERISTIC AND SOIL DESCRIPTION

Ivory figures of Twelve Apostles, property of Wawel Cathedral Treasury in Cracow, were sculpted in 18th

215

Table 1.

Specifications of ReNOVALaser systems.

Parameter

ReNOVA-Laser1 ReNOVA-Laser5

Wavelength [nm]

1064

Output energy [mJ] Pulse duration [ns] Repetition rate [Hz] Beam diameter [mm] Beam delivery

120 8 1–20 6 Optical fibre

1064; 532; 355; 266; 214 700 (1064 nm) 15 1–10 8 Pantograph

Figure 1. Photographs of fragments of the statues before restoration. Left side – bust of St. John; right side – pedestal of St. Philip.

century by an unknown artist. Sculptures stand on ivory pedestals, mounted with iron screws. Photographs of fragments of the statues before restoration are shown in Figure 1. Soil deeply penetrated all scratches, fractures and slots. Moreover, in many places thin layers of dirt formed pits into ivory structure. Yellowing, although present at the whole sculpture surface, is mostly visible at its backside. Longitudinal pits and local branching were caused by ivory anisotropy and hygroscopicity, accelerated due to thermal-humidity air variations. Hair cracks occur around neck, hands, legs and trunk of figures. Dark grey-green soils are deep seating in the slots of pits, which contributes to further ivory disintegration as well as disturbs aesthetic reception. Typical soil includes corrosion products of iron screws and nails, discolorations caused by metal joints and wax, glue splashes. Moreover, the whole object is covered with a thin layer of grey inorganic patina and aliphatic organic compounds.

3

LASER CLEANING AND DETERMINATION OF IVORY DAMAGE THRESHOLDS

As it was stated earlier, except some experimental results for different lasers (Nd:YAG, excimer), wavelengths and pulse durations, we did not found in the literature investigations of bone and ivory damage thresholds dependence on laser fluence and wavelength for Q-switched, Nd:YAG laser. Such measurements were the first step preceding cleaning of the ivory figures of Twelve Apostles. The ivory damage thresholds for different Nd:YAG laser harmonics (1064, 532 and 355 nm) were determined by means of tunable ReNOVALaser5 system, installed in a laboratory stand also equipped with diagnostic optical microscopes. Except for visual damage of the ivory surface, the laser irradiation can also influence on the protein collagen in superficial layers. Burning of ivory has been reported (Madden et al.

Figure 2. Experimental setup for the determination of ivory damage threshold.

2003) and observed far above visual damage threshold. This effect was not registered in the present study. The laser used in cleaning was a Q-switched Nd:YAG system ReNOVALaser1 (1064 nm). Equipped with a system of optical fibre beam delivery, this laser head was much more suitable for precise cleaning of the small figures of Twelve Apostles. Specifications of both laser systems used are shown in Table 1. In all cleaning procedures, laser fluence was kept below 4 J/cm2 , four times below the ivory damage threshold (Table 2). This safety margin was the result of a twice shorter pulse duration of ReNOVALaser1 as compared to ReNOVALaser5 system used in the determination of ivory damage threshold. The absence of damage was confirmed during the initial set of comparative laboratory measurements. The value of laser fluence which causes damage to the surface of the ivory sample, was determined by means of the Z-scan method with the experimental setup shown in Figure 2 (Ostrowski et al. 2007). Figure 3 shows ivory samples. The sample under investigation, placed in the path of the laser beam close to the lens, was then moved 5 mm each step in the direction of the beam waist plane (thus increasing energy density on the sample surface) and exposed to one pulse, until clear damage

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Figure 3. Ivory samples used in damage threshold determination. Table 2. Damage thresholds of ivory for different Nd:YAG laser wavelengths. Generation

Single pulse

10 Hz

Wavelength [nm] Threshold [J/cm2 ] (sample 1) Threshold [J/cm2 ] (sample 2)

1064 >20

532 18.6

355 3

1064 7

532 5

355 0.9

−

>> 9

–

–

3.5

0.9

to the surface was observed with the naked eye. The sample was then removed and examined with a microscope to check if the damage indeed occurred and to determine exactly where. Then, the sample was placed again in the laser beam path in a position shifted by 5 to 10 mm from initial position, where damage was detected, and the sample was moved 1 mm each step in the direction of the beam waist plane in order to locate the exact position, and therefore exact laser fluence, where damage occurs. It has to be mentioned that every sample was also moved crosswise to the axis of laser beam in order to expose a fresh unprocessed sample surface to the laser pulse and, on the other hand, to average on the surface, thus minimising the influence of potential heterogeneity or the presence of surface cracks on the determined damage thresholds. Results of measurements are presented in Table 2. Results of laser cleaning are shown in Figures 4, 5. Soil clusters inside 0.1–1 mm slots were several times cleaned using focused laser beam, in all cases not exceeding a fluence of 4 J/cm2 . 4

Figure 4. Statue of St. John (a) and pedestal of St. Philip (b) after laser cleaning. The small photograph at the top shows the pedestal before restoration (Fig. 1).

DIAGNOSTICS

It is known, that encrustation is non-homogeneous across the whole object surface, and does not possess the same thickness, structure and even colour. One of the few physical parameters allowing description of encrustation characteristics is the average reflection-backscattering coefficient of white light (or laser light). Spectrometric measurement of the amplitude of the backscattered white light as a function of the wavelength represents synonymous and objective

Figure 5. View of the collection of the ivoryTwelveApostles after laser cleaning.

colorimetry, frequently used for fast and suitable determination of the cleaning level of different substrates, included ivory (e.g. Marczak 2001). The task of the fibre optics spectrometer shown in Figure 6 was to detect the amplitude of the

217

depending on, for example, the level of cleaning of the investigated surface (Fig. 7). 5

Figure 6. Scheme of the diagnosis system with fibre optics spectrograph for the investigation of reflection (scattering) coefficient of superficial layers.

CONCLUSIONS

The results of laser cleaning of the Twelve Apostles collection were surprisingly good. The use of a laser beam allowed to remove soil from almost all hard to get relief areas. The condition of the ivory surface after laser treatment has been tested using a standard microscope (50× magnification). Thanks to earlier measurements of ivory damage thresholds and careful adjustment of laser fluence, there were no observable destructions of the primary ivory substrate. It is difficult to compare the obtained results with the results of other methods of ivory cleaning, although comparative chemical tests with detergents and mechanical cleaning with the use of glass fibre stick were both arduous and time consuming. It should be stated that proposed laser technology is fast, ecological (no solvents) and safe for the object, if we take into consideration the problems connected with the hygroscopicity and anisotropy of ivory, signalised in the introduction of this paper. Another advantage is the absence of contact of the cleaning tool (laser beam) with the delicate object surface. ACKNOWLEDGEMENTS Work has been supported by the Ministry of Science and Higher Education, Poland, project 120/E-410/ SPB/EUREKA/KG/DWM 97/2005-2007.

Figure 7. Results of measurements of light amplitude scattered from ivory for different laser cleaning levels.

REFERENCES

backscattered light where cleaning was carried out. Light emitted by an halogen lamp (mercury, sodium lamp or even another laser) is delivered to the cleaned surface of the object by the central optical fibre. Backscattered light is collected by six other fibres surrounding the central one. Collected light is then transmitted through the optical system to the diffraction grating of the spectrometer and, after dispersion, to the linear matrix of CCD detectors. Finally, it is displayed on the monitor of a computer. The distance of the measurement tip from the examined surface was selected in such a way to obtain a maximum of scattered light for ivory fracture. In the present case, fresh ivory determined a reference for comparison to other values of light scattering

Landucci, F. et al. 2000. Laser cleaning of fossil vertebrates: a preliminary report. J. Cult. Heritage 1: S263–S267. Landucci, F. et al. 2003. Toward an optimized laser cleaning procedure to treat important paleontological specimens. J. Cult. Heritage 4: S106–S110. Madden, O. et al. 2003. Removal of dye-based ink stains from ivory: evaluation of cleaning results based on wavelength dependency and laser type. J. Cult. Heritage 4: S98-S105. Mann, E. O. & Espinoza, M. J. (ed.) 1992. Identification Guide for Ivory and Ivory Substitutes. 2nd edition. Baltimore: WWF Publications. Marczak, J. 2001. Surface cleaning of art work by UV, VIS and IR pulse laser radiation. Proceedings of SPIE 4402: 202–209. Ostrowski, R. et al. 2007. Laser damage thresholds of bone objects. Proc. SPIE 6618. In print. Strzelec, M. et al. 2005. Results of Nd:YAG laser renovation of decorative ivory jug. Springer Proceedings in Physics 100: 163–168.

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Laser Cleaning of Paintings and Polychromes

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Investigating the use of the Nd:YAG laser to clean ancient Egyptian polychrome artefacts C. Korenberg, M. Smirniou & K. Birkholzer The British Museum, London, United Kingdom

ABSTRACT: The aim of the present study was to investigate whether laser cleaning using a Nd:YAG laser would be a suitable technique to treat ancient Egyptian polychrome artifacts. Red ochre, realgar, calcite, gypsum, yellow ochre, orpiment, green frit, malachite and Egyptian blue pigments were mixed with gum arabic. Samples were prepared to investigate whether parameters such as the binder concentration or the nature of the substrate affect the damage threshold of the paints. For most of these paints, none of these factors was observed to have a significant effect. The ablation thresholds of lamp black and five consolidants were measured and found to be higher than the damage thresholds of most paints. Also, when removing lamp black, yellowing was observed on some of the substrates studied. It was concluded that laser cleaning at these laser parameters would not be a suitable method to remove consolidants or soot from ancient Egyptian polychrome artefacts.

1

INTRODUCTION

The British Museum holds an extensive collection of ancient Egyptian polychrome coffins and wall paintings. These artifacts usually have a surface that is absorbent and vulnerable, attracting dust particles into the porous painted layer. Deposits on wall paintings and painted coffins can include: dust, soot, salts, pollutants, old conservation materials and residues from previous restorations. Conservators employ different methods to clean painted surfaces, depending on the physical structure and stability of the pictorial layers and the composition and thickness of the deposits to be removed. Dry erasers and sponges, such as vulcanised latex or smoke sponges of vulcanised natural rubber, are used to remove dry dust and loose soot or grit, but they are not appropriate for heavy deposits. Mechanical cleaning using scalpels or dental tools is a common way to remove solid encrustations and salts. However, this can cause scratches and surface damage and the outcome depends on the skills and experience of the conservator. Wet cleaning is used to remove soot, grime, soluble salts or organic deposits from surfaces that tolerate wet treatment. This technique is not entirely satisfactory as applying a solution may redeposit dirt instead of removing it. Many polychrome Egyptian artifacts in the British Museum are very difficult to clean using the conventional methods mentioned above as these risk damaging the delicate pictorial layers. These artifacts include an Egyptian coffinAES 6690, which is covered

Figure 1. Fragment of the Nebamun mural, EA 37976, on which a past conservation coating has become dark and unsightly.

by soot from fire damage, and a fragment of the Nebamun mural, EA 37976, on which a past conservation coating has become dark and unsightly (Fig. 1). As reviewed by Fotakis et al. (2007), there have been several case studies reporting successful laser cleaning of painted surfaces covered with contaminants, such as soot or aged resins. However, laser irradiation risks altering the colour of certain paints due to phase changes or decomposition reactions (Chappe et al. 2003, Pouli et al. 2003, Sansonetti & Realini 2003) and it is necessary to assess the effect of laser irradiation on every paint on which it is to be used. The effect of laser irradiation on ancient Egyptian paints has not been previously studied and the

221

aim of the present research was to investigate whether laser cleaning using a Q-switched Nd:YAG laser would be suitable for cleaning these artifacts. As binder concentration in ancient Egyptian paints is probably variable and the paints were diluted to different extents and applied on different substrates, it was first investigated whether these factors have an effect on the damage thresholds of the paints. Further, the ablation thresholds of lamp black and five consolidants that are currently used or were used in the past to treat painted surfaces were determined to assess whether such contaminants could be removed safely from ancient Egyptian polychrome artifacts using laser light. Some samples were also thermally aged to investigate the effect of ageing on the values of the damage thresholds of the paints and ablation thresholds of the consolidants. The results are presented and discussed below.

2 2.1

EXPERIMENTAL Paint samples

This project is focussed on the main paints used by the ancient Egyptians before the Roman period and the following pigments were selected for study: red ochre, realgar, calcite, gypsum, yellow ochre, orpiment, ‘green frit’ and Egyptian blue. Although malachite was rarely used as a pigment by the ancient Egyptians, it was included in this study as it is present on polychrome artifacts as a degradation by-product of green frit and Egyptian blue (Lee & Quirke 2000). All the pigments except green frit were purchased from Kremer and Cornelissen artists materials suppliers; green frit was synthesised in the laboratory following the method published by Pages-Camagna & Colinart (2003). The pigments were analysed prior to use using Raman spectroscopy to confirm their composition and the spectra obtained were found to match the reference spectra of the corresponding pigments. Some pararealgar was detected in the realgar pigment, probably formed upon exposure to light during the manufacturing process -realgar is known to transform into pararealgar when exposed to light (Douglass et al. 1992). When investigating the laser cleaning of painted artifacts, it is necessary to conduct tests on paints made with the same binder as the nature of the binder has an effect on the damage threshold of paints (Hildenhagen et al. 2005, Schnell et al. 2005). Gum arabic dissolved in water was used in the present work as this was the binder the most widely used by the ancient Egyptians (Newman & Serpico 2000). It should be noted that the effect of the binder on the damage threshold of paints is complex. It has been shown that there is no common principle that allows predictions to be

made for the damage threshold of a paint made with a specific binder and pigment (Hildenhagen et al. 2005). Paints were prepared with different concentrations of gum arabic to determine the effect of the binder concentration on the damage threshold. The paints were also diluted with increasing amounts of water to investigate whether thin layers of paint have a different damage threshold than thicker layers. 2.2 Substrates The nature of the substrate has been reported to affect the damage threshold of paints and this is thought to be due to the different thermal conductivity and diffusivity of different substrates (Gordon Sobott et al. 2003). In the present study, different substrates representative of Egyptian artifacts were used. Paints on ancient Egyptians artifacts were generally not applied directly on stone or on wood, but on a ground of gypsum or crushed calcite (Lee & Quirke 2000, Middleton 1999, Middleton 2000). Unlike gypsum, calcite cannot be used on its own as a ground and it was probably mixed with a binder. To date, no information is available on the nature of the binder in calcite grounds of ancient Egyptian artifacts and in the present study gum arabic was used to make calcite and gypsum grounds. Thin layers of paints were applied on slabs of gypsum, cardboard covered with gypsum mixed with gum arabic and cardboard covered with calcite mixed with gum arabic. To investigate the effect of the laser irradiation on the paints with a negligible contribution from the substrate, thick and opaque layers of paints made with a low binder concentration were also applied on cardboard. 2.3 Materials to be removed The removal of lamp black and five consolidants was investigated. The consolidants selected for the present project were animal glue, soluble nylon (N-methoxymethyl nylon), microcrystalline wax, Primal AC33 (ethylacrylate methyl methacrylate copolymer), Paraloid B72 (ethyl methacrylate copolymer) and Mowital B30H (polyvinyl butyral polymer). Animal glue, soluble nylon and microcrystalline wax were commonly used in the middle of the past century for consolidating flaking paint. Since then, it has been shown that these materials are not suitable for long-term conservation treatments and should be removed from artifacts. For example, soluble nylon yellows upon ageing (Sease 1981), while animal glue discolours, shrinks and becomes brittle resulting in potential damage to paint layers. Upon ageing these consolidants tend to become very difficult to remove using conventional techniques. Primal AC33, Paraloid B72 and Mowital B30H are currently used in conservation and, as it may be desirable to remove them in some instances, it was

222

assessed whether laser cleaning would be a suitable technique. Lamp black was applied on a slab of gypsum, cardboard covered with gypsum mixed with gum arabic and cardboard covered with calcite mixed with gum arabic, while the consolidants were applied on slabs of gypsum. Paraloid B72 was prepared as a 2.5% (by weight) solution diluted in a 1:1 acetone/IMS (industrial methylated spirits) mixture, Primal AC33 as a 5% (by weight) dispersion in water and Mowital B30H as a 3% (by weight) solution in a 1:1 acetone/IMS mixture.

Table 1. Damage threshold at 1064 nm of the red ochre paint prepared with increasing amounts of gum arabic solution and diluted to different extent. The paints were applied on cardboard strips covered with either gypsum or calcite grounds.

2.4 Ageing tests To assess the effect of thermal ageing on the values of damage and ablation threshold, sets of samples of paints and consolidants were aged at 60◦ C and 70◦ C for 28 days in a dessicator in which the relative humidity was controlled at approximately 50% using a glycerol solution. 2.5

Laser cleaning tests

Tests were conducted using a Lynton Phoenix Q-switched Nd:YAG laser emitting 5–10 ns pulses at wavelengths 1064 nm and 532 nm. The average fluence was calculated by dividing the energy per pulse by the area of the laser spot. The size of the laser spot was estimated by taking a burn pattern on a photographic paper. Results were assessed visually, using a magnifier and using an optical microscope. 3

3.1

EFFECT OF THE BINDER CONCENTRATION, DEGREE OF PAINT DILUTION, SUBSTRATE AND AGEING

Composition of the paint

Gypsum J/cm2

Calcite J/cm2

0.1 g in 0.2 mL solution 0.1 g in 0.4 mL solution 0.1 g in 0.8 mL solution 0.1 g in 1.6 mL solution 0.1 g in 3.2 mL solution 0.1 g in 6.4 mL solution Same as above, but diluted Same as above, but diluted Same as above, but diluted

0.24 0.24 0.20 0.27 0.16 0.14 0.33 0.32 0.22

0.34 0.34 0.30 0.24 0.23 0.24 0.32 0.21 0.16

Table 2. Damage thresholds of the paints applied on different substrates at 1064 nm. The substrates employed were A: slabs of gypsum, B: cardboard covered with gypsum mixed with gum arabic, C: cardboard covered with calcite mixed with gum arabic and D: cardboard. Damage threshold at 1064 nm J/cm2 Paints

A

B

C

D

Red ochre Realgar Yellow ochre Orpiment Egyptian blue Malachite

0.45 * 0.46 0.42 0.71 0.25

0.24 * 0.30 0.23 0.64 0.23

0.26 * 0.44 * 0.40 0.15

0.21 * 0.24 * 0.60 0.32

* Alteration was noted at the lowest fluence allowed by the set up (approximately 0.14 J/cm2 ).

Effect of the binder concentration and degree of paint dilution

As illustrated in Table 1 for red ochre, there was no discernible trend in the value of the damage threshold when the paints were prepared with various concentrations of gum arabic and diluted to different concentrations. Similar results were obtained for the other paints. This suggests that the binder concentration and degree of dilution of the paint had no detectable effect on the damage threshold. It was noted that there was a large variability in the value of the damage threshold of the paints. This was thought to be due to a certain extent to the heterogeneity of the paints which were mixed and applied by hand, but also to the laser itself. The fluence of the Nd:YAG laser is not uniform over the area of the beam with some hot spots present in the beam and damage to the paints seemed to occur at the location of the hot spots. The performance of the employed laser is optimized by

the manufacturer at the maximum energy output such that the beam quality is relatively poor at low energy levels. Since the energy used in the experiments was very low for most of the paints, it is suspected that at a given average fluence, the local fluence at the location of the hot spots was quite variable from pulse to pulse, contributing to the observed variability of the damage threshold. 3.2 Effect of the nature of the substrate The values of the damage threshold of the paints applied on different substrates are given in Tables 2 and 3. These values varied between different substrates, but the variations were within the experimental variations noted earlier and it was concluded that the nature of the substrate had no detectable effect on the damage thresholds of the paints.

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Table 3. Damage thresholds of the paints applied on different substrates at 532 nm. The substrates employed were A: slabs of gypsum, B: cardboard covered with gypsum mixed with gum arabic, C: cardboard covered with calcite mixed with gum arabic and D: cardboard.

Table 4. Damage thresholds of unaged and thermally-aged paints applied on cardboard. (Ageing was conducted at 60◦ C.) 1064 nm J/cm2

532 nm J/cm2

Damage threshold at 532 nm J/cm2

Paints

Unaged

Aged

Unaged

Aged

Paints

A

B

C

D

Red ochre Realgar Yellow ochre Orpiment Egyptian blue Malachite

0.04 0.09 0.06 * 0.27 0.23

* 0.03 0.04 0.06 0.39 0.10

* 0.05 * 0.06 0.43 0.13

* 0.03 0.04 0.03 0.25 0.12

Red ochre Realgar Yellow ochre Orpiment Malachite

0.21 * 0.24 * 0.32

0.16 * 0.24 * 0.34

0.02 0.03 0.04 0.03 0.12

0.02 0.02 0.04 0.03 0.20

* Alteration was noted at the lowest fluence allowed by the set up (approximately 0.01 J/cm2 ).

However, it was noticed that discolouration was perceived more easily on some substrates for some of the paints and this may affect the experimental determination of the damage threshold. For instance, orpiment tends to become paler when irradiated at 1064 nm and it was easier to perceive fading on the white calcite ground than on the off white gypsum ground. Also, for some paints, such as Egyptian blue, laser irradiation caused the removal of pigment particles rather than discolouration and this was in general more easily perceived on the thick paint layers applied on cardboard than on the thin layers applied on the other substrates. This highlights the limitations of visual assessment and future work should focus on developing methods to assess laser-induced damage on paints in a systematic way.

3.3

Effect of thermal ageing

To investigate the effect of thermal ageing on the damage thresholds of the paints, two sets of samples made with the same concentrations of gum arabic and diluted to the same extents were applied on cardboard and one set of samples was aged at 60◦ C for 28 days. The appearance of the paints did not change upon ageing. The values of the damage thresholds for the unaged and aged paints are shown in Table 4. No significant change in the damage threshold was observed. To further investigate the effect of thermal ageing, two additional sets of paint samples were prepared for ageing at 70◦ C. Since cardboard may be affected at this higher temperature, the paints were applied on gypsum slabs. The Egyptian blue and malachite paints acquired a brown tinge upon ageing. Egyptian blue paints on ancient artifacts are often found to have become brown with time and this has been attributed to the browning of gum arabic combined with the poor hiding power

* Alteration was noted at the lowest fluence allowed by the set up at 1064 nm (approximately 0.14 J/cm2 ). Table 5. Damage thresholds of unaged and artificially aged paints applied on gypsum slabs. (Ageing was conducted at 70◦ C.) 1064 nm J/cm2

532 nm J/cm2

Paints

Unaged

Aged

Unaged

Aged

Red ochre Realgar Yellow ochre Orpiment Egyptian blue Malachite

0.45 * 0.46 0.42 0.71 0.25

0.34 * 0.28 0.29 0.68 0.29

0.04 0.09 0.06 0.01 0.27 0.23

0.02 0.04 0.04 0.02 0.28 0.21

* Alteration was noted at the lowest fluence allowed by the set up (approximately 0.14 J/cm2 ).

and transparency of the pigment (Daniels et al. 2004). It is likely that the same factors are responsible for the browning of the malachite paint observed here. The appearance of the other paints did not change upon ageing. The values of the damage thresholds for the unaged and aged paints are shown in Table 5. The damage threshold values were observed to decrease after ageing for some of the paints, however these changes were within the experimental variations. From these two series of tests, it was concluded that thermal ageing did not affect the damage thresholds of the paints detectably.

4

EVALUATING THE SUITABILITY OF LASER CLEANING FOR EGYPTIAN POLYCHROME ARTEFACTS

4.1 Threshold values of the paints Table 6 compiles the values of the damage thresholds of the paints from all the tests. The damage thresholds are generally higher when using the 1064 nm wavelength than the 532 nm wavelength.This trend has been

224

Table 6. Damage threshold ranges of the paints measured at 1064 and 532 nm.

Table 8. Ablation thresholds of the thermally-aged consolidants.

Paints

1064 nm J/cm2

532 nm J/cm2

Consolidants

1064 nm J/cm2

532 nm J/cm2

Red ochre Realgar Yellow ochre Orpiment Egyptian blue Malachite Green frit* Calcite* Gypsum*

0.14–0.45 <0.14–0.19 0.20–0.46 0.10–0.42 0.40–0.71 0.16–0.32 1.10 1.92 0.96

<0.01–0.04 <0.01–0.09 0.01–0.06 0.01–0.10 0.27–0.42 0.08–0.23 1.02 1.39 0.69

Soluble nylon Animal glue Microcrystalline wax Primal AC33 Mowital B30H Paraloid B72

1.7 2.5 2.0 2.6 2.4 2.4

1.2 1.5 1.2 1.5 1.4 1.4

* The damage threshold was only measured for paint applied on cardboard. Table 7. Ablation thresholds of the unaged consolidants. Consolidants

1064 nm J/cm2

532 nm J/cm2

Soluble nylon Animal glue Microcrystalline wax Primal AC33 Mowital B30H Paraloid B72

1.9 1.8 1.8 2.1 1.9 1.9

1.1 0.9 0.9 1.0 1.0 1.0

observed by other authors working on different paints, see for instance Abraham et al. (2005). Except for Egyptian blue, calcite, gypsum and green frit, the values of the damage threshold of the paints are relatively low. In particular, laser cleaning would be unsuitable for realgar at 1064 nm and red ochre, realgar, yellow ochre and orpiment at 532 nm as the paints were altered by the laser irradiation at very low fluences. 4.2

Removal of consolidants

Two sets of consolidants were applied on slabs of gypsum and one set was thermally-aged at 70◦ C for 28 days. After ageing, the animal glue and soluble nylon samples had yellowed considerably and this is usually observed for naturally-aged animal glue and soluble nylon samples. The appearance of the other consolidants was the same as the unaged consolidants. The values of the ablation thresholds of the unaged and thermally-aged consolidants are shown in Tables 7 and 8 and, except for the calcite paint at 532 nm, they were higher than the values of the damage threshold of the paints under study, suggesting that the laser would be unsuitable to remove consolidants from Egyptian polychrome surfaces. It is interesting to note that the removal of thermallyaged consolidants systematically required a higher fluence than unaged consolidants and further work is planned to investigate this effect. It was observed,

Figure 2. The surface of the gypsum slab was pitted after removing animal glue using the laser at 1064 nm (horizontal field of view: 9 mm).

however, that at both wavelengths the surface of the laser cleaned slab was pitted (Fig. 2). The values of the damage threshold of gypsum at 1064 and 532 nm were measured and found to be 1.4 J/cm2 and 0.9 J/cm2 respectively. This is in good agreement with the values quoted in the literature for plaster: Tanguy et al. (2005) reported damage at 1064 nm at fluences greater than 1.41 J/cm2 , whereas Sokhan et al. (2003) found that the values of the damage threshold were 1.0 J/cm2 at 1064 nm and 0.8 J/cm2 at 532 nm. The higher values of the ablation threshold of the consolidants as compared to the value of the damage threshold of gypsum explain why the laser cleaned surface of the slab was pitted. 4.3 Removal of lamp black It was possible to remove lamp black from the gypsum slab at a fluence of 1.0 J/cm2 at 1064 nm and at 0.34 J/cm2 at 532 nm. However, the surface that was laser cleaned at 1064 nm had a slightly yellow hue and the ablation threshold of lamp black at 532 nm was higher than the damage thresholds of most paints. Lamp black could be removed from the cardboards covered with calcite and gypsum mixed with gum

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arabic at a fluence of 0.48 J/cm2 at 1064 nm and 532 nm, but yellowing was observed on both substrates at 1064 nm and on the calcite substrate at 532 nm. This series of experiments suggests that laser cleaning would be unsuitable to remove soot from Egyptian polychrome artefacts. Tanguy et al. (2005) have also reported yellowing when laser cleaning plasters at 1064 nm. This yellowing is possibly due to the presence of carbon residues left on the surface (Verges-Belmin & Dignard 2003).

5

CONCLUSIONS

The damage thresholds of modern replicates of Egyptian paints were measured at 532 and 1064 nm. It was attempted to assess the effects of the binder concentration, degree of paint dilution, nature of the substrate and thermal ageing on the damage threshold. Due to the large variability of the damage threshold, it was not possible to detect any discernable effect of these factors. The ablation thresholds of five different consolidants were determined and found to be higher than the damage thresholds of most paints at 532 and 1064 nm. Tests were conducted to assess the removal of lamp black from different substrates and yellowing was observed on several of the substrates as a result of laser cleaning. It was concluded that laser cleaning at these laser parameters would not be a suitable method to remove consolidants or soot from ancient Egyptian polychrome artefacts. REFERENCES Abraham, M., Madden, O., Learner, T. & Havlik, C. 2005. Evaluation of the effects of laser irradiation on modern organic pigments. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Chappe, M., Hildenhagen, J., Dickmann, K. & Bredol, M. 2003. Laser irradiation of medieval pigments at IR, VIS and UV wavelengths, Journal of Cultural Heritage 4: 264–270. Daniels, V., Stacey, R. & Middleton, A. 2004. The blackening of paint containing Egyptian blue. Studies in Conservation 49: 217–230. Douglass, D. L., Shing, C. & Wang, G. E. 1992. The light induced alteration of realgar to pararealgar. American Mineralogist 77: 1266–1274.

Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2007. Lasers in the preservation of cultural heritage, principles and applications. New York and London: Taylor and Francis. Gordon Sobott, R. J., Heinze, T., Neumeister, K. & Hildenhagen, J. 2003. Laser interaction with polychromy: laboratory investigations and on-site observations. Journal of Cultural Heritage 4: 276–286. Hildenhagen, J., Chappe, M. & Dickmann, K. 2005. Reaction of historical colours and their components irradiated at different Nd:YAG laser wavelengths (w, 2w, 3w, 4w). In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Lee, L. & Quirke, S. 2000. Painting materials. In P. Nicholson and I. Shaw (eds.), Ancient Egyptian materials and technology. Cambridge: Cambridge University Press. Middleton, A. 1999. Polychromy of some fragments of painted relief from El-Bersheh. In W.V. Davies (ed), Studies in Egyptian antiquities: a tribute to T.G.H. James. London: British Museum Press. Middleton, A. 2000. Report on the identification of some pigments from two 21st Dynasty coffin lids, British Museum internal report. Pages-Camagna, S. & Colinart, S. 2003. The Egyptian green pigment: its manufacturing process and links to Egyptian blue. Archaeometry 45: 637–658. Pouli, P., Emmony, D. C., Madden, C. E. & Sutherland, I. 2003. Studies towards a thorough understanding of the laser-induced discoloration mechanisms of medieval pigments. Journal of Cultural Heritage 4: 271–275. Sansonetti, A. & Realini, M. 2000. Nd:YAG laser effects on inorganic pigments. Journal of Cultural Heritage 1: 189–198. Schnell A., Goretzki, L. & Kaps, Ch. 2005. IR-laser effects on pigments and paint layers. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Sease, C. 1981. The case against using soluble nylon in conservation work. Studies in Conservation 26: 102–110. Sokhan, M., Gaspar, P., McPhail, D. S., Cummings, A., Cornish, L., Pullen, D., Hartog, F., Hubbard, C., Oakley, V. & Merkel, J. F. 2003. Initial results on laser cleaning at theVictoria &Albert Museum, Natural History Museum and Tate Gallery. Journal of Cultural Heritage 4: 230–236. Tanguy, E., Huet, N. & Vincotte, A. 2005. Lasers cleaning of patrimonial plasters. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Verges-Belmin, V. & Dignard, C. 2003. Laser yellowing: myth or reality? Journal of Cultural Heritage 4: 238–244.

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Laser cleaning as a more culturally appropriate treatment option for Native American pictographs and pictograms M. Abraham Los Angeles County Museum of Art, Los Angeles, USA

C. Dean Dean and Associates, Portland, Oregon, USA

ABSTRACT: This paper will discuss specific programs to establish a dialogue with the Native American community in the Pacific Northwest of the United States ofAmerica. The authors attempt to find culturally appropriate methods for the treatment of vandalized rock images in this region. The work details comparative treatments at sites vandalized by modern paints. The work further covers attempts to convey treatment options to the local Native American Tribal entities and to elicit their input on the selection of treatment approaches using a survey.

1

BACKGROUND

For some time now, the use of lasers in conservation of works on stone has been studied in terms of the best way to clean unwanted layers from art objects. The criteria have been the most efficient cleaning of pollution or graffiti, while creating a minimum of damage to the substrate that forms the art object. Over the years, it has become clear that lasers are often a very good method of achieving this goal. In the area of cultural resource management (and indeed in some instances in the area of arts management) the most scientifically efficient method of cleaning may not be the only consideration. Within their cannon of ethics, professional conservators are also tasked with taking into account the wishes of the owner of the work, the artist or artisans intentions, and the history of the piece. These considerations may become very important in cases where the objects are of significant cultural or religious importance to a group of people. Balancing the needs of the “owners” of a piece or of a site, with the wishes of the cultural group who claim a historical sociological bond with, the piece can create grave difficulties for conservation. The Native American Community in the Pacific Northwest has a rich and varied history of producing pictographs and pictograms, often commonly referred to simply as rock art. Within the post contact context of the European based culture, these objects are usually viewed as a beautiful and intriguing indication of an indigenous past. Within the Native American context, these objects are part of a living tradition of beliefs and practices still in use today. For many Native

American groups, sacred sites are considered to have a living spirit. For this and other reasons (including site use by tribal members), many Native Americans have expressed a dislike of the use of solvents to remove graffiti, further, solvent use exposes conservators to health hazards. The authors believe that the use of lasers as a tool to treat graffiti, and other senseless desecration of sacred Native American sites (and potentially sacred sites in other parts of the world) offers a means of meeting the needs of the site managers (often Park Services or other land use management organizations) to clean and remove the unwanted blight of graffiti. At the same time the not chemical nature of the method may, in some instances, be more sympathetic to the belief structure of the people who still hunt, fish, congregate and worship at or near these sites. Specifically, we attempt to address: the options for and logistics of cleaning of graffiti at these sites, the aim of the conservation and the degree to which the objects should be treated, the responsibility of the conservator to meet the needs of a number of interested parties, and the difficulty of achieving dialogue with the all parties concerned. Case studies using lasers are presented to illustrate the problems and successes of this approach within a U.S. Park Service sponsored programs to investigate the issue in detail. 2

CASE STUDIES USING A LASER

Conservation efforts using lasers have been carried out at two sites. In both cases, paint (due to graffiti

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Figure 2. After cleaning: h cleaned by laser while i, t and e cleaned by solvent.

Figure 1. Graffiti before cleaning.

or the making of impressions) was removed. The laser was effective in removing some, but not all, unwanted paint. Masking was used to protect surfaces pigmented by Native artisans, as the laser can alter the iron oxide pigment. We found that in some cases it was possible to retain natural growths on the stone. In both cases, the work was carried out with the consent of local tribal entities. It is also worth noting that there are other possible techniques that are more environmentally friendly. One of the most often used methods for cleaning graffiti is the use of micro blasting of either sand or walnut shell. This is a less desirable option in the case of rock images as it removes the “desert varnish” patina formed by years of organic growth. This patina is considered protective of the images and worth conserving if possible. Further, the use of solvents or micro-blasting can reduce the chances for carbon dating of the site if it is deemed appropriate at a later date. Still, the authors acknowledge that there are certainly other means of cleaning without the use of solvents and encourage research in those areas that might yield other solvent free methods for conserving these sites.

2.1

John Day Park, Eastern Oregon

The site had been vandalized by graffiti consisting of pink spray paint. A pulsed Nd:YAG laser operation in the fundamental wavelength and Q-switched was used at the site. Additionally, three courses of poultices using methyl ethel ketone (MEK) were used to clean separate areas of the graffiti. The laser system is made by New Wave Research (model sold under the name “Tempest”) and was powered by a small generator. The results can be seen in Figure 2. The h in the name White was removed using the laser, while the ite group was removed using the solvent pultacing. In the case of the pultacing, there was a clear ghosted image

Figure 3. Rock image repository at Horse Thief Park.

left after treatment. The laser left very little residue behind. 2.2 Horse Thief Park, Washington State The Horse Thief Park in Washington State functions as a repository for many of the rock images moved during the construction of a local hydro electric dam. Figure 2 shows many of the rocks along the river which bare images. Many of the images have been painted during the process of making prints (a practice of tourists in the past). The local tribal entities still consider these images sacred and would like to see them cleaned. With the consent of tribal elders and the local park managers, test cleanings were preformed at the site. The laser was a Nd:YAG Continuum Surlite 1 operating in the fundamental and Q-switched mode. While many of the paints were easily removed, others were more intractable. Still it was clear that the laser was able to remove enough of the paint to minimize the disruption to the quality of the pictographs and pictograms. Further, the laser left many natural features on the rock such as lichens and molds. The tribal elders involved with monitoring the cleaning

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from seven communities that are independent entities with their own unique cultural identities. However, their answers to the questions suggest some commonly held concerns and approaches to the care of rock image sites, especially where the treatment of graffiti is concerned. All felt that graffiti and vandalism at rock image sites needs to be addressed when it occurs and with the aim of removing or reducing it. All considered the preservation of the cultural and ecological integrity of the site to be of utmost importance when considering how to treat a site. Finally, all considered the financial cost of treatment to be of least importance. Most considered consultation with tribal communities (beyond existing legal requirements) through the appropriate land managing agency about proposed treatments to be of outmost importance, along with the involvement of tribal monitors to observe the treatment process. A majority of respondents felt that all sites should be considered of equal importance when planning for conservation. All indicated that treatments should be chosen using the least invasive or aggressive options first, and that these should be a “natural” as possible. Based on these results, the authors feel that there is some basis for the notion that laser cleaning may function as a more ecologically and therefore culturally appropriate method for cleaning modern paint damage from sacred sites in the region. Figures 4 and 5. Before (up) and after (down) cleaning of paint by laser. Note the minimal cleaning to achieve visual continuity and the fact that natural mold and lichen remain on the rock surface.

expressed satisfaction with the more natural patina left by the laser cleaning. Also per the express wishes of the tribal entities involved, the intervention was keep to a minimum. Only the most visually disruptive modern paints were removed while smaller overprinting was left to erode over time.

3 THE SURVEY The U.S. Park Service funded a survey of local Tribal Organizations regarding conservation practices. The survey included a description of various cleaning methods and a discussion of the environmental consequences of cleaning using each. Tribes were asked to comment on their impressions of the importance of conservation of sites, their own involvement with the process and the importance of environmentally friendly methods. The aim of the survey was to determine what the Native community considered important regarding best conservation practices. The responses come

4

CONCLUSIONS

The use of laser is one method of reducing solvent applications at sacred or culturally important sites. The efficiency of the laser in removing graffiti from stone substrates has been shown in the above described tests as well as in many other research programs. (See cleaning tests in this volume and previous LACONA publications). Still it is the apparent wishes of the involved parties to find environmentally acceptable conservation techniques that make the laser so much more appropriate in these cases.This desire for an environmentally “clean” technology has been elucidated in a concise survey of the local Tribal entities. It is the authors hope that this work will aid in bringing more dialogue and understanding between conservators, the users of sacred sites and the other governmental agencies responsible for maintaining these sites.

ACKNOWLEDGEMENTS The authors would like to acknowledge the following for their contributions to this work: The National Park Service, Center for Preservation Training and Technology. Managers of The John

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Day Park. Confederated Tribes of Warm Springs, Cultural Resources Department. Wanapam Band, Cultural Representative. Nez Perce Tribe, Cultural Resources Program. Coeur d’Alene Tribe, Lake Management Depart. Cultural Resources. Confederated Salish and Kootenai Tribes, Tribal Preservation Department.

Yakama Nation, Cultural Resources Program. Confederated Tribes of the Colville Reservation, History/Archaeology Program. Spokane Tribe, Culture & Heritage Program. Kalispel Tribe, Cultural Resources Program. Confederated Tribes of the Umatilla Indian Reservation, Cultural Resources Protection Program.

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The Arca Scaligera of Cansignorio della Scala by Bonino da Campione: Cleaning of the polychrome and gilded decorations V. Fassina Soprintendenza al Patrimonio Storico Artistico ed Etnoantropologico per le province di VE, PD, BL e TV, Venezia, Italy

G. Gaudini Soprintendenza ai Beni Architettonici e il Paesaggio di VR, VI, e RO, Verona, Italy

S. Siano Istituto di Fisica Applicata “Nello Carrara” – CNR, Sesto Fiorentino, Italy

R. Cavaletti RWS, Padova, Italy

ABSTRACT: A widespread blackening was covering the ornamental stone and the sculptures of Cansignorio della Scala’s funerary monument (Verona, 1374-76) thus reducing the enjoyment of the sculptures. A careful inspection carried out during preliminary investigations allowed us to discover some traces of polychromy and gilding. As it is important to protect and preserve these traces during any cleaning or restoration processes, our primary goal was to devise the most suitable cleaning methodology. In the present work, we report the optimisation study of the laser approach, aimed at achieving a careful cleaning of gilded and polychrome areas, as well as a suitable cleaning result for the decoration background. After a thorough stratigraphic characterisation using spectroscopic methods, microscopy, and X-ray diffraction, a set of preliminary irradiation tests using Nd:YAG lasers were carried out. Special attention was devoted to determine the operative fluence ranges and define a suitable irradiation protocol allowing controlled degrees of cleaning and preventing any damaging or discolouring phenomena associated with laser cleaning.

1

INTRODUCTION

The ornamental stone and the sculptures of Cansignorio della Scala’s funerary monument built in Candoglia marble between 1374 and 1376 by Bonino da Campione were almost completely covered by a widespread black stratification, which prevented the enjoyment of the sculptures (Fig. 1). For this reason the Verona Superintendence to Monuments care decided to plan a restoration project to remove as much as possible the black crusts without damaging the surface of the marble itself. Formerly the project forecasted to adopt the following cleaning techniques: a) washing of the surfaces with deionised water, b) application of basic solutions chemical poultices, c) localised use of high-control micro-sandblasting. Preliminary investigations included a careful mapping of the various forms of alteration affecting the marble surfaces of the monument primarily to

study the deterioration mechanisms and hence adopt the most suitable methodologies for the removal of the stratified products without damaging the marble substrate. According to the original project, at first, on some not carved areas, the above-mentioned methods were tested with a view to select the most suitable means for achieving the desired results. Successively, a careful inspection carried out during preliminary investigations allowed us to discover some traces of polychromy and gilding almost completely covered by the black crusts and grey airborne particulate deposits. In such circumstances cleaning methods involving the application of chemical poultices or spraying with nebulised deionised water are not only ineffective but even invasive since they tend to increase the volume of the cavities and have little impact on the crusts. Airbrasive techniques are also inadequate in this case and difficult to apply, owing to the very small areas on which one operates.

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2

MATERIALS AND METHODS

To assess the different alteration products the following analytical methods were used. i) Optical microscopy of cross-sections to identify the different layers of the surface stratification. ii) Scanning electron microscopy (SEM) providing morphological information on crystals. iii) Energy Dispersive X-ray Spectroscopy (EDS) for achieving elemental composition distributions, which allow associating the composition to the conservation treatments carried out in the past or to atmospheric pollution decay. iv) Ion chromatography to determine qualitatively and quantitatively the water-soluble anions most harmful for stone decay.

Figure 1. Cansignorio della Scala’s funerary monument.

Contrarily to the previous methods optimised laser ablation does not involve relevant interactions with the marble. A suitable selection of the irradiation parameters can allow a gradual and selective removal of the black crusts. Furthermore, the laser technique offers a greater degree of control over the cleaning process as compared with chemical methods as well as it is less aggressive than mechanical techniques. Each cleaning technique has particular features that can be properly considered according to the typology of decay, the minero-petrographic properties of the single lithotype, the presence of protective films, as well as of paint layers on marble sculptures and ornamental decorated marble surfaces. In order to assess the different forms of decay a wide sampling was carried out taking into account the weathering of different areas in relation with the exposure to natural agents, as well as atmospheric pollutants (Amoroso & Fassina, 1983). In addition to the formation of black crusts we focused our attention on the following concerns. i) Existence of different types of polychromy and gilding. ii) Identification of old treatments carried out on the surface as maintenance operation. iii) Thin yellow patina extensively present on the surface.

Two commercial laser systems were used throughout the present work. 1) Smart Clean II (El.En. SpA). It is Short Free Running (SFR) Nd:YAG (1064 nm) laser with pulse duration between 40–120 µs depending on the energy set (0.2-2 J/pulse). The output beam is fibre-coupled and imaged on the surface under treatment through an optical handpiece allowing for an easy setting of the irradiation spot (typically set between 3–5 mm Its average power is about 20 W. 2) Michelangelo (Quanta System SpA). It is a Q-switched (QS) Nd:YAG (1064 nm) laser with pulse duration between 5–10 ns. The output beam is coupled into a 7 mirrors articulated arm, which is terminated with a handpiece including a long focal (1 m) lens. Pulse energy up to 0.9 J and average power about 20 W. 3 3.1

RESULTS Decay patterns

The principal decay patterns observed are the following: i) Washing areas are characterised by the action of driving rain or surface water flow. According to the porosity of the marble these areas appeared whitish in the case of compact marble or alternatively greyish feature in the case of more porous marble. The latter is characterised by significant erosion phenomena and a biological infestation produced by the running water and moisture stagnation subsequent to rain events, respectively. ii) Black stratifications, classified as dirt accumulation, were located in sheltered areas far from surface water flow. Black carbonaceous particles are accumulating on the stone surface, mainly in dry condition, and are associated with low amount of gypsum, which in turn affects the stone only in the superficial layer without penetrating into depth (Fassina, 1993). Cross-section analyses revealed the presence of complex stratigraphies

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Figure 2. Representative stratigraphy evidencing the presence of three intentional applications underneath a thin black crust layer (50 µm).

where only the outermost layer was a properly called black crust, whereas the layers underneath are attributable to intentional applications of the past. One representative example is displayed in Figure 2. Three intentional patination layers are clearly recognisable, because of the different relative amounts of inert component and pigment load (ochre and black carbon). The diffuse presence of Ca-oxalates indicates the treatment also included organic components. iii) Black areas characterized by dendrite-shaped crust were classified as dirt wetting. They are generally present on sheltered areas, far from driving rain, and have a rough and spongy appearance and thickness of some millimetres. The black crust was mainly composed by carbonaceous particles and gypsum crystals. Under the black crust a marked decohesion of marble is frequently occurring due to the formation of gypsum crystals inside the calcite grains of marble. The presence of gypsum crystals inside the texture-structure of the marble is due to the transformation of calcium carbonate crystals into gypsum, as a consequence a loss of cohesion occurs, which causes the detachment of small marble fragments. Under the optical microscope, a close observation of the crust revealed the formation of many gypsum crystals growing perpendicularly to the stone surface (Fassina, 2002). iv) Granular disaggregation associated with microcracks formation on the boundary areas between sheltered and exposed areas. This was typically observed on the lower part of legs were water splash-back is depositing on stone surface. This indirect action of water is cyclically investing the surface. The loss of granular cohesion on areas directly exposed to rainwater (white washing) is mainly

Figure 3. Cross section of painted surface. a) Preparation (missing). b) Azurite paint layer. c) Patinations applied during previous restoration works. d) Deposits.

ascribed to the thermal changes and it is strongly accelerated by the decreasing pH of acid rain of urban polluted environment. 3.2 Polychrome and gilded surfaces During a careful surface examination of the Apostles of the lantern, the ground of the shell of the six Virtues, the dresses and armours of the six Saints, some remains of polychromy were discovered. They were almost completely covered by surface treatments and black deposits more or less adherent to the marble surface. By examining the cross section of sample 6, took from the back of S. Sigismondus, we observe a blue painted layer, 150–200 µm thick, in contact with the marble surface and formed by azurite crystals. The paint layer was covered by a patination layer composed of gypsum, silicates, iron oxides an black carbon (50–200µm) and, at the outermost level, by deposits of carbonaceous particles (Fig. 3). A similar situation was present also in other areas showing remains of polychromy. Paint layer traces are mainly concentrated on the sheltered areas where surface water flow is absent. In the washing areas the mechanical action of driving rain has caused the progressive detachment of the colour and it is impossible to reconstruct the original painting. Another important outcome of the preliminary characterisation was the discovery of gilding traces on the edges of the dresses and of the armours. As for the polychromy some remains were visible by naked eye, but the most parts were completely covered by the stratification (Fig. 4). Under the optical microscope it was possible to investigate on the preparation layer of the gilding as well as on the deposits of black carbonaceous particles.

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the polyammino-polycarboxilic acids, which often contain six and sometimes even more donor atoms. The reason for the additional stability due to chelation frequently depends more on geometric than on electronic factors.As previously explained the reactivity of chelating agents is calcium-based instead of sulphate-based. This is very important because in that case the reagent does not discriminate the calcium of the gypsum crust from the one of calcium carbonate-based substrate. Such an approach is hence more harmful than the previous one. On the contrary these chelating compounds are more reactive and therefore more efficient to remove gypsum-matrix stratifications. They can be used after a careful preliminary optimisation carried out by a skilled operator. Figure 4. Gilding of San Giorgio. From the bottom to the top: lead white and oxalates, gold leaf, lead oxide, and gypsum layer with ochre and black carbon.

3.3

Preliminary cleaning tests

To achieve the desired combination of technical efficiency and aesthetic result in dealing with these various situations, the following techniques were employed: a) The use of chisels to remove cement grouting, among stone blocks, and of scalpels to remove excess mortar from the tiny recesses created by tooth-chiselling process. b) The widespread biological infestation was removed using specific biocides applied with cellulose poultices for twelve hours. Poultices applications were repeated 3–4 times according to the degree of dirt and successively swab-rinsing with deionised water. c) Manual and mechanical removal of deposits of organic material from horizontal surfaces using soft and hard brushes and spatulas, and successively swab-rinsing with deionised water to neutralise the residues of pigeon droppings. d) More complex was the cleaning of the black deposits of the statues. First attempts to remove the gypsum matrix stratification were performed using different chemical poultices. The use of the traditional poultices, such as ammonium carbonate and the modified version of AB 57 revealed a very low efficiency for the removal of thick crusts. This high resistance to ammonium compounds led us to test some chelating agents, multi-dentate reagents, whose chemical action is to bind the calcium ion by means of two or more “teeth”, whereupon a ring structure is formed. A multi-dentate chelating agent may be compared to an octopus, which with its many arms grasps its prey, the metal ion. Particularly stable complexes are formed by

Here, repeated applications from 20 to 60 minutes were carried out using DTPA diethylene-triaminepenta-acetic acid neutralized with ammonium hydroxide and NTA Nitril-triacetic-acid neutralized with ammonium hydroxide (Fassina & Mazza, 2006). Despite several tests for optimising the poultice preparation were carried out the final effectiveness was very scarce and no one of the tests provided satisfactory results. After this unsuccessful experimentation of the chemical poultices it was urgently necessary to find a methodology for the selective removal of the stratification without producing any serious damage to the underlying fragile paint and gilded layers. 3.4 Laser cleaning Preliminary tests were carried out on encrusted marble, polychrome, and gilded areas using both QS and SFR Nd:YAG lasers in order to select the most suitable irradiation parameters providing the best compromise between efficiency and selectivity. According to the results of previous studies (Siano et al. 2003), the SFR laser provided a gradual and selective removal of the stratification and it was very effective on gilded areas (Fig. 5). In gilded areas with a weak adhesion to the substrate a fluence of 1.8 J/cm2 was used. In gilded areas with a good adhesion to the substrate the fluence was increased up to 2.5 J/cm2 . In the black deposits areas without any underlying gilding layer the fluence was increased up to 4 J/cm2 . However in the latter case its efficiency appeared insufficient to approach the overall cleaning of the present monument within the available restoration budget. At the same time the SFR laser was too aggressive on azurite already at the cleaning threshold because of the high optical absorption of this pigment in the near infrared (Siano et al. 2006). The removal of the stratification on the most of the monument area was carried out using the QS laser whereas the use of SFR laser was confined to the area presenting traces of gilding. In the black areas

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after 4 applications of 30 minute each one. Nevertheless, since the moisture deriving from the application of poultices was very harmful because of the weak adhesion of the paint layer to the substrate it was decided to use an optimised standalone laser approach, which could provide the selective removal of the stratification and the complete safeguard of polychrome and gilded surfaces. In particular, the SFR laser was more appropriate for the removal of the stratification on gilded areas, while the QS laser showed good results on the blue coloured layer azurite-based, as well as, on the red ground of the niches ochre-based. ACKNOWLEDGEMENTS Figure 5. The bas-relief decorating the sarcophagus: laser cleaning using SFR Nd:YAG laser, which allowed safeguarding the gilding of the leafs and of the bridles of the donkey.

The present restoration work was financed by the Italian Ministry of Cultural Heritage. The authors wish to thank the team participating at the restoration: Claudio Modena, Piero Cevese, Mauro Cova. A special thanks to Paola Zoroaster, Emanuela, and Elvira Boglione who carefully and patiently carried out the laser cleaning. We are also grateful to the Superintendent arch. Sabina Ferrari who gave the permission for the publication of the preliminary results. REFERENCES

Figure 6. St. Aluisius before and after laser cleaning using QS Nd:YAG laser, which allowed safeguarding the azurite background of the gilded decoration motives of the armour treated using SFR laser.

without any underlying colour layer the fluence of the QS laser was around 1 J/cm2 while on azurite it was lowered to about 0.5–0.7 J/cm2 , according to the different adhesion of the paint layer to the substrate.

4

CONCLUSIONS

A careful examination of the present funerary monument surfaces revealed the presence of some remains of paint layers on the marble surface of the statues, which were partially or completely covered by a widespread black stratification. Such a situation represents a serious obstacle for the traditional chemical approaches. They were hence tested only on not painted areas where satisfactory results were achieved

Amoroso, G.G. & Fassina V. 1983. Stone decay and conservation, Amsterdam, Elsevier. Fassina, V. 1993. The weathering mechanisms of marble and stone of Venetian monuments in relation to the environment. In Proceedings of the 10th Triennial meeting of ICOM, Committee for Conservation: 345–351, Washington, 22–28 August, 1993. Fassina, V., Favaro M. & Naccari A. 2002. Principal decay patterns on Venetian monuments. In Siegesmund S., Weiss T. & Vollbrecht A., (eds.) Natural Stone, Weathering Phenomena, Conservation Strategies and case Studies: Special publication 205, 381–391. London: Geological Society. Fassina, V. & Mazza, M. 2006. Proposta per una metodologia di rimozione degli ossalati sulla facciata della casa dei Cavalieri del Podestà a Conegliano. In Spiazzi A.M., Fassina V. & Magani F. (eds.) Facciate dipinte: verifiche sui protettivi e metodologie innovative di pulitura a Feltre e nel Veneto Orientale, 119–134. Siano, S., Casciani, A., Giusti, A., Matteini M., Pini, R., Porcinai, S., Salimbeni, R. 2003. The Santi Quattro Coronati: cleaning of the gilded decorations. Journal of Cultural Heritage 4, Supplement 1: 123s–128s. Siano, S., Brunetto, A., Droghini, F., Guasparri, G., Scala, A. 2006. Cappella del Manto e Sagrestia Vecchia in Santa Maria della Scala, Siena: rimozione laser di scialbature su dipinti murali. In: V. Dell’Aquila (ed.), Atti del IV Congresso Nazionale IGIIC–Lo Stato dell’Arte: 295–302. Firenze: Nardini Editore.

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Assessment of laser cleaning on a polychrome Islamic ceramic B. Sáiz Department of Graphic Expression in Architecture, Institute for the Restoration of Heritage, Universidad Politécnica de Valencia, Spain

E. Aura & M.T. Domenech Department of Conservation and Restoration of Cultural Heritage, Institute of the Restoration of Heritage, Universidad Politécnica de Valencia, Spain

A. Domenech Department of Analytical Chemistry, Universitat de València, Spain

ABSTRACT: Laser cleaning was tested in order to evaluate the effects of irradiation on polychrome ceramics from the Islamic Period. Before using the laser method, other cleaning techniques were tested as well but without success. Laser irradiation seems to be the only method capable of eliminating the crust of hardened dirt strongly adhered to the ceramic substrate. The laser system used was the Maestro, a portable diode-pumped Q-switched Nd:YAG laser operating at a wavelength of 1064 nm, from MPA, SL. Optical Microscopy (OM), Scanning Electron Microscopy/Energy Diffraction X-ray (SEM/EDX) and voltammetry have led to significant results in characterising the material and evaluating the process of laser cleaning. Optimising the laser parameters according to the nature of the ceramic materials has allowed defining a protocol to recover the original layers.

1

2

INTRODUCTION

The work describes the research carried out to assess the effects of laser irradiation on a very damaged Islamic ceramic piece of MadinatAl-Zahara style from the 11th century (Figs. 1, 2). The ceramic piece was found in 2004 during the archaeological excavations carried out on the plot of the new headquarters of the “Colegio Oficial de Ingenieros de Caminos, Canales y Puertos de Valencia”, located at Luis Vives Street in Valencia. Several cleaning methods, before laser irradiation, were tested. Mechanical and chemical cleaning techniques were applied but results were not successful and the risk of leading to superficial abrasions or chemical side effects was too high, so they were discarded. The effect of laser irradiation was analysed on both faces of the ceramic piece, evaluating the results on the black and green enamels and on the white glazed front side as well as on the honey colour glaze and the ceramic biscuit from the back face. The laser used for the experimental procedure and the parameters tested have allowed to define a cleaning procedure in order to achieve a safe intervention in most of the tested areas. This has not been possible with other traditional cleaning techniques.

2.1

PRELIMINARY STUDIES State of conservation

The ceramic piece showed an incomplete profile with a rounded contour, and consisted of a total number of seven fragments, with a decoration, probably epigraphic, in black and green colours. The ceramic glaze was made up of thin layers presenting low adherence to the ceramic substrate. Loss of shine and a worn surface were visible to the naked eye. Corrosion and alteration of the glaze caused by ageing were identified as well as medium-size cracks, vitreous flakes or even loss of material. The alteration that was observed all over the piece was attributed to the non-existent conservation and to a ceramic-making process with particular problems in the vitreous coating (González et al. 1992). Furthermore, at the time the ceramic was made, tin had a moderate use because of its price, so it was applied in low densities and onto the front side only, applying a honey colour glaze made from lead to the back side (Escudero et al. 1990). The superficial layers presented a mixture of superficial deposits and hard concretions of earthy and calcareous nature adhered to the surface and stuck into the cracks.

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Table 1. Chemical composition of the ceramic biscuit, honey glaze and white glaze (% in oxides).

Figure 1. Front side of the Islamic ceramic piece with the decoration in black and green before cleaning.

Figure 2. Back side of the Islamic ceramic piece before cleaning.

The dirt from both the surface and the cracks appeared as carbonate concretions and compact layers of dirt, which concealed the original Islamic ceramic decoration. Figure 1 shows the front side ceramic piece with the enamel decoration in black and green colours and the white glazed part. The earthy layer and hard concretion deposits can be seen in more detail in Figure 3. Figure 2 shows the back side of the ceramic piece, which has a honey colour glaze with strong signs of damage. The loss of the glaze leaves the ceramic biscuit visible in a superficial layer intermixed with rests of particles from the altered glaze. Over these two layers, the honey colour glaze and the ceramic biscuit, the concretion shows higher adherence to the honey colour glaze (Fig. 7) in relation to the ceramic biscuit. 2.2

Chemical analysis

Characterisation of the chemical composition of the different constituent materials of the ceramic and the dirt layers was carried out.

Chemical comp. (%)

Ceramic biscuit

Honey glaze

White glaze

SiO2_ Al2 O3 CaO MgO FeO SnO2 PbO Na2 O K2 O TiO2

42.22 14.57 30.46 2.37 4.21 – – 1.49 1.71 1.67

39.30 6.84 6.83 1.36 2.21 – 37.92 1.03 4.06 0.46

70.39 12.97 2.34 0.26 0.31 4.67 2.08 0.47 6.58 0.22

For the electrochemical study, the method of voltammetry of microparticles through PIGEs (paraffin-impregnated graphite electrodes) transference was applied with the aim to identify electroactive compounds.The square wave of the voltammetric electrochemical measures was obtained with CHI420 and BAS CV 50 W equipment. Scanning Electron Microscopy (SEM-EDX) (JEOL, JSM 6300 model with Link-Oxford-Isis microanalysis system) was also employed. Samples of white glaze, black and green enamels, crust taken from over the green enamel and the earthy dirt layer (front side) and from the white and honey glaze (back side), as well as of ceramic biscuit were studied. The electrochemical analysis concluded that the green enamel layer was formed due to the addition of iron (Fe) and copper (Cu). The same analysis for the black enamel concluded that the colour layer was obtained due to the presence of manganese (Mn). Table 1 shows the chemical analysis results expressed in % for the ceramic biscuit and the honey and white enamels. Analytical results were quite useful to consider laser parameters before irradiation in order to avoid any damage to the constituent materials. 3

CLEANING OF CERAMIC PIECE

3.1 Assessment of traditional cleaning techniques The cleaning works were initially carried out through mechanical techniques. By using a scalpel, and with very smooth movements, concretions were attempted to be removed, either by making superficial abrasions or by trying to remove flakes or dirt. Mechanical cleaning was not able to eliminate all the concretions because of their hardness but it was possible to eliminate 5% of the total dirty surface. Other abrasion techniques as the use of multi Dremel were discarded because the concretion layer was very thin, hardly

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Table 2. Energy per pulse at the output beam achieved at 10 Hz repetition rate. Positions 1, 2, and 3 were tested onto the black and green enamel, white and honey glaze and ceramic biscuit respetively. E per pulse (mJ) Repetition rate (Hz)

1

2

3

4

5

Max.

10

1.5

6.0

8.0

9.0

11.0

12.0

1 mm, and the risk of damaging the glaze layer after having eliminated the dirt was higher. Chemical cleaning was tried by means of ethanol and ethyl acetate with cotton and cellulose pulp napkins. None of them had any effect on the crust deposits. 3.2

Laser system

The laser system used for the cleaning was the Maestro, from MPA. SL, a portable diode-pumped Q-switched Nd-YAG, operating at a wavelength of 1064 nm, pulse duration of 4 ns, repetition rate range 10–200 Hz; pulse energy range 5–30 mJ/pulse; maximum average power 6 W. The laser beam is delivered onto the surface by means of an optical fibre. The average working distance was calculated to be around 2.4 cm. This means a focus beam diameter of 1.5 mm, and working spot size of ∼1.8 mm2 . 3.3

Figure 3. Front side layers. A: Black enamel, B: Green enamel, C: White glaze, D: Earthy layer, and E: Hard concretions over black manganese enamel before irradiation. The white square indicates the irradiated area.

Laser cleaning procedure

The black and green enamel layers, white and honey glaze and ceramic biscuit were irradiated at a repetition rate of 10 Hz to evaluate the feasibility of the parameters selected to eliminate the strongly stuck crust (Chlouveraki et al. 2003). Cleaning tests were done both in dry and wet conditions. Table 2 shows the energy per pulse at the output beam achieved at the repetition rate of 10 Hz. Preliminary irradiation tests, in dry and wet conditions, at 10 Hz repetition rate and energy per pulse ranging from 1.5 to 6 mJ, during 1 s, were done on the black and green glazed areas without crust to evaluate the colour variation. No changes were observed. By observing its effect under an optical microscope (OM) the laser irradiation was controlled. A binocular magnifying glass was used, the Nikon model SMZ800, with a zoom of 10X up to 63X, equipped with the digital photographic camera Nikon model COOLPIX950. It was decided to irradiate the ceramic piece at the lowest repetition rate given by the laser, 10 Hz, to preserve the chemical composition of the constituent materials and colour layers. Attempts to irradiate at

higher repetition rates were discarded for not allowing the intended cleaning control. Laser irradiation was evaluated directly on the surface with the aim to determine ablation thresholds for the crust.

4

RESULTS AND DISCUSSION

4.1 Laser irradiation on the front side The irradiation results obtained on the crust over the black enamel characterised as manganese black, were able to eliminate the hardest concretions without damaging the black manganese. Figure 3 shows the black enamel decoration with the crust strongly adhered before irradiation. Figure 4 shows the black enamel decoration revealed after crust elimination. An initial fluence of 0.35 J/cm2 , with the crust previously moistened, was employed for the first irradiation. Once the crust was partially eliminated, and with a dry surface, a lower fluence of 0.08 J/cm2 was applied to avoid unnecessary risks. Complete removal of the concretion was achieved in this way, the elimination of crust was confirmed using OM technique. Laser irradiation was also effective on the green enamel decoration characterised as copper green. The crust layer was slowly treated, irradiating it under optical microscope during a few seconds and stopping in order to control the effectiveness of crust elimination before reaching the glaze. Figure 5 shows the green copper glaze with the crust strongly adhered before irradiation while Figure 6 shows the effects of the irradiation at an initial fluence of 0.35 J/cm2 on the wet surface. Like in the case of black glaze cleaning, a lower fluence of 0.08 J/cm2 was

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the harder concretions yielded a satisfactory result. In this case and in a very specific area, the white glaze was irradiated at 8 mJ per pulse in dry conditions to try to remove some minuscule spots of concretion which offered resistance. Visualisation under OM allows appreciating rests of the earthy layer, as it was decided not to reach the white glaze in order to avoid any damage. This is not visible to the naked eye, so the result was considered to be very good since the contaminating crust was removed and the visualisation of the original glaze was achieved. 4.2 Laser irradiation on the back side Figure 4. The squared area shows black manganese enamel decoration revealed after irradiation controlled by OM.

Figure 5. Concretions over copper green enamel before irradiation. The square indicates the area to be irradiated.

The ceramic biscuit and the honey colour glaze from the back side were irradiated at a repetition rate of 10 Hz and low fluence with different results (Hildenhagen at al. 2005). The dirt from the ceramic biscuit was not made up of hard concretion as in other areas. Dirt was easily eliminated but the irradiation could not be controlled once it was removed. An irradiation fluence of 0.35 J/cm2 did not result in a selective cleaning. The ablation threshold for the concretions was similar to the ablation threshold for the ceramic biscuit, and a dark coloration could be noticed on the irradiated area, so cleaning was not recommended because of the non-selective results. No topographic changes were observed. Irradiation at a fluence of 0.08 J/cm2 was not enough to eliminate concretions. The laser cleaning of the ceramic biscuit was not suggested because of the absorption and colour changes induced by laser irradiation. The concretions that were found in the honey colour glaze presented higher hardness, similar to those from the front side. Figure 7 shows earthy dirt and harder concretion layers over the honey colour glaze. Figure 8 shows the result obtained by irradiating at a fluence of 0.34 J/cm2 on a dry surface. A fluence of 0.5 J/cm2 was only employed in areas, which presented a higher resistance. It was possible to practically remove all of the concretions without damaging the original glaze. It was decided not to reach a total removal as the result was considered to be visually optimal to the naked eye. 5

Figure 6. The square area shows copper green enamel decoration revealed after irradiation controlled by optical microscopy.

applied without moistening the surface, immediately after the superficial crust elimination was observed. Under the same conditions, the irradiation of the white glaze to eliminate both the earthy layer and

CONCLUSIONS

Laser cleaning results, both positive for the black manganese and copper green enamels and on the white and honey colour glazes, and negative for the ceramic biscuit, were better than any other results achieved by using traditional cleaning techniques. Laser irradiation has been considered as being the only selective technique for concretion removal, which does not damage the revealed original layers of decoration. Irradiation with a Q-switched Nd:YAG laser

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10 Hz has been essential for controlling the stopping time once the elimination of the crust was visualised under OM. The spot size has also been a valuable tool proving to be quite selective regarding dirt elimination and avoiding irradiation of unnecessary areas. It has to be said that the results achieved are due to both the careful adjustment of laser parameters employed and the skilled hand of the restorer responsible for the cleaning.

ACKNOWLEDGEMENTS

Figure 7. Concretions (A) over honey colour glaze (B) and earthy dirty layer (C) before laser irradiation. Back side.

Thanks are given to the Institute for the Restoration of Heritage for supporting restoration works for the conservation of the heritage of Universidad Politécnica de Valencia and to the R&D&I Linguistic Assistance Office at the Universidad Politécnica de Valencia for their help in revising and correcting this paper.

REFERENCES

Figure 8. Glazed layer revealed after irradiation controlled by optical microscopy.

system offers the possibility of working with low fluences, thus reducing the risk of laser irradiation damage. Low cleaning velocity of the repetition rate of

Chlouveraki, S., Pouli, P., Melessanaki, K. & Yiannasaki, M. 2003. Laser cleaning studies of hard insoluble aluminosilicates crust on Minoan (LM IIIC) Pottery. In K. Dickman, C. Fotakis & J.F. Asmus (eds.), Lasers in the Conservation of Artworks, LACONA V; Proc. Intern. Symp., Osnabrueck, 15–18 September; 143–148. Escudero, J. 1990. La Cerámica decorada en verde y manganese de Madinat Al-Zahra. Cuadernos de la Alambra 2, Córdoba; 127–161. González, G., González, M., González, C. & Vallejo, A. 1992. Estudio arqueométrico de algunas cerámicas medievales de Madinat Al-Zahra (Córdoba). Boletín de la Sociedad Española de Cerámica y Vidrio, 31. 491–498. Hildenhagen, J., Dickmann, K. & Hartke, H.-G. 2005. Removal of strong sinter layers from ceramic artworks with Nd:YAG-laser. Lasers in the Conservation of Artworks, LACONA VI; Book of Abstracts, Vienna, 21–25 September; 113.

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Laser cleaning of stucco’s fragments from an early middle age bas-relief A. Sansonetti, C. Colombo & M. Realini ICVBC-CNR Istituto per la Conservazione e la Valorizzazione de i Beni Culturali, Sezione di Milano “Gino Bozza”, Italy

M. Palazzo & M. De Marchi Direzione Generale per i Beni Culturali e Paesaggistici della Lombardia, Milan, Italy

ABSTRACT: A complete conservation program on a set of stucco fragments coming from a middle age basreliefs has been planned by Direzione Regionale per i Beni Culturali e Paesaggistici della Lombardia with the scientific support of ICVBC-CNR. The particular conservation history of the fragments produced pulverized surfaces and substantial problems occurred. Chemical and mechanical cleaning systems have been tested without successful results due to stucco gypsum composition and to its state of conservation. Laser cleaning has been much more promising since the beginning of the tests, operated with Quanta System Palladio Nd:YAG, both at wavelengths of 1064 and at 532 nm. The correct fluence to ensure the best balance between harmfulness and effectiveness has been defined using non-destructing methods (Reflectance Colorimetry and Stereo-Microscopy) and micro-destructive analyses (Polarised Microscopy). The most useful fluences have been found in the range 450–475 mJ/cm2 , as to diverse soiling thickness, using 1064 wavelength. Laser cleaning allowed the discovery and safeguard of yellowish patinas and of very small finishing remnants; on this remnants, in some cases with polychromy, cleaning with an Er:YAG laser has been tested with the aim to preserve them without any discoloration. Harmfulness criteria had the priority over effectiveness granting a good conservation of stuccos patinas and finishing.

1

INTRODUCTION

The church of Saint Maria Maggiore in Lomello, a village close to Pavia in the north of Italy, is an important example of early Romanesque architecture being probably built around the year 1000. After having undergone many rebuilding works during the centuries, in the 18th century, the church assumed a new baroque aspect and a new vault was built, hiding an extraordinary stucco relieves cycle, which decorated the main nave. The cycle probably represented a procession of saints dressed with armours and loricaes, inscribed into niches with phytomorphic motifs. In the 40’s of the 20th century, during conservation works, the medieval aspect of the church was restored; during the demolition of the 18th century vault, stucco relieves were brought to light, even if most of them, in the meanwhile, were collapsed on the extrados of the vault itself. The collection of stucco fragments was collected together. Only one headless figure, probably a saint with weapons, is still standing on the masonry. The rest of 160 fragments were picked up during these conservation works held in 1940–42, and they were collected in a damp storehouse for the last 60 years: at

the moment the Direzione Regionale per i Beni Culturali e Paesaggistici della Lombardia undertakes a complex conservation plan in order to clean and consolidate the fragments. Moreover an attempt to bring the frieze to its former harmonious and coherent shape will be carried out. This work is to be considered very important because just a few stucco cycles coming from so early ages are conserved in the whole of Europe. For these reasons a careful cleaning was needed, aimed at removing soiling, but with the complete safeguard of finishing tool marks, still present on the surfaces and useful to reconstruct original craftsmanship. Fragments are different in size, ranging from a few square centimetres to about some square decimetres; they mainly come from friezes and from a figure, probably a saint or a soldier, similar to the one still on site. During the long period of time after the collapse and even in the damp and unsuited storage, decay mechanisms lead to surface pulverisation, probably due to the presence of a high relative humidity, which partly solubilised the matrix of the material; moreover a quite homogeneous dusty soot deposited over the stucco surfaces, both the external finished ones, and the ones which originally were located in the

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interface between the stucco body and the masonry. The stucco body is mainly composed of gypsum with a low amount of calcite and quartz as aggregate fraction. Total open porosity is in the range 20–25% (analyses carried out at ICVBC-CNR laboratory by means of mercury porosimeter Pascal 140 e 240 Thermoquest). The presence of high relative humidity levels in the storehouse environment, induced the dissolution of gypsum. These phenomena produced crystallization cycles causing a new gypsum phase on the surface, which, in its growing processes, cemented soiling and dust. At the moment the planned conservation works, data collection and the previous considerations advised a laser cleaning procedure. A wet agent traditional cleaning was attempted but immediately abandoned because of its scarce effectiveness and the production of a spotty surface. Unfortunately no reference in the scientific literature helped to foresee the correct laser cleaning parameters. The reference found (Calcagno et al. 2001; Delivré et al. 2001, Doganis et al. 2005) do not provide any operative fluence or damage threshold. For these reasons a complete diagnostic survey was undertaken mostly focused on stereomicroscopy. Actually, this was a quite rare occasion to investigate large portions of artistic surfaces with the precision provided by laboratory conditions. Nevertheless, sampling limited to a few representative items was carried out at the uncleaned-cleaned border line and thin cross sections were obtained in order to observe the effect of laser cleaning. 2 2.1

MATERIALS AND METHODS Laser devices

Two laser sources were used for cleaning the whole stucco surfaces: a) A Palladio Nd:YAG Laser from Quanta System emitting at 1064 nm (450 mJ/cm2 , 20 Hz, 7 mm diameter) and frequency doubled, provided with an articulated arm containing 7 mirrors. b) A pulsed Er:YAG laser from MonaLaser, LCC, Orlando, Fla., emitting at 2.940 µm (20–40 mJ/cm2 , 10 Hz) used for some special tests on polychrome areas and white finishing. A comparison between the two laser sources was carried out on soiling dusty surfaces. 2.2

Evaluation methods

In order to evaluate the harmfulness of the laser radiation the following analytical procedures were used. Color of the surface was measured using a MINOLTA CR 200 colorimeter; data were collected in CIE L∗ a∗ b∗ system before and after the laser irradiation. Leitz Wild

Figure 1. Stucco fragment used for cleaning tests (cleaning fluence values expressed in mJ/cm2 ).

M420 Stereomicroscope was used to observe surface morphology before and after irradiation. Thin cross sections were observed by Nikon Eclipse E400Pol microscope in polarized light; both microscopes were equipped with a digital image capturing system. 3

EXPERIMENTAL

3.1 Laser tests A fragment with a flat surface was chosen to carry out evaluation tests: this fragment allowed a series of significant cleaning tests, due to the homogeneity of soiling and to the geometry of surfaces. Figure 1, shows the sample treated; in the upper part it is possible to notice the soiling, while in the lower part the surfaces cleaned at different fluences. The laser tests were carried out at 1064 nm, after having tried with 532 nm radiation, which did not provide any remarkable positive effects. 3.2 Effectiveness and harmfulness evaluation Following the current debate (Mecchi et al., in press) effectiveness and harmfulness are here discussed, linking together obtained scientific data, as in a circumstantial survey. A greyish/black irregular deposit is well evident on the fragments external surface, it covers the outer profile of the plastic decoration and gives a grey homogeneous aspect to the stucco. In Figure 2 it is possible to distinguish this decay phenomena: the black deposit, observed by optical microscopy in polarized light, shows an irregular thickness (ranging from 50 µm to 200 µm) and a darkening on the outer portion, due to the presence of many black round particles. In Table 1 chromatic values, measured on the greyish/black soiled and on the various laser cleaned surfaces, are presented.

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Table 1.

Table 2. Differences in chromatic features calculated √ for cleaned surfaces (E = L∗2 + a∗2 + b∗2 ).

Chromatic features of laser cleaned surfaces. ∗

∗

∗

Area

L

a

b

Soiled 375 mJ/cm2 425 mJ/cm2 450 mJ/cm2 475 mJ/cm2 500 mJ/cm2

61.88 70.71 76.14 81.19 80.41 84.72

1.36 3.26 3.39 2.61 2.56 1.73

8.56 15.68 16.70 15.67 16.36 12.31

Fluences used for cleaned surfaces

E

375–425 mJ/cm2 425–450 mJ/cm2 450–475 mJ/cm2 475–500 mJ/cm2

5.52 5.22 1.05 5.98

Figure 2. Stucco finishing layer characterized by a black deposit. (nicol 45◦ , bar = 100 µm).

Figure 3. Boundary line between areas soiled and cleaned at 450 mJ/cm2 (bar = 1 mm).

Analysing data in Table 1, it is evident that brightness is directly proportional to laser fluence; on the contrary as regards the b∗ component (yellow/blue axis), a top value is reached quite immediately (16.70 at 425 mJ/cm2 ), while at the highest fluence, a smart decreasing is detected (12.31 at 500 mJ/cm2 ). A very similar trend is evident for the a∗ parameter (red/green axis), even if absolute values are much lower with respect to b∗ parameter. Comparing the colorimetric data with microscope observations it is possible to affirm that a “patina”, yellowish in colour, has been immediately brought to light, even at a fluence of 425 mJ/cm2 , but that a higher fluence is needed to clean it at the best. Actually surfaces are still quite dark at 375 and 425 mJ/cm2 (L∗ 70.71 and 76.14 respectively), reaching a higher value of brightness (L∗ = 81.19) at 450 mJ/cm2 . A very small colorimetric difference was obtained between areas cleaned with 450 and 475 mJ/cm2 (E = 1.05, Table 2). Actually a difference around 1 point in absolute value is not distinguishable by the naked eye. Using fluences out of the range 450–475 mJ/cm2 , E values obtained were around 5 points, giving rise to striking changes in the chromatic features. Actually a yellowish "patina" is evident observing cleaned surfaces at the naked eye and with low magnification at the stereomicroscope (Figs. 3 and 4). This patina is quite homogeneous in colour and it shows a uniform structure all over the investigated areas. Only

in small crevices and craters it seems to be a little bit thicker. Thickness, micro-structure and state of adherence to the substrate are well visible in Figures 7 and 8, which display thin cross sections. In Figure 7 after laser cleaning at 450 mJ/cm2 , the stucco external surface, appears very different respect to the one visible in Figure 2. Figure 8 shows the thin cross section of a sample coming from a surface cleaned using 475 mJ/cm2 . The use of the lower of these two fluences (450 mJ/cm2 ) has already entirely removed the black deposit, showing the presence of the patina as an ochre layer, 50 µm thick (Fig. 7), perfectly adherent to the stucco profile. The use of the higher fluence (475 mJ/cm2 ) has produced the same effect as regard the black deposit, but it has reduced to 20 µm the thickness of the ochre layer (Fig. 8). An impression of reducing in the thickness of the patina is evident even comparing Figures 3 and 4. In this latter image, the patina, directly observed at a low magnification, seems to be lighter, thinner, even if still complete. Examining surface cleaned at higher fluences it is possible to put in evidence that marks of detachments and of loss of material are more frequent. This is probably the case of the mark which is visible at the centre of Figure 5, where patina has been detached, bringing to light the white, dusty bulk of the gypsum body underneath. Cleaning at 500 mJ/cm2 involves the loss of the patina as shown in Figure 6. Only some residues of

245

Figure 7. Presence of an ochre film on the stucco outer profile after a laser cleaning with 450 mJ/cm2 (nicol 45◦ , bar = 100 µm).

Figure 4. Boundary line between areas soiled and cleaned at 475 mJ/cm2 (bar = 1 mm).

Figure 5. Surface cleaned at 475 mJ/cm2 . The loss of surface material is shown by the arrow (bar = 1 mm).

Figure 8. Presence of an ochre film on the stucco outer profile after a laser cleaning with 475 mJ/cm2 (nicol 45◦ , bar = 100 µm).

yellow/ochre material are still present in the bottom of small crevices as shown by the arrow; surface seems to be, in the whole, as abraded; moreover the data presented in Table 1 shown that brightness has been increased, but the yellow component has been decreased, as above remarked; these data should be interpreted has the evidence of over-cleaning. 3.3 Er:YAG laser tests on finishing and polychrome residues

Figure 6. Surface cleaned at 500 mJ/cm2 . Residues of ochre material is shown by the arrow at the bottom of crevices (bar = 1 mm).

Some polychrome and very small finishing residues are still present on Lomello stuccoes surfaces; they show red, ochre, blue and white colors. They are important witnesses of craftsmanship and they could give information about symbolic content of the artworks. With these considerations in mind, it was very important to carefully conserve these small residues. The availability of Er:YAG laser allowed some cleaning tests focused on polychrome residues and even a comparison carried out by cleaning stucco body in

246

Figure 9. Residue of red painting layer cleaned with Er:YAG laser. Figure 11. Comparison between areas irradiated with Er.YAG and Nd:YAG lasers (bar = 1 mm).

White finishing, which are composed by lime wash (calcite detected by XRD analysis and thin section observations), are present in small remnants on some surfaces; they have been effectively cleaned with Er:YAG radiation (Fig. 10); a very thin soiling residue is still present underneath scales where laser radiation has not reached it. Quartz aggregate grains have been bring to light by laser radiation; no other morphological changes as for example detachments are visible even at low magnification. In the same fragment already used for Nd:YAG tests (see Fig. 1), and in an area very next to the 450 mJ/cm2 cleaning test area (as discussed above), an Er:YAG laser test has been carried out. Results are visible in Figure 11. It is evident that Er:YAG laser does not preserve yellow patina, but it produces a whitish surface. In the hypothesis that the yellowish patina is composed by gypsum, this data could be explained remarking the absorption of the -OH bond present in CaSO4 2H2 O structure.

4 Figure 10. Residue of white finishing before cleaning (a) and after Er:YAG laser cleaning (b) (bar = 500 µm).

between Er:YAG and Nd:YAG sources. Er:YAG tests were made in the range 20–30 mJ/cm2 with a repetition rate of 10 Hz; tests have been carried out either in dry conditions or with water as a wetting agent; only these latter have been effective. No discoloration appeared on red remnants after laser irradiation, as remnants were too small to allow a colorimeter measure. Observations were carried out at the naked eye (Fig. 9).

CONCLUSIONS

In the case study of Lomello stucco bas-reliefs, the laser cleaning operating fluence has been found in the range 450–475 mJ/cm2 . The fluence of 450 mJ/cm2 allows the conservation of a yellowish patina present underneath the soiling. A fluence of 475 mJ/cm2 is more effective, and is more advisable especially if the soiling is thicker, but it probably induces some small detaching detectable with a stereomicroscope at low magnification. Er:YAG laser radiation could be used to clean white calcite and polychrome finishing. Further investigations are in progress to better understand the compositional nature of the patina.

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REFERENCES Calcagno, G., Bristol, A. 2001, Laser beam cleaning of decorated plaster surfaces of the monumental XVIth century staircase of the Grimani Palace in Venice, Italy. In LACONA IV Book of Abstract, Paris. Delivré J. 2001, Le Laser de nettoyage de la pierre, adapté au plâtre. In Georges Barthe (ed.), Le Plâtre, l’art et la matière, CREAPHIS editions, Paris.

Doganis, Y. 2005. Laser cleaning of stone, stucco and plaster ornamental reliefs in the Benaki Islamic art collection, Athens, Greece. IN LACONA VI Book of Abstract, Vienna. Mecchi A.M., Poli T., Realini M., Sansonetti A., 2007, Problems in drawing up standards to evaluate effectiveness and harmfulness in cleaning operation, in Conservation 2007 Proceedings, Milan, 10–11 May 2007. In press.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Soot removal from artificial fresco models by KrF excimer laser J. Hildenhagen & K. Dickmann Laser Center (LFM), Münster University of Applied Sciences, Steinfurt, Germany

W. Maracineanu & R. Radvan INOE/CERTO, Platforma Magurele, Bucharest, Romania

ABSTRACT: Mechanical cleaning of strongly polluted frescoes is a difficult task for restorers. The application of laser radiation for cleaning of frescoes is usually limited due to the hazard of colour modification of the integrated mixture of pigments and binders. However frescoes represent a special case where pigments build a mixture with the binders and the ground layer. Studies on artificial frescoes with UV laser radiation (KrF excimer, λ = 248 nm, 35 ns, 40–400 mJ/cm2 ) and subsequent color measuring analysis were carried out in a cooperative project. The results show a spectrum of different reactions: 40% of the samples could be cleaned without appreciable discoloration, 40% with minimal discoloration and the last 20% were damaged.

1

INTRODUCTION

Orthodox Churches in East Europe house a large number of frescoes (often in poor condition) polluted with sticky soot mainly caused by candle smoke (Fig. 1). Finding a gentle solution for this problem is not easy since the cleaning of fragile frescoes by mechanical techniques without damaging the objects is not trivial. The normal layer composition of frescoes is as follows: underground layer are bricks or stones, a ground layer from lime, sand and sometimes other components ensure the adherence between the paint and the support layer and produce a smooth surface. For the mixture of the paint layer different techniques exist. The studied samples imitate the old style of the Byzantine and post Byzantine school. Thereby the pigment and Ca(OH)2 were applied on the wet support layer, which means that the artist has to work just in time on the fresh (ital. fresco) prepared wall. Ca(OH)2 from the paint and the CO2 from the air react to produce CaCO3 and H2 O. The calcium carbonate is an insoluble substance and incorporates the pigment, the result is a colored plaster. This unique mixed composition nowadays leads to a problem in the field of restoration. For example the analysis of the used pigments is critical and it is impossible to distinguish the lime used as binder for the pigments and the already applied lime plaster from the support layer. The common cleaning problem, elimination of the black candle soot, will be hindered very often by the mechanical sensitive of the surface: cracks and flakes

Figure 1. Frescoes in the Monastery of Bucovina (Romania).

soak the frescoes. Hence, conventional cleaning techniques are unusable, often dangerous and very slow. As well known, laser cleaning often influences colour pigments and each application has to be tested in more

249

Figure 2. Photograph of the applied excimer laser cleaning system (Lambda Physik LPX 305i) with XYZ stage.

detail (Gaetani et al. 2000, Zafiropulos et al. 2003, Chappé et al. 2003, Dragasi et al. 2005, Andrettoni et al. 2006). This study concentrates on UV laser irradiation (KrF excimer, λ = 248 nm) which has a short absorption range limited to the surface and thereby a limited modification depth.

2

STUDIES

For this cooperatively systematic study, artificial frescoes models (4 × 4 × 0.5 cm3 , sandstone carrier, pigments generally used from companies which are specialized on French Charbonnel and Italian Maimeri in a traditional technique) were created by mural painting restorers from the Bucharest National University of Fine Arts. To simulate intensive pollution, half of each sample surface was exposed 30 seconds close to soot candles while the other half were covered to keep them as reference. The method of preparation of samples was the final result of long-time investigations onsite, performed by art restorers and scientists. The corresponding originals decorate the interior and sometimes the exterior walls of Orthodox churches in Romania. The used KrF excimer laser (λ = 248 nm, 35 ns) was combined with a computer controlled XYZ stage (Fig. 2). The surface was treated with a laser spot of 1.3 × 1.3 mm2 , created via a mask projection technique. Energy densities between 40 and 400 mJ/cm2 were used in a scan mode with 80% overlap in X andY directions. The number of repetitions (1–6) varied per sample whilst the frequency was kept at 10 Hz. The sooty and clean sample areas were exposed to UV laser radiation in order to test the pigment chromatic stability and the selectivity of UV laser irradiation. First, each energy density was tested without scan mode (square area of 1.3 × 1.3 mm2 ) and afterwards promising results with no or no significant

Figure 3. Frescos sample “Terra verde”, Ital. Maimeri pigment, mixed in aquarelle technique, right side artificial soot, spots A and B scanned 1 × with λ = 248 nm, 120 mJ/cm2 , 80% overlap, the colour measuring confirmed no significant modification.

discoloration were transferred to squares of 6 × 6 mm2 in scan mode on the sooty and clean site. Colour measurements (L∗ a∗ b∗ method, measured with GretacMacbeth Spectrolino) were carried out to obtain relative colour modification.The untreated frescoes colour was measured and stored as reference. Afterwards the soot and the laser irradiated test squares on the sooty and clean side were measured and set in proportion to the reference. Thereby each color variance could be assessed very clearly: if L∗ value decreases, the sample becomes darker than the original/reference and an increase means an alteration to more brightness. Rising of the a∗ , value indicates a higher proportion of red and a low a∗ value of more green. A higher yellow level is indicated as a higher b∗ value and a reduced b∗ value means a more blue colour in the spectrum than measured at the reference sample. 3

RESULTS

This study uses the visible and measurable colour modification as single criterion to estimate cleaning success. Particularly a chemical analysis is complicated because of the mixture composition. Three different types of reactions can be generated by laser irradiation: Case 1. Ideal case, shows no modifications at the irradiated clean square (Spot B, Fig. 3) and has no or only slight differences with the cleaned square (Spot A, Fig. 3). Case 2. No modification at the irradiated clean square (Spot B) and discoloration at the irradiated soot square (Spot A), which means residues of soot on the surface (Fig. 4). Case 3. Color modifications on both areas due to an excess of applied laser fluence. The brightness level of

250

Table 1.

Figure 4. Fresco sample “Oltremare celeste”, Ital. Maimeri pigment, mixed in aquarelle technique, right side artificial soot, spot A and B scanned 1x with λ = 248 nm, 170 mJ/cm2 , 80% overlap, the color measuring shows no modification at the unsoiled side (Spot B) but only a medium cleaning effect at the soot side (Spot A)

Figure 5. Fresco sample “Terre d’ombre bruleé”, French Charbonnel pigment, iron oxide (Fe2O3) and Manganese Dioxide (MnO2), right side artificial soot, spot A and B scanned 1× with λ = 248 nm, 170 mJ/cm2 , 80% overlap, the color measuring confirmed a flare effect on both sides

discoloration can be darkening or flare, depending on the physical/chemical reaction. Flare usually means a removal of the pigmented layer (Fig. 5), while darkening indicates a modification of the pigment or binding media. The enumerated results (Table 1) show that a comprehensive removal of the soot was possible on four samples with laser fluency under the modification threshold and without visible discoloration of the fresco layer. Four other samples show a minimal discoloration or incomplete removal of the soot layer. The remaining three samples react with massive discoloration on UV-laser irradiation or with a removal of parts of the upper layer.

Overview of tested pigments.

Name

colour

style

cleaning quality

Noir de vigne Nerro d’avorio Oltremare celeste Bleu de cobalt Outremer clair Terre d’ombre brulée Ocre jaune Giallo indiano Vert emeraude Terra verde Scarletto

black black blue blue blue brown ochre ochre green green red

fr.Ch. it.Ma. it. Ma. fr. Ch. fr.Ch. fr.Ch. fr.Ch. it.Ma. fr.Ch. it.Ma it.Ma.

++ + + O O − − O O ++ −−

4

CONCLUSIONS

The removal of pollutants on frescoes is a special application for laser cleaning, whereby the pigments compose a conglomerate with the ground layer and binders. Thus UV laser radiation can be a powerful tool only if the present pigments and binders are colour resistence to laser irradiation. Otherwise only a reduction of soot is possible, which can also pass as an acceptable result. For this study an industry excimer laser was used, which can not be placed on site. However, the system can demonstrate the possibilities of the UV radiation, which newest and future systems will deliver in a more compact size (e.g. 4th harmonic of Nd:YAG laser). Also other spectral ranges and pulse durations will have good results in special cases. REFERENCES Gaetani M.C. & Santamaria U. 2000. The laser cleaning of wall paintings, Journal of Cultural Heritage 1: S199–S207. Zafiropoulos V., Balas C., Manousaki A., Marankis G., Maravilla-Kalaitzaki P., Melesanaki K., Pouli T., Stratoudaki T., Klein S., Hildenhagen J., Dickmann K., Luk’Yanchuk B.S., Mujat C. & Dogariu A. 2003, Journal of Cultural Heritage 4: 249–256. Chappé M., Hildenhagen J., Dickmann K. & Bredol K. 2003. Journal of Cultural Heritage 4: 264–270. Dragasi E., Minos N., Pouli P., Fotakis C. & Zanini A. 2005, Laser cleaning studies on wall paintings; a preliminary study of various laser cleaning regimes, Book of Abstracts of LACONA VI. Andreotti A., Colobini M.P., Nevin A., Melessanaki K., Pouli P. & Fotakis C. 2006, Laser Chemistry, Volume 2006, Article ID 39046, 11 pages, doi:10.1155/2006/39046.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

The interaction of laser radiation at 2.94 µm with azurite and malachite pigments M. Camaiti, M. Matteini & A. Sansonetti ICVBC-CNR Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, Florence, Italy

J. Striová & E. Castellucci Department of Chemistry, University of Florence, Italy

A. Andreotti & M.P. Colombini Department of Chemistry and Industrial Chemistry, University of Pisa, Italy

A. deCruz & R. Palmer Department of Chemistry, Duke University, Durham, NC, USA

ABSTRACT: We present a detailed analysis of the behaviour of azurite and malachite pigments exposed to laser radiation. A pulsed, free-running, Erbium:YAG laser (2.94 µm) has been used. Pure pigments, laboratory models and original wall paintings fragments have been exposed to the laser in order to establish the conditions at which the alteration of colour or binder occurs. Energy density variation, consecutive pulses and various surface wetting agents have been considered. Micro-Raman spectroscopy, chromatic measurements and optical microscopy are employed to evaluate the interaction of the laser radiation with the pigments; gas chromatography/mass spectrometry follows the changes in the organic substances. Malachite, as a pure pigment, is more sensitive than azurite although partial darkening of both pigments, due to the formation of CuO, is observed in absence of surface wetting agents. However, in the presence of the latter, no colour alteration occurs for the testing fluences (7–10 J/cm2 ).

1

malachite (CuCO3 · Cu(OH )2 ). The reasons for choosing these pigments were:

INTRODUCTION

Er:YAG laser has been proved a successful tool for cleaning of canvas polychrome surfaces and panel paintings (Bracco et al. 2003, Andreotti et al. 2006a). On the contrary, only a few experiments have been reported in the case of mural paintings. The availability of fresco fragments from the Camposanto Monumentale in Pisa, allowed tests on surfaces with a dramatic and complex conservation history (Baldini et al. 2005). On the basis of the tests performed, it was concluded that the Er:YAG laser radiation at 2.94 µm exhibits good properties for removing animal glue, gypsum, whewellite, residues of polysaccharide substances and even remnants of metallic lead, present on the surface of problematic mural paintings (deCruz et al. 2006). Numerous pigments have been used for the realization of these paintings and, although there is a lack of knowledge in the literature about the effects of the 2.94 µm laser radiation on any of these pigments, our attention was focused on the interaction of the Er:YAG laser radiation with azurite (2CuCO3 · Cu(OH )2 ) and

– presence of –OH groups in their crystal structure and, therefore, they might be sensitive to 2.94 µm radiation; – they are widely distributed on the Camposanto Monumentale mural paintings, and – cleaning by chemical treatment (e.g., ammonium carbonate) is problematic because of their possible discoloration in an alkaline environment. A comparison between tests executed on laboratory specimens and on original fresco surfaces is reported.

2

MATERIALS AND METHODS

2.1 Specimens preparation Pigments used were supplied by Zecchi, Florence. For the tests on the pure pigments, they were placed into a plastic holder; no pigment manipulation was

253

performed, so their active surface area was not changed. Purity of the malachite and azurite pigments was checked by X-ray diffraction (XRD analysis. For a series of specimens simulating mural painting 5 × 5 × 1 cm slabs of soft limestone were used as support. A mortar composed of aerial lime and siliceous sand (1 : 2 in volume) was applied over the limestone support in a layer 1 cm thick. The mortar was kept in contact with a damp environment to allow a complete setting of the material, which was then checked by XRD analysis. After 1 month of curing, a tempera binder was prepared and mixed with the pigment, following an ancient recipe, using a 1 : 1 : 1 ratio of egg yolk; egg albumen and pigment, with four drops of white vinegar for 3 ml of mixture. The paint layers applied on the mortar substrate were artificially aged in a Suntest CPS+ apparatus (Heraeus) equipped with a 1500 W Xenon lamp light source (λ > 295 nm); the mean irradiance was 50 W/m2 , for a period of 300 hours.

2.2

Laser device

The experiments were carried out with a pulsed, free-running Er:YAG laser (emission wavelength, 2.94 µm), specifically the High Power Er CrystaLase, manufactured by MonaLaser (LLC, Orlando, Florida). The pulse is a 350 µs “macro-pulse” that consists of a train of 1–2 µs micro-pulses about 1–4 µs apart. The pulse is delivered through an articulated arm, producing a 1–3 mm diameter spot size. The macro-pulse repetition rate can be varied from 1 to 10 Hz. In a typical procedure, the energy threshold for efficient removal of the material is first established by preliminary tests (deCruz et al. 2003). The experiments reported here were carried out with a pulse repetition rate of 10 Hz and a spot diameter of 1 mm.

2.3 Ablation mechanism Er:YAG laser exhibits several properties which make it an ideal tool for cleaning of delicate painted surfaces (deCruz et al. 1999a, Bracco et al. 2003). Laser radiation at 2.94 µm corresponds to a strong absorption peak in the infrared spectra of organic and inorganic substances containing –OH or –NH groups. The energy of photons at this wavelength excites O–H and N–H bond vibrational stretching modes. Any substance containing a high concentration of O–H bonds has a strong affinity for photons at 2.94 µm and confines the absorption of these photons to a surface layer not deeper than a few microns (deCruz et al. 1999b). The pigments investigated in this paper are potentially highly sensitive to laser radiation at 2.94 µm because of their –OH content. However, the following observations may justify the application

of Er:YAG laser to the cleaning of these copper pigmented surfaces: a) Organic compounds containing –OH groups or thin liquid films of water or alcohol deposited on the painted surface as contaminant or just before the laser irradiation, act as a stain of relatively high concentration and very high radiation absorption, providing a natural barrier to energy penetration into the underlying layers. b) The selectivity of laser absorption due to the strong –OH absorption peak makes the use of 2.94 µm radiation effective even at relatively low pulse energies, generally between 5 and 20 mJ/pulse. For a spot size of 1 mm diameter, this corresponds to an irradiance of approximately 2.5–10 kW/cm2 . c) The photon energy of Er:YAG radiation is not high enough to break bonds. The energy required for O-H bond dissociation ranges from 3.4 to 4.5 eV, while photons of 2.94 µm only have 0.4 eV. Furthermore, the irradiance of the laser is too small to allow for multiphoton effects, which might provide the necessary dissociation energy (deCruz et al. 2000). 2.4 Evaluation In order to evaluate the effects of laser radiation, the following analytical procedure was carried out: 1) Micro-Raman spectroscopy (Renishaw 2000, 785 nm laser excitation source) and FTIR (Perkin Elmer, System 2000) were used to identify the laser-induced changes in the chemical composition of the pigments. The details of the Raman Renishaw System 2000 spectrometer coupled to a Leica optical microscope used to study the samples was described elsewhere (Striova et al. 2005). 2) Surface colour was measured using the CIE L∗ a∗ b∗ system (MINOLTA CR 200 colorimeter). Optical microscopy was used to observe surface morphology (Leitz Wild M420 Stereomicroscope equipped with a digital image capturing system). Colour measurements and microscopic observations were performed before and after irradiation, with the aid of a grid, in order to identify, even microscopically, exactly the same micro-areas. In CIE L∗ a∗ b∗ colour space, the L∗ value ranges between 100 (white, perfect diffuser) and 0 (black); a∗ and b∗ values range between −60 and +60; positive a∗ is red, negative a∗ green; positive b∗ yellow and negative b∗ blue. Total colour difference (E) is calculated according to:

254

where L∗ = L∗ − L∗ , a∗ = a∗ − a∗ , b∗ = b∗ − b∗ and L∗ , a∗ , b∗ values were obtained from the measurements of paint surface before irradiation, while L∗ , a∗ , b∗ after irradiation.

3

RESULTS AND DISCUSSION

3.1 Laser tests on pure pigments As a preliminary test to evaluate the pigment sensitivity to laser radiation at 2.94 µm, experiments with pure pigments were performed. Pigment crystals were placed into a plastic holder and their careful observation, by means of optical microscopy before and after the irradiation, was a main tool to detect the onset of the colour alteration. With increasing fluences, the formation of a black substance, initially on the edges of both malachite and azurite pigment crystals, was observed. The pigment alteration in dry conditions (no wetting agent was used) begins at a fluence of 0.6 J/cm2 (5 mJ/pulse) when applying 120 consecutive pulses on the same spot for malachite. The azurite threshold was found to be 1.3 J/cm2 (10 mJ/pulse) employing 300 consecutive pulses on the same spot. It is necessary to point out that such change in the pigment was only detected by optical microscope and with relatively high accumulation of pulses. The threshold values corresponding to the changes visible by the naked eye are 1.3 J/cm2 and 1.9 J/cm2 (15 mJ/pulse) for malachite and azurite, respectively, applying ten consecutive pulses on the same spot. The black substance, which is formed on the surface of pigment crystals above the alteration threshold, was identified as tenorite (copper (II) oxide - CuO) by means of micro-Raman spectroscopy (Goldstein et al. 1990), as the Raman spectrum in Figure 1 illustrates. With regard to the pigments’ alteration, the mechanism and rate of thermal decomposition of hydroxycarbonate minerals of copper might be quite complex and depends on the atmosphere in which the dissociation takes place, temperature and grain size (Seguin et al. 1974, Mansour et al. 1994). In general, the water molecules get released at about 100◦ C, and

294 345

614

Intensity (a.u.)

3) To follow changes in binder composition, gas chromatography/mass spectrometry (GC/MS) was performed (6890N GC system gas chromatograph coupled with a 5973 mass selective detector, Agilent Technologies). The analysis of both the lipid and proteinaceous component confirmed the good level of aging of the specimens’ binder. The characterization of the proteinaceous material (egg) was made by Principal Component Analysis (PCA) of the amino acidic percentage content data, using a reference data set of 80 reference samples containing egg, casein, and animal glue (Andreotti et al. 2006). The analysis of the lipid fraction of the binding media on the basis of characteristic parameters: A/P = 0.1 (azelaic over palmitic acid ratio), P/S = 3.4 (palmitic over stearic acid ratio), D = 3 (sum of dicarboxylicacids) agrees with the literature values (Colombini et al. 2002).

200

400

600

800

cm-1 Figure 1. Raman spectrum of the black substance formed upon irradiating the azurite crystals using fluences above the pigment alteration threshold.

pigment decomposition continues with release of CO2 molecules at about 350◦ C (Seguin et al. 1974). In this study, no traces of other alteration products such as cuprite, red copper(I) oxide (Cu2 O), were detected by optical or spectroscopic techniques. Therefore, it may be concluded that the end-product on the surface of pigment crystals is tenorite. Infrared photons are known to act as a heat source and heat transfer is the most likely mechanism causing the alteration of the pigment. It is well known that Er:YAG laser is often used together with –OH containing wetting agents. The high temperature gradients and explosive vaporization of the wetting agents ensures that the contaminants are ejected from the paint surface, taking much of the heat with them (deCruz et al. 2000). In fact, the wetting agents containing –OH groups proved to be also essential in protecting the azurite and malachite pigments, thanks to the strong absorption of these –OH groups at 2.94 µm. In fact, in presence of wetting agents no pigment damage was observed at all tested fluences (up to 7.0 J/cm2 , 60 mJ/pulse). 3.2

Laser tests on specimens prepared with egg binder

Malachite and azurite pigments were usually applied in the mural painting technique with a binding medium (the so called “a secco” technique). The experimental procedure of laser irradiation on samples simulating ancient mural paintings was performed according to the following points: 1) A 25 cm2 area of each specimen was divided into 9 equal sectors; 2) Three sectors (no. 1, 2 and 3) were treated in dry conditions with laser radiation of increasing fluence, firstly in order to identify the damage threshold, then to increase the radiation effects;

255

Table 2. Chromatic variations of azurite specimens’ surfaces irradiated with Er:YAG laser at different fluences (F).

Table 1. Chromatic variations of malachite specimens’ surfaces irradiated with Er:YAG laser at different fluences (F).

−0.30 −5.06 −3.26 −2.23 −2.25 −1.74 −1.58 −1.08

i-PrAl Water

2.11 10.37 10.01 −0.75 0.10 0.20 −2.24 −0.80

b∗

E

F(J/cm2 )

Sector

L∗

a∗

b∗

E

F(J/cm2 )

−0.26 −1.26 −0.68 0.09 0.02 −0.22 2.81 1.58

2.15 11.61 10.55 2.35 2.25 1.77 2.81 1.58

1.3 1.9 2.5 0.6 7.6 12.7 1.3 3.8

1 2 3 4 5 6 7 8

3.55 2.14 1.84 −3.50 −1.86 −3.76 −1.38 −0.73

−1.82 −1.85 −2.00 1.18 0.53 1.98 0.29 −0.12

5.59 6.60 6.80 −3.21 −1.48 −4.25 −1.48 0.08

6.87 7.18 7.32 4.90 2.43 5.98 2.04 0.74

1.3 1.9 2.5 0.6 7.6 12.7 1.3 2.5

Dry

1 2 3 4 5 6 7 8

a∗

i-PrAl Water

L∗ Dry

Sector

3) Three sectors (no. 4, 5 and 6) were irradiated with the same procedure, but using water as a wetting agent; 4) The remaining three sectors (no. 7, 8 and 9) were treated as mentioned above, but using pure isopropyl alcohol (i-PrAl) as wetting agent. Chromatic values for malachite and azurite samples, measured before and after laser irradiation are reported in Tables 1 and 2, respectively. Remarkable E values were found from the measurements on surfaces before and after irradiation in dry conditions at fluences of 1.9 (malachite sample) and 1.3 (azurite sample) J/cm2 . A spotted darkening at these fluences was produced especially on the malachite sample surface (Fig. 2). The main contribution to E in dry conditions is due to a∗ (decrease of the green component in absolute value) in the case of the malachite sample, and to b∗ (decrease of the blue component in absolute value) in the case of the azurite sample. In the case of malachite specimen a loss of brightness was detected (negative values of L∗ ). On the contrary, brightness was slightly increased (positive values of L∗ ) in the case of the azurite specimen. Laser irradiation with a wetting agent does not cause a remarkable change in the colour of the malachite sample. In the case of azurite, an evident E is obtained when using water as wetting agent, but this seems independent of the fluence. It is possible to assume that water produces a slight settling of the paint layer, which took place independently of the laser irradiation. The 9th sector of the sample (designated for the tests with isopropyl alcohol) was not irradiated because this solvent quickly evaporates above the fluence threshold of 3.8/2.5 J/cm2 (35/25 mJ/pulse), and the surface behaved as if no wetting agent were used: the paint layer was directly exposed to the radiation as shown in Figure 3. The different threshold identified for malachite (3.8 J/cm2 ) with respect to azurite (2.5 J/cm2 ), when isopropyl alcohol was used as wetting agent, may be attributed to different physical properties of the paint layers. In the presence of azurite, the layer is more absorbent and allows a faster drying of the surface.

Figure 2. Microscopic observations of malachite surface (sector 2). On the left, before irradiation; on the right, the same area after irradiation in dry conditions with 1.9 J/cm2 . Scale = 5 mm.

Figure 3. Microscopic observations of azurite surface (sector 8). On the left, before irradiation; on the right, the same area after irradiation using isopropyl alcohol as wetting agent and 2.5 J/cm2 fluence. Scale = 5 mm.

In some tests a slight whitening of the binder has been observed, especially at low fluences. Since no modifications were detected by GC/MS or Fourier Transform Infrared (FTIR) analysis (results not shown), the slight whitening could be a result of protein media denaturation, which is manifested by changes in protein structure rather than in its composition. Water and/or alcohol effectively protect the painting layers from an eventual laser damage because the radiation is preferentially absorbed by these agents. In this process, water has been proved to be much more effective than isopropyl alcohol (Table 3). Values with an asterisk in Table 3 represent the operative fluences over which the surfaces dried, for the horizontal setup of the specimen. In general, these values depend on the properties of the substrate and on the evaporation and/or absorption velocity of the wetting agent. Working with isopropyl alcohol, lower

256

Table 3. Damage thresholds for specimens simulating mural paintings. The value is expressed as fluence (pulse energy (E) per unit area (A) according to F = E/A).

3 2 animal glue

egg

database

1

Damage threshold (J/cm2 )

0

Dry conditions

Water

i-PrAl

-8

-7

-6

-5

-4

-3

-2

-1

1

2

-1

Malachite Azurite

1.9 1.3

7.6∗ 7.6∗

3.8∗ 2.5∗

-2 -3

threshold values were detected. This is probably due to the lower boiling point of this wetting agent (82◦ C) and to the higher volatility or vapor pressure (33 mmHg at 20◦ C for isopropyl alcohol and 17.5 mmHg at 20◦ C for water) that leads to faster evaporation from the substrate as compared to water. Other higher boiling alcohols such as 2-butanol (boiling point 100◦ C, vapor pressure 12.5 mmHg at 20◦ C) proved to be unsuitable for this experiment due to the permanent oily residues which remained on the surface. The binding media of azurite and malachite were collected after laser irradiation at a fluence of 2.5 J/cm2 in dry conditions (sector 3), for which the most evident darkening effect of the pigment was reached. In these conditions, the organic material was not degraded, as evidenced by the analysis of the proteinaceous component. In fact, the aminoacid percentage content in the sample collected after laser irradiation is not significantly different from the sample taken before laser irradiation of the azurite specimen. This is also clear in the PCA score plot (Fig. 4), where both samples are located very close one to the other in the egg cluster. Moreover, the analysis of the lipidic fraction of the egg binder in both the specimens of malachite and azurite also show that below a fluence of 7.6 J/cm2 , the characteristic ratios of the fatty acids remain almost the same as those of the material before laser exposure (P/S: 2.9 ÷ 3.5, A/P: 0.1, D: 2.7 ÷ 3.1). These results are consistent with the threshold limits of Er:YAG laser at 2.94 µm when used on painted surfaces containing lipid, proteinaceus and resinous materials, which are reported in the literature (M. P. Colombini et al. 2003). 3.3 Laser tests on Camposanto mural paintings An original fragment from Camposanto Monumentale was available for experiments thanks to the Soprintendenza ai Beni Architettonici e al Paesaggio, al Patrimonio Storico Artistico e Demoantropologico, Pisa (Italy). The sample, designated CS9 (Fig. 5), was first analysed by means of FTIR and GC/MS for the presence of organic materials. No traces of any organic matter were evidenced by either technique. The high content of oxalates detected by FTIR might suggest the oxidative degradation of an original binder component.

-4

3

before irradiation

casein sector 3after irradiation

Figure 4. PCA score plot relative to the aminoacid percentage content of the binding media of azurite specimen before and after laser irradiation at 2.5 J/cm2 in dry conditions.

Figure 5. Fragment CS9 coming from Camposanto Monumentale Pisa.

FTIR in diamond anvil cell and micro-Raman analysis identified the nature of the pigments as azurite with a brush stroke containing malachite pigment. This fragment was used to obtain the alteration threshold values of azurite and malachite pigment when irradiated in dry conditions, which gave identical results to those obtained for the pure pigments in the case of malachite (1.3 J/cm2 ). For azurite, the alteration threshold results were found to be higher (2.5 J/cm2 ) with respect to pure pigment (1.9 J/cm2 ). Such a finding emphasises the importance of direct in situ laser tests, because the original mural paintings might have different composition and thus different threshold values than laboratory prepared samples. When using the wetting agents the results are the same as those obtained for the laboratory prepared samples, i.e. damage does not occur until the evaporation/absorption of the wetting agent.

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Another laser cleaning test was performed in situ on the light blue drapery area using the following conditions: 2 passes, isopropylalcohol as wetting agent, fluence: 1.9 J/cm2 (15 mJ/pulse), 505 pulses per cm2 . In this case, the superficial white layer (under investigation) was removed without any damage to the paint layer.

4

CONCLUSIONS

Pure malachite pigment irradiated without use of wetting agents has proved to be more sensitive to the pulsed laser energy at 2.94 µm than the azurite pigment. Tenorite is as an alteration product for both pigments over the damage threshold 1.3 J/cm2 for malachite and 1.9 J/cm2 for azurite. In tests performed on the original mural painting surface, the damage threshold resulted 1.3 J/cm2 for malachite and 2.5 J/cm2 for the azurite pigment, also without the use of wetting agents. When a paint layer with a proteinaceous binding medium is irradiated, the malachite pigment is effectively protected and the damage threshold is increased to 1.9 J/cm2 . The same protective effect was, however, not demonstrated when the azurite paint layer was exposed to the radiation. For the laboratory prepared tempera paint layers irradiated with Er:YAG laser, a reverse behaviour with respect to the pure pigments has been identified, azurite layers being slightly more sensitive than malachite layers. Wetting agents proved to be very efficient in protecting the pigments. In general, a layer of an optically absorbing agent protects the pigment until these agents are absorbed or evaporated. In this study, water proved to be much more effective than isopropyl alcohol, allowing operators to use fluences as high as 7–10 J/cm2 , depending on the pigments and the painting layer physical properties. Our investigation will continue with an in-depth study of the role of the binding materials, with specific attention to damage thresholds of other pigments. The efficacy of Er:YAG laser in removing inorganic and organic treatments from the surface of mural paintings is the subject of an ongoing project. Moreover, a comparison between laser and chemical methods in cleaning the frescoes from the Camposanto Monumentale Pisa, is being carried out to better understand the benefits and possibilities offered by the Er:YAG laser, which are, presently, very promising.

ACKNOWLEDGEMENTS One of the authors (J. S.) wishes to acknowledge the ATHENA project (contract MEST-CT 2004-504066) within Marie-Curie actions.

REFERENCES Andreotti, A., Bracco P., Colombini, M. P., deCruz, A., Lanterna, G., Nakahara, K. & Penaglia, F., 2006a. Novel Applications of the Er:YAG Laser Cleaning of the Old Paintings. Lacona VI Proceedings, in press. Andreotti, A., Bonaduce, I., Colombini, M. P., Gautier, G., Modugno, F. & Ribechini, E. 2006b. Combined GC/MS Analytical Procedure for the Characterization of Glycerolipid, Waxy, Resinous, and Proteinaceous Materials in a Unique Paint Microsample. Anal. Chem. 78: 4490–4500. Baldini, U., Baracchini, C., Bonaduce, I., Caleca, A., Caponi, G., Colombini, M. P., Luppichini, E. & Spampinato, M. 2005. Una storia complicata: gli affreschi del camposanto monumentale di Pisa. In Biscontin & Driussi (ed), Sulle pitture murali, Proc. of XXI Con. Scienza e Beni Culturali, Bressanone 12–15 July 2005. Venezia: Arcadia. Bracco, P., Lanterna, G., Matteini, M., Nakahara, K., Sartiani, O., deCruz, A., Wolbarsht, M. L., Adamkiewicz, E. & Colombini, M. P. 2003. Er:YAG laser: an Innovative Tool for Controlled Cleaning of Old Paintings: Testing and Evaluation. J. Cult. Heritage 4: 202s–208s. Colombini, M. P., Modugno, F., Fuoco, R. & Tognazzi, A. 2002. A GC-MS Study on the Deterioration of Lipidic Paint Binders. Microchem. J. 7: 175–185. Colombini, M. P., Andreotti, A., Lanterna, G. & Rizzi, M. 2003. A Novel Approach for High Selective Microsampling of Organic Painting Materials by Er:YAG Laser Ablation. J. Cult. Heritage 4: 355s–361s. deCruz, A., Hauger, S. A. & Wolbarsht, M. L. 1999a. The Role of Lasers in Fine Arts Conservation and Restoration. Optics & Photonics News 10: 36–40. deCruz, A., Wolbarsht, M. L. & Hauger, S. A. 1999b. Laser Removal of Contaminants from Painted Surfaces. J. Cult. Heritage 1: S173–S180. deCruz, A., Wolbarsht, M. L. & Hauger S. 2000. The Introduction of Lasers as a Tool in Removing Contaminants from Painted Surfaces. Art et Chimie: La couleur Proceedings, Paris – Louvre, France. deCruz, A., Wolbarsht, M. L., Palmer, R. A., Pierce, S. E. & Adamkiewicz, E. 2003. Er:YAG Laser Applications on Marble and Limestone Sculptures with Polychrome and Patina Surfaces. LACONA V Proceedings, Osnabrück, Germany. deCruz,A.,Andreotti,A., Colombini, M. P., Castellucci, E.M., Striova J., Matteini, M. & Sansonetti A. 2006. Design of a Research Program on the Effects of Er:YAG Laser Radiation on Mural Painting: Preliminary Results on the case study of the “Cimitero Monumentale” in Pisa. 12th Laser Conference on Laser Optics St.Petersburg, Russia. Goldstein, H. F., Dai-sik K., Yu, P. Y., Bourne, L. C., Chaminade J. P. & Nganga L. 1990. Raman Study of CuO Single Crystals. Phys. Rev. B 41: 7192–7194. Mansour, S. A. A. 1994. Thermoanalytical Investigations of Decomposition Course of Copper Oxysalts. J. Therm. Anal. 42: 1251–1263. Seguin, M. K. 1974. Thermogravimetric and Differential Thermal Analysis of Malachite and Azurite in Inert Atmospheres and in Air. Canadian Mineralogist 13: 127–132. Striova, J., Lofrumento, A., Zoppi, A., Castellucci, E. M. 2005. Prehistoric Anasazi Ceramics Studied by microRaman Spectroscopy. J. Raman Spectrosc. 37:1139–1145.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Conservation of medieval polychromed wooden sculpture of Madonna and Child K. Chmielewski, A. Koss & M. Mazur Inter-Academy Institute for Conservation and Restoration of Works of Art, Academy of Fine Arts in Warsaw, Poland

J. Marczak & M. Strzelec Institute of Optoelectronics, Military University of Technology, Warsaw, Poland

ABSTRACT: This paper presents, step by step, the complete conservation procedure of the sculpture Madonna and Child (turn of 15th to 16th century) belonging to the collection of sacred sculptures of Diocesan Museum “Długosz House” in Sandomierz, Poland, utilizing laser as a cleaning tool. Q-switched Nd:YAG ReNOVALaser 2 system using near infrared radiation at 1.064 µm was employed; it was particularly very successful at removing dirt concretions adhering directly to the whole figure face surface and also removed pollution reaction products from the specimens when present.

1

INTRODUCTION

The polychromed wooden sculpture of Madonna and Child (turn of 15th to 16th century) belongs to the collection of sacral sculptures of Diocesan Museum “Dlugosz House” in Sandomierz, Poland (Fig. 1). The sculpture height of 1 m and the plane development of its back side indicated an original arrangement in altarpiece. The stylistic analysis, based on the characteristic configuration of figure fragments allowed to date its origin at the decline of 15th or beginning of 16th century and to localize the studio in Lesser Poland. More detailed determination of the Madonna’s provenance was impossible due to a very bad sculpture condition – losses of sculptural form, face damages and seizure of details, usually characterizing medieval art origin. Damages and strong soil included all technological layers: linden wood, chalk-glue ground, red clay, silver and polychrome. The cleaned surface was fractured and it was composed of many concave, bowl shaped flakes. This fact made cleaning substantially difficult as well as it forced the minimization of solvent treatment. Protruding flakes edges were easily eluted, still leaving dirt in flake hollows. A method has been developed, which consisted in delicate dirt softening with the use of solvents and subsequent mechanical thorough cleaning with a scalpel. After softening, dirt formed a gelatine layer, which could be partially removed but also it partially penetrated

Figure 1. Photograph of Madonna and Child before conservation.

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Figure 3. Results of stratigraphy of Madonna’s coat and dress. Sample number refers to area from Figure 2.

Figure 4. Results of stratigraphy of Madonna’s dress and Child’s complexion.

Figure 2. Sampling areas in stratigraphy of painting layers and binders identification.

the underlying original surface structure. The surface plane after cleaning was “broken” by dark clusters of soil created in it. It was disturbing, particularly for the aesthetic reception of Madonna’s and Child’s faces. Selection of laser as a complementary cleaning tool for the present wooden sculpture was the best and only one solution, characterized also by the smallest interference with the original object structure.

STRATIGRAPHY OF PAINTING LAYERS AND BINDER IDENTIFICATION

Figure 5. Binder identification in Madonna’s coat lining. Sample number refers to area in Figure 2. A – denotes results of dying in Ponceau S solution; B – dying in Black Sudan B solution.

Conservation started with chemical investigations, aimed at determination of sculpture technological structure. It included pigments and binders analyses in samples of colour layers and determination of quantity and stratigraphy of technological layers. Sampling points are shown in Figure 2. Stratigraphic structure microscope analyses (max. magnification 256x) followed sealing of all samples in Premacryl Plus resin and polishing of observation cross section (Figs. 3, 4). Binder identification was based on microscopic observations (magnification 100× and 250×) of samples, dying in saturated solutions Ponceau S in 30% of CH3 COOH for protein identification and Black Sudan B in C2 H5 OH for searching of lipids. Some selected, representative results are shown in Figures 5-7.

Stratigraphic and identification analyses were supplemented by dendrology studies as well as X-ray and UV light imaging. Conducted studies confirmed a lack of original sculpture’s polychrome. Fragments of painting layers found at Madonna’s coat, dress, and both complexions were made using oil technique. Utilized pigments (coat blue – Pruss Blue with lead white; complexion – lead white with vermillion; dress white – lead white with barium white) gave the evidence the origin of painting layers as not earlier than the 19th century. Medieval polychrome usually contained paints with tempera binder.

2

260

Figure 8. Photographic illustration of the results of chemical cleaning: a) tests performed at Child’s face and trunk, Madonna’s hand, neck and coat (rectangles); b) overall view after chemical cleaning.

Figure 6. Binder identification in Madonna’s complexion.

Gothic origin. Hence, the main restoration assumption included: – Complete sculpture clearance, – Removal of partially preserved oil overpaintings, – Rebuilding/completion of sculptural form.

Figure 7. Binder identification in Child’s complexion.

The sculpture had been made of linden wood and sized using glutoline glue. The whole sculpture surface, except the reverse side, was covered with gluechalk ground. Red bole layer was spread on the dress and at the external side of the coat. A small fragment of silver, found in Madonna’s dress hollow, indicated original dress’s silver plating. The coat could have been silver plated or gilded. Traces of silver were not enough basis to decide about further reconstruction of this layer.

3

RESTORATION PROCEDURE

Taking into account the overall bad preservation of the figure sculptural form and the polychromy, it was difficult to determine the range and kind of conservation work. The lower part of Madonna’s dress and coat folds, cut off in whole together with the figure basement, disturbed the correct sculpture proportions. The overall reception of the artwork was difficult due to other damages like the cut of Madonna’s hair locks, fractures of many protruding elements, wood cracks, overpaintings, and strong soil. The main priority became to place the object into optical order and restore its

Chemical cleaning with solvents was preceded by careful resistance tests of painting layers (Fig. 8a). It was found, that two cleaning solutions are optimal: 3% ammonia liquor and conservation soap, based on 1:1 mixture of turpentine oil and typical cleaning liquid. Solvent action was interrupted with the use of white spirit. Fragmentarily preserved, contrast planes of oil overpaintings, observed after cleaning of soil, had negative influence on the artwork perception. This can be seen in Figure 8b. Colour interference created the contrasts, which improperly directed the observer’s attention. It was considered, that a better solution would be the presentation of the sculpture with preserved layers of ground and bole together with uncovered wooden fragments. The layers of oil overpaintings were removed using chemical methods with subsequent thorough mechanical scalpel cleaning of individual sculpture parts. As it was stated earlier, the red bole layer coated with oil overpaintings was moisture-sensitive and easy to elute during dirt removal. Oil paint layers on bole were cleaned mainly using scalpels. They were initially softened with the use of 2.5% ammonia water and subsequently removed with a scalpel. Due to the application of this safe, but time-consuming method, it was possible to uncover the preserved silver fragments under white overpainting and red lead layers. Figure 9a shows view of Madonna and Child figure after chemical removal of overpaintings. Tests of laser cleaning around faces and at red bole are presented in Figure 9b.

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Figure 9. Madonna and Child figure in transient stage of restoration: a) after chemical removal of overpaintings; b) during laser cleaning tests.

Figure 11. Final view of Madonna and Child wooden sculpture after the restoration process.

Figure 10. Sculpture after laser renovation and reconstruction (a); after completion of mortar and pulment layers (b).

Laser renovation proved to be very precise and uniform, and has been applied to clean the wooden substrate, ground and red bole layers. Results are shown in Figure 10a. The laser used in the procedure of cleaning was a Q-switched Nd:YAG system ReNOVALaser2 with a pulse width of about 10 ns, output energy up to 600 mJ, repetition rate 1–10 Hz, wavelength of 1064 nm, beam diameter 10 mm and beam delivery through pantograph. Typical average fluence was lower than 0.350 J/cm2 . Laser cleaning finally integrated artwork surface. Figures complexion became uniform, introduced harmony and improved whole aesthetic reception.Additional advantage of this coherent “tool” was its efficiency, far higher than in the case of traditional cleaning methods. The final stage of conservation included the completion of ground and red bole layers at recovered wooden surfaces, reconstruction of small Madonna’s and the Child’s face fragments (Fig. 10b) as well as ultimate retouch of the whole surface presented in

Figure 11. The sculpture is now exhibited in the Hall VII of Sandomierz Diocesan Museum. 4

CONCLUSIONS

The laser cleaning technique has been thoroughly compared to traditional chemical and mechanical procedures in the case of a wooden artwork, a medieval sculpture of Madonna and Child. Conventional cleaning tests showed that none of the methods known to experienced restorers met all the requirements. Moreover, laser cleaning appeared to be much faster and safe for original linen wood substrate and technological layers, particularly around all fractures and inside hollows. The process was fully under so called selfcontrol of the ablation phenomenon, based on the much smaller absorption of wood in comparison with almost all typical surface layer materials. ACKNOWLEDGEMENTS Work has been supported by the Ministry of Science and Higher Education, Poland, project 217/E284/SPUB-M/EUREKA/T-11/DZ 203/2001–2003.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Advanced laser renovation of old paintings, paper, parchment and metal objects J. Marczak, M. Strzelec, R. Ostrowski, A. Rycyk, A. Sarzy´nski & W. Skrzeczanowski Institute of Optoelectronics, Military University of Technology, Warsaw, Poland

A. Koss & R. Szambelan Inter-Academy Institute for Conservation and Restoration of Works of Art, Warsaw, Poland

R. Salimbeni & S. Siano Istituto di Fisica Applicata “Nello Carrara”, Florence, Italy

J. Kolar Morana Rtd, Ivancna Gorica, Slovenia

M. Strlic University of Ljubljana, Slovenia

Z. Márton & I. Sánta PANNONLASER, University of Pecs, Hungary

I. Kisapáti University of Fine Arts, Budapest, Hungary

Z. Gugolya & Z. Kántor University of Pannonia, Veszprém, Hungary

S. Barcikowski Laser Zentrum Hannover, Germany

P. Engel University of Hildesheim, Germany

M. Pires National Institute for Engineering, Technology and Innovation, Lisbon, Portugal

J. Guedes, A. Hipólito & S. Santos Arqueologia Lda, Porto, Portugal

A.S. Dement’ev, V. Švedas, E. Murauskas & N. Slavinskis Institute of Physics, Vilnius, Lithuania

K. Jasiunas EKSPLA, Vilnius, Lithuania

M. Trtica VINCA Institute of Nuclear Sciences, Belgrade, Serbia

ABSTRACT: The main aim of the presented EUREKA E!3483 EULASNET LASCAN project, signed by 20 Institutions from 9 Countries and coordinated by the Institute of Optoelectronics from Warsaw, is to construct, develop, and investigate laser systems and technologies for renovation of paintings, antique books and metal works of art. The Project’s Consortium intends to accomplish it through the implementation of advanced laser cleaning

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heads, including generation of ultra-short pulses with high repetition frequency and tunable wavelength; laser renovation technologies and complementary technologies; artwork diagnostics methods particularly methods of “on-line” cleaning process diagnostics and control; techniques of automatic laser beam scanning for application to flat objects (2D) and for irregular surfaces (3D); methods of diagnostics for renovation of the workplace environment (eye safety and air pollution control). This paper presents current developments described by project partners as well as results of events and joint experiments conducted during the project realization.

1

INTRODUCTION

Taking into account the specialisation of the consortium partners as well as the agreed cooperation between individual institutions, development and implementation of the following technologies were expected in the frame of EULASNET LASCAN project: – Renovation of historical paintings and painted coatings, including not only classical paintings but also decorated stones, plaster, glass and wood substrates (frescoes, stained-glass windows, icons, polychrome, etc.). – Cleaning of old printings on parchment and paper, books and maps (long-term encrustation with composition depending on exhibition/storage place, biological originated infections, unwanted inscriptions and drawings). – Renovation of historical objects made of metals and alloys, including artworks in which other substrate materials are covered with metallic layers (e.g. gildings) as well as artworks, in which metal elements are parts of the structure (e.g. fabrics with metal threads). The project plan included also R&D work on artworks diagnostics and particularly diagnostics of laser renovation processes to preserve object original characteristics, to protect it against damage as well as to realize the option of laser cleaning automation. Diagnostic methods were based on different non-destructive or “micro-destructive” optoelectronic techniques and analytical methods of nuclear physics and chemistry. This paper presents results of R&D work in the frame of EULASNET LASCAN project, selected among the newest developments of project partners.

2

CLEANING OF PAINTINGS AND PAINTED SURFACES

Laser cleaning can be a promising method for fragile painted objects, due to the lack of mechanical contact. Nevertheless, preliminary consolidation of the objects may be necessary, which can affect the cleaning efficiency.

Figure 1. Excavated fresco fragment from Roman age Pannonia, covered by thick encrustation, consolidated with acrylic dispersion and cleaned with KrF laser (right side).

Ancient Roman fresco fragments (consolidated by different consolidants and untreated) were cleaned by KrF excimer laser at 248 nm (Fig. 1). This wavelength was chosen because of its short absorption depth, compared to the commonly used 1064 nm (Nd:YAG laser). The obtained results are summarized as follows: a) Choosing the appropriate KrF laser parameters, encrustations of different thickness and composition could be removed and the underlying paint was not damaged. Above ∼0.5 mm contamination thickness, the most effective way of cleaning was to focus the beam on the surface, thus the fluence was over 20 J/cm2 . In this case, the photomechanical effect of the laser radiation is dominant. The contamination splinters off from the surface, due to the micro-explosion caused by the high energy density. For the thin contamination layers, the cleaning was done using fluences between 0.5 and 3 J/cm2 . b) The consolidants used, Syton X30, Primal WS24 and Paraloid B72, made the surface cleaning a bit less effective, but not impossible. For the surfaces which were covered by a thin layer of contamination and a film of Paraloid B72, both the film and the contamination layers could be removed using fluences around 2 J/cm2 , although a slight discoloration occurred on certain pigments. c) The interaction of the different pigments with the 248 nm laser beam was also investigated. It can be concluded that the presence of cinnabar excludes the applicability of the KrF laser, since it blackens irreversibly even after a single laser pulse. Yellow ochre became brown, except one case. Red ochre is

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less sensitive than yellow ochre, it darkened slightly only once, during the experiments. The black paint on a sample was removed from the untreated surface, and did not change colour where the sample was consolidated by Paraloid B72. Terre Verte and a white pigment were not discoloured. d) Wetting by distilled water improved the cleaning process. e) Repeating the cleaning procedures at 1064 nm with a fluence of 0.6 J/cm2 (Nd:YAG laser) showed that all the pigments were more sensitive to laser irradiation, while at this fluence the contamination did not leave the surface. The thick encrustation could not be removed by the Nd:YAG pulses, and the pigment was removed by the first pulse in areas where the sample was treated by Paraloid B72.

Figure 2. Q-switched laser irradiation of gold amalgam film at relatively low fluence (365 mJ/cm2 ) induces surface micro-melting effects (left side – 200 µm).

Results of other works are included in two separate papers (Ostrowski et al. & Márton et al., in this Volume). 3

Figure 3. Temperature rise upon laser irradiation for various pulse duration. The gilded area on the right is well preserved at 70 ns.

LASER CLEANING OF METALS

Metals and metal alloys suffer heavy degradation effects caused by environmental conditions. Laser cleaning may provide in these cases a progressive and controlled removal of corrosion layers. Activity of the team from Istituto di Fisica Applicata “Nello Carrara”, Florence on this topic found a strong interest in studying gilded bronzes (Siano et al. 2001). The studies carried out at IFAC indicated that using Nd:YAG lasers with pulse width in the range of 70–100 ns were more suitable than Q-switched lasers with pulse width in the range of 5–10 ns, in order to avoid excessive heating of the 2–10 µm gold amalgam film, leading to local melting or wrinkling (Figs. 2, 3). A Short Free Running mode Nd:YAG lasers operating at 35 µs and relatively high fluence (1.5–2 J/cm2 ) was built and employed for the cleaning of all the freezes around the main panels, later in the case of the David by Verrocchio, a bronze statue host in the Museum of Bargello in Florence and the Attis by Donatello. In all cases the gold leaf was perfectly preserved. Negative effects of melting, discoloring or damaging can be avoided by using femtosecond (fs) laser irradiation when the laser fluence is below the threshold of plasma formation (according with investigations by Laser Zentrum Hannover). Figure 4 shows confocal microscopy pictures of a fs laser treated outdoor bronze object (the state of Friedrich III statue) showing the surface topography at different laser fluences (150 fs, focus diameter 50 µm, 10 Hz). Laser cleaning of bronze Roman coins from the 3rd century have been tested in VINCA Institute of Nuclear Sciences, Belgrade, using picosecond (ps) Nd:YAG laser (40 ps, 1064/532 nm) and TEA CO2 laser (120 ns, 10.6 µm). As it can be seen in Figures 5

Figure 4. Confocal Microscopy pictures of a femtosecond laser treating outdoor bronze object (right corner – contour view).

and 6 above, both lasers modified the coin surface with much more local interaction area in the case of ps pulses. Laser removal of oxides from archaeological bronzes from the medieval Oporto Monastery has been performed by Arqueologia Lda, Porto with the consultancy of Dr. M. Pires from INETI, Lisbon, Portugal (Fig. 7). Photographs A and C (Fig. 7) shows results obtained with a Palladio Compac laser (1064 nm, Q-switched, 400 mJ, 1 Hz), photograph B (Fig. 7) shows the interaction of a Smart Clean 2 laser (1064 nm, Short Free Running, optical fibre delivery, 500 mJ, single pulse, 2 Hz).

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Figure 5. Irradiation of a Roman coin by picosecond Nd:YAG laser operating at 1064 (a) and 532 nm (b). Cumulative action after 15 pulses. Fluences, 3.7 (a) and 4 J/cm2 (b). Left corner: original coin view; LI: laser illumination area.

Figure 9. Photographic illustration of laser cleaning of an iron ancient object.

Figure 6. Irradiation of a Roman coin by nanosecond TEA CO2 laser operating at 10.6 µm. Cumulative action after: (a) 60 and (b) 200 pulses; fluences: (a) 7.1 and (b) 5.5 J/cm2 .

Figure 7. Laser removal of oxides from archaeological bronzes from medieval Oporto Monastery.

Figure 8. Photographic illustration of EDTA chemical cleaning of iron samples for different treatment periods.

Portuguese partners realized also comparative tests of conventional and laser cleaning of iron artefacts (Figs. 8-11). For the chemical cleaning of corrosion (Fig. 8), iron artefacts were immersed in aqueous solution of EDTA at 150◦ C. The evaluation of cleaning level was done by Mössbauer analysis (Tables 1, 2). Laser cleaning was performed using Q-switched Nd:YAG laser at low pulse repetition rate (single pulse, 1 or 2 Hz) and energy density of 0.14 J/cm2 . It was verified that, although not as effective as chemical or mechanical cleaning (Table 2), laser radiation

Figure 10. SEM micrograph (above); and EDX spectra of cleaned surface of an iron artefact (below).

removed selectively the iron corrosion products, without damaging the substrate. Lack of cementite (Fe3 C), which is a compound usually found in the bulk of the iron objects, demonstrates the selectivity of laser

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Figure 11. Reflectance spectra of differently treated “Data Copy” office-type paper. Reflectances of untreated paper, contaminated and soft cleaned, and laser-modified paper are shown. The resolution is 4 cm−1 . Offset of spectra is made for clarity.

4

Table 1. Quantitative results of Mössbauer analysis of samples removed from the surface of iron objects shown in Figure 8. Content [%] Component

a)

b)

c)

Fe-α Fe3 C Fe(OH)3 α – Fe2 O3 Fe3 O4 (A) Fe3 O4 (B)

11.8

98.9 1.1

96.2 3.8

60.3 6.0 8.9 13.0

Table 2. Quantitative results from Mössbauer analysis of samples removed from the surface of an iron object shown in Figure 9 for mechanical and laser cleaning procedures. Content [%] Component Fe-α Fe3 C Fe(OH)3

Original material 6.7 93.3

Mechanical cleaning

Laser cleaning

71.0 18.0 11.0

52.3

Figure 12. Difference of laser modified and untreated paper spectra.

47.7

cleaning process. Moreover laser cleaning showed to be a faster process, possible to be directed to the desired area or parts of the object and usable indoors or outdoors. Quantitative evaluation of surface chemical microcomposition of laser cleaned iron artefacts was also realized using SEM and EDX techniques (Fig. 10).

LASER CLEANING OF PAPER, PARCHMENT AND TEXTILES

The aim of the work conducted in the Institute of Physics, Vilnius, was to process the artificially contaminated office-type paper by subnanosecond (0.15 ns) pulse Nd:YAG laser radiation (1064 nm and 532 nm), and to investigate laser caused modifications of the paper substrate by three Fourier Transform Infrared (FTIR) spectra sampling methods: reflectance, transmittance and photoacoustic detection (Švedas et al. 2007). Laser energy density cleaning threshold with the 1064 nm subnanosecond laser pulse is almost one order smaller than the cleaning threshold with the ordinary ns duration laser pulses. Laser cleaning recovered more than 80% of the paper initial brightness observed in the visible range. The laser fluences above the optical breakdown threshold of the paper surface resulted in the uplift of the paper surface and thinning out of cellulose fibres in the breakdown zone, accompanied by the FTIR detected compositional changes of the substrate: the intensity of three CaCO3 infrared peaks (2514, 1793, and 874 cm−1 ) decreases after laser treatment (Figs. 11, 12). Images presented in Figure 13 show results of fs laser cleaning of paper, realized in Laser Zentrum Hannover. Paper samples were prepared in University of Hildesheim. Figure 14 presents results of measurements of size distribution of micro and nanoparticles, generated during paper cleaning. Results show clearly dependence of particle sizes on the type of paper. The set of photographs presented in Figures 15 and 16 illustrates the laser cleaning of paper and parchment experiments conducted in the Institute of Optoelectronics MUT, Warsaw. Textile laser cleaning results are presented in (Strliˇe et al., in this Volume).

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Figure 16. Left side, cleaning test of old copper plate engraving for paper with Nd:YAG laser at 532 nm. Right side, office-type paper covered with standard artificial dirt. Nd:YAG laser at 1064 nm. Each row (2 cm): one more pulse of 280 mJ/cm2 .

Figure 13. Results of fs laser paper cleaning.

Figure 17. Photograph of 3D scanning system.

Figure 14. Micro- and nanoparticle size distribution of fs laser cleaning of two types of paper, applying a fluence of 50 J/cm2 .

Figure 15. Nd:YAG laser cleaning of 15th century old parchment.

5

SCANNING SYSTEMS

Since for accurate laser cleaning the fluence needs to be well controlled, the laser cleaning stations should be equipped with an automatic distance sensing and positioning unit for cleaning objects with great plasticity. A prototype of such a unit was developed in the University of Pannonia, Veszprém (Fig. 17).

The surface tracking automatics ensures the constant distance between the beam shaping optics and the specimen surface. The actual stroke length of the unit is 120 mm; however, depending on the geometrical characteristics of the samples to be treated, larger strokes lengths are also available, up to approximately 350 mm. The optical table on the top of the unit is actuated by a spindle driven by a high-speed step motor. In the actual design, the sensitivity of the spindle drive is 2 mm/rev., which can be changed to a lower value for higher resolution of to a higher value for increased scanning speed. The surface distance is sampled by a laser distance sensor applying a 670 nm red diode laser light.The basic resolution of the sensor is 80 µm, while the sampled spot size is approximately 0.8 mm. The distance measurement is based on optical triangulation, using the light spot formed by the scattered light at the sample surface, thereby, the precise positioning requires a light scattering, non-translucent surface (typical for paintings and wooden surfaces). The measuring laser is directed to the surface by a mirror, to avoid sensor contamination, as well as for approaching the measurement beam as close to the processing beam as possible. The measuring red beam arrives to the processing point from bottom, at an angle of 15 degrees with the processing beam. Within the lateral dimensions of the red laser spot (i.e. 0.8 mm), the distance measurement is sensitive to the contrast of the surface, but the error resulting from contrast does not exceed 1% of the sensing range. The sensor signal is digitized and fed back to the motion by an 8-bit

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Table 3. Specifications of the surface tracking automatics of sensing and positioning unit.

Stroke length (mm) Spindle tap size (µm) Resolution (µm/step) Maximum tracking speed (mm/s) Static table dimension (mm × mm) Moving table dimensions (mm × mm) Total height (mm) Positioning accuracy (static and dynamics) (mm) Measured spot size (mm)

Min.

Actual

Max.

50 1 2.5 2.5

120 2 5 5

350 10 25 10

60 × 100

60 × 200 60 × 200

100 × 200

0.15

approx 75 0.2

0.8

Figure 20. Ranges of movements (in mm) for IOE MUT X-Y scanning table.

Figure 18. Photographs of linear scanning systems at IOE MUT. Left side, scanning system without safety housing. Right side, testing of paper cleaning with ps Nd:YAG laser at 532 nm.

parchment and small paintings (Fig. 18) using picosecond Nd:YAG laser (Fig. 19), developed by EKSPLA, Vilnius. Figure 20 shows a scheme of IOE MUT scanning table with attainable ranges of movement. Cleaning level control is achieved by acoustic measurements (Strliˇe et al. 2005) as well as LIBS process monitoring, which is under development.

6

Figure 19. View of uncovered ps laser (left side) and optical autocorrelator, designed for measurements of ultrashort laser pulse (right side).

analogue microcontroller, offering an operation panel with simple controls and indicators of the actual status of positioning. The feed back cycle time is kept below 50 ms, thus up to approximately 2.5 mm/s target velocity, the tracing capacity of the unit ensures at least 0.2 mm positioning accuracy. Specifications are summarized in Table 3. Linear (2D) scanning systems are under investigations in the University of Pecs (and developed at University of Pannonia) for laser cleaning of paintings using excimer laser, and, in the Institute of Optoelectronics MUT, Warsaw, for laser cleaning of paper,

CONCLUSIONS

Widely presented results of previous EUREKA project E!2542 RENOVA LASER (Strzelec 2003, Marczak et al. 2003), arouse interest of conservators and the laser community. It is therefore understandable, that a proposal of a new project, which extended R&D work on application and optimisation of laser renovation techniques to clean paintings, historical books and metal objects has been addressed to a number of significant centres and enterprises from West and Central-East Europe. As a result, LASCAN project consortium gathers private SMEs, scientific institutes and R&D centres from nine European countries. Results presented above describe only a small part of the conducted research and some of them should be particularly emphasised:

269

– Successful application of ps Nd:YAG lasers (Vinca, IOE MUT) which can replace expensive fs lasers and conventional ns Q-switched Nd:YAG lasers, – First attempts to develop a 3D scanning head (Pecs, Hungary), – Development of Q-switched Er:YAG laser for varnish removal (Ostrowski et al., this issue).

It should be also stated and appreciated, that despite of the lack of national funds, project partners from Serbia, Germany and Portugal continued the realization of their tasks in the years 2006–2007 in the frame of different projects. Some earlier results of our R&D work can be found among presentations prepared for POLLASNET-HULASNET Workshop in Pecs, 9–11 November 2006 (see Internet 1 in the references). REFERENCES Internet 1. http://www.pollasnet.org.pl/konferencje_eng.php. Marczak J. Et al. 2003. E!2542 RENOVA LASER – Laser renovation of monuments and works of art. Proceedings of 5th EC Conference, Cultural Heritage Research: a pan-European Challenge. Cracow. May 16–18, 2002 (Co-published by Directorate General Research and The Official Publication Office of the European Communities in Luxembourg, Poland, 2003).

Márton, Z. Makkai, G. Galambos, É. Bóna, I. Jébert, I. Szentkirályi, M. 2007. Comparative study of laser treatment on aged paint-varnish layer systems, in this Volume. Ostrowski, R. Marczak, J. Rycyk, A. Strzelec, M. Koss, A. 2007. Influence of Er:YAG laser pulse duration on varnish ablation. Book of abstracts of the Conference on Lasers in the Conservation of Artworks, LACONA VII, September 17–21, Madrid, page 71. Siano, S. & Salimbeni R. 2001. Studies in Conservation 46, 269. Švedas, V. et al. 2007. Cleaning of contaminated paper with the subnanosecond Nd:YAG laser pulses. Lithuanian Journal of Physics. 47 (2). 221–228. Strliˇc, M. et al. 2005. Optimisation and on-line acoustic monitoring of laser cleaning of soiled paper. Appl. Phys. A: Materials Science & Processing A81. 943–951. Strliˇc, M. et al. 2007. A comparison of laser and traditional cleaning of historical textiles, in this Volume. Strzelec, M. 2003. Rénovation de laser des monuments et des objets d’art, Polish Science and Technology Forum, Paris, 15–16 Sept. 2003, Digest of Papers.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Comparative study of laser varnish removal from historical paintings Z. Márton & I. Sánta University of Pécs, Hungary

É. Galambos Museum of Fine Arts, Hungary

C. Dobai & Á. Dics˝o Association of Hungarian Restorers, Hungary

Z. Kántor University of Pannonia, Hungary

ABSTRACT: Application of lasers for cleaning, conservation or restoration of artworks has little history in Hungary, despite of the promising international results. However, the correct evaluation of the method in paintings needs vast amount of case studies, because of the huge variety of possible varnish-paint systems and their potential damages. In the present study we account on systematic experiments aiming at removal of different aged/damaged varnish layers from old paintings chosen from the storage of Museum of Fine Arts, Budapest. A KrF excimer laser was used for laser surface treatment, while the results were tested by microscopic study of the epoxy resin cross-section by multi-spectral photography and by LIBS spectroscopy.

1

INTRODUCTION

In September 2006, a consortium of five Hungarian institutes started cooperation on laser applications in restoration. The particular focus of the joint work is practical testing of laser varnish removal from paintings. This technique is not a novelty (see Fotakis et al. 2007). and references therein), but it is still not part of the restorers’ everyday routine. Considering the complexity of the material composition of the painting, the contamination and the possible over-painting or retouching layers, and the various types of damages, it is obvious that it takes a lot of case studies to be able to decide under which conditions is laser varnish removal the best available practice. Evaluation of the cleaning results is most difficult to define objectively. A painting is a unique art object, as a whole, with its layer structure, its history of life and previous interventions. Ideally, stratigraphic studies should precede any restoration process, providing information about the layers to be removed. Traditionally, restorers use solvents of different concentration, or scalpel for mechanical treatment, where the chemical method does not work, and they evaluate their results visually, by naked eye or under an optical microscope. Multi-spectral photography is also available. However, even if the stratigraphy of

a microscopic sample is well known, the painting can show completely different characteristics one centimeter further from the place of sampling. The aim of the present work was to test the laser varnish removal technique on different types of historical paintings, and to found a database containing the documentation of the experimental parameters and the results. Furthermore, we intend to call attention on those characteristics of real paintings, which make them to respond to laser cleaning differently than evenly painted test samples. We expect that this work will encourage the restorers to involve lasers in varnish removal where the usage of the method is well established. 2 TEST PAINTINGS AND EXPERIMENTAL METHODS 2.1

Paintings chosen for test laser treatments

The test paintings were chosen from the collection of the Museum of Fine Arts, Budapest. All of them are oil paintings on different supports from the 18th–19th centuries. – Sample No. 1 (inventory code 52,673) is oil on wooden panel, in style of D. Teniers, Junior, from the 19th century. The varnish layer is even, apart

271

–

–

–

–

from the crackle pattern consisting of horizontal cracks following the fibre direction of the panel, and a perpendicular, much thinner crack system. The varnish is thick and dark, and its lowest layer contains pigment particles. Sample No. 2 (inventory code 65,2) is a 18th century German painting. It was seriously damaged by hot water. The varnish and the pigmented layer have shrunk, and even burnt at some spots. It is very difficult to distinguish the whitened varnish from the underlying white colours. Sample No. 3 (inventory code 840) is one of the copies of the Madonna with Child by Lucas Cranach, the Elder. Its support is a thin tinned iron plate. The pigmented layers, as well as the varnish, are very thin. Red overpaint was found on top of the varnish. Sample No. 4 (inventory code 1378) is a German wooden panel from the 18th century. Over the thin pigment layer there is an old and a recent thin varnish layer. Sample No. 5 (inventory code 6339) was painted on canvas, in the style of Magnasco, with rich impasto textures. A 30–50 µm thick, darkened and polluted varnish layer covers the surface.

2.2

Experimental methods and techniques

Laser ablation is the physical mechanism behind laser varnish removal. It is known that polymers are ablated very well with the different UV lasers, but ablation can occur at other wavelengths as well (Bäuerle, 2000). The physical parameters of the laser irradiation, like wavelength, fluence, pulse width, repetition rate, are responsible for the characteristic phenomena causing material removal, and these circumstances influence the final visual impression after the laser treatment. Hereby we present the results of cleaning experiments with a KrF excimer laser at 248 nm, the results obtained with other wavelengths will be reported elsewhere. For scanning the laser beam over the painting surface, a 2D scanner was built. A surface-tracking device for the 3rd dimension has also been developed for cleaning objects with great plasticity, but was not used in the experiments presented here. (Marczak et al. 2008). Epoxy resin cross section microscopy, multispectral imaging, and high resolution LIBS were used to analyse the cleaning results. 3

Figure 1. a) Microscopic image of a laser cleaned spot (at 248 nm, 184 mJ/cm2 , 20 shots, sample No.3). b) the original layer structure, c) the upper varnish was thinned by the laser d) Laser-cleaned window and damaged varnish on sample No. 2.

RESULTS

3.1 Varnish removal Figure 1 a) shows a laser cleaned spot on sample No. 3. The samples for cross section micrography were taken near a spot where the paint pealed off earlier.

Figure 2. a) The area scanned with 248 nm laser pulses, at 750 mJ/cm2 , sample No. 1, b) untreated cracks, c) cracks on the laser cleaned area.

After laser treatment the topmost contamination is completely removed, the thick secondary varnish layer is thinned (to different thicknesses, corresponding to the different original layer depths). Comparing Figure 1 b) and c), note that the thin, original varnish layer, under the thick upper varnish (which glued together the weak original structure) remained intact. Figure 1 d) shows a cleaning window on sample No. 2. Although the opaque varnish could be removed, the underlying, very sensitive, heat damaged pigmented layer peals off at weak spots, due to the mechanical recoil accompanying laser cleaning. Sample No. 1 shows deep cracks following the direction of the wood fibres. The varnish layer is broken along those cracks. This is a peculiar danger in case of laser cleaning, because already the first laser pulses affect or even remove the pigment layer from the depth of the cracks. The window on Figure 2 is still under-cleaned, while the cracks are deepened and widened by the 248 nm laser radiation compared to those at the lower, untreated part of the picture. Sample No. 5 is a good example for the case where laser cleaning, in spite of being not ideal, is a better alternative than chemical solvents.

272

Figure 3. Solvent (a) and KrF laser (b) cleaned windows on sample No. 5.

Figure 4. The red over-painting was removed from above the aged varnish. Sample No. 3.

Figure 5. a) LIBS signal during subsequent laser pulses (26–30th) b) spots cleaned with 20–35 laser pulses, respectively. Sample No. 4.

Figure 3 a) shows windows cleaned with acetone (left, undercleaned) and dimethyl formamide (right, over-cleaned) while Figure 3 b) depicts a window cleaned with scanned laser pulses, at 248 nm, 323 mJ/cm2 . The latter shows a better overall visual impression, in spite of traces of varnish that remain in the trenches of the rough surface. 3.2

Removal of over-painting

On different areas on the robe of the Madonna on sample No. 3, a red retouch could be seen over the aged varnish. With 20–25 shots of 185 mJ/cm2 KrF pulses the retouch could be removed, so that the underlying varnish was not destroyed significantly. With 30 further pulses also the varnish could be thinned to the desired depth (See Fig. 4). Removal of dark brown retouch from sample No. 2 was much less selective, due to the similarity of the over-painting and the lower layers. 3.3 Correspondence between LIBS data and cleaning results LIBS is widely used for the analysis of artwork (Castillejo et al. 2000), and is considered a good candidate for online control of the laser varnish removal. On Figure 5 a) we present a part of the LIBS signal, which is characteristic of the pigment layer, and is in perfect correspondence with the level of

cleaning, which can be seen on Figure 5 b). The reference specimen is sample No. 4. The pigments are smalt and lead white, while the yellowish patina is a resinous varnish, most probably dammar or mastic according to FTIR measurements. The Co and Pb peaks show inevitably that the cleaning beam reached the pigment layer. However, Figure 5 b) also shows that the cleaning is not perfectly homogeneous, due to the inhomogeneities of both the laser beam and the varnish. Lead white was temporarily discoloured, as it is described also in the literature (Bordalo et al. 2006).

4

CONCLUSIONS

The present work aimed at representing the realistic problems that can affect the results of laser varnish removal. The applicability of the method was found to be strongly dependent on the surface quality of the painting. Inhomogeneous depth of the varnish (e.g. due to cracks) can decrease the cleaning efficiency, and question the applicability of on-line LIBS control too. These drawbacks are unavoidable, and should be considered before cleaning. However, if no selective solvent can be found, and the surface characteristics allow, the laser is a better alternative to mechanical scratching. The proper on-line control of the method is still to be developed.

273

ACKNOWLEDGEMENTS

Bordalo, R. et al. 2006. Laser Cleaning of Easel Paintings: An Overview. Laser Chemistry Vol. 2006, Article ID 90279. Castillejo M. et al 2000. Laser-induced breakdown spectroscopy and Raman microscopy for analysis of pigments in polychromes. J. Cult. Heritage 1, S297–S302. Fotakis, C. et al (eds.) 2007. Lasers in the preservation of Cultural Heritage, New York: Taylor & Francis Group. Marczak, J. et al 2008.Advanced laser renovation of old paintings, paper, parchment and metal objects, Eureka initiative E!3483 EULASNET LASCAN project, in this Volume.

The authors are indebted for the support of the Agency for Research Fund Management and Research Exploitation (KPI) under the grant No. OMFB01147/2006 and OMFB-01698/2006. REFERENCES Bäuerle, D. (ed.) 2000. Laser Processing and Chemistry, Berlin: Springer.

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Laser Cleaning of Metal Objects

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Laser interactions with copper, copper alloys and their corrosion products used in outdoor sculpture in the United Kingdom M. Froidevaux Haute Ecole d’Arts Appliqués Arc, La Chaux-de-Fonds, Switzerland

P. Platt University of Liverpool, Department of Engineering, Brownlow Hill, Liverpool, UK

M. Cooper Conservation Technologies, National Conservation Centre, National Museums Liverpool, Whitechapel, Liverpool, UK

K. Watkins University of Liverpool, Department of Engineering, Liverpool, UK

ABSTRACT: Laser interactions with copper and copper alloys used in outdoor sculpture in the UK were studied in order to characterise a discoloration phenomenon appearing after laser cleaning of cuprite layers: turning from the typical red-brown colour of cuprite to a grey-purple tinge. Non-corroded, naturally and artificially oxidised copper, brass and bronze samples, as well as natural brochantite layers on copper, were irradiated with Nd:YAG laser radiation (λ = 1064 nm, pulse duration 10 ns) at various fluence levels. The discoloration effect induced by laser radiation on these surfaces was then investigated using a range of analytical techniques: Optical Microscopy, Visible Spectrophotometry, Optical Metallography, X-Ray Diffraction, Scanning Electron Spectroscopy, Energy Dispersive X-Ray Spectroscopy and X-Ray Photoelectron Spectroscopy. An initial study was also carried out to compare laser, abrasive (JOS) and steam (DOFF) cleaning techniques on various copper surfaces. Laser cleaning of the cuprite corrosion layer was shown to be not self-limiting but was found to provide a more sensitive method of removing unwanted paint than JOS cleaning.

1

INTRODUCTION

Laser cleaning has been successfully used on a wide range of materials for over 30 years (Larson et al. 2000, Asmus 1976, Asmus 1978). Laser cleaning of metals, however, has received relatively little attention within the conservation field during that time (Larson 1995, Cooper 2001, Siano & Salimbeni 2001, Fotakis et al. 2007). Today copper alloy outdoor monuments strongly suffer from the effects of the polluted environment to which they are exposed in all urban and industrial areas in the world (Selwyn 2006) and there is a strong need for preserving those monuments using the most sensitive conservation techniques available. During the past 5 years, the various conservation works conducted at the National Conservation Centre (National Museums Liverpool) have included the laser cleaning of several copper alloy outdoor sculptures, such as the Monument to Lord Nelson in Liverpool,

designed by Matthew Cotes Wyatt and sculpted by Sir Richard Westmacott (1813) and the Monument to Queen Victoria in Southport (Fig. 1), sculpted by George Frampton (1912). Laser cleaning was chosen as the principal cleaning method in each case as it was believed to be the most effective and controllable method available at that time. These treatments were successful in removing active corrosion products, pollution deposits and unwanted paint layers, thereby greatly improving the long-term stability and aesthetic appearance of the sculptures. However, laser cleaning of copper alloys is not usually self-limiting and some discoloration in the surface of the cuprite layer (Cu2 O) was observed, as the original red-brown colour turned to a grey-purple tinge. This phenomenon seemed to affect only a very thin layer of material at the surface, to reverse with time and was not visible once the statues had been waxed. This work presents initial investigations, on real and artificial samples, into several areas of interest

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Laser output

Circular template

Control PC

Laser beam Sample XY table Figure 1. Monument to Lord Nelson, Liverpool, UK (left); monument to Queen Victoria, Southport, UK (right). Both monuments were laser cleaned as part of the conservation work.

in the laser cleaning of such sculptures: discoloration effects, material ablation rates, surface analysis and analysis of the metal microstructure. The results will go some way towards improving the understanding of the laser cleaning of copper-alloy outdoor statues, allowing conservators to improve the quality of cleaning undertaken and minimise the risk of damaging the surface of the sculpture.

2 2.1

EXPERIMENTAL Samples

Work was undertaken primarily on artificially corroded samples, which allowed the study of the effects of laser cleaning on a ‘pure’ corrosion layer free from any interference brought by a mixture of other corrosion products and pollutants, such as soot, salts, sand and organic deposits resulting from exposure to an outdoor environment. Copper, bronze and brass samples with no corrosion layers were also studied. Tests were also undertaken on a piece of naturally corroded copper roofing, which had formed a surface layer of bluish-green brochantite (CuSO4 · 3Cu(OH)2 ), a corrosion layer typically found on copper and bronze in urban areas. Copper (99% pure), brass (copper 86.8%; zinc 11.9%; tin 0.8%) and bronze (copper 85.6%; tin 7.4%; zinc 4.7%; lead 2.2%) were selected for this work as these metals are commonly used in monumental sculpture in the United Kingdom. Copper and brass were chosen as their elemental composition is very similar to the alloy compositions of two laser-cleaned outdoor sculptures in the north-west region of England: the Lord Nelson Monument in Liverpool and the Queen Victoria statue in Southport (Fig. 1). Bronze was selected because of its common use as a casting metal for outdoor statuary in general.

Figure 2. Schematic representation of experimental setup.

Artificial cuprite (Cu2 O) layers were formed by immersing the abraded and cleaned metal samples in a boiling solution of copper nitrate for 30 minutes (Cottam 1998). 2.2 Sample preparation and analysis Samples were laser cleaned using a Q-switched Nd:YAG laser (Lynton Lasers Phoenix 2+) emitting laser radiation at a wavelength of 1064 nm in pulses of 10 ns duration (repetition rate 0.63 Hz). Tests were carried out at a range of average fluence levels (0.36, 0.61, 0.85, 1.12 J/cm2 , ± 25%) designed to cover the range that may be used in a ‘real’ laser cleaning project on a copper-alloy monument. The beam was delivered through an articulated arm and focusing lens. The aperture of the articulated arm was clamped in position above the sample, which was mounted on a computer-controlled XY translation stage (Fig. 2). The laser fired at a constant rate of 0.63 Hz while the sample was moved beneath the beam in a rastering pattern. Samples were moved at a constant rate so that each part of the surface was exposed to an average 12.3 pulses, which was felt to be generally in line with the number of pulses to which the surface of a monument might be exposed during cleaning once the unwanted layers have been removed. A beam homogeniser was not available during the course of this work, which meant that some parts of the surface received slightly laser radiation at slightly higher fluence levels than those quoted here due to hotspots within the multimode beam. A mask with a 6 mm circular aperture was used to remove the lower energy outer part of the beam and produce a circular beam. Sample areas between 15 × 15 mm2 and 25 × 25 mm2 were prepared and analysis was carried out away from the edge of the treated region to ensure consistency in terms of number of pulses received by the surface. This type of experimental setup was used as it allowed sample preparation in a reproducible manner under conditions that are close to those encountered in

278

2.3

Figure 3. Cross-section through naturally corroded copper roofing. Green brochantite layer approximately 35–55 µm thick; brown cuprite layer approximately 5 µm thick.

a ‘real’conservation treatment on large outdoor bronze and copper sculpture. Sample analysis was undertaken using a range of techniques. Optical Microscopy (OM) (up to 200X magnification) was initially used to examine the surfaces of each sample. Scanning Electron Microscopy (SEM) was also used to characterise the surface topography before and after exposure to laser radiation. Energy Dispersive X-Ray analysis (EDX) and X-Ray Diffraction (XRD) were used to investigate any changes to the elemental composition of laser irradiated samples on a qualitative level. X-Ray Photoelectron Spectroscopy (XPS) was undertaken on selected samples to examine possible changes to the composition of the corrosion layer at and just below the surface. The microstructure of the cuprite (on copper) samples was examined by using OM (400X magnification) to look at cross-sections through the samples, etched using alcoholic ferric chloride (Platt 2007). Initial work using a benchtop visible spectrophotometer was undertaken to see if this technique might be able to record any discoloration caused to the corrosion layers as a result of laser irradiation and any changes over a period of a few weeks after irradiation. A detailed description of the techniques used here is beyond the scope of this paper and can be found elsewhere (Platt 2007, Froidevaux 2007). An initial investigation on the ablation rate of brochantite at different fluence levels was carried out using OM to examine cross-sections taken through samples of the naturally corroded copper roof sample (Fig. 3). Several measurements of the thickness of the brochantite layer on the reference sample were taken and the mean thickness calculated. Similar measurements were taken for samples exposed to a known number of laser pulses. The difference in thickness was then used to calculate the mean ablation rate per pulse.

Comparative cleaning tests

Finally, comparative cleaning tests were carried out on the artificially prepared cuprite samples and on artificially prepared brochantite samples as well. Laser cleaning was compared with alternative cleaning methods that have been used on public monuments in the UK: JOS (an abrasive technique mainly used to remove acrylic paints, carbon deposits, dirt and scale; it uses a mixture of water, low air pressure and fine inert abrasive in a swirling vortex) and DOFF (a steam cleaning technique mainly used to remove greasy deposits, chewing gum, algae, epoxy and polyester paints). Both JOS and DOFF can be employed using a range of nozzles to control the sensitivity of cleaning. In the past, public copper alloy monuments in Liverpool have been covered with black coatings in an attempt to mask corrosion and reduce maintenance work. Analysis of a black coating sample from the monument to King Edward in Liverpool revealed it was composed of a mixture of an acrylic/styrene copolymer medium and animal charcoal (possibly bone black), extended with dolomite and clay. A top layer of the same acrylic/styrene polymer had been applied as a varnish. An artificial black coating (very similar to the original coating) was applied to the samples tested here. The medium used was Soluble Gloss Varnish for Acrylics Cryla® mixed with ivory black and kaolin. A non-pigmented layer of the same medium was applied on the top as a varnish. A small part of the sample was kept uncovered with the black coating, for reference purposes. Accelerated ageing of the paint layers was carried out by heating the samples in an oven for 24 hours at 75◦ C. Laser cleaning was carried out at a fluence of 0.4 J/cm2 (the lowest fluence that achieved an acceptable level of paint removal) and a repetition rate 1.25 Hz. The laser delivery arm was held by hand to match the method used in ‘real’ cleaning work. JOS was used with calcite (the softest abrasive) at a pressure of 1.5 bar (21.7 psi), with a 4 mm nozzle (the smallest available). DOFF was used at a pressure of 75 bar (1088 psi) at 150◦ C, with the smallest available nozzle. The aim was to remove the black coating whilst minimising any effect on the underlying corrosion layers. 3

RESULTS

3.1 Analysis of metal samples The results of the OM examination on the metal samples are shown in Table 1. SEM analysis revealed signs of localised melting around imperfections in the surface (e.g. scratches) for each sample at 0.61 J/cm2 . The signs of melting were found to be more evident (for each sample) as the fluence increased. The brass sample was found to

279

Table 1.

Table 2.

Results of OM examination for metal samples.

Results of OM examination for cuprite samples. Average fluence (J/cm2 , ±25%)

Average fluence (J/cm2 , ±25%) Sample

0.61

0.85

1.12

Sample

0.61

0.85

1.12

Copper

No change

Very slight ‘grainy’ aspect to surface

Cuprite (on copper)

Grey discoloration

Overall slightly darker, matt appearance. ‘Grainy’.

As for 0.61, but more pronounced. Individual impact spots visible.

Cuprite (on brass)

No change

Very slight ‘grainy’ aspect to surface

Slight purplish discoloration. Slightly shinier. Very slight discoloration to pinkishorange. A few localised grey spots. Slightly shinier.

As for 0.61, but surface lighter and shinier. Appearance of blue marks. Matt surface appearance. Similar to 0.61, but surface shinier and grey spots no longer present.

Similar to 0.85.

Brass

Dark marks visible to naked eye. Matt appearance. Similar to 0.85 but much stronger effects. Light and dark spots visible showing uneven energy distribution in beam. Light marks visible to naked eye. More ‘grainy’ aspect than for 0.85.

Bronze

Cuprite (on bronze)

Brownishpurple discoloration. Matt surface. Similar to 0.85.

Figure 5. Photograph showing discoloration of cuprite (on copper) at 0.61 and 0.85 J/cm2 . Figure 4. SEM micrograph of brass after exposure to laser radiation at 1.12 J/cm2 , showing extensive surface melting.

demonstrate the most extensive melting (across most of the surface) at 1.12 J/cm2 (Fig. 4). 3.2 Analysis of cuprite samples The results of the OM examination on the cuprite samples are shown in Table 2. Discoloration of the cuprite layer from its original reddish-brown colour was observed for all the substrate types at each fluence level (Fig. 5). The cuprite layer on the copper substrate appeared to be the most sensitive to laser radiation. SEM analysis showed signs of melting in the surface at all fluence levels for all substrate types. At 0.61 J/cm2 , melting was only partial and the cuprite crystals could still be seen, but melting

across the whole surface was observed at and above 0.85 J/cm2 (Figs. 6, 7). EDX analysis revealed a slight decrease in the oxygen content and an increase in the relative concentrations of copper (all samples), tin (bronze sample) and zinc (brass sample) on samples irradiated at 0.6 J/cm2 . This suggests that there was partial removal of the cuprite layer at 0.6 J/cm2 and higher fluence levels. Similar results were obtained for the naturally occurring cuprite layer irradiated at 0.36 J/cm2 . Results of the analysis undertaken of the microstructure of cuprite (naturally occurring) samples (on copper) irradiated at 0.36 J/cm2 , 0.61 J/cm2 and 0.85 J/cm2 are shown in Figures 8 and 9. A random arrangement of grains (of varying sizes) throughout the microstructure was observed for all samples. No heat-affected zone was seen beneath the cuprite layer

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Figure 6. SEM micrograph showing cuprite crystals on copper substrate.

Figure 7. SEM micrograph showing evidence of melting of cuprite (on copper substrate) across surface, resulting from exposure to laser radiation at 0.85 J/cm2 .

(approximately 5 µm thick), indicating that there was no thermal damage beneath the layer of corrosion or that the thickness of the affected layer was too small to be observed in this experiment. This would suggest that the thickness of any affected region in the metal beneath the corrosion layer is less than 10 µm. Initial colour measurements on the cuprite samples using a basic visible spectrophotometer confirmed a shift in reflected light towards the blue end of the spectrum and revealed that after 35 days exposed to air the colour of the surfaces had reverted slightly towards their initial colour (i.e. acquired a more reddish tone). It was noted that the samples had acquired a more matt appearance over this time, likely due to the formation of ‘new’ cuprite. Further work is required to establish whether the surfaces would ever revert back to their original colour and, if so, over what timescale. Initial XRD analysis was undertaken on an artificially prepared cuprite sample (copper substrate) irradiated at 0.61 J/cm2 . This sample was chosen as

Figure 8. Microstructure of copper before laser irradiation (naturally occurring cuprite on copper substrate).

Figure 9. Microstructure of copper after irradiating a naturally occurring cuprite sample at 0.85 J/cm2 .

it showed strong discoloration (it was not possible to analyse all samples within this project). No changes to the crystalline structure of the sample were observed. The most likely explanation for this is that XRD probes to a depth (several microns) significantly deeper than the discoloured region. XPS is a much more sensitive technique (probing to a depth of several nanometres) and was used here in an attempt to characterise the discoloration further. The results of this initial XPS work are presented in Froidevaux (2007). In summary, the discoloured surface was found to contain both cuprite and a small amount of tenorite (CuO). Tenorite is black and can be formed by heating cuprite to high temperatures. It is plausible, therefore, that the discoloration could be partly due to transformation of a small amount of cuprite into tenorite as a result of the heating effect induced by laser irradiation. Further work is required to confirm how much tenorite is produced, whether

281

Table 3. Mean ablation rate of brochantite (natural) at different fluence levels. Average Fluence (J/cm2 , ±25%)

Mean ablation rate (µm/pulse)

0.36 0.61 0.85

3.2 4.4 4.0

Figure 11. SEM micrograph showing brochantite surface after irradiation at 0.36 J/cm2 .

Figure 10. SEM micrograph showing brochantite surface.

this effect is seen at different fluence levels and on cuprite layers on bronze and brass substrates as well. 3.3 Analysis of brochantite samples

Figure 12. SEM micrograph of untreated copper surface.

Table 3 shows the results obtained from measurements of the mean ablation rate of brochantite. OM revealed that the brochantite layer was completely removed from some areas of the samples exposed to 0.61 and 0.85 J/cm2 . This suggests that the ablation rates calculated at these fluence levels are in fact underestimated. Therefore, these results simply show that approximately 3.2 µm of material was removed per pulse at 0.36 J/cm2 and that at least 4 µm per pulse was removed at the higher fluence levels tested. SEM analysis of the surface of the brochantite layer showed that ablation at 0.36 J/cm2 resulted in a roughening of the surface topography. This is illustrated in Figures 10 and 11. OM showed that after irradiation the surface of the brochantite appeared darker, possibly due to the roughening of the surface. 3.4

Comparative cleaning tests

Laser cleaning at 0.4 J/cm2 resulted in almost complete removal of the model paint layer on each of the samples: copper; cuprite (copper substrate); brochantite (copper substrate). The copper surface (examined by OM and SEM) appeared unaffected, but the cuprite

surface showed evidence of partial melting (Figs. 12– 14) and the brochantite surface showed signs of slight damage to the crystals. JOS cleaning removed the paint layer completely from the copper and cuprite samples but caused significant damage to the underlying surface (Fig. 15). The cuprite layer was completely removed and the copper surface abraded by the impact of the cleaning medium. The brochantite layer was almost completely removed and damage was clearly visible on the copper surface beneath. DOFF cleaning resulted in complete removal of the paint layer from the copper sample without any visible damage. Paint removal from the cuprite and brochantite samples was partial (Fig. 16), but no damage was observed on either of the corrosion layers. Further tests were carried out on a decorative element covered by black paint from the copper alloy monument of King Edward, situated on Liverpool’s Pierhead since 1921 (alongside the River Mersey). Laser cleaning resulted in removal of the black paint at fluence levels ranging from 0.4 to 1.4 J/cm2 . The remaining cuprite layer showed a slight purplish discoloration. In some areas a dark greenish sulphate layer was present. JOS and DOFF cleaning were carried

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Figure 13. SEM micrograph of untreated cuprite layer on copper substrate.

Figure 15. SEM micrograph showing JOS-cleaned cuprite sample. Paint layer and cuprite layer have been removed and the underlying copper damaged.

Figure 14. SEM micrograph of laser cleaned cuprite sample (0.4 J/cm2 ). Paint layer removed but melting of the cuprite is evident.

out using the same conditions as for the testing on the model samples. DOFF cleaning only partially removed the paint layer and was less effective than when removing the paint from the model samples. This was probably due to the age of the coating, which had been exposed outdoors for several decades and was harder and more brittle than the paint used on the model samples. It was not possible to completely remove the paint using DOFF alone. JOS cleaning proved to be too aggressive and removed both the paint layer and underlying corrosion layers, exposing the raw metal beneath. 4

SUMMARY AND CONCLUSIONS

This initial study has shown that laser radiation (1064 nm, 10 ns) at fluence levels above 0.61 J/cm2 causes some melting to copper, bronze and brass. The melting is localised at 0.61 J/cm2 and restricted to imperfections in the surface. Brass appeared to be the most sensitive of the metals tested. Irradiation of cuprite (on all metals tested) resulted in discoloration

Figure 16. SEM micrograph of DOFF-cleaned cuprite sample. Paint layer has been partially removed (remnants still evident on right side) without any visible damage to the remaining cuprite crystals.

of the corrosion layer, partial removal and some melting. The extent of discoloration and melting depends on the type of substrate and fluence level, with copper appearing to be the most sensitive. Discoloration of the cuprite layer appears to fade over time as a new cuprite layer forms. No change to the microstructure of the copper substrate beneath the cuprite layer was observed after laser irradiation, suggesting that any changes to the surface are restricted to a very thin layer, of the order of 10 µm. However, a small amount of tenorite was detected in the cuprite layer after exposure to laser radiation, indicating a slight change of the chemical structure of the corrosion layer. Laser irradiation of brochantite at 0.36 J/cm2 resulted in a slight darkening and roughening of the surface, and removal of the corrosion layer at a rate of just over 3 µm/pulse. An understanding of the effects of laser radiation on cuprite and brochantite (and other corrosion layers) is important because

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the aim of cleaning copper-alloy monuments will often be to remove unwanted layers, such as paint and active corrosion products, whilst minimising any damage/removal of stable corrosion layers (such as cuprite and brochantite). It is very rare that the aim of cleaning would be to remove all corrosion layers and expose the raw metal beneath. Laser cleaning, JOS cleaning and DOFF cleaning were all able to remove the black paint layer to some extent. Laser cleaning and JOS cleaning were the most effective in terms of paint removal, but laser cleaning left a slightly discoloured surface with some evidence of melting in the surface region and JOS cleaning removed all corrosion and damaged the metal beneath. DOFF cleaning caused the least damage but was less effective in removing the paint, especially on the naturally weathered sample taken from the King Edward statue. In this instance paint removal by laser cleaning was found to be less aggressive than by JOS cleaning. JOS cleaning completely removed the underlying cuprite layer and damaged the metal surface beneath, whereas laser cleaning enabled the cuprite layer to be partially preserved. It is clear, however, that laser cleaning of copper-alloy monuments is unlikely to be self-limiting (unless the paint layer rests directly on top of the metal surface, which may be the case if aggressive cleaning treatments have been carried out previously resulting in the loss of corrosion layers). It is, therefore, important that adverse side-effects are minimised by controlling laser beam parameters such as fluence and repetition rate. In some cases, optimum cleaning may be obtained by using the DOFF technique initially to remove as much unwanted material as possible and then employing laser cleaning at relatively low fluence to remove remnants of the dirt/paint/active corrosion that DOFF cleaning is unable to remove. As it is often the case in conservation, a combination of techniques may provide the optimum solution. There is still much work to be done in this area. One area of interest remains the discoloration of cuprite exposed to laser radiation. Quantification of the amount of tenorite created within the cuprite layer and its significance in the discoloration process would be of interest. It is unlikely that it is the only contributing factor and light scattering effects (possibly due to a change in the surface morphology or the formation of thin oxide films on the surface) may play an important role. Usually, a coating (possibly with a small amount of pigment) is applied to the clean surface of the monument to prevent the ingress of moisture and further corrosion. This coating should be reversible, i.e. can be safely removed, and must be maintained at regular intervals. The coating has the effect of saturating the surface and, once applied, any discoloration to the cuprite layer caused by laser cleaning is no longer visible. Further work could be undertaken to investigate what effect such a coating has on the fading of the discoloured layer that is normally witnessed over time.

ACKNOWLEDGEMENTS This project was conducted at the National Conservation Centre (National Museums Liverpool) in the frame of the Master projects of two students from the University of Liverpool (UK) and the Haute Ecole d’Arts Appliqués Arc (Switzerland). Many thanks are given to Annemarie LaPensée and Siobhan Watts from the National Conservation Centre, Prof. Ken Watkins, Prof. Gordon Tatlock and Peter Beehan from the University of Liverpool, Thomas Sidler and Patrik Hoffmann from the Swiss Federal Institute of Technology (Lausanne, Switzerland), for their advice, help and support during this research project. The authors are also grateful to Peter Northover (University of Oxford) for his generous help with the XRD work.

REFERENCES Asmus, J. 1976. The Development of a Laser Statue Cleaner. 2nd International Symposium on the Deterioration of Building Stones, Athens, 21st Sept. – 1st Oct. 1976: 137–141. Asmus, J. 1978. Light cleaning: Laser technology for surface preparation in the Arts. Technology & Conservation 3: 14–18. Cooper, M. 2001. Laser removal of paint layers from corroded copper: possible applications to bronze sculpture cleaning. Monuments and the Millenium, Proceedings of the conference organised by English Heritage and UKIC, V & A Museum, London, 20–22nd May 1998: 109–119. Cottam, C. 1998. TEA CO2-Laser Treatment of Coated and Corroded Metals. PhD thesis, Department of Physics, University of Loughborough: 113. Fotakis, C. et al. 2007. Lasers in the Preservation of Cultural Heritage: Principles and Applications: 306–315. Taylor and Francis. Froidevaux, M. 2007. Characterization of laser radiation interactions with copper alloys used in outdoor sculpture in the United Kingdom, Travail de mémoire, Haute Ecole d’Arts Appliqués Arc, Filière conservation-restauration, orientation objects archéologiques et ethnographiques. Larson, J. 1995. Eros: The Laser Cleaning of an Aluminium Sculpture. From Marble to Chocolate: the conservation of modern sculpture, London, Tate Gallery, 18–20 Sept. 1995. Archetype publications: 53–58. Larson, J. et al. 2000. Developments in the Application of Laser Technology for Conservation. IIC, Tradition and Innovation. Advances in Conservation. Contributions to the Melbourne Congress, 10th–14th October 2000 : 107–110. Platt, P. 2007. Laser Interactions with Copper and its Corrosion Products, Industrial MSc. Project, Department of Engineering, University of Liverpool. Selwyn, L. S. & Roberge, P. R. 2006. Corrosion of MetalArtefacts Displayed in Outdoor Environments. ASM Handbook, Volume 13C, Corrosion: Environments and Industries (#05145G): 289–305. Siano, S. & Salimbeni, R. 2001. The gate of paradise: physical optimization of the laser cleaning approach. Studies in Conservation 46: 269–281.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Investigating the laser cleaning of archaeological copper alloys using different laser systems C. Korenberg & A.M. Baldwin The British Museum, London, United-Kingdom

P. Pouli Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas (IESL-FORTH), Heraklion, Crete, Greece

ABSTRACT: This study investigates the laser cleaning of archaeological copper alloys. Tests were conducted on archaeological coins using lasers emitting at different wavelengths and pulse durations. It was not possible to remove burial deposits at 248 nm using pulse durations ranging from 500 fs to 30 ns and laser cleaning at 355 nm and 15 ns caused melting of the surface. The most promising results were obtained using wavelengths of 532 and 1064 nm. Although pulses of 130 µs at 1064 nm led to discolouration and melting of the surface, using lasers with shorter pulse durations reduced this. For both 1064 and 532 nm, the results were similar at ns and ps pulses durations. However, uneven cleaning and discolouration of the cuprite were observed on the majority of coins. This indicates that laser cleaning of archaeological copper alloys is complex and further work is required to refine the useable laser parameters for practical applications.

1

INTRODUCTION

Archaeological copper alloy artefacts are usually covered by corrosion crusts and burial deposits that may require removal. Conservators do not normally clean an object back to its metal surface as, in the majority of cases, the metal core will be featureless, but instead uncover the ‘original surface’. In conservation, the term original surface denotes a layer within the corrosion products where decoration, tool marks or evidence of wear can be found.The position of the original surface in the copper alloy corrosion crust varies depending on the condition and state of mineralisation of the object. It can be difficult to detect the original surface in the layers of corrosion, but it is often a more or less continuous compact layer, which differentiates it from the less dense secondary corrosion products. No cleaning method currently in use is entirely successful and conservators have to select a cleaning method for each particular object in terms of risk and benefit. Copper alloy objects cleaned using chemicals can be pitted with loss of form, surface detail, decoration or other evidence preserved within the corrosion products. Also, chemical residues are hard to remove from the object and can lead to future deterioration, making this cleaning method unsuitable in most cases. Air abrasive, whilst useful, is often difficult to control and tends to produce a matt surface.

The preferred conservation method for archaeological copper alloys is mechanical cleaning using a scalpel as it is accurate and adaptable. However, mechanical cleaning can take a long time and damage can occur either through breakage due to pressure applied to the object or scratching of the surface. Laser cleaning has gained considerable success as a valuable method of conservation and the present research project is focussed on the use of the laser to clean archaeological copper alloys. There have been several studies investigating the removal of burial deposits and corrosion layers from archaeological copper alloy artefacts (Cottam et al. 1995, Pini et al. 2000, Drakaki et al. 2004, Batishche et al. 2005, Korenberg & Baldwin 2006), but the results have been varied. For example, Pini et al. (2000) reported successful cleaning of archaeological bronze objects using a Nd:YAG laser emitting at 1064 nm and 20 µs, while recent work (Korenberg & Baldwin 2006) has found that laser cleaning at 1064 nm and 10 ns removes burial deposits from archaeological copper alloy objects but does not preserve the original surface. Laser parameters such as wavelength and pulse duration affect the laser cleaning process. Systematic studies using different wavelengths both on artificially corroded samples (Siatou et al. 2006) and naturally corroded non-archaeological objects (Mottner et al. 2005) have shown that the damage thresholds of the

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Table 1. Composition of the uncorroded metal cores of the coins determined using XRF. Coins

Core composition (% wt)

EE3 EE6 EE8 EE10 EE11 EE14 EE17 EE18 EE19

92% Cu, 3% Pb, 3% Sn, 1% Ag 88% Cu, 9% Pb, 2% Sn 66% Cu, 34% Pb 93% Cu, 3% Sn, 3% Ag, 1% Pb 89% Cu, 7% Pb, 2% Sn, 1% Ag, 1% Fe, 1% As 89% Cu, 9% Pb, 1% Sn 88% Cu, 8% Pb, 2% Sn, 1% Fe 86% Cu, 14% Pb 74% Cu, 26% Pb

metal substrates and the ablation thresholds of the corrosion products can be very close. Therefore, careful optimisation of the laser parameters is required. Ultra short laser pulses reduce thermal alterations to the substrates and non-archaeological bronze and copper objects have been cleaned successfully using a laser emitting at 780 nm and 140–1500 fs (Burmester et al. 2005). The aim of the present study was to explore the effects of laser parameters on the laser cleaning of archaeological copper alloys. Tests were performed on archaeological objects using nine laser systems emitting at wavelengths ranging from 248 nm to 1064 nm and pulse durations ranging from 500 fs up to 130 µs. The corrosion crust on each object was analysed prior to laser cleaning to locate the position of the original surface. The effects of the wavelength and pulse duration on the cleaning process were compared and it was carefully assessed whether the laser treatment preserved the original surface of the objects.

Figure 1. Cross section of EE14 viewed using SEM (length of scale marker: 100 microns). Table 2. Composition of the green corrosion layers, as determined using SEM-EDX. Coins

Composition of the green corrosion layer (% wt)

EE3 EE6 EE11 EE14 EE17

59% Cu, 16% Sn, 13% Si, 12% Pb 58% Cu, 13% Sn, 14% Si, 15% Pb 59% Cu, 8% Sn, 25% Si, 6% Ag, 2% Ca 66% Cu, 9% Sn, 25% Si 88% Cu, 5% Sn, 3% Pb, 2% P, 2% Fe

ArtAX spectrometer (voltage 50 kV, current 0.80 mA and beam size 0.65 mm) to determine the composition of the metals and the results are shown in Table 1. 2.2

2

SAMPLES

The formation of corrosion products on archaeological copper alloys can be complex and differs according to the burial environment and composition of the metal (Robbiola et al. 1998, Scott 2002, Selwyn 2004). As it is difficult to artificially produce corrosion crusts similar to those present on archaeological objects in the laboratory, tests were performed on nine unregistered Roman copper alloy coins of unknown provenance, listed in Table 1. Prior to cleaning, sections across the objects were removed using an Isomet saw. The samples were embedded in an epoxy resin, ground on carborundum paper and polished with diamond pastes of 6 µm and 1 µm. The composition of the metal core was determined and the corrosion structure examined to identify the location of the original surface. The results are discussed below. 2.1 Composition of the uncorroded metal core X-ray fluorescence (XRF) analyses were carried out on the uncorroded metal core of the objects using an

Location of the original surface

The cross sections removed from the objects were examined using polarised light microscopy, Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) to help locate the position of the original surface. Five of the coins had an original surface located just above or within a layer of green corrosion products. These were: EE3, EE6, EE11, EE14 and EE17. Of these EE3, EE6, and EE14 had a cleavage plane at the original surface and this is illustrated in Figure 1 for EE14. The green layers were analysed using X-ray diffraction, but either no lines or only very faint lines were obtained, which did not match any of the well-defined copper compounds often found on archaeological copper-alloys, such as malachite or atacamite. No spectra were obtained using Raman spectroscopy, which suggests that the corrosion products were poorly crystallised. The occurrence of poorly crystallised corrosion products on archaeological metals is quite common (Robbiola et al. 1998, Robbiola, pers. comm.). The elemental compositions of the green layers are summarised in Table 2.

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Table 3. Working parameters of the lasers set-ups used in the study.

Laser system

Wavelength (nm) Pulse

SFR∗ Nd:YAG 1064 (El.En., EOS1000) Q-switched Nd:YAG 1064 (Lynton Phoenix) Q-switched Nd:YAG 1064 (EKSPLA, SL312) Q-switched Nd:YAG 532 (Lynton Phoenix) Q-switched Nd:YAG 532 (EKSPLA, SL312) Q-switched Nd:YAG 355 (Spectron, SL805 mod.) KrF Excimer 248 (Lambda Physik, LPX200) KrF Excimer 248 (Laser Lab. Goettingen) KrF Excimer 248 (Laser Lab. Goettingen) ∗

Fluence (J/cm2 )

130 µs 3.1–20.4 10 ns

4.8–6.4

150 ps

0.58–3.1

10 ns

0.70–2.8

150 ps

0.11–0.40

15 ns

1.7

30 ns

1.8

5 ps

0.35

4

500 fs

0.35

4.1 Nd:YAG laser emitting at 1064 nm

Figure 2. Evidence of melting on the laser cleaned surface of EE10 irradiated at 1064 nm and 130 µs (fluence = 12.2 J/cm2 ). (Length of scale marker: 100 µm).

Short free running.

There were traces of a silver wash on EE10, but most of the original surface was lost. The three coins which contained only lead as an alloying element (EE8, EE18 and EE19) had lost their original surface through the corrosion process. However, tests were conducted on them to assess whether the burial deposits could be removed using the laser without affecting the corrosion products underneath. 3

LASER CLEANING TESTS

A range of laser parameters were used to optimise the cleaning process as summarised in Table 3. The wavelengths available were 248 nm, 355 nm, 532 nm and 1064 nm. The pulse durations employed varied from 500 fs to 130 µs, depending on the laser. It was not possible to test all the lasers on every coin due to their small size (the diameter of the largest coin was less than 20 mm). The average fluence was calculated by dividing the energy per pulse by the area of the laser spot. The size of the laser spot was estimated by measuring the laser irradiated area on the coins. Water was applied to the surface of some of the coins while using the laser, as the use of a wetting agent has sometimes been reported to improve the laser cleaning of metals (Cottam et al. 1995, Koh & Saradi 2003, Burmester et al. 2005, Dickmann et al. 2005, Koh 2006). However, no discernible difference was noted and no distinction is made in the text between the experiments carried out with or without water.

RESULTS OF THE CLEANING TESTS

Three pulse durations were tested at the 1064 nm wavelength: 130 µs, 10 ns and 150 ps. Laser cleaning tests at 130 µs were conducted on EE8 and EE10 and the results were similar for both coins. The effect of the laser varied across the surface at a given fluence and cleaning was uneven. For example, at a fluence of 12.2 J/cm2 the laser was ineffective at removing burial deposits in one area, while in another the cuprite layer was revealed. Also, the cuprite was discoloured and melting and the formation of small prominent particles occurred, which were visible using optical microscopy and SEM (Fig. 2). It was possible to uncover traces of the silver wash on EE10, but the cuprite in adjacent areas was melted. Tests at 10 ns were conducted on EE3, EE6, EE8, EE10 and EE11. Good results were obtained on EE3 when one pulse was applied at 4.8 J/cm2 : the burial deposits were removed successfully and the green corrosion layer containing the original surface was uncovered. However, the presence of a cleavage plane at the original surface of this coin may account for the good surface cleaning (note that copper alloy objects with a cleavage plane are usually easily cleaned using a scalpel). With similar tests on EE6, which has a less pronounced cleavage plane, it was not possible to remove the burial deposits from the areas of low relief without removing some of the green corrosion containing the original surface on the raised design of the coin. Results on the other coins were disappointing. Little burial deposit was removed from EE8 at 6.4 J/cm2 and discolouration of the cuprite was observed. Tests carried out at a fluence of 5.8 J/cm2 on EE10 resulted in uneven cleaning: burial deposits remained in the lower areas, while the cuprite was revealed and discoloured on the raised areas. Tests on EE11 were not successful

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Figure 3. Diagonal striations on EE11 within three spots after applying 450 pulses at 0.35 J/cm2 using the excimer laser emitting at 248 nm and 5 ps. (Horizontal field of view: 6 mm).

as it was not possible to remove burial deposits at a fluence of 6.4 J/cm2 without revealing the metallic core. Laser tests at 150 ps were carried out on EE6 and EE18. The results on EE6 were not very successful as it was not possible to remove the burial deposits without removing the green corrosion layer containing the original surface. Note, however, that the cuprite layer was not discoloured on this coin. On EE18, using a fluence of 0.92 J/cm2 revealed the cuprite layer, but laser irradiation had discoloured the cuprite. The results were different on different areas of the coin: in one area, burial deposits were not removed when applying four pulses at 1.1 J/cm2 , while in another area the metallic core was revealed after three pulses at the same fluence. This is probably due to differences in composition or thickness of the burial deposits and corrosion crust. 4.2

Nd:YAG laser emitting at 532 nm

At the 532 nm wavelength, the effect of two pulse durations, 10 ns and 150 ps, was assessed. Tests were carried out on EE3, EE14, EE17 and EE19 at 10 ns. On EE3, it was not possible to preserve the green corrosion layer that contained the original surface. After applying 8 pulses at 2.4–2.8 J/cm2 , the original surface was still covered by burial deposits; however, 10 pulses removed the green corrosion layer and uncovered the metallic core. On EE14, uneven cleaning was obtained: after 5 pulses at 2.7 J/cm2 , the green corrosion layer was uncovered in some places along the cleavage plane, while in other places burial deposits were still present in the lower areas and the cuprite layer was revealed and discoloured on the raised areas. Similar results were obtained on EE17. For EE19, which has lost its original surface, uneven cleaning was observed and the cuprite layer was discoloured. Laser cleaning at 532 nm and 150 ps was evaluated on EE3, EE11, EE14, EE17 and EE19. A fluence of 0.45 J/cm2 was used on EE11 and EE14. On EE14, less than 10 pulses were required to remove the burial

deposits and reveal the green corrosion layer along the cleavage plane. On EE11 after applying 40 pulses in one spot, a thick layer of burial deposits remained in areas whilst metal was exposed in the centre of the beam. This could be due to inhomogeneities in the laser beam. A lower fluence, 0.1 J/cm2 , was used on EE3, EE17 and EE19. On EE17, there was very little removal of the burial deposits after 20 pulses and the irradiated areas had discoloured. On EE3 and EE19, the burial deposits were removed after 20 pulses, but cleaning was uneven. It was noted that the cuprite layer underneath the green corrosion was discoloured after laser irradiation on EE3, EE14 and EE19, but not on EE11. 4.3 Nd:YAG laser emitting at 355 nm EE11 and EE18 were irradiated using the third harmonic of a Nd:YAG laser (355 nm) emitting 15 ns pulses. At 1.7 J/cm2 , burial deposits on EE11 were removed only after applying 82 pulses, but the green corrosion products were largely removed at the same time. The burial deposits adjacent to the irradiated area were discoloured and melting seemed to have occurred, although there was no discolouration of the cuprite layer. The burial deposits were removed from EE18 after applying 20 pulses at 1.7 J/cm2 . However, as for EE11, the green corrosion layer containing the original surface was removed at the same time. Melting and discoloration of the corrosion layers were also observed. 4.4 Excimer laser emitting at 248 nm Laser cleaning tests using an excimer laser emitting at 248 nm and 30 ns were carried out on EE11 and EE14 at a fluence of 1.8 J/cm2 . The results were not satisfying: the areas irradiated by the laser darkened, the burial deposits remained and appeared to have melted to some extent. Tests at 500 fs were carried out at the maximum energy delivered by the laser, which corresponded to a fluence of 0.35 J/cm2 . At this fluence, very little was removed from EE11 even after 140 pulses. The same tests were repeated with the same laser at the pulse duration of 5 ps. Up to 500 pulses at 0.35 J/cm2 were applied on EE11 and some of the burial deposits were removed, but the original surface was not revealed. In addition, striations were visible on the surface, as illustrated in Figure 3. The formation of these features is most probably due to inhomogeneities in the laser beam profile. 5

DISCUSSION

5.1 Pulse duration From the experiments, it is possible to draw some conclusions on the effect of the pulse duration on the

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laser cleaning process. At 1064 nm and a pulse duration of 130 µs, extensive melting of the cuprite was observed on the surfaces of EE8 and EE10 using an optical microscope. However, for shorter pulse durations, the cuprite was discoloured but there was no sign of melting using optical microscopy. This indicates that, as reported by other authors, short laser pulses reduce thermal alterations to the substrates. In contrast, Pini et al. (2000) have used a Nd:YAG laser emitting at 20 µs to clean 15 archaeological bronze objects and reported no melting or discolouration of the cuprite layer on any of the objects. This does not appear to be in agreement with the present study; however, differences in the compositions of the alloys and burial deposits may account for their results. There was no detectable difference between the results obtained using the Nd:YAG lasers emitting at 10 ns pulses and 150 ps pulses. It was difficult to assess the benefits of using the excimer laser emitting at 500 fs as the energy output was very low. 5.2 Wavelength As expected, the wavelength of the laser beam strongly affected the cleaning process. Removal of the burial deposits was limited with the 248 nm wavelength for all pulse durations tested. This is probably because this wavelength is not strongly absorbed by the corrosion crusts. At 355 nm, burial deposits were removed but melting was observed. The most promising results were obtained using 532 and 1064 nm and it is recommended that future work is focussed on the use of these wavelengths and possibly longer wavelengths. 5.3

Composition of burial deposits

The composition of the burial deposits appeared to have an effect on the fluence and number of pulses required to remove them. For example, both EE14 and EE11 were cleaned at a fluence of 0.45 J/cm2 using a laser emitting at 532 nm and 150 ps. For EE14, less than 10 pulses were necessary to remove the burial deposits, whereas on EE11 the green corrosion layer was not uncovered after 40 pulses. The burial deposits on both samples had a different appearance and were analysed using SEM-EDX: on EE14 they were very rich in lead, whereas on EE11 they were found to contain high amounts of copper and silicon. The difference in composition probably accounts for the differences in laser absorption and removal of the burial deposits. 5.4 Discolouration of cuprite On many of the coins the cuprite layer was discoloured by the laser irradiation, becoming darker and sometimes more purple in hue. This was observed at all wavelengths and pulse durations. Copper compounds are known to decompose to tenorite when heated in

air between 400 and 600◦ C (Scott 2002) and the discolouration of cuprite is probably linked to the partial transformation of cuprite to tenorite. It is, however, difficult to confirm this as tenorite gives a weak Raman signal compared to cuprite and it is not possible to detect its presence on cuprite-rich surfaces. Also, the layer of discoloured cuprite is very thin making sampling for XRD impractical. It is interesting to note that the cuprite was not discoloured on two of the coins, EE6 and EE11, regardless of the wavelength and pulse duration used. This suggests that the discolouration is related to the object itself, rather than laser parameters. However, the reason for the occurrence of discolouration is not obvious. It does not appear to be related to the composition of the alloys as EE14 and EE17 have a similar composition to EE6 (see Table 1) and, unlike EE6, the cuprite layers on these two coins were discoloured. It is not linked to the thickness of the cuprite layer either as it was not significantly different on EE6 and EE11 to those present on the other samples. EE6 and EE11 have a green corrosion layer just above the cuprite layer, but the composition of this layer does not seem to play a role in preventing the discolouration, since the two coins have green corrosion layers of different composition (see Table 2). Also, the cuprite layer of EE3, which has a green layer similar in elemental composition to EE6, was discoloured by the laser. Further work would be required to elucidate the factors affecting the discolouration of the cuprite layer.

6

CONCLUSIONS

Different laser systems emitting at different wavelengths and pulse durations were tested on a range of archaeological copper alloy coins. Ultraviolet irradiation at 248 nm with pulse durations ranging from 500 fs to 30 ns was found to be ineffective at removing the corrosion crusts, which was thought to be due to poor absorptivity of the crusts at 248 nm. Laser tests at 355 nm and 15 ns were not satisfactory as melting was observed. Only the 1064 nm and 532 nm wavelengths were effective at removing burial deposits from the coins. Although pulses of 130 µs at 1064 nm led to discolouration and melting of the surface, better cleaning was obtained using lasers with shorter pulse durations. For both 1064 and 532 nm, the results were similar whether 10 ns or 150 ps pulses were used. However, several difficulties were encountered. Cleaning was uneven on surfaces with a shallow relief, as raised areas were over cleaned and burial deposits remained in lower areas. Uneven cleaning was also observed on flat areas and this was thought to be due to slight variations in the composition of the corrosion crust or burial deposits. On the samples where

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the original surface was contained in a green corrosion layer, it was not always possible to preserve this layer and the cuprite layer underneath was often partially or totally uncovered. For most of the samples, cuprite was discoloured by the laser irradiation. The occurrence of discolouration seems to be related to the sample itself, rather than the laser parameters, but further work is required to elucidate this. These difficulties highlight that laser cleaning of archaeological copper alloys is complex and may require online monitoring to be successful. ACKNOWLEDGEMENTS This work was funded in part by the European Commission through the Research Infrastructures activity of FP6 (‘Laserlab-Europe RII3-CT-2003-506350’). REFERENCES Batishche, S., Kouzmouk, A., Tatur, H., Gorovets, T., Pilipenka, U. & Ukhau, V. 2005. Laser cleaning of metal surface – laboratory investigations. In K. Dickmann, C. Fotakis & J.F. Asmus, (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 September 2003. Berlin: Springer. Burmester, T., Meier, M., Haferkamp, H., Barcikowski, J., Bunte, J. & Ostendorf, A. 2005. Femtosecond laser cleaning of metallic cultural heritage and antique artworks. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 September 2003. Berlin: Springer. Cottam, C.A., Emmony, D.C., Larson, J. & Newman, S. 1997. Laser cleaning of metals at infra-red wavelengths. In W. Kautek & E. König (eds), Lasers in the conservation of artworks: LACONA I proceedings, Heraklion, 4–6 October 1995. Vienna: Mayer and Comp. Dickmann, K., Hildenhagen, J., Studer, J. & Musch, E. 2005. Archaeological ironwork: removal of corrosion layers by Nd:YAG laser. In K. Dickmann, C. Fotakis & J.F. Asmus

(eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Drakaki, E., Karydas, A.G., Klinkenberg, B., Kokkoris, M., Serafetinides,A.A., Stavrou, E.,Vlastou, R. & Zarkadas, C. 2004. Laser cleaning on Roman coins. Applied Physics A 79: 1111–1115. Koh, Y. & Sarady, I. 2005. Surface cleaning of iron artefacts by lasers. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Koh, Y.S. 2006. Laser Cleaning as a Conservation Technique for Corroded Metal Artefacts, Doctoral Thesis, Lulea University of Technology. Korenberg, C. & Baldwin, A. 2006. Laser cleaning tests on archaeological copper alloys using a Nd:YAG laser. Laser Chemistry 2006: article ID 75831. Mottner, P., Wiedemann, G., Haber, G., Conrad, W. & Gervais, A. 2005. Laser cleaning of metal surface – laboratory investigations. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the conservation of artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Pini, R., Siano, S., Salimbeni, R., Pasquinucci, M. & Miccio, M. 2000. Tests of laser cleaning on archaeological metal artefacts. Journal of Cultural Heritage: 129–137. Robbiola, L., Blengino, J.M. & Fiaud, C. 1998. Morphology and mechanisms of formation on natural patinas on archaeological Cu-Sn alloys. Corrosion Science 40: 2083–2111. Robbiola, L. 2006. Ecole Nationale Supérieure de Chimie de Paris, Personal communication. Scott, D.A. 2002. Copper and bronze in art – Corrosion, Colorants, Conservation. Los Angeles: Getty Publications. Selwyn, L. 2004. Metals and Corrosion – A Handbook for the Conservation Professional. Ottawa: Canadian Conservation Institute. Siatou, A., Charalambous, D., Argyropoulos, V. & Pouli, P. 2006. A comprehensive study for the laser cleaning of corrosion layers due to environmental pollution for metal objects of cultural value: preliminary studies on artificially corroded coupons. Laser Chemistry 2006: article ID 85324.

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Investigating and optimising the laser cleaning of corroded iron C. Korenberg & A.M. Baldwin The British Museum, London, United-Kingdom

P. Pouli Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas (IESL-FORTH), Heraklion, Crete, Greece

ABSTRACT: The aim of the present study was to investigate and optimise the laser cleaning of historical iron objects. Experiments were carried out on artificially corroded iron coupons using lasers emitting at different wavelengths (248 nm, 355 nm, 532 nm and 1064 nm) and pulse durations varying from 500 fs to 130 µs. The most suitable wavelength for removing corrosion was found to be 1064 nm. Using a Nd:YAG laser emitting 150 ps pulses at 1064 nm at 0.17 J/cm2 most of the corrosion products were removed, the surface of the metal was not discoloured and no melting was observed. When using ultraviolet lasers emitting ns pulses, there was little removal, the corrosion products left on the surface darkened and magnetite was found to have formed. At all wavelengths, short pulses were more effective at removing corrosion products while reducing thermal alterations to the underlying metal.

1

INTRODUCTION

In conservation, active corrosion products are generally removed from the surface of historical iron artefacts using wire wool and a solvent. This method is not entirely satisfactory as it can alter the surface of the metal (either polishing textured surfaces or etching smooth surfaces) and may leave some corrosion, especially on rough surfaces. Laser cleaning has gained considerable success as a valuable method of conservation and its potential use for cleaning metal artefacts has been the subject of many publications since 1995. In particular, several recent case studies have reported trials on excavated iron artefacts and shown that the laser cleaning of archaeological iron is complex and laser parameters need to be selected carefully (Koh & Sarady 2003, Dickmann et al. 2005). Lightly corroded historical objects differ significantly from archaeological iron objects, with different corrosion products and corrosion structures, and this is expected to affect their laser cleaning. To date, there has only been one case study published on the laser cleaning of a historical iron object (Biedermann et al. 2005) and very good results were reported. Unfortunately, this study was not systematic as corrosion products were not identified prior to laser cleaning and the results were only assessed visually. There has been research on corroded mild

steel using different lasers and the results have been varied (Cottam et al. 1995, Cottam & Emmony 1999, Koh et al. 2006, Siatou et al. 2006). Also, since samples were corroded to different levels using a variety of methods in each study, this makes it difficult to compare results and draw conclusions on which laser parameters are the most suitable for cleaning corroded iron. The aim of the present study was to investigate and optimise the laser cleaning of historical iron objects. Tests were performed on artificially corroded iron coupons using lasers emitting at different wavelengths (248 nm, 355 nm, 532 nm and 1064 nm) and pulse durations varying from 500 fs to 130 µs. The effects of the wavelength and pulse duration on the cleaning process were compared and it was carefully assessed whether the laser treatment altered the metal surface. 2

SAMPLES

The samples used for the experiment consisted of 25 × 50 × 1 mm3 coupons of rolled sheets of iron (99.5% purity with traces of manganese, silicon, carbon, phosphorus and sulphur). The coupons were corroded artificially for four weeks in a chamber humidified at 75% RH (±5%) using a saturated aqueous solution of NaCl. This produced an even layer of corrosion products with spots of shallow pitting across

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Table 1. Working parameters of the laser set-ups used in the study.

Laser system

SFR∗ Nd:YAG 1064 (El.En., EOS1000) Q-switched Nd:YAG 1064 (Lynton Phoenix) Q-switched Nd:YAG 1064 (EKSPLA, SL312) Q-switched Nd:YAG 532 (Lynton Phoenix) Q-switched Nd:YAG 532 (EKSPLA, SL312) Q-switched Nd:YAG 355 (Spectron, SL805 mod.) KrF Excimer 248 (Lambda Physik, LPX200) KrF Excimer 248 (Laser Lab. Goettingen) KrF Excimer 248 (Laser Lab. Goettingen)

Figure 1. SEM image of the cross section of an artificially-corroded iron coupon showing corrosion and pitting of the surface (length of scale marker: 200 microns).

the surface of the metal. As measured using scanning electron microscopy (SEM), the corrosion layer was about 5–15 microns thick with shallow pitting of the metal surface to a depth of approximately 20–30 microns (Fig. 1).The corrosion products were analysed using Raman spectroscopy and ferrihydrite (hydrated iron (III) oxy-hydroxide) and goethite (α-FeOOH) were identified as the main constituents. Akageneite (β-FeOOH) was also detected. These corrosion products are typical of those formed on iron exposed to atmospheric conditions (Biasini & Cristoferi 1995, Wang 2007).

Wavelength Pulse (nm)

∗

Fluence (J/cm2 )

130 µs 9.6–14.3 10 ns

0.34–1.5

150 ps 0.14–3.1 10 ns

0.95–2.6

150 ps 0.11–0.45 15 ns

0.85–1.7

30 ns

1.7

5 ps

0.35

500 fs

0.28

Short free running.

carried out using SEM to assess whether melting had occurred on a microscopic level.

4

RESULTS OF THE CLEANING TESTS

4.1 Tests at 1064 nm 3

LASER CLEANING TESTS

The laser systems used in this study and their working parameters are listed in Table 1. The wavelengths available were 248 nm, 355 nm, 532 nm and 1064 nm. The pulse durations employed ranged from 500 fs to 130 µs depending on the laser. For each combination of wavelength and pulse duration used, the number of pulses applied to the surface and the value of fluence were varied to optimise cleaning. The average fluence was calculated by dividing the energy per pulse by the area of the laser spot. The size of the laser spot was estimated by measuring the laser irradiated area on the coupon. Spot tests were carrried out on both wet and dry surfaces. Preliminary tests showed that the nature of the wetting agent did not appear to affect the cleaning and a solvent (either ethanol or industrial methylated spirits) was used as a wetting agent rather than water to prevent any subsequent corrosion to the samples. After the laser cleaning tests, the iron coupons were examined using optical microscopy to determine the efficacy of the laser for removing corrosion and levels of alteration of the surface. Further examination was

Three pulse durations were tested at the 1064 nm wavelength: 130 µs, 10 ns and 150 ps. For all pulse durations the use of a wetting agent was beneficial: more corrosion could be removed at a given fluence and discolouration of the metal was lessened. This is illustrated in Figure 2. The results discussed in this section refer to tests conducted on a wet surface. At 130 µs, most of the corrosion products were removed at a fluence of 9.6 J/cm2 , but darkening of the metal was observed. At 10 ns, it was possible to remove the corrosion products at a fluence of 1.3 J/cm2 , but the metal uncovered had a slightly grey colour, which suggests that melting had occurred. Only when using the lowest fluence obtained by the experimental set-up, i.e. 0.8 J/cm2 , the metal uncovered appear unaffected both visually and using optical microscopy. At this fluence, however, some corrosion products remained on the surface and examination of the metal using SEM indicated that there was slight melting of the iron (Fig. 3). Cleaning at a fluence of 0.17 J/cm2 using 150 ps pulses gave a visually similar level of cleaning to a fluence of 0.80 J/cm2 using 10 ns pulses. In contrast

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Figure 2. Surfaces cleaned by applying one 130 µs pulse at 1064 nm and 9.6 J/cm2 with a wetting agent (left), and on a dry surface (right). Laser spot diameter: approximately 2 mm. Figure 4. SEM photograph of the surface after applying one pulse at 0.17 J/cm2 using the Nd:YAG laser emitting at 1064 nm and 150 ps with a wetting agent: the surface was not melted or altered (length of scale marker: 20 microns).

Figure 3. SEM photograph showing slight melting of the surface after laser cleaning at 0.80 J/cm2 using the Nd:YAG laser emitting at 1064 nm and 10 ns with a wetting agent (length of scale marker: 50 microns).

to the 10 ns tests, examination using the SEM found no evidence of melting (Fig. 4). This is consistent with studies showing that shorter pulse durations limit thermal alterations to the substrate (Burmester et al. 2005, Fotakis et al. 2007). It should be noted that Siatou et al. (2006) reported effective laser cleaning of artificially corroded low carbon steel coupons with no melting of the underlying metal at a microscopic level using a Nd:YAG laser emitting at 1064 nm and producing 5–7 ns pulses. However, the coupons were corroded following a different corrosion protocol (dry-humid cycles without the presence of NaCl), which produced a much more superficial layer of corrosion than in the present study, and this is believed to account for the successful removal of corrosion. Indeed, Koh et al. (2006) observed that laser cleaning is not as effective on heavily corroded steel samples as it is on lightly corroded surfaces. Also, preliminary experiments have found that a laser emitting 10 ns pulses at 1064 nm was effective at removing localised corrosion spots from a superficially corroded iron coupon without causing melting.

Figure 5. Surfaces cleaned by applying five 150 ps pulses at 532 nm and 0.45 J/cm2 . In the photograph on the left, a wetting agent was used, while the surface was kept dry on the right. Laser spot diameter: approximately 2 mm.

4.2 Tests at 532 nm Tests using the 532 nm wavelength were conducted at pulse durations of 10 ns and 150 ps. The use of a wetting agent aided cleaning at both pulse durations. At a given fluence, when no liquid was used, discolouration occurred at the edges of the laser cleaned area and more corrosion was left compared to wet cleaning. This is illustrated in Figure 5. The best results at 532 nm were achieved using a wetting agent and applying four 150 ps pulses at 0.45 J/cm2 . Under these conditions, most of the corrosion was removed and no melting could be seen using optical microscopy. However, examination of the laser cleaned surface using SEM indicated that melting of the metal had occurred at a microscopic level and small prominent particles had formed on the surface of the metal (Fig. 6). The formation of such particles on laser cleaned metal surfaces has been reported in the literature (Degrigny et al. 2003, Dickmann et al. 2005, Korenberg & Baldwin 2006, Koh 2006) and it has been

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In agreement with these results, Siatou et al. (2006) reported insufficient removal and alteration of the corrosion products when using the same laser system. 4.4 Tests at 248 nm

Figure 6. SEM photograph showing the formation of small prominent particles after applying four 150 ps pulses at 0.45 J/cm2 using the 532 nm wavelength; a wetting agent was used (length of scale marker: 50 microns).

Very little corrosion was removed from the iron coupon using the KrF excimer laser emitting 30 ns pulses. The corrosion products were blackened on the irradiated surface and the underlying metal was discoloured suggesting melting. Applying a wetting agent to the surface did not improve the results. Siatou et al. (2006) also observed blackening and insufficient removal of corrosion products from corroded low carbon steel coupons using a KrF excimer laser producing 10 ns pulses. In contrast, it was possible to remove some corrosion at 248 nm using 5 ps and 500 fs pulses; but, as the maximal energy output delivered by the laser system was limited, it was necessary to apply up to thirty 500 fs pulses and ten 5 ps pulses to remove the corrosion. The application of a wetting agent was beneficial as it reduced discolouration along the edges of the irradiated surface. However, even when using a wetting agent, the cleaning was uneven and the surface of the underlying metal was discoloured slightly. 5 ANALYSIS OF THE ALTERED CORROSION PRODUCTS

Figure 7. Blackening of the corrosion and discolouration along the edges of the irradiated surface observed after applying ten 15 ns pulses at 355 nm with a fluence of 1.7 J/cm2 . No wetting agent was used. Horizontal field of view: 3.5 mm.

suggested that this is due to the metal vaporising and being redeposited. 4.3 Tests at 355 nm Results obtained using the Nd:YAG laser emitting 15 ns pulses at 355 nm were disappointing. Two fluences, 0.85 and 1.7 J/cm2 , were tested and with both there was little removal of corrosion from the surface. In addition, the corrosion products left on the surface blackened and the surface of the metal and the edges of the irradiated surface discoloured (Fig. 7). The blackened corrosion was examined using an optical microscope and small spherical particles of 1–5 µm in size were observed at high magnification. Applying a wetting agent reduced discolouration along the edges slightly but did not improve removal of the corrosion.

Darkening of the corrosion occurred on many of the samples but was most pronounced at 248 nm and 355 nm for ns pulses. The darkened corrosion products were analysed using Raman spectroscopy and magnetite, Fe3 O4 , was detected (note that the Raman spectroscopy analysis must be conducted using a low laser power as irradiation can cause magnetite to transform to hematite, α-Fe2 O3 ). Magnetite is black and this would explain the change in colour of the corrosion after laser irradiation. The darkened corrosion on the coupon irradiated at 532 nm was also analysed using Raman spectroscopy and lepidocrocite, γ-FeOOH, was detected. Lepidocrocite has an orange-brown colour, which does not account for the dark colour of the irradiated corrosion. On a corroded coupon, when the superficial layers of corrosion were removed using wire wool and solvent, lepidocrocite was detected, showing that it is formed during the corrosion process near the metal. Thus, it is likely that the lepidocrocite detected on the coupon irradiated at 532 nm was uncovered by laser irradiation as the superficial layers of corrosion were removed. The Raman spectrum for magnetite is characterised by an intense and broad peak at 663–676 cm−1 (de Faria et al. 1996) and the spectrum of lepidocrocite has a broad peak at 658 cm−1 . In the spectrum obtained for

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the darkened corrosion products, the intensity of the peak at 658 cm−1 was significantly higher than the reference spectrum for lepidocrocite and this could indicate that magnetite was also present, mixed with lepidocrocite. 6 6.1

DISCUSSION Effect of the working parameters

The best results were achieved at 1064 nm using the shortest pulses available at this wavelength. Iron has a low absorption at infrared wavelengths and this explains why fewer alterations to the substrate took place at 1064 nm than at the other wavelengths. Koh et al. (2006) report better cleaning results at 532 nm than at 1064 nm when cleaning corroded grooved steel surfaces. However, the results were assessed in terms of which wavelength was the most effective at removing corrosion and it was not investigated whether the laser had altered the surface at a microscopic level or not. This probably explains why the 532 nm wavelength was deemed more successful than 1064 nm. In the present study, the 532 nm wavelength also removed more corrosion than 1064 nm when using 150 ps pulses, but it altered the underlying metal and the use of this wavelength is therefore unsuitable for cleaning museum objects. Short pulses are reported to reduce thermal effects to substrates (Burmester et al. 2005, Fotakis et al. 2007), and this was observed in this study. Using short pulses allowed the removal of corrosion while limiting alterations to the underlying metal. At 248 nm, short pulses also appeared to be more effective at removing corrosion: at 30 ns very little corrosion was removed, while most of the corrosion was removed at 5 ps or 500 fs (although the underlying metal was discoloured). In most tests the application of a wetting agent to the surface of the samples increased the cleaning ability of the laser whilst reducing discolouration to the underlying metal. This is in agreement with the work of several researchers (Koh & Sarady 2003, Dickmann et al. 2005, Koh et al. 2006). In particular, at 1064 and 532 nm, using a wetting agent meant that cleaning could be conducted at a lower fluence than on a dry surface and this reduced thermal alteration to the underlying metal. The beneficial effect of wetting agents is generally attributed to the mechanical action caused by the rapid expansion and vaporisation of the liquid, which penetrates into the oxide layer. 6.2 Alteration of the corrosion Darkening of the corrosion occurred after applying ns pulses at 248 and 355 nm and magnetite was detected, accounting for the dark colour.

The darkening of iron corrosion products has been reported by several authors (Cottam & Emmony 1999, Koh & Sarady 2003, Dickmann et al. 2005, Koh & Sarady 2005), but the mechanism is not fully elucidated. Cottam & Emmony (1999) analysed corrosion products from two naturally corroded samples that had discoloured to slate grey after laser cleaning at 10.6 µm and at a pulse duration of 100 ns. For one object, the corrosion originally comprised of maghemite (γ-Fe2 O3 ), magnetite (Fe3 O4 ), iron oxyhydroxide (FeOOH) and goethite (α-FeOOH) and no change in chemistry was detected before and after laser irradiation. On the second object, akaganeite (ß-FeOOH) and magnetite were initially present in the corrosion layer, while only maghemite (γ-Fe2 O3 ) was detected after laser irradiation. Maghemite has a dark brown colour and the authors speculated that iron oxyhydroxide had decomposed to maghemite and water under the effect of heat. In the present study, the corrosion on the coupons was mostly ferrihydrite, goethite, akaganeite and lepidocrocite and it appears that laser irradiation has caused their decomposition to magnetite. Note that according to the literature (de Faria et al. 1996), magnetite can transform into maghemite at 200◦ C and then into hematite at 400◦ C. The presence of small particles on the surface of the blackened corrosion suggests that much higher temperatures were reached during laser irradiation and it seems surprising to detect magnetite, rather than maghemite or hematite. An explanation for the presence of magnetite on the laser irradiated surfaces may be that the conversion of magnetite to maghemite and then hematite does not take place with very short pulse durations. 6.3 Melting of the underlying metal Melting of the underlying metal took place on most of the samples. As the melting point of iron is 1538◦ C, this shows that laser irradiation generates considerable heat. However, this is a localised phenomenon and the size of the heat-affected zone is thought to be proportional to the pulse duration (Fotakis et al. 2007). Evidence for this could be observed on the laser cleaned coupons: melting was extensive for long pulses as shown by the visual discolouration of the metal, but was much reduced for short pulses and only visible at very high magnification. 7

CONCLUSIONS

Laser tests at 248 and 355 nm using pulse durations of 15–30 ns were not successful for cleaning artificially corroded iron coupons as the corrosion products were discoloured and largely remained on the surface. Analysis of the darkened corrosion indicated that magnetite

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had formed, accounting for the dark colour. The use of 5 ps and 500 fs pulses at 248 nm improved the removal of corrosion, but the surface of the underlying iron was discoloured. Better results were obtained using the 1064 and 532 nm wavelengths, except for the laser emitting 130 µs pulses at 1064 nm, which caused discolouration of the underlying metal. Most of the corrosion products were removed at these wavelengths and the surface of the metal was not discoloured. However, when the laser cleaned surfaces were examined using SEM, it was observed that only the laser emitting 150 ps pulses at 1064 nm did not cause melting of the iron surface at a microscopic level. Further experiments will be carried out on iron at the 1064 nm wavelength to explore the practical use of the laser and the effect of laser irradiation on the metal surface. Future investigations will focus on the re-corrosion rate of laser cleaned surfaces in comparison to uncorroded and mechanically cleaned samples. The surface morphology of laser cleaned and mechanically cleaned samples will also be characterised and compared. ACKNOWLEDGEMENTS This work was funded in part by the European Commission through the Research Infrastructures activity of FP6 (“Laserlab-Europe RII3-CT-2003-506350”). REFERENCES Biasini, V. & Cristoferi, E. 1995. A study of the corrosion products on sixteenth- and seventeenth-century armour from the Ravenna National Museum. Studies in Conservation 40: 250–256. Biedermann, M., Kempe, A. & Panzer, M., 2005. Lasercleaning of an ancient menial armour. Lasers in the Conservation of Artworks: LACONA VI book of Abstracts, Vienna, 21–25 Sept. 2005. Burmester, T., Meier, M., Haferkamp, H., Barcikowski, J., Bunte, J. & Ostendorf, A. 2005. Femtosecond laser cleaning of metallic cultural heritage and Antique artworks. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the Conservation of Artworks: LACONA V Proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Cottam, C.A., Emmony, D.C., Larson, J. & Newman, S. 1997. Laser cleaning of metals at infra-red wavelengths. In W. Kautek & E. König (eds), Lasers in the Conservation

of Artworks: LACONA I Proceedings, Heraklion, 4–6 October 1995. Vienna: Mayer and Comp. Cottam, C.A. & Emmony, D.C. 1999. TEA-CO2 laser surface processing of corroded metals. Corrosion Science 41: 1529–1538. de Faria, D.L.A., Venancio Silva, S. & de Oliveira, M. T. 1997. Raman microspectroscopy of some iron oxides and oxyhydroxides. Journal of Raman Spectroscopy 28: 873– 878. Dickmann, K., Hildenhagen, J., Studer, J. & Müsch, E. 2005. Archaeological ironwork: removal of corrosion layers by Nd:YAG laser. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the Conservation of Artworks: LACONA V proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2006. Lasers in the Preservation of cultural heritage. New York: Taylor and Francis. Koh, Y.S. & Sarady, I. 2003. Cleaning of corroded iron artefact using pulsed TEA CO2 and Nd:YAG lasers. Journal of Cultural Heritage 4: 129–133. Koh, Y. & Sarady, I. 2005. Surface cleaning of iron artefacts by lasers. In K. Dickmann, C. Fotakis & J.F. Asmus (eds), Lasers in the Conservation of Artworks: LACONA V Proceedings, Osnabrück, 15–18 Sept. 2003. Berlin: Springer. Koh, Y.S., Powell, J. & Kaplan, A.F.H. 2006. The removal of layers of corrosion from steel surfaces: a comparison of laser methods and mechanical techniques, Laser cleaning as a conservation technique for corroded metal artefacts: 45–60, Doctoral thesis, Lulea University of Technology. Koh, Y.S., Bergström, D., Powell, J., Åberg, G., Grahn, J. & Kaplan, A.F.H. 2006. Cleaning oxides from copper artifacts using a frequency-double Nd:YAG laser, Laser Cleaning as a ConservationTechnique for Corroded Metal Artefacts: 85–95, Doctoral thesis, Lulea University of Technology. Korenberg, C. & Baldwin, A. 2006. Laser cleaning tests on archaeological copper alloys using a Nd:YAG laser. Laser Chemistry 2006: article ID 75831. Neff, D., Bellot-Gurlet, L., Dillmann, P., Reguer, S. & Legrand, L. 2006. Raman imaging of ancient rust scales on archaeological iron artefacts for long-term atmospheric corrosion mechanisms study. Journal of Raman Spectroscopy 37: 1228–1237. Siatou, A., Charalambous, D., Argyropoulos, V. & Pouli, P. 2006. A comprehensive study for the laser cleaning of corrosion layers due to environmental pollution for metal objects of cultural value: preliminary studies on artificially corroded coupons. Laser Chemistry 2006: article ID 85324. Wang, Q. 2007. Effects of relative humidity on the corrosion of iron: an experimental view. The British Museum Technical Research Bulletin 1: 65–73.

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Nd:YAG laser cleaning of heavily corroded archaeological iron objects and evaluation of its effects J. Chamón, J. Barrio, E. Catalán, M. Arroyo & A.I. Pardo Departamento de Prehistoria y Arqueología, Universidad Autónoma, Madrid, Spain

ABSTRACT: Archaeological iron objects are subject to corrosion under the conditions of their burial, and it is rather common to find that they preserve no metallic core at all. They develop an expansive corrosion crust that is extremely brittle and thus fragile. This makes the application of standard cleaning process very difficult, because of the stress induced by the mechanical cleaning methods that can produce fractures, and eventually collapsing or breaking the object.This work offers an evaluation of heavily corroded archaeological iron objects cleaning using a short free running Nd:YAG laser. It also evaluates the effectiveness of laser treatment combined with mechanical cleaning methods, carried out by experienced curators. X ray diffraction, scanning electron microscope and optical microscopy have been used to observe and analyse the surface of the objects. The results show the capacity of laser treatment to recover the original surface, revealing details of the objects that were hidden under the corrosion layer. It is a very valuable technique in the archaeological conservation realm, a cleaning technique that preserves archaeological data and details of the original surface.

1

INTRODUCTION

The deterioration of buried archaeological objects has certain characteristics that are hard to foresee. The study of the corrosion processes of archaeological metals has produced interesting work in recent years on a diverse restoration-conservation realms. The oxygen transport phenomena related to the corrosion of archeological iron objects has been studied by Vega (Vega et al. 2005) and the corrosion mechanism of archeological metals has been researched by Chitty (Chitty et al. 2005) to solve the modern problem of the long time stability of metallic reinforcements embedded in concrete. Chitty proposes a corrosion strata scheme as follows: a metallic core zone (M), a dense product layer located around the core (DPL) which they identify with the original surface/patina, a transformed medium/distorting corrosion (TM), and an outer soil crust (S). This terminology will be employed in our study, as we find it useful. The standard procedure for cleaning archaeological metallic objects, which is commonly accepted (Berducou, M. 1990), consists in the removal of the TM, a mixture of soil and oxides, until the layer of DPL, mainly a mixture of goethite, magnetite and maghemite, is reached and the original surface is exposed. The original surface is composed of oxides and hydroxides which are so dense they reduce oxygen diffusion and thus provide higher corrosion stability.

It also preserves the original shape of the object to a certain extent. An initial study of the object is performed employing non destructive techniques, X-ray plates for example, and a subsequent manual mechanical cleaning is carried out using scalpels, micro-drills and/or microsandblasters. The corrosion crust that distorts the object (S and TM layers) is removed to a greater or lesser extent, leaving the patina or original surface (DPL). Although this is an attested method that works properly with most objects, it has certain drawbacks: – Fragile objects without a metallic core can be cracked or broken due to the mechanical pressure. Preexisting cracks can become longer and/or deeper. The application of consolidants and adhesives during this kind of cleaning is an imperative need to prevent further deterioration. – The complete recovery of the original surface depends greatly on the skill, knowledge and experience of the conservator. The use of micro-drills can cause unnoticed losses of the original surface. – Niello, damascene, copper-plated ornaments are always covered by iron corrosion products. In this case, controlling the cleaning can be very difficult when using a micro-drill. Moreover, ductile metal ornaments, such as gold or silver, are quite susceptible to scratches and detachments.

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– The morphology of certain objects (riveted, drilled, or carved objects) can make this kind of cleaning a very hard task as there is a difficult access to corrosion. Laser ablation is being tried as an alternative to manual cleaning of archaeological metals that can solve these problems. Laser has been commonly employed for cleaning stone and architectural elements for decades now, and there are abundant references on this issue. Nonetheless, very few studies on laser cleaning of archeological metals have been published thus far as this subject has been researched only in recent years (Cooper, 1998, Siano et. al. 2001a and 2001b, Dickmann et al. 2001, Koh et al. 2003, Barrio et al. 2006). In this paper we present the results of laser cleaning on a variety of archaeological iron objects that were chosen for the difficulties their mechanical cleaning presented. At the same time, the effect of radiation on diverse layers of oxides will be discussed. The following objects will be studied: a soliferrum with no metallic core and very susceptible to breakage, an iron knife with copper plating on the handle, very affected by corrosion, the hilt of an iron biglobular dagger decorated with silver niello, and as an example of the results of laser cleaning on other metals, a gilded copper plate. 2 2.1

MATERIALS AND METHODS Historical context

The following objects have been studied: – Knife from the Hispanic Muslim city of Qalat Rabah, Calatrava la Vieja (8th–15th century AC, belonging to the Parque Arqueológico de Alarcos, located at Ciudad Real, Castilla la Mancha, Spain. – Soliferrum: it is an Iberian pole weapon with the peculiarity that both the pole and tip were made of a single piece of iron. Its place of origin is the Iberian necropolis of “El Salobral” (4th century b. C.) which is located at Albacete, Castilla la Mancha, Spain. – Biglobular dagger hilt with silver niello decoration from the Hispanic Roman site of “La Bienvenida” (1st century a. C), Ciudad Real, Castilla la Mancha, Spain. – Gilded copper plate from the Hispanic Muslim city of Qalat Rabah, Calatrava la Vieja (8th–15th century a. C), belonging to the Parque Arqueológico de Alarcos, located at Ciudad Real, Castilla la Mancha, Spain. Analyses from these objects has been published elsewhere (Barrio 2005). 2.2

Laser treatment and surface characterization

A Short Free Running mode (SFR) Nd:YAG laser with wavelength of 1064 nm has been employed. Two

Figure 1. Knife handle from Calatrava la Vieja: A) conservation state of the end of the freshly excavated handle, B) first cleaning stages. The metallic grey colour of the spinel layer can be seen, C) last cleaning stages. The shape of the rivet and the general outlines of the handle are revealed, D) final state. The reddish colour corresponds to the copper plate underneath the spinel layer.

different settings were used, we will refer to first one as “high energy conditions”: energy per pulse 0.8 J, fluence 1 J/cm2 , frequency 6 Hz.The second one is named “low energy conditions”: energy per pulse 0.2–0.4 J, fluence 0.3–0.5 J/cm2 , frequency 6 Hz. X-ray plates, X-ray diffraction, and optical microscopy analyses were performed prior to the intervention in order to evaluate the cleaning. A cross section SEM-EDX analysis was made on a detached fragment from the soliferrum. 3

RESULTS

3.1 Knife from Qalat Rabah This object has a crack located in the joint of the blade and the handle. The corrosion crust that covered the handle was removed by laser cleaning and revealed the cylindrical rivets with a smooth and well preserved surface (Fig. 1). The cleaning also revealed a thin copper plate that covered both sides of the handle which was obscured by iron corrosion products. This copper plating was probably located underneath the handle, lost nowadays. The efficiency of laser cleaning, its capability of cleaning zones with difficult access and the advantages of a pressure-free cleaning have been proven with this study.Although laser cleaning is somewhat slower than manual cleaning, it also provides better control which is useful when the operator is cleaning zones closer to the original surfaces or very complicated areas. Laser

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Figure 3. Biglobular hilt. It is an object decorated with silver niello that was hidden by iron corrosion:A) conservation state before treatment, B) outlines of the ivory hilt, C) conservation state after treatment, the niello was recovered.

Figure 2. This Iberian Pole weapon has no metallic core: A) cross section, B) section treated with low energy settings, C) section treated with high energy settings, D) SEM cross section image (backscattering mode) after cleaning, E) SEM cross section image (backscattering) before cleaning.

cleaning also provides a fine control of the cleaning level, as it has been proven with the thin copper plating that was freed from iron corrosion without causing damage or loss to its surface. 3.2

Soliferrum from El Salobral

Another test has been performed on this outstanding Iberian weapon. A fragment of the soliferrum was employed for our research. A metallographic study of the object showed that there was no metallic core. The stratigraphic scheme can be outlined as S/TM/DPL/Hydroxides. The corrosion has an appearance similar to puff pastry. The intervention criterion was to remove the corrosion up to the DPL layer and preserve the hydroxides that were located in the interior of the object. The lack of metallic core does not interfere with laser performance, although it makes the object more susceptible to breakage. Figure 2 shows the soliferrum and also a section of the object at three different moments: prior to treatment (2a), cleaned at low energy (2b), cleaned at high energy (2c). On this occasion, laser cleaning has proven its efficacy in removing iron rust. A disadvantage of

laser cleaning applied to large objects as the soliferrum is the slowness of cleaning. In spite of that, we strongly recommend laser cleaning as an alternative technique to remove rust, especially in objects with a conservation state similar to the soliferrum. 3.3 Biglobular dagger hilt from La Bienvenida This object had a silver niello decoration that was hidden by iron corrosion. Laser cleaning removed the hard corrosion crust that covered it without damaging the niello so we could recover this outstanding decoration without endangering it. Very probably, the efficacy of the cleaning is due to the radiation absorption rate of silver, which is lower than the crust absorption rate, thus preserving the silver and removing the corrosion (Fig. 3). A mechanical cleaning with micro-drill was performed by an experienced restorer before the object arrived to our laboratory. The niello was unnoticed by the restorer, who removed it within the crust. Laser is the best option to recover this kind of very delicate decoration, as the niello is made of a very thin and fragile silver thread and is often almost detached from the original surface of the object. 3.4 Gilded copper plate from Qalat Rabah This is an example of laser cleaning of a non-iron metallic object. This gilded copper object was laser cleaned selecting very low energy parameters. The gilding suffered ablation if several pulses hit at the same point, so a frequency of 1 Hz was employed

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Table 1. X-ray characterization of studied objects, after and before treatment. Before treatment

Low energy treatment

Knife from Qalat Rabah. Handle Calcite, Cuprite Cu2 O magnesium Copper Cu carbonate Magnetite Fe3 O4 (Mg0.06 Ca0.94 )CO3 Copper sulfide Cu2 S Copper Cu Knife from Chalet Rabah. Blade Triiron (III) Magnesioferrite phosphate (V) MgFe2 O4 trioxide Fe3 PO7 Hematite Fe2 O3 Jarosite Moissanite SiC KFe3 (SO4 )2 (OH)6 Soliferrum from El Salobral Goethite Magnesioferrite MgFe2 O4 Fe2 O3 · H2 O Hematite Fe2 O3

Figure 4. Gilded copper plate from Calatrava la Vieja belonging to the Parque Arqueológico de Alarcos: A) Conservation state before treatment, B) image of the plate undergoing treatment, C) conservation state after treatment.

in order to avoid irradiating the same spot on more than one occasion. With these settings, the plate was successfully cleaned, removing the corrosion copper crust without altering the gilding and the underlying corrosion that acts as a substrate and maintains the coherence of gold (Fig. 4).

4 4.1

DISCUSSION Laser ablation and side effects

High energy laser cleaning removes sulphates, carbonates and phosphates. These kinds of chemical species undergo complete ablation and are eliminated without problem. Nonetheless, silicates, sand and quartz embedded in TM, absorb little radiation at this wavelength and photo-ablation phenomena, such as spallation and ionization, do not occur. Silicates can be eliminated by removing the corrosion crust around them so they detach or are ejected due to mechanical pressure or tensions induced by the different thermal expansion rates of the materials irradiated.

High energy treatment

No analysis

Magnesioferrite MgFe2 O4 Hematite Fe2 O3

Jacobsite (Mn, Mg, Fe) (Mn, Fe2 )O4

The crust of corrosion products and soil materials is ablated at both high and low energy conditions, producing a plasma plume. The irradiated surface reaches extremely high temperatures (>1400◦ C) in microseconds. Apart from the ablation itself, the radiation produces also a change in the surface, which looks grayish and show traces of micro-smelting. An X-ray analysis has been performed on the irradiated surface (Table 1). The results show that a spinel type iron oxide structure has been produced by the irradiation. The presence of magnetite in the spinel structures is proven by analysis, as it was expected, but there also appears magnesioferrite, and jacobsite. All these structures have an Fd3m (F41 /d 3 2/m) symmetry and they are [M3+ ]Td [M2+ Fe2+ ]Oh O4 spinel group oxides. The temperature reached by the surface of the magnetite is high enough to produce a Fe3+ metallic cations exchange, substituted by other metallic cations coming from TM or S zone, such as Mg or Mn. On a first cleaning stage, the object surface (S and TM) is irradiated to remove the carbonates and sulfates by photo-ablation and the silicates and soil particles by mechanical action and ejection. As the cleaning progresses it gets closer to the iron oxides-hydroxides rich zone of TM and a few microns layer of spinel type iron oxides is generated as a side effect. This species has a low absorption rate at 1064 nm, so the effectiveness of laser cleaning lowers dramatically once the spinels have formed. Thus, it is mandatory to remove the layer employing a brush or a scalpel before carrying on the cleaning. A cleaning that combines the use of scalpel and laser quickens the treatment.

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3) Removal of the spinel layer with brush or scalpel, preferably under an optical magnifier and being cautious about the arising of the original surface. 4) Repeat steps 2 and 3 as many times as necessary to recover the original surface (DPL).

5

Figure 5. Diagram of the cleaning methology proposed: A) ideal scheme of iron corrosion in archaeological objects S/TM/DPL/M, B) material removal and generation of spinel layer, C) mechanical removal of spinel layer employing a brush or a scalpel, D) material removal and generation of spinel layer, E) end of cleaning DPL/original shape reached, F) application of an acrylic protective coating to prevent oxygen and moisture from reaching the surface of the object.

If DPL is mainly composed of goethite, laser radiation will transform it into a few microns thick magnetite spinel layer, and this layer will improve the resistance to corrosion of the object, as spinel type compounds are less permeable to oxygen. This transformation of the surface, together with the application of protective layers is a very promising conservation treatment for iron objects.

4.2

We have found evidence of the formation of iron spinels oxides on the laser irradiated surface. These spinels oxides are generated by high temperatures and metallic cations from the soil crust can be introduced into its structure during their formation. The restorer must be able to adequately change the cleaning parameters. Different parameters must be employed when cleaning different metals but also when facing different cleaning stages. High energy parameters have been useful to clean iron objects at first, but they have been switched to low energy parameters when we have come closer to the original surface. We propose to clean employing laser and mechanical cleaning in synergy to increase the efficacy, improve results and quicken the treatment. It is better, for example, to remove soil and loose corrosion by mechanical means, as laser is a slower technique that should be employed where necessary.

ACKNOWLEDGEMENTS We are grateful to the Parque Arqueológico de Alarcos, located at Ciudad Real, Castilla la Mancha, Spain. Thanks are also given to EL. EN. (Italy) and Laser Tech Ibérica.

Mechanical cleaning and laser cleaning. Proposed treatment

We propose a cleaning intervention employing a SFR 1064 nm laser radiation with a fluence of about 1 J/cm2 and a frequency of 6 Hz. Steam cleaning is employed in the first cleaning stage, when our aim is to remove the S and TM layer. The sudden evaporation of water will help the mechanical forces with the removal of concretions and cool down the surface, thus impeding the generation of spinels. When the irradiated surface darkens, due to spinel formation, the spinel layer will be removed with a brush or a scalpel. This first cleaning stage will be finished when the original surface of magnetite or goethite appears. The second stage consists in the application of low fluences and higher frequencies to generate the protective spinel layer. Therefore the proposed treatment is as follows: 1) Mechanical cleaning, removal of soil and loose corrosion with micro-drill, brushes, and scalpel. 2) Laser cleaning with steam cleaning and high fluences.

CONCLUSIONS

REFERENCES Barrio, J., Climent, A., Enguita, O., Pardo, A. I., Arroyo, M. Migliori & A. Ferretti, M. 2005. Aplicación de la técnica de haces de iones (IBA) en la investigación para la conservación de los dorados medievales islámicos de Qalat Rabah (Calatrava laVieja). II Congreso del Grupo Español del IIC. Barcelona. 21–32. Barrio, J., Arroyo, M., Chamón, J., Pardo, A.I. & Criado, A. 2006. Laser cleaning of archaeological metal objects. Proceeding of Heritage, Weathering & Conservation. Madrid. 699–706. Berducou, M. 1990. La conservation en archéologie, Méthodes et pratique de la conservation-restauration des vestiges archéologiques. Masson (eds). Cooper, M. 1998. Laser cleaning in conservation, an introduction. Oxford. Butterworth-Heinemann (eds). Dickmann, K., Hildenhagen, J. & Studer, J. 2001. Laser removal of corroded layers from archaeological ironwork. Lacona IV. Paris: 71–74.

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Chitty, W.J., Dillmann, P., L’Hostis, V. & Lombard, C. 2005. Long-term corrosion resistance of metallic reinforcements in concrete, a study of corrosion mechanisms based on archaeological artefacts. Corrosion Science 47: 1555–1581. Koh,Y. & Sárady, I. 2003. Cleaning of corroded iron artefacts by using pulsed TEA CO2 and Nd:YAG-lasers. Journal of Cultural Heritage 4: 129–133. Siano, S., Salimbeni, R., Pini, R., Pasquinucci, M., Bianchini, S., Facella, A. & Miccio, M. 2001 a. Studi sui manufatti metallici di provenienza archeologica.

Teniche e sistema laser per il restauro del beni culturali. 71–104. Siano, S., Salimbeni, R., Pini, R., Margheri, F., Mazzinghi, P., Modi, S. & Checchi, C. 2001 b. Progetto e realizzazione di un sistema laser per il restauro. Teniche e sistema laser per il restauro del beni culturali. 105–116. Vega, E., Berger, P. & Dillmann, P. 2005. A study of transport phenomena in the corrosion products of ferrous archaeological artefacts using 18 O tracing and nuclear microprobe analysis. Nuclear Instruments and Methods in Physics Research B 240: 554–558.

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Laser as a cleaning tool for the treatment of large-scale bronze monuments A. Dajnowski CSOS Inc. Forest Park, Illinois, USA

ABSTRACT: This paper discusses the level of corrosion removal, concentrating on a chloride problem, and the precision of metal cleaning using a laser ablation process. Various laser systems and their properties will be discussed. The effectiveness of laser cleaning will be compared with blasting and chemical methods. Safety and other aspects of laser usage will be compared to traditional cleaning techniques. The treatment of bronze monuments of General Corse (from Burlington, Iowa), E. B Wolcott (from Milwaukee, Wisconsin), various tests on copper alloys, and the treatment of (about 6000 square feet of bronze surface) four 15 ft and four 24 ft monumental bronzes by Alexander M. Calder from the top of Philadelphia’s City Hall Tower (400 ft high) will be discussed. Unusual conservation treatment aspects of working on large-scale sculptures at unusual heights and new possibilities provided by lasers will be discussed in this paper. 1

INTRODUCTION

The use of lasers provides a conservator with a very unique opportunity (Fotakis et al. 2006). Now, monuments can be treated just above pedestrians or in the vicinity of pristine surfaces. Cleaning can be performed outdoors in the wintertime. The ablation process can be controlled and all the by-products of the surface cleaning can be extracted by vacuuming and, as a result, prevent contamination of the surrounding area. Laser ablation can be performed with great precision what is important for the next steps of a treatment. The treatment of eight monumental bronzes at the top of the Philadelphia City Hall tower (Fig. 1) presented unusual problems because of the height of the building (400 feet/122 m) and the fact that the white marble walls of the tower had just been cleaned. The size and shape of the sculptures added difficulty to the project. The use of traditional surface cleaning methods would not provide the desired level of corrosion removal without damaging the substrate or contaminating the surrounding area. All the sculptures had typical bronze corrosion products plus a large amount of chlorides that were found in many green areas and in almost all the tested black areas. Black corrosion, in general, is classified as copper sulphide. However, in this case, it was much more complicated. When the black crust was examined, large amounts of carbon, silica, sulphides and sulphates, as well as tar-like materials were found. This “corrosion” product was created by environmental pollution. By-products from factories producing cement or plastics, petroleum refineries,

Figure 1. Philadelphia tower sculptures.

chemical plants, the large number of cars constantly moving around the tower and dust particles created a very hard and dense corrosion crust. The bronze sculptures of E. B. Wolcott (Fig. 2) and General Corse (Fig. 3) are located in parks, but tests performed on corrosion samples taken from these sculptures also showed chloride contamination of the surfaces.

2

EFFECTIVENESS OF LASER CLEANING IN COMPARISON TO TRADITIONAL METHODS

Due to the fact that the surface of the sculptures in Philadelphia was heavily contaminated with chlorides

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Figure 4. Mass loss of bronze sample 85% Cu, 5% Sn, 5% Zn and 5% Pb exposed to typical parameters used during tests of corrosion removal.

Figure 2. Statue of E. B. Wolcott, Milwaukee.

Figure 3. Statue of General Corse, Burlington.

and the location of the sculptures required major expenses to erect the scaffolding, a decision was made to remove as much corrosion as possible (preferably 100%). This is not the preferred conservation choice, but it was necessary from a practical and economical point of view. In most cases, localized chloride removal is possible, but when most of the surface is contaminated, localized treatment is not practical. In order to select the best approach, traditional cleaning methods were investigated and laser-cleaning tests were performed. – All blasting methods using solid media were discarded due to the possible surface damage and contamination of the surroundings. When used in chloride-contaminated areas, blasting removed only the pinned superficial corrosion and sealed the corrosion in the crevices. – Chemical methods required a lot of time and did not provide the desired level of chloride removal. In addition, poulticing could potentially cause run-offs that could stain the building below. Treatment with

Potassium Sodium Tartrate removed some of the chlorides. However, all test samples showed residue of chlorides after treatment. – Electrolytic reduction, although very successful in chloride removal from small artifacts, is not usually practical for treatment of large monuments. – Water jet blasting provided a similar level of chloride removal as the laser, but it was the most surface damaging technique that was tested. In order to remove the corrosion entirely, a pressure of 40000 psi was needed. This process would also have been difficult to control in terms of contamination of the surrounding area. In addition, there were numerous crevices, undercuts and tight spots where cleaning using this technique would not be possible. This process does not allow partial cleaning. Even though it is very aggressive, this technique does not provide complete chloride removal. However, it is the only method that provides a level of corrosion removal comparable to the laser when used in the field on a large monument. The worst side effect of this process is the removal of all loose particles and the possibility of surface damage. Although this process is very aggressive, selecting the appropriate water flow, distance from the surface and good skill of the operator can produce relatively satisfactory results.Tests showed surprising results with fast surface cleaning and relatively low (in comparison to what might be expected) level of surface damage. After testing, it was determined that laser ablation would be the best choice for the Philadelphia project. It was possible to remove most of the chlorides. The process was clean. There was no risk of staining the building, and most of the surface of the sculptures could be cleaned. In some cases, the sculptures were so close to the walls of the tower that dismantling large parts of the sculpture or the building would have been necessary to reach and clean those areas (This was done in some cases). Figure 4 illustrates loss of mass caused by tested surface cleaning methods. Percentage of mass loss better represents the changes resulting from treatment

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Figure 5. This picture illustrates the necessity of using a laser with optical fibre for this project.

Figure 6. Safety equipment necessary during laser treatment: goggles, respirator, extraction system and protective barrier around the work area.

than absolute weight loss since the tested samples weight different. The following lasers were tested:

3

– Palladio and Michaelangelo lasers manufactured by Quanta (Italy) – 120 W Q-switched laser manufactured by Clean (German made laser sold in USA by Adapt Laser Systems) – 100 W, 200 W and 400 W Q-switched lasers manufactured by GLC (General Lasertronics Co. from California, USA). Both Italian lasers successfully ablated green corrosion but removed only very thin layers of black crust. Due to the fact that these systems transfer light through articulated arms, this instrument design was found to be not practical for cleaning large and complicated shapes on a scaffold. The mirrors within the arms were easily contaminated and had to be often replaced. It was decided that a laser with an optical fibre would be better (more practical) for this project. Adapt 120 W and GLC 200 W lasers proved to be very effective. Both lasers successfully removed all layers of corrosion. Adapt 120 W with one optical fibre (stylus hand piece) and another 120 W with two optical fibres (stylus and blue gun – larger hand piece) were tested. While using these lasers, it was possible to set appropriate parameters and clean the surface of the sculptures without damaging the substrate. It was also important that the “blue gun” of Adapt Laser Systems was able to extract (by vacuuming) a lot of the ablated particles, which was particularly important when work was done in very tight spots. However, this design often caused contamination of the lens at the hand piece. The GLC laser does not extract the ablated particles. Instead, it blows air out providing positive pressure and, as a result, prevents the lens from becoming contaminated. Both lasers use long (30 m Adapt, 45 m GLC) optical fibres, making work in hard to reach areas possible without moving the laser units.

SAFETY ASPECTS OF LASER USAGE IN COMPARISON TO TRADITIONAL CLEANING TECHNIQUES

The most common problems when using solid media blasting are usually the inhalation of dust, the possibility of physical injury to the operator as well as damage to the artifact, and the contamination of surrounding areas. High pressure water jet blasting creates similar problems. Physical damage of the artifact or personnel is usually the worst potential outcome from using this technique. Chemical treatments carry common problems of contamination, staining, burning, etching of the surface and, obviously, numerous potential harmful effects on the people performing the treatment as well as the observers. Laser treatment, in general, has just three main hazards: – Inhalation: Ablated particles can create health hazards (Barcikowski 2005). Due to their small size, it is important to extract the ablated material, because even HEPA (High Efficiency Particulate Air) filtered masks are not rated for particles smaller than 3 micrometers in size. Smaller particle size often makes up a large portion of ablated material and therefore, effective extraction of ablated material is technically more important than wearing a respirator during treatment. Respirators should still be worn, and be equipped with OV (Organic Vapor) – HEPA filters. HEPA vacuums should be used to extract the ablated material right at the spot being cleaned and, based on experience, the use of strong fans blowing air across the treated surface, away from the conservator, should minimize the risk of inhalation of ablated particles. The size of the particles and their chemistry should be carefully studied when cleaning is performed in order to

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avoid unnecessary risk and to provide the necessary protection for conservators. – Eye damage: This danger is easily resolved by always wearing appropriate goggles that should be selected according to the wavelength of the laser used. Physical barriers must always be erected to protect people in the surrounding area from exposure. – Skin damage: It is highly unlikely to hurt one’s skin with such lasers. Considering that most lasers incorporate two safety switches, one would have to consciously point the laser apparatus to oneself while firing. The effects of a laser on skin are not well known and need further study. Lasers would probably burn the skin. 4

Figure 7. This diagram shows EDS results of analysis of a chloride-contaminated black corrosion layer taken from proper left leg of the Wolcott monument.

DISCUSSION OF LASER TREATMENT OF BRONZE MONUMENTS

Lasers provide conservators with the possibility of partial or complete cleaning of corrosion particles from a bronze surface (Sook Koh 2005). Three treatments of monuments that have been contaminated with chloride corrosion products are examined: – Eight sculptures at the top of the Philadelphia City Hall, USA – Equestrian Statue of General Corse from Burlington, Iowa, USA – Equestrian Statue of E. B. Wolcott from Milwaukee, Wisconsin, USA Corrosion containing chlorides is very damaging to bronze substrate. To date, the following methods have been used to address this problem: electrolytic reduction, chemical stabilization, blasting with a wide range of media, and corrosion inhibitors. Using 120 W Q-switched Nd:YAG optical fibre lasers made it possible to successfully clean the surface of corrosion layers and remove most of the chloride contamination on the three projects. The material left on the surface after laser ablation had identical composition in the green and black zones. Those areas were heavily contaminated with chlorides before laser ablation. Most of the chlorides have been removed. Obviously, the ablation process cannot remove the corrosion and chlorides in areas where light cannot reach them. Undercuts and deep crevices have corrosion that was not removed, but its amount was so small that one can say that laser ablation provides the best surface cleaning results of all presently known techniques. This treatment, if followed by the application of corrosion inhibitors, and appropriate coatings, should provide long-term stability of the treated surface. Test results illustrated in Figure 7 of a sample taken prior to laser treatment when compared with analysis of a sample taken from the same spot after ablation

Figure 8. Overlaid results of EDS analysis of four different tests performed on samples taken from (Wolcott) green and black corrosion zones.

(Fig. 8) show considerable reduction in sulphur and chloride picks. The analysis shows that although the corrosion products on the sculpture consisted of mixtures of several types of corrosion prior to ablation, the composition of the laser cleaned areas is essentially identical and consistent after ablation treatment. 5

COMPARISON OF LASER CLEANING TESTS PERFORMED ON VARIOUS COPPER ALLOYS

In order to determine if the laser beam has a damaging effect on the substrate, hundreds of tests were performed on various samples of raw (as cast free of corrosion) samples, corroded samples prepared in the lab, samples taken from Wawel Castle in Poland (copper roofing material) and samples from the Philadelphia tower.

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Figure 9. 200x magnified surface of polished silicone bronze (97% Cu, 3 % Si). This surface was treated with a GLC laser set to a safe level required to remove black corrosion from the surface of sculptures in Philadelphia.

Figure 11. 1600x SEM image of a cast sample 85, 5, 5, 5 treated with a GLC laser set to the level necessary to remove black corrosion during treatment of Philadelphia sculptures. There is a slight melting effect (2 µm) on the surface, but the lead inclusion in the middle of the surface line is intact.

Figure 10. View of 200x magnified surface of a polished bronze sample composed of 85% Cu, 5% Sn, 5% Zn and 5% Pb.

Figure 12. 1000x SEM image of 120 W Adapt laser cleaned sample of the original bronze surface from the hatch of one of the Philadelphia Tower figures.

Some of the test results proved that the laser beam could potentially have a melting effect on the surface if the energy level is set too high. The melting effect, however, was measured to be not deeper than 2 µm. Visual microscopy (up to 1000X) as well as SEM analysis (up to 10000X) strongly supports that the melting effect is very superficial. It also appears to be related to the delta phase of metal composition where lower melting point elements are predominant. Lead, tin, and zinc might melt when the raw samples are exposed to a powerful ablation process. When corroded samples were studied, it was observed that much of the Pb, Sn and Zn were removed first by the environmental corrosion process. In addition, in the case of excessive ablation, most of these elements could be removed and leave behind a copper rich surface. The sample illustrated in Figure 10 was intentionally exposed to a very high level of laser energy. Polished samples were used in order to better evaluate the effect of the ablation process on the surface of

metal. Corroded samples or samples with a coarse finish (dendritic structure typical after casting) would not show these surface changes due to an original surface topography that would make it difficult to distinguish changes. All the surface corrosion is removed (Fig. 12) and there is no visible melting although this image is highly magnified. If the parameters of the ablation process are set correctly, there is no adverse effect on the cleaned bronze substrate. Corrosion absorbs most of the energy and is consequently removed from the surface. The melting effect is easily avoided as seen in Figure 13 by lowering the energy level and pulse frequency of the laser. The parameters of a laser beam can be adjusted to such a level that even gilded surfaces can be safely cleaned (Radke). The melting effect might have a positive future influence on the corrosion resistance of the cleaned metal due to the fact that the dendritic structure of cast metal is originally porous in nature. The molten metal stays in place and seals the surface porosity. This melting phenomenon, although

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6

CONCLUSIONS

Laser ablation is appropriate for the treatment of copper alloy sculptures and has significant advantages over other techniques. If needed, corrosion products can be removed partially or entirely. Ablation is a very effective technique for addressing corrosion and chloride contamination on bronzes of any size. The laser is a very dynamic tool that can be adjusted to produce a variety of treatment results. ACKNOWLEDGEMENTS Figure 13. 7500x SEM image of polished bronze sample (82% Cu, 15% Sn, 3% Pb) exposed to the ablation level required to remove corrosion layers entirely and safely, showing no sign of surface melting.

Figure 14. This diagram shows the surface topography of polished metal sample 85, 5, 5, 5 treated with GLC laser that was set to safe parameters for corrosion removal. The studied area was 1.25 mm × 1.25 mm and the height is measured in µm.

technically not desired, could be considered as a positive side effect of powerful laser ablation, because it is not noticeable to the eye. In order to better understand this problem, surface topography was also studied and measured. The surface topography shows that the laser did not cause any structural disturbance in the metal. Further research is needed to fully understand this phenomenon and its potential long-term effects on the corrosion resistance of bronze.

The author wants to thank A. Lins, Head of Conservation Department at Philadelphia Museum of Art, and A. Lasseter Clare, also from the Philadelphia Museum of Art for the tests on black crust. The corrosion samples were also tested together with I. Kobla by means of SEM/EDS at the Polish Academy of Sciences in Gdansk. Tests of corrosion during treatment of the statue of E. B. Wolcott (Milwaukee, WI, USA) were performed for CSOS Inc. by M. Miller from MVA, Atlanta. Surface topography analysis performed by J. Marczak, Chief of Laser Devices Section, IOE Inst. Optoelectronics, Warsaw. Invaluable support and collaboration was provided by A. Lins and A. Lasseter Clare (PMA), M. Miller (MVA), J. Marczak (IOE, Warsaw), I. Kobla (Warsaw), Prof. G. Sliwinski (PAS, Gdansk), M. Sawczak (PAS, Gdansk). This research was directly related to the monuments CSOS treated in cooperation with D. Back from Milwaukee, P. Collier from Burlington, J. Schlotterback, Y. Wise, M. Berg and H. Boise from Philadelphia. Special thanks to Prof. J. Krauze, Torun. REFERENCES Barcikowski, S. 2005. Generation of Nano-Particles During Laser Ablation; Risk Assessment of Nano-Beam Hazards During Laser Cleaning. Lasers in the Conservation of Artworks, LACONA VI Proceedings, Vienna 2005. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2006. Laser in The Preservation of Cultural Heritage Principles and Applications. Taylor & Francis. Radke, G. M. The Gates of Paradise, Lorenzo Giberti’s Renaissance Masterpiece, Yale University Press. Sook Koh, Y. 2005. Laser Cleaning as a Conservation Technique for Corroded Metal Artifacts. Lulea University of Technology, Sweden.

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Experimental study on the use of laser cleaning of silver plating layers in Roman coins A.A. Serafetinides, E. Drakaki & I. Zergioti Physics Department, National Technical University of Athens, Athens, Greece

C. Vlachou-Mogire Numismatic Museum of Athens, Athens, Greece

N. Boukos Institute of Materials Science, National Center for Scientific Research “Demokritos”, Paraskevi, Athens, Greece

ABSTRACT: The use of laser technology in the cleaning of artworks has a wide range of applications, including the cleaning of metallic objects. The main aim of this work was to investigate the use of lasers in the cleaning of the thin silver plating layers found in late Roman copper alloy coins. Previous work showed that corroded silvered copper alloy coins required different cleaning conditions than other corroded copper coins. In this paper, comparative cleaning tests by means of a Nd:YAG laser (1064 nm, 532 nm and 266 nm) were performed in order to minimize the thermal effects and to minimize the laser absorption depth to the thin layer of corrosion near the surface. The laser-treated surfaces were characterised using Optical Microscopy and Scanning Electron Microscopy (SEM).

1

INTRODUCTION

The political problems in Late Roman Empire caused significant changes in coin technology. The silver content dropped severely and a new technology was introduced, which was applied in all the mints operating around the Empire. For the production of these coins, copper based quaternary alloys were now used and their surface was covered by silver amalgam plating layer a few microns thick (Vlachou 2004). Hoards of these coins have been recovered across the Empire; however, their treatment has been problematic. Both mechanical and chemical cleaning results in the damage or the complete destruction of the thin silver layer. The use of laser technology in the cleaning of artworks has a wide range of applications which includes the laser cleaning of metallic objects (Kearns et al. 1998, Drakaki et al. 2004). The main aim of this work was to investigate the use of lasers in the cleaning of the thin silver plating layers found in late Roman coins. Previous work has shown that the case of corroded silvered copper alloy coins requires different cleaning conditions than other corroded copper coins. In our case, from the employed laser wavelengths the second

harmonic of Nd:YAG was found to be more effective (Vlachou-Mogire et al. 2007). In this paper, additional work was performed using a wide range of laser wavelengths, in order to minimize the thermal effects during the laser cleaning process, and to minimise the laser absorption depth to the thin layer of corrosion near the surface. The optimum laser parameters were achieved through comparative cleaning tests by employing Nd:YAG (1064 nm, 532 nm and 266 nm) laser pulses, on dry, wet and submerged surfaces, in order to enhance the efficiency and limit the penetration of the laser beam. The application of simultaneous microscopic monitoring during the experimental procedure helped in the successful cleaning of the coin surface. The laser treatment under an optical microscope was shown to be very useful for the in situ control of the cleaning progress. The laser-treated surfaces were characterised using optical microscopy and Scanning Electron Microscopy-SEM. Successful oxide removal was achieved above certain thresholds, which defined the lower end of the process operating window for single-pulse operation.

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Table 1. The alloy contents of the coins according to Cope’s analytical results (data from Cope’s Archive). Coin

Ag

Cu

Sn

Pb

Coin 1 (BM422)∗ Coin 2 (R2)∗ Coin 3 (R3)∗

1.6 2.87 2.95

90.76 91.39

2.79 2.65

3.47 2.66

2

MATERIAL AND METHODS

Three Roman coins from Cope’s Archive were used for the purpose of this research (Cope 1972). Coin 1 was issued in 307 AD in the mint of London and coins 2 and 3 were issued at the time of Licinius, in 315 AD in the mint of Alexandria. The composition of their substrate alloys, according to Cope’s analytical results, is shown in Table 1. For coin 1, Cope had analysed only the silver content but coin BM53, issued by the same mint and in the same year had 1.84% Ag, 5.39% Sn and 6.56% Pb. Their diameters were from 18.5 to 20 mm. Optical microscopy examination revealed that the surface of the coins was covered with a corrosion layer of mainly copper corrosion products. Coins 1, 2 and 3 had almost a uniform corrosion layer and a silver plating layer was observed under or between the corrosion. The metallographic examination of coins dating from the same period as the coins 1, 2 and 3 showed that they consist of quaternary copper alloy, Cu/Sn with Ag and Pb separately segregated at the grain boundaries. The plating was an applied layer with a thickness of 1 µm which in some areas has suffered from corrosion (Vlachou 2004). The composition of their substrate alloys shown in Table 1, exhibits just slight differences in composition, especially in the copper content (∼1%). Copper, the main element of those alloys, corrodes preferentially and forms the corrosion products, and thus this constituent is the one that affects the laser cleaning procedure most. The laser cleaning systems consisted of a Q-switched Nd:YAG laser operating at 266 nm, 532 nm and 1064 nm with pulse duration of 6 ns at maximum repetition rate of 10 Hz and energy fluence, ranging from 0.1 J/cm2 to 7 J/cm2 . The beam was focused through a lens of 50 mm focal length, fixed in position, onto the sample, which was located on an X-Y-Z micro-adjustable stage. The monitoring of the cleaning depth and the damage laser energy thresholds of the silver plating were assessed through various irradiation trials at different test sites. Optical microscopy was applied in order to examine the surface features of the coins and the state of their preservation before the cleaning trials and to evaluate the efficiency of each cleaning treatment after the applied procedure.

Figure 1. Microscopy photos of the untreated (A) surface and (B) the cross section of the corroded coins, with a magnification bar of 2 mm shown at the bottom-right side of the photos.

A scanning electron microscope (FEI Quanta Inspect operated at 25 kV) with an EDAX Genesis ultra-thin film window energy dispersive X-ray microanalyser was used after the cleaning experiments to examine the surface microstructure of the coins and to obtain quantitative analysis of the cleaned areas. The elements, which were detected in the analysis, were Ag, Cu, Sn, Mg, Fe, Pb, Si and Cl.

3

RESULTS

Figures 1A-B illustrate an untreated view of the coins surface which is defect-rich and inhomogeneous, has a porous corrosion product layer, which was formed over the surface of the silver plating in a more loose manner, as reported by bibliography (Saettone et al. 2003). The deterioration effect of time upon the silver plating can be seen in the Figure 1A, where fragments of the silver are evident. Different thickness of the encrustation between the peripheral area of the coin, its centre and the letter area was also noticed. Figure 1B shows the thick and porous corrosion layer, above the silver plating. Cleaning procedure was found to be more difficult than expected due to the inhomogeneity of the corrosive layer. The cleaning of silver plating layers in metal coins is a sensitive task, as treatments applied to the corroded surface of the coin could severely damage or remove the silver layer. Common treatments like mechanical stripping and chemical cleaning agents may be used, but it is difficult to avoid some surface damage, since they are difficult to control and have poor precision [Siano et al, 2001]. These problems have led us to look for other cleaning techniques, more appropriate for our case. Laser cleaning is a high precision method for removing unsightly and potentially damaged layers from extremely fragile surfaces of museum artefacts. This cleaning process can remove the corrosion and the unwanted encrustation without loss of precious

310

Table 2.

Experimental conditions of laser cleaning of coin 1 (Nd: YAG laser at 1064 nm, 1 pulse).

Trials

Fluence J/cm2

Corrosion removal

Silver plating is observed

Condition of the plating layer

Blackening

Melting

1 2 3 4

0.68 0.75 0.82 1.57

some many many many

traces traces traces traces

damaged damaged damaged damaged

no no no no

no no no no

Table 3. Quantitative analysis of the percentage elemental concentration of coin 1, irradiated by 1064 nm with 1 laser pulse, for the above irradiation tests (Table 2). Fluence Ag J/cm2

Cu

Sn

Pb

Cl

Si

Fe

0 0.68 0.75 0.82 1.57

47.63 81.03 80.65 63.23 79.14

4.83 4.88 5.90 8.77 6.41

9.86 3.23 3.65 14.80 3.33

3.86 1.03 1.01 0.76 0.37

12.34 0.65 1.48 0.64 0.53

2.32 0.51 0.55 1.11 0.73

3.61 7.59 6.02 9.35 9.07

material, patina and fine surface detail, therefore the old coating remains intact. The tests were carried out using different wavelengths of a Nd:YAG laser and useful indications of their suitability were deduced. First, the silver plated coin 1 (BM 422) of copper based quaternary alloy was cleaned through several irradiation tests with one laser pulse of the 1064 nm Nd:YAG laser. In Table 2 the experimental conditions of laser treated areas are depicted. SEM-EDAX analysis was used for further quantitative research of the laser treated surfaces. In an uncleaned area of coin 1 a small quantity of Al (∼10%) and traces (up to 2.5%) of Ca, P, and K were observed. On the other hand, copper and tin were observed to increase on the cleaned surfaces by up to 81% and 11% respectively, since they are part of the quaternary alloy of the coin. The SEM-EDAX quantitative analysis of the surface of coin 1 after laser cleaning was found to be efficient in the removal of the corrosion layer (Table 3), but this was accompanied by a major destruction of the silver layer. A representative overview of irradiation tests, of cleaning versus different fluences at 532 nm, with one laser pulse in coin 2 is shown in Table 4. For further quantitative research of the laser cleaning process with the 532 nm wavelength, SEM-EDAX analysis was employed. The cleaning efficiency was defined as the ratio of the quality of the cleaned corroded surface after the irradiation of 1 laser pulse, to the quality of the total corroded surface before laser cleaning.

The cleaning efficiency was expected to increase with an increase of laser fluence, but the results showed an optimum fluence window between 0.98 J/cm2 and 1.10 J/cm2 . Therefore, we focused our attention on three fluence values (0.93 J/cm2 , 0.98 J/cm2 and 1.00 J/cm2 ), at which we obtained a better cleaning behaviour. Quantitative analysis by SEM-EDAX of the surface of coin 2, after laser cleaning in three areas, where the plating layer was in very good condition, showed that the cleaning was successful (Fig. 2, Tables 4, 5). The morphology of coin 2 surface, after the irradiation with the selected fluences, is shown in Figure 2, with a magnification bar (–) of 100 µm shown at the bottom-right side of the photos, while Table 5 presents a quantitative analysis of the element concentration after laser cleaning. According to Table 5, the high concentration of silver is indicative of the presence of the silver layer on the surface, while chlorides corrosion products are minimised. In the same areas the composites of copper alloys are in low concentration. We have also confirmed that the corrosion products are removed successfully, without any damage to the silvering at the applied laser fluences. Silver appears in the range of 68%–83%, while copper is around 10%– 24%. It is shown that at the fluence of 0.93 J/cm2 we have a satisfactory cleaning, but a small percent of the silver layer is revealed, while at 0.98 J/cm2 and at 1.00 J/cm2 the ratio of the Ag/Cu increases, with the presence of chlorides being less at the 0.98 J/cm2 fluence value. Table 6 represents irradiation trials from the laser irradiation process of coin 3, with the 266 nm of Nd:YAG laser, with thirteen laser pulses. From the results, the laser fluence of 0.35 J/cm2 was found to be more satisfactory, in relation with the other fluence effects on that coin. But when we tried to repeat the laser scanning, melting of the surface was observed. From the laser irradiation process of coin 3 with the 266 nm of Nd:YAG we came to the conclusion that 1–12 pulses showed no removal of the corrosion layer. Thirteen pulses and above produced some more satisfactory results, but the removal of the corrosion was accompanied by undesirable side effects.

311

4

Figure 2. SEM photographs of the morphology of coin 2 surfaces cleaned at 532 nm, a) 0.93 J/cm2 , b) 0.98 J/cm2 and c) 1.00 J/cm2 .

DISCUSSION AND CONCLUSIONS

The aim of this paper was to find the optimum laser wavelength and energy fluence conditions for safe cleaning of the plated Roman coins’ surfaces. Damage thresholds were defined from the analytical methods used, where melting, removal, or other damage was estimated and quantified. At the fundamental wavelength of Nd:YAG laser, the energy fluence values, from 0.7 J/cm2 till 1.6 J/cm2 , turned out to be suitable for the removal of the corrosion encrustation. In some cases, where the inhomogeneity of the surface layer was apparent, e.g. the periphery of the coin, experimental results at 1064 nm on coin 1 showed an effective cleaning of the corrosion layer, although in most of the cases the silver layer was diminished, as we can conclude from the SEM-EDAX quantitative analysis. Nevertheless, we can explain this due to the variation of the silver quantity across the silver plated coin. We have also noticed the influence and predominance of the copper under these thin silver layers during the laser cleaning process. The target is affected differently at the three laser wavelengths due to the different surface reflectivities. Indeed, other researchers have reported that the reflectivity of Cu is 0.97 for 1064 nm (Cabalin et al. 1998, Conesa et al. 2004, Lide et al. 1996). Hence, at 1064 nm, only 3% of the laser energy can penetrate into the copper target, resulting in heating, melting and vaporization, and the remaining energy is simply reflected from the surface. On the other hand, the lower reflectance of the inhomogeneous corrosion layer introduced uncontrolled side effects to the cleaning process. Experiments were also performed with more recent silver plated coins and the cleaning results with 1064 nm were more successful. In these coins the encrustation was typically less compact and coherent than the underlying cuprite layer, thus mechanical effects induced by laser irradiation are typically larger on the former, contributing to a higher ablation rate with respect to the latter. In addition the surface specific ablation rate mainly depends on the thickness of the respective corrosion layer and of the underlying silver plating. As a result we noticed that the higher thickness of the more coherent corrosion layer of the thinner silver plated Roman coin dramatically changed the surface ablation rate with that wavelength. At 532 nm laser cleaning, an optimum fluence window between 0.93 J/cm2 and 1.00 J/cm2 was defined. According to Tables 4 and 5 and Figure 2, at those fluences the high concentration of silver is indicative of the presence of the silver layer on the surface. In these areas corrosion products diminished and more homogeneous cleaning was evident. Experimental results at 532 nm had shown us the variability of the corrosion reaction due to the different

312

Table 4.

Experimental conditions of the cleaning of coin 2 (Nd:YAG at 532 nm, 1 laser pulse).

Trials

Fluence J/cm2

Corrosion removal

Condition of the plating layer

Blackening

Melting

1 2 3 4 5 6 7 8 9 10 11 12 13

0.39 0.86 0.90 0.93 0.94 0.98 1.00 1.02 1.10 1.25 1.27 1.34 7.10

good partial partial good good good good partial good good partial good partial

traces damaged good very good very good very good very good damaged good good damaged damaged damaged

a little a little traces traces a little no no a lot traces a little a lot a lot a little

no no no no no no no no no no no no no

Table 5. Quantitative analysis of the percentage elemental concentration of coin 2, irradiated by 532 nm with 1 laser pulse, for the above irradiation tests (Fig. 2). Fluence, J/cm2

Area

Ag

Cu

Sn

Mg

Pb

Cl

Si

Fe

0.93

A B C D A B C D A B C D

72.66 65.31 52.30 82.20 83.72 83.62 86.21 75.62 82.79 85.48 84.20 79.26

18.91 29.20 37.98 11.35 09.16 10.89 06.31 14.69 11.52 10.89 08.38 11.44

2.52 1.58 4.74 1.92 2.55 2.84 1.43 1.17 1.99 0.89 2.11 1.17

0.95 0.55 1.01 1.44 1.37 1.58 1.29 1.16 1.30 1.58 1.7 2.22

3.85 1.95 2.06 2.07 1.63 0.00 2.18 3.52 1.16 0.00 2.18 3.19

0.16 0.31 0.52 0.20 0.31 0.33 0.79 1.34 0.14 0.25 0.19 0.33

0.51 0.59 0.75 0.41 0.51 0.23 0.35 0.96 0.49 0.49 0.69 0.90

0.44 0.52 0.66 0.42 0.75 0.68 0.41 0.47 0.61 0.42 0.55 0.49

0.98

1.00

Table 6.

Experimental conditions of the cleaning of coin 3 (Nd:YAG at 266 nm, 13 laser pulses).

Trials

Fluence J/cm2

Corrosion removal

Silver plating is observed

Condition of the plating layer

Blackening

Melting

1 2 3 4

0.35 0.54 0.82 0.95

little traces traces traces

no no no no

no plating no plating no plating no plating

no little little some

some some a lot a lot

composition and thickness of the irradiated area of coin 2. Other researchers have reported that an average corrosion thickness, on a great number of bronzes from the Bronze Age to historical periods, is in the range 10–250 µm when the alloy core is still present. In most of the cases, the corrosion consists of a well adhering single layer, the appearance of which is variable

depending on the sample and the location of the corrosion on the sample (Robbiola et al. 1998). Although the thermal diffusion lengths of pure Cu and Ag for a pulse duration of 4 ns are approximately 1.7 µm and 2 µm respectively, the co-existence of corrosion products ofAg and Cu in some areas, where the encrustation layer is thinner, increased the thermal diffusion

313

length, and the material removal was less confined on the corrosion surface (Drakaki et al. 2004, Siano et al. 2001). Experiments at 532 nm irradiation with low fluence didn’t reveal any reaction with the surface, while after a large number of laser pulses a blackening of the corrosion area was observed. Below the cleaning threshold a blackening with a faint greyish tinge was observed. A possible explanation for this behaviour below the cleaning threshold is the oxidation of the Cu2 O to CuO at these low fluences (Kearns et al. 1998). Experimental results at 266 nm on coin 3 showed us the unsuitability of that wavelength to clean the corrosion layer. It is known that most metals strongly absorb at ultraviolet (UV) wavelengths, as compared to the infrared (IR) wavelengths. Therefore, irradiation at UV wavelengths might lead to heating of the metal artefacts, which is not appropriate in the case of an artwork of copper alloy with a fragile silver layer. The reflectance of Cu and Ag at that wavelength is much lower (0.33 and 0.24 respectively) than those at the wavelengths of 532 nm and 1064 nm (Lide et al. 1996). As a result, although the irradiation process didn’t reach the melting point of the alloy metals, even in very low fluences partial melting of the corrosion occurs, which caused difficulties in the removal of that layer. The use of pulsed 266 nm laser for cleaning corroded silver-plating metals is thus limited by the risk of surface melting and blackening due to thermal and photochemical effects. From our preliminary experiments we conclude that corroded silvered copper alloy coins require different cleaning conditions than other corroded copper coins. From the three laser wavelengths 266 nm, 532 nm and 1064 nm, which were employed, the second harmonic of Nd:YAG seemed to be more controllable and promising. Comparison between 532 nm and 1064 cleaning procedures, through the quantitative SEMEDAX analysis revealed that the silver layer could remain in a better condition with the second harmonic of Nd:YAG laser cleaning. In addition the much higher ratio of Ag to Cl at that wavelength indicated the success of the cleaning of the corrosion products, without damaging the surface silver layer. One of the main aims of our experiments was the selection of the laser wavelength which results in the protection of the silver plating. The compositions of the coins’ substrate alloys, shown in Table 1, exhibit slight variations in the copper content, which is the main element that affects the laser cleaning procedure. Although these differences are small, they create some limitations for common conclusions for all three coins (the use of three coins was inevitable as the surface of one or even two coins was small to accommodate all experiments described in this article).These limitations arise from the possible variability of the experimental data obtained from more

than one sample, which limits the modelling of the laser metal processing (Bergström 2005). In most cases the corrosion consists of a well adhering single layer, whose appearance is variable depending on the sample and the location of the corrosion on the sample. This limitation is taken into account not only when we treated different coins, but even on the surface of the same coin. Selective removal, which has important implications for improving the control of the cleaning, could be based on the differences in structures and optical behaviour of the materials involved. On top of the previous limitations, post-treatment examination of the treated areas by surface techniques such as scanning electron microscopy (SEM) are commonly used to evaluate the removal efficiency, which brings poor or no information regarding the process dynamics and the good condition of revealed silver layer. This restriction of “the after cleaning” characterisation techniques is especially severe for laser-induced removal since the process is normally induced upon exposure of the surface to many pulses. Future work is planned at different laser wavelengths, in order to minimize the thermal diffusion effects during the laser cleaning process. Therefore, there is a great need to continue the investigation of cleaning techniques that will not only improve the cleaning efficiency of those silver plated artworks but also minimise damage to the critical surfaces of silver during the cleaning process.

ACKNOWLEDGEMENTS This research effort is financially supported by the General Secretariat of Research and Technology of Greece, Joint Research and Technology Programmes, Greek-Italian Cooperation Program 2006– 2008 (and particularly the Project “Characterizations and cleaning of metal artefacts by lasers”). We would like to acknowledge also the Department of Coins and Medals, British Museum for allowing access to their Cope’sArchive. Finally thanks are due to Dr.A.Travlos for his kind cooperation with the SEM measurements.

REFERENCES Bergström D. 2005. The Absorbance of Metallic Alloys to Nd:YAG and Nd:YLF Laser Light, Phd Thesis, Division of Manufacturing Systems Engineering Department of Applied Physics and Mechanical Engineering Luleå University of Technology Luleå, Sweden. Cabalin, L.M., Laserna, J.J. 1998. Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q:switched laser operation, Spectrochim. Acta, Part B: Atom. Spectrosc. 53: 723–730.

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Conesa, S., Palanco, S., Laserna, J.J. 2004. Acoustic and optical emission during laser-induced plasma formation, Spectrochim. Acta, Part B: Atom. Spectrosc. 59: 1395–1401. Cope, L.H.1972. “The metallurgical analysis of Roman imperial silver and aes coinage. In: E.T. Hall and D.M. Metcalf (eds.): “Methods of chemical and metallurgical investigation of ancient coinage”, Royal Numismatic Society, London. Drakaki, E., Karydas, A. G., Klinkenberg, B., Kokkoris, M., Serafetinides, A.A., Stavrou, E., Vlastou, R. & Zarkadas, Ch. 2004. Laser cleaning on Roman coins. Appl. Phys. A, 79: 1111–1115. Kearns, A., Fischer, C., Watkins, K.G., Glasmacher, M., Kheyrandish, H., Brown, A., Steen W.M., & Beahan, P. 1998. Laser removal of oxides from a copper substrate using Q-switched Nd:YAG radiation at 1064 nm, 532 nm and 266 nm. Appl. Surf. Sci, 124–129: 773–780. Lide D.R. & Frederikse H.P.R. (Eds.) 1995–1996. Handbook of Chemistry and Physics, 76th edn., CRC Press, Boca Ratom, FL. Robbiola L., Blengino J.M. and Flaud C. 1998. Morphology and mechanisms of formations of natural

patinas on archaeological Cu-Sn Alloys. Corros. Sci.. 40: 2083–2111. Saettone E.A.O., da Matta1 J.A.S, Alva1 W., Chubaci J.F.O., Fantini M. C. A., Galvao R.M.O., Kiyohara P. and Tabacniks M.H. 2003. Plasma cleaning and analysis of archeological artefacts from Sipan., J. Phys. D:Appl. Phys. 36: 842–848. Siano S. & Salimbeni R. 2001. The gate of paradise: physical optimization of the laser cleaning approach. Studies in Conservation. 46: 269–281. Vlachou C. 2004. The manufacturing and plating technology used in the production of mid-3rd/4th century AD Roman coins – An analytical study, Ph.D thesis, Dep. Arch. Sciences, Chapters 11–12, University of Bradford, Bradford, England. Vlachou-Mogire, C., Drakaki, E., Serafetinides, A. A., Zergioti, I., Boukos, N., 2007. Experimental study on the effect of wavelength and fluence in the laser cleaning of silvering in late Roman coins (Mid 3rd/4th centuryAD). In Peter A. Atanasov, Tanja N. Dreischuh, Sanka V. Gateva, Lubomir M. Kovachev (eds), Proc. SPIE 14th International School of Quantum Electronics “Laser physics and Applications”, 6604: 240–248.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Morphological and colorimetric changes induced by UV laser radiation on metal leaves S. Acquaviva, E. D’Anna & M.L. De Giorgi Department of Physics, University of Salento, Italy

A. Della Patria C.N.R.-I.N.O.A., Arnesano (Lecce), Italy

L. Pezzati C.N.R.-I.N.O.A., Arcetri-Firenze, Italy

ABSTRACT: The morphological and colorimetric changes induced by UV laser radiation (248 nm) on wooden artifacts coated with different metal leaves were studied. For the purpose of investigating the applicability of UV laser treatments on metallic surfaces, a set of samples at different fluences and number of pulses was irradiated. The morphological alterations were evidenced by scanning electron microscopy. The colour data were obtained from the spectral reflection factors taken by an integrating sphere spectrophotometer and then expressed with the CIE L∗ a∗ b∗ cylindrical colour coordinates. Specular included and specular excluded acquisition modes were considered in the interpretation of the results.

1

INTRODUCTION

The study of gilded artifacts is of great interest for conservation purposes. Mechanical and chemical restoration interventions may affect the gilded surfaces in an undesirable and non-reversible way. Commonly, the control of these traditional techniques is up to the conservator’s ability. Laser technology is greatly eligible as a safe procedure for conservation, being controllable and reproducible. Before any laser intervention, a preliminary study is mandatory to avoid surface modification and damage; this is accomplished to establish the best working conditions and to infer the laser parameters (wavelength, fluence and number of pulses) for a safe procedure. Throughout the last decade several researches (Hildenhagen & Dickmann 2003, Pouli et al. 2003, Sansonetti & Realini 2000) have been carried out on the interaction of laser radiation with different artworks and artistic surfaces (varnishes, pigments, etc.). Cleaning of paintings and stones is a field where intense research activities are performed (Castillejo et al. 2002) and procedures are well-established. However, interest in the treatment of wooden artifacts (Panzner et al. 1998, Wiedemann et al. 2000), in

particular of gilded wood (Gaspar et al. 2000), is growing. The main advantage of laser cleaning of gilded wood is in the prevention of damage due to the action of chemical solvents that are generally used in traditional cleaning procedures. In this paper the interaction of laser radiation with gilded surfaces is studied by reflectance spectroscopy. Indeed, it has been shown that it is a powerful tool in support of restoration interventions allowing the characterization of the different techniques of surface gilding (Elias & Menu 2001) and the monitoring of change, in a non-invasive way (Bacci et al. 2003, Dupuis et al. 2002). In particular, we report on systematic investigations of the effect of KrF excimer laser irradiation on a well-defined set of samples, representative of gilded wood objects. Laser-induced chromatic variations were measured through non-destructive colorimetric analyses. Particular attention is required when reflectance spectroscopy is applied to reflecting surfaces, as metallic leaves are. Since sample gloss can significantly affect measurements, spectral reflectance is acquired by taking into account also the specular component. The employed instrumentation was able to discriminate between the specular and diffuse components.

317

2

SAMPLE PREPARATION

Traditionally, artifacts were gilded by using different procedures and materials; the choice was generally influenced by the costs. So various metallic leaves could be employed and the surfaces were subsequently treated with different finishing according to the desired visual effects. We prepared the samples for the irradiation tests with traditional techniques for works on wood. A representative sample was a layered structure consisting of a flat wooden support covered with gypsum and bole layers. On the top thin layers of the gold, pinchbeck (Zn-Cu compound) or aluminium leaves were put down. Then shellac, bitumen and/or mecca∗ were spread over the leaves to provide the desired colour effect. In particular, for each type of metallic leaf, samples coated with shellac and bitumen were investigated; moreover a pinchbeck sample was covered with a single layer of shellac and two aluminium samples were covered with bitumen and shellac together with mecca, respectively. The aim of the laser intervention was the partial or total removal of the surface finishing to restore the original metallic leaves without any damage. 3

EXPERIMENTAL

For the irradiation tests, a KrF excimer pulsed laser (LEXtra 50, Lambda Physik, λ = 248 nm, τ = 20 ns) was used and its beam profile homogenized by an optical system. The areas of interest on the irradiated surfaces were selected through a clear circular (8 mm diameter) aperture. The fluence, F, was tuned with an attenuator, for fixed values of laser energy and spot size, and its uncertainty was of about 5%. Colour analysis was effectuated with a portable commercial Minolta CM-2600d contact spectrophotometer, which recorded the spectral reflection factor (SRF) in order to monitor laser-induced chromatic changes on the sample surfaces. The instrumentation is equipped with a 52 mm diameter integrating sphere and it provides colour data in d/8 geometry. The spectral acquisition comes from a 3 mm diameter circular measurement area and the recording wavelength range is from 360 to 740 nm, with 10 nm spectral steps, both in specular excluded (SPEX) and specular included (SPIN) modes. The distinctiveness of this colorimeter lies in its ability to return SPEX and SPIN colour data in a single run, by simply managing the specularly reflected light via software. Recommendations of the Commission Internationale de l’Eclairage (CIE) 1964 were followed as far as concerns the colour matching functions under the standard illuminant D65 . To ∗

Mecca is an ancient orange transparent varnish for gilding

prevent undesired thermochromism effects, the environmental temperature was kept constant at 22◦ C (Hunt 1998). The instability of the illuminating system and the colorimeter contribution to the statistical fluctuations of colour coordinates were taken into account by recording ten spectra for each investigated area on any sample; every single spectral acquisition was averaged over five illumination flashes. Changes in the SRF spectra were interpreted in the CIE L∗ a∗ b∗ 1976 (Berns 2000, Oleari 2002) colorimetric space, a rectangular coordinate system with L∗ , a∗ , b∗ axes describing the differences in lightness L∗ (ranging from 0-black to 100-white), rednessgreenness a∗ , and yellowness-blueness b∗ (both unbounded), between two points. In this space the metric for the colour differences is the Euclidean distance:

Our results were expressed with the CIE L∗ a∗ b∗ cylindrical coordinates, where the perceptual attributes of chroma, C ∗ , and of hue, h, substitute for a∗ and b∗ in a constant lightness plane. The chroma is an indicator of the colourfulness, so it accounts for the effects of discoloration; chroma difference, C ∗ , between two areas is defined as:

where the subscript r indicates the point taken as reference. The hue angle, h, expressed in degrees, is:

and it ranges from 0◦ to 360◦ . The difference in hue angle is h = h − hr . h is seldom used for hue difference communication; instead, the hue difference H ∗ is more frequently reported and is calculated according to the following formula:

In spite of this, it is our firm belief that it is advisable to report the value of h as well, because undoubtedly it represents a more immediate and intuitive way of determining the direction of a hue shift. Indeed, it is easier to realize which fundamental hue (red, yellow, green or blue) the irradiated area has approached, simply from the knowledge of the sign of h and of the quadrant that contains hr .

318

The relation between h and H ∗ is given by the formula:

H ∗ gives no direct information about the hue shift, except in the case of little colour differences. In this case, the equation (5) turns into a direct proportionality with respect to h:

In this paper the colour coordinates of the bare metallic samples are taken as reference for the chromatic variations induced by laser irradiations on the covered samples. Microscopy observations were carried out by means of a scanning electron microscope (JEOL JSM6480LV operating in low vacuum mode) to assess the morphologic modifications within the irradiated areas. 4

RESULTS AND DISCUSSION

The effects of UV pulsed laser radiation on metallic layers (gold, aluminium and pinchbeck), either bare or with different finishing coats, were investigated by colorimetry. For each irradiated area the contributions of the single coordinates to the colour alterations were inferred. The use of different laser fluences, F, and number of pulses, N , yielded the determination of the laser parameters that minimize the colour differences with respect to the coordinates of the sample taken as reference. The first stage of the study involved the colorimetry of the bare samples to calculate the reference data points for later use. Moreover, the degree of internal colour non-uniformity was evaluated through a colour mapping over sixteen regularly-arranged areas on each sample surface. It represents the average of the colour differences of every single area with respect to the average colour coordinates of the investigated sample. The results in SPIN and SPEX modes are summarized in Table 1. For a correct interpretation of the results with highly reflecting surfaces, it must be pointed out that SPEX and SPIN color coordinates relate to different physical phenomena; while in SPEX mode the light is uniformly illuminating the sample, in SPIN mode the highly Table 1. Degree of internal non-uniformity of the bare samples (E ∗ units) in the two acquisition modes.

SPIN SPEX

Gold

Aluminium

Pinchbeck

0.7 2.1

0.5 11.2

3.7 3.5

directional illumination source strongly affects color results. In the real world completely diffused illumination is not so frequently encountered, so SPEX mode colour coordinates would be inadequate for a correct communication of colour perception. However, the SPIN mode does not fully describe the behavior of metal surfaces, as the colour perception is a function of surface scattering and of the illuminating angle. The high value for the aluminium sample in SPEX mode may be concerned with the instability of the specular light management algorithms when used with almost neutral highly reflecting surfaces. The distance between the colour coordinates in SPEX and SPIN modes, defined as

was also calculated for the same samples; it is an indicator of the altered perception of the colour induced by the reflected specular light. The obtained results of 15.2 ± 0.6, 33 ± 3, 21 ± 1 for gold, aluminium and pinchbeck, respectively, were proportional to the average reflectance of the samples in SPIN mode and they were mainly due to the increased lightness, as detailed in Table 2. Then the colour damage threshold, CDT, defined as the laser fluence value at which a single laser pulse induces a colour difference with respect to a reference area, was evaluated for the bare samples. As a rule, this colour difference is assumed to be significant either above unity or above the sample non-uniformity value, if the latter is higher than one. A CDT value of 35 mJ/cm2 was found for gold (Acquaviva et al. 2007); it is quite low if compared with that at 100 mJ/cm2 for pinchbeck. The upper laser fluence limit, in our experimental configuration, did not allow the evaluation of CDT for aluminium, so it should be considered to exceed 150 mJ/cm2 . Indeed, none of the permitted fluences induced significant colour variation. At fluences higher than CDT, irradiated surfaces presented corrugations, cracks and partial peeling-off of the leaves, as observed by SEM. To assess the laser ability to remove the finishing from the coated metallic leaves, irradiations were performed at F = 110 mJ/cm2 with increasing number of pulses. For each value of N the colour data differences were recorded with respect to the corresponding ∗ Table 2. Components of the difference ESPIN ,SPEX for each bare sample.

L∗

C∗

H∗

Gold 14.4 ± 0.7 4.8 ± 0.3 0.1 ± 0.1 Aluminium 33 ± 3 −0.7 ± 0.2 1.6 ± 0.2 Pinchbeck 20 ± 1 6.5 ± 0.9 0.2 ± 0.3

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h (◦ ) 0.2 ± 0.2 161 ± 4 0.4 ± 0.8

0.60

90 80

0.55

60 0.50

50 f

SRF (%)

70

40

0.45

30 20

0.40

(a)

10

350 400 450 500 550 600 650 700 750

0.35 0

90

400

600

800

1000

N

80 70 SRF (%)

200

Figure 2. f parameter vs laser pulse number for the shellac and bitumen coated gold sample irradiated at 110 mJ/cm2 .

60 50 40 30 20 (b)

10

350 400 450 500 550 600 650 700 750 wavelength (nm)

Figure 1. SRF values of the bare non-irradiated gold sample (solid line) and of the shellac and bitumen coated gold sample irradiated at 110 mJ/cm2 with 1000 pulses (dashed line) in (a) SPIN and (b) SPEX mode.

bare sample. For the shellac and bitumen coated gold 1000 pulses were not sufficient for the removal of the coat since the residual E ∗ was unacceptably high (17.0 ± 0.1, 17.9 ± 0.3 in SPIN and SPEX mode, respectively). SRF plots of the shellac and bitumen coated sample, irradiated with 1000 pulses and of the reference bare gold, both in SPIN (Fig. 1a) and SPEX (Fig. 1b) modes, are reported. The behaviours are rather different because in SPIN mode the residual shellac keeps the SRF values below that of the gold all over the spectrum, while in SPEX mode a reversal occurs above 540 nm. In both cases, the highest contribution to the discrepancy arises from the difficult matching of the chroma of the reference sample. A more complete communication of colour perception is possible giving also the parameter

assumed as an indicator of the gloss amount in the actual sample (subscript s) with respect to another sample chosen as reference (subscript r). In this work f parameter was studied as a function of the number of laser pulses, N , for differently coated metal surfaces

Figure 3. SEM micrograph of the shellac and bitumen coated gold sample; the area on the right side was irradiated with N = 1000 at 110 mJ/cm2 .

and hypotheses were made about the surface alterations, with respect to the corresponding bare samples, at the highest number of laser pulses. Some of these conjectures were later validated by SEM observations. At increasing number of pulses the shellac and bitumen coated gold sample experienced a moderate increase in gloss with respect to the bare nonirradiated sample taken as reference until N = 300, but at N = 1000 its surface was made more diffusive (Fig. 2). Indeed the irradiated area was very irregular in morphology with respect to the surrounding surface (Fig. 3). The effects of the irradiations on the bitumen coated aluminium sample, at the same fluence as before, were quite difficult to interpret. In fact, although the colour coordinates of the aluminium were correctly restored in SPEX mode with N = 100 (residual E ∗ = 2 ± 1) in SPIN mode the minimum discrepancy was poor however. Indeed, residual E ∗ was 4.42 ± 0.08 with

320

1.1 1.0

f

0.9 0.8 0.7 0.6 0.5 0

200

400

600

800

1000

N

Figure 4. f parameter vs laser pulse number for the shellac and bitumen coated aluminium sample irradiated at 110 mJ/cm2 .

Figure 5. SEM graph of the area irradiated with N = 1000 at 110 mJ/cm2 on shellac and bitumen coated aluminium sample. 0.85 0.80 0.75 0.70 f

N = 60, almost equally shared among the three components. Perhaps more than 100 pulses could further lower the just reported values, but the risk of peel-off should be seriously taken into account, as its onset was apparent with N = 160.The f parameter does not imply remarkable modifications of the surface because the associated uncertainties are too large; nonetheless, it is interesting to note that the gloss amount was very close to that of the bare aluminium, regardless of N . The irradiations on the shellac and bitumen coated aluminium sample were effective in SPEX mode starting from N = 500, since they induced a residual E ∗ of 6.4 ± 0.3, even if a minimum residual (E ∗ = 4 ± 3) was recorded at N = 1000. In SPIN mode also at N = 1000 the residual difference is significant (residual E ∗ = 3.95 ± 0.08), as lightness and hue are the most difficult parameters to be matched. The overall effect of the residual finishing on the SRFs was a considerable reduction in the first half of Vis range. Different from the gold equivalent samples, in the case of the aluminium the gloss amount of the notirradiated sample was closely matched with 1000 pulses (Fig. 4); it may be supposed that the sample surface was made increasingly mirror-like with irradiations. Indeed in SEM observations (Fig. 5) the surface seemed very smooth, despite the high number of pulses. The shellac and mecca coated aluminium sample did not match the reference colour coordinates in SPIN mode even with N = 1000; in fact, the residual was found to be E ∗ = 5.72 ± 0.08 (in SPEX mode the agreement was good, namely E ∗ = 5 ± 2). The contribution of the three coordinates to this difference was almost equal. The SRF of the coated sample demonstrated that also in this case a residual shellac layer prevented the complete restoration of the underlying aluminium and

0.65 0.60 0.55 0.50 0

100

200

300

400

500

N

Figure 6. f parameter vs laser pulse number for the shellac and bitumen coated pinchbeck sample irradiated at 110 mJ/cm2 .

no significant changes in gloss occurred after irradiations at F = 110 mJ/cm2 of the shellac and mecca coated aluminium sample. Analogous irradiations were performed on the shellac and on the shellac and bitumen coated pinchbeck samples. The latter was severely peeled at N = 1000 and the residual E ∗ was unacceptably high even at many less pulses in both acquisition modes (E ∗ ≈ 10). The f parameter was increasing with N , slowly approaching the value of the bare pinchbeck (Fig. 6). A closer approach was noticed with the shellac coated pinchbeck sample because the residual colour distance was E ∗ = 4.9 ± 0.7 in SPIN mode and E ∗ = 6 ± 4 in SPEX mode. These discrepancies were mainly attributable to the inability to match more closely the chroma. The gloss parameter value (f = 0.88 ± 0.05), calculated at N = 1000, indicated that the shellac coated sample was a little less glossy than the bare pinchbeck.

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5

CONCLUSIONS

Wooden models coated with different metal leaves were irradiated with excimer laser radiation and the induced morphological and colorimetric modifications were investigated. The morphological alterations and the damage thresholds were evidenced by scanning electron microscopy. In non-destructive colorimetric analyses, performed to measure chromatic variations induced by the laser beam, it is mandatory to consider both the specular and diffuse components to attain a whole comprehension of the colour perception. So SPIN and SPEX modes were considered in the interpretation of the spectra since it is known that particular attention is required when reflectance spectroscopy is applied to metallic surfaces. SFRs and the gloss parameter, taking into account the specular component of the scattered light, allowed the inferring of helpful information about the laser irradiation effects on the models. The d/8 geometry of the spectrophotometer, providing colour data limited to light at 8◦ , did not fully describe the behaviour of metal surfaces. In the future, for a better insight of the laser action onto the gilded samples, a gonio-spectrophotometer will be used to analyse the scattered light at different angles. ACKNOWLEDGEMENTS The authors acknowledge Prof. R. Lerede of Accademia delle Belle Arti (Lecce, Italy) for the model sample preparation and fruitful discussions. REFERENCES

Bacci, M. et al. 2003. Non-invasive spectroscopic measurements on the Il ritratto della figliastra by Giovanni Fattori: identification of pigments and colourimetric analysis. Journal of Cultural Heritage 4: 329–336. Berns, R.S. 2000 Principles of colour technology. New York: John Wiley & Sons. Inc. Castillejo, M. et al. 2002. Analytical study of the chemical and physical changes induced by KrF laser cleaning of tempera paints. Analytical Chemistry. 74: 4662–4671. Dupuis, G. et al. 2002. Pigment identification by fiber-optics diffuse reflectance spectroscopy. Applied Spectroscopy 56: 1329–1336. Elias, M. & Menu, M. 2001. Characterization of surface states on patrimonial works of art. La Revue de MétallurgieCIT/Science et Génie des Matériaux: 777–782. Gaspar, P. et al. 2000. A study of the effect of the wavelength in the Q-switched Nd-YAG laser cleaning of gilded wood. Journal of Cultural Heritage 1: 133–144. Hildenhagen, J. & Dickmann, K. 2003. Nd:YAG laser with wavelengths from IR to UV (ω, 2ω, 3ω, 4ω) and corresponding applications in conservation of various artworks. Journal of Cultural Heritage 4: S174–S178. Hunt, R.W.G. 1998 Measuring Colour. London: Fountain Press. Oleari, C. 2002. Misurare il colore. Spettrofotometria, fotometria e colorimetria. Fisiologia e percezione. Milano: Hoepli. Panzner, M. et al. 1998. Experimental investigation of the laser ablation process on wood surfaces. Applied Surface Science 127–129: 787–792. Pouli, P. et al. 2003. Studies towards a thorough understanding of the laser-induced discoloration mechanism of medieval pigments. Journal of Cultural Heritage 4: S271–S275. Sansonetti, A. & Realini, M. 2000. Nd:YAG laser effects on inorganic pigments.Journal of Cultural Heritage 1: S189–S198. Wiedemann, G. et al. 2000. Laser cleaning applied in the restoration of a medieval wooden panel chamber at Pirna.Journal of Cultural Heritage 1: S247–S258.

Acquaviva, S. et al. 2007. Laser cleaning of gilded wood: a comparative study of colour variations induced by irradiation at different wavelengths. Applied Surface Science 253: 7715–7718.

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Application of Ion Beam Analysis (IBA) techniques for the assessment of laser cleaning on gilded copper M. Barrera & C. Escudero Centro de Conservación y Restauración de Bienes Culturales, Junta de Castilla y León, Simancas, Valladolid, Spain

M.D. Ynsa & A. Climent-Font Centro de Micro-Análisis de Materiales (CMAM), Universidad Autónoma de Madrid, Madrid, Spain

ABSTRACT: The aim of this research has been to perform a complementary study using several non destructive analytical techniques such as PIXE (Proton Induced X-ray Emission) and RBS (Rutherford Backscattering Spectrometry) along with SEM (Scanning Electron Microscopy) in order to characterize materials, manufacture techniques and corrosion compounds and to check the suitable cleaning in a cross made of gilded copper belonging to Vado-Cervera church (Palencia, Spain). This research includes a comparative study of traditional cleaning methods with laser cleaning. The results show that Q-switched Nd:YAG laser a 1064 nm, at the fluence of 0.130 J/cm2 and with a pulse duration of 6 ns is not satisfactory for our purposes because of a increasing the diffusion of the surface gold towards the support due to the thermal effect generated by transient heating of the metal substrate, which occurs during the irradiation of cleaned areas.

1

INTRODUCTION

The cross of Vado-Cervera (Palencia, Spain) is a magnificent example of gilt work typical of the 14th century, prior to the later development of much more ostentatious works in silver (Fig. 1). The materials employed (gilded copper with small insertions of enamel) were processed by means of diverse techniques, stamped, beaten and chiselled, with the inclusion of small figures in gilded bronze obtained by moulding and smelting processes. The most important problems of conservation of this object are those related to previous repair jobs which are most serious in the case of the macolla (the support in which the cross is inserted), both structural and as a result of the dirt deposited on the surface. Thus, we have verified how the cross itself showed a coating composed principally of a greasy product (aged siccative oil) applied as a protection, together with copper oxides and remains of old abrasive cleaning products, all of which were easy to eliminate. The problem with the macolla (Fig. 2) was especially dramatic since it was completely covered by a thick, homogeneous layer of black smoke, deposited during an inadequate soldering procedure which was carried out to repair the separation of the two hemispherical pieces of which it is composed.

The soldering material employed is extra-hard, made from brass, with a melting point of over 800◦ C, which produced gases and smoke during the process leading to darkening of the gilded surface. The elimination of the mentioned coating by traditional restoration procedures was slow and difficult, thus we proceeded to perform a comparative study between the results, and possible secondary effects, obtained with a chemical process versus a laser cleaning procedure. Previous experiments with regard to the application of laser cleaning of metals have developed different lines of work which evaluate the possible effects of different wavelengths using a diverse types of lasers (Nd:YAG, CO2 , Er:YAG) (Cottam et al. 1997, Pini et al. 1988). The most promising ones are those which have focussed on the effects derived from the duration of the pulse (Siano 2001, Burmester et al. 2003). Almost all these works have in common their special attention to the surface and the effects of the laser on it whether they deal with patinas or decorative surfaces, such as applied gilts (Siano et al. 2001). In our case, and taking into account the consideration of the variety of metals composing the cross under study, it appeared to us important not to focus so much on the more external aspects of the gilt layer as on the evaluation of the interface between the metal

323

Figure 2. Macolla (cross support) and cleaning tests: a) area chemically cleaned, b) area untreated and c) area laser cleaned.

Figure 1. Gilded copper cross of Vado-Cervera.

support (copper) and the metal supported (gold). This requires an analytical technique such as RBS, which allows a detailed study of the thickness of the gilt layers, necessary for carrying out the restoration of this type of objects with confidence, in general terms, and the possible modifications caused by the laser, in particular. The RBS study was backed up by the (PIXE) technique. Three cleaning tests were carried out on areas with dimensions of 2 cm2 : 1) Area laser cleaned using a Nd:YAG laser with an optical arm emitting at 1064 nm, 20 Hz, fluence of 0.130 J/cm2 and pulse duration of 6 ns (Fig. 2c). 2) Area chemically cleaned using: H3 PO4 (10% vol/vol) + H2 O2 (5 vol) + tensoactive + methylcellulose as a gelificant) (Fig. 2a). 3) Untreated area (Fig. 2b).

2

EXPERIMENTAL

Ion beam analysis (IBA) techniques are based on an analysis of the photons and particles which are generated when an ion beam is made to fall on the

sample. In RBS the number and energy of the backscattered particles are analyzed. These depend on the composition and structure of the layers, obtaining a deep distribution of elements at each point irradiated. The PIXE technique represents an alternative to the commonly used SEM/EDX, for the detection and processing of the X-ray spectra. The composition of the sample is obtained from the X-rays emitted by the sample during irradiation. The different cleaning tests were irradiated in air using an external microprobe line. A beam of 3 MeV protons focused in the sample position to a diameter of less than 60 µm was used. The incident proton beam was normal to the sample surface and the beam current was in the range of 100–150 pA.The beam was extracted from the vacuum line through a 200 nm thick Si3 N4 window to minimize lateral straggling. The samples were placed at a distance of 4 mm from the window in a He atmosphere in order to minimize the energy loss of the beam and the lateral straggling. Two Gresham Si(Li) detectors, placed at 45◦ and −45◦ to the beam direction were used to detect X-rays (PIXE). One of these detectors, with an active area of 80 mm2 , a 25 µm thick Be window and a Mylar filter (350 µm in thickness), were used for the detection of medium and heavy elements. The other Si(Li) detector with an active area of 10 mm2 and a 12 µm thick Be window and provided with helium gas flow through a nozzle, was used for the detection of elements light emission. For RBS analysis, backscattered protons were detected using a surface barrier blind detector with an active area of 50 mm2 located at

324

an angle of 40◦ below the beam in Cornell geometry. This detector was also equipped with a nozzle to create a He atmosphere between the detector and the sample. All these signals were collected simultaneously using the Genie gathering system. The RBS spectra determine the structure of the layers of the different areas analyzed. These spectra were adjusted using the SIMNRA program (Mayer 1997). The quantitative analysis of the PIXE spectra was carried out with the GUPIX program (Campbell 2000) using the information previously obtained from the RBS analysis. The quantitative results were tested by using standard reference materials. 3

METHODOLOGY

Specific analyses were performed both on the cross, which serves as the reference element, as well as on the macolla: four points in an upper corner of the cross, two points in the soldering material of the macolla, three points in the area cleaned by laser and one point in the area cleaned chemically. As already noted, the PIXE and RBS techniques were performed simultaneously. The calculation of PIXE concentrations by means of GUPIX is rather complicated when the sample is constituted with various layers. The principal limitation is that the same element can not be found in different layers in order to obtain an unequivocal result of the quantification. In the present case, we have presumed that the different samples (cross and macolla) were composed by three layers. The first and external one is a thin layer formed of dirt and oxides, the second, is composed of Ag, Au and Hg (gilded stratum) and finally the third layer is composed of Cu, Fe, Zn and Pb. This hypothesis does not take into account the diffusion phenomena observed by RBS, but it was necessary given the limitation of GUPIX method. 4

RESULTS AND DISCUSSION

The evaluation of the PIXE analysis gives us the following data regarding the composition of the objects. The cross (the reference element) is basically composed of copper with slight impurities of Fe, Zn and Pb. The gilt layer shows a composition of Au (83.3 ± 0.3%), Ag (2.0 ± 0.3%) and Hg (15.8 ± 0.4%) (Fig. 3). The composition of the macolla is similar to the corresponding to the cross, with slightly different results due to the repair actions and its liturgical use. The external black layer is mainly composed of smoke. The analysis of soldering material of the macolla shows a high concentration of Cu and Zn, and the presence of Sn and Pb in minor concentration. On the cross, using the RBS technique, it was observed a slight diffusion of the gilt layer towards the copper substrate. The extent of diffusion changes

Figure 3. RBS spectrum of the cross.

clearly when the concentration of Au is less than 50% (Fig. 3). The gilded surface layer is shown to be relatively homogeneous with a thickness varying from 3 to 7 µm on the obverse side and less than 1.2 to 3 µm on the reverse one. Using PIXE and RBS techniques for the analysis of different areas of the macolla, the area laser cleaned (area1), the area chemically cleaned (area 2) and the untreated area (area 3), we can draw the following conclusions: The area 1 is composed mainly of Au although its quantity decreases slowly as we penetrate deeply into the sample. In contrast, the quantity of Cu increases without observing a perfectly defined interface. The diffusion of gold is therefore gradual (Fig. 4). For the area 2, the spectra are similar to those obtained for the cross without blackening (reference situation). The interface between the substrate and the gilt zone can be observed (Fig. 5). In the area 3, the two layer structure is maintained although the gilt surface is thinner as it corresponds to a zone with considerable wear. The presence in the PIXE analyses of C (carbon) together with Au is due to the presence of carbon black caused by the possible proximity of a heat source (the flame of a blowtorch). This is the cause of the blackening of the macolla in this place (Fig. 6). At this time, we should point out some theoretical considerations to explain this behaviour. In the first place, one has to take into account the basic mechanisms involved in the laser cleaning process in view of two parameters: the fluence (F) and the laser pulse duration (t). On the other hand, we have to remember that the support is a metallic surface (in this case gold) with a high reflectivity. This reflectivity decreases because of the incrustations of carbon black particles giving a greater optical absorption. Thus, at first sight, the Q-switched Nd:YAG laser emitting at 1064 nm represents a good choice. However, the pulse duration used (6 ns) is not the ideal one for allowing a rapid elimination of the dirt by avoiding heat conduction towards the underlying gold layer.

325

Figure 6. RBS spectrum of the untreated area.

Estimates carried out by certain authors assign values close to a 4 µm thickness for the diffusion process due to the effects of the heat from the thermal conductivity values of the gold. In accordance with these results, the heating would be confined, in our case, to the gold layer. There are some factors which have prevented the success of the cleaning performed by the Nd:YAG laser on the macolla. One of these factors is the limited pulse duration (fixed in 6 ns). The thermal effects associated with the laser irradiation increase the diffusion process of the gold to the copper substrate and the consequence of this effect was the progressive disappearance of the interface between both metals when the laser cleaning was performed.

Figure 4. RBS spectrum of the laser cleaned area.

Figure 5. RBS spectrum of the area chemically cleaned.

5

This is reflected in the well known equation (Internet site I) which expresses the transitory increase in the surface temperature:

Independently of the well-known and studied microfusion processes on gilded surfaces, the RBS technique was shown, in this case, to be an excellent tool for a detailed knowledge of the stratigraphic morphology of the surface. This knowledge has allowed an additional study of the non-surface process developed after the use of a Nd:YAG laser at 1064 nm, with fluence of 0.130 J/cm2 and a pulse duration of 6 ns for the cleaning of a gilded copper object. The non-desired effects are associated with the transitory temperature increase, the diffusion of the gold surface towards the interior with the consequent disappearance of the copper-gold interface. The unfavorable results obtained for laser cleaning using this type of laser (Nd:YAG at 1064 nm, 6 ns pulse duration) suggest the need to use lasers with longer pulse duration to minimize the associated transitory thermal effect.

where F = fluence; t = pulse duration; K = thermal conductivity; and D = gold diffusivity D = K/ρCp with ρ = density and Cp = specific heat of the gold. According to the values assigned to these parameters (Lide 2001) and to the working conditions, a fluence of 0.130 J/cm2 and a pulse duration of 6 ns, a value of 522◦ C for the maximum temperature peak is obtained. The equation (1) establishes an inversely proportional dependence between the temperature and the pulse duration which means that the surface temperature rises when the pulse duration is shorter. The Gaussian profile of the laser pulse and its relation with the temperature reached in the gold-air interface are in good agreement with those obtained by Siano et al. (2003). The thermal decrease on the surface is estimated to reach about 300◦ C when the pulse duration is increased to 100 ns. Satisfactory experiments have been performed using lasers with pulse duration of 28 ns (Siano et al. 2001).

CONCLUSIONS

REFERENCES Burmester, T. et al. 2003. Femtosecond Laser Cleaning of Metallic Cultural Heritage and Antique Artworks, Proceedings of LACONA V, 61–70.

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Campbell, J.L. et al. 2000. Nuclear Instrumental and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 170: 193–204. Cottam, C.A. et al. 1997. Laser Cleaning of Metals at Infrared Wavelengths, Proceedings of LACONA I, 95–98. Internet site I, Siano, S. Principles of laser cleaning, in www.science4heritage.org/COSTG7/booklet/chapters/ prin_cle.htm. Lide, D.R. 2001. Handbook of Chemistry and Physics.Boca Raton, Florida 33431: CRC Press LLC. Mayer, M. 1997. SIMRA User Guide Technical Report. MaxPlanck Institut Fur Plasmaphysik. Garching, Germany IPP 9/113.

Pini, R. et al. 1988. Tecniche Laser per il Restaur dei Metalli Archeolotgici, Kermes, 35: 14–17. Siano, S., & Salimbeni, R. 2001. The gate of Paradise: physical optimization of the laser cleaning approach. Studies in Conservation, 46: 269–282. Siano, S., & Salimbeni, R. 2003. Optimised laser cleaning of gilded surfaces. Conference on Laser Ablation, Heraklion, Crete 2003.

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Laser cleaning: Influence of laser beam characteristics M. Pires INETI – Instituto Nacional de Engenharia, Tecnologia e Inovação, Lisboa, Portugal

C. Curran, W. Perrie & K. Watkins Laser Group, University of Liverpool, United Kingdom

ABSTRACT: Since the beginning of the use of laser radiation for the cleaning of deteriorated surfaces, by John Asmus in 1972, it was understood that the characteristics of the incident laser were determinant not only to the mechanisms associated with the cleaning action but also for the quality and efficiency of the intervention. In the present work precious metal surfaces (silver and gold), affected by tarnishing, industrial oil contamination or overpainting were treated by laser irradiation with different amount of overlapping, from infrared to ultraviolet wavelength, and micro, nano and picoseconds duration. The improvement of quality and/or efficiency of laser cleaning technique applied to the surface of precious metals was analysed through the comparison of the results experimentally obtained by the use of beam shaping and adjustment of beam scanning parameters, as well as the choice of laser wavelength and pulse duration.

1

2

INTRODUCTION

Since the beginning of the use of laser radiation for the cleaning of deteriorated Cultural Heritage surfaces, by John Asmus (1972), it was understood that the characteristics of the incident laser were determinant not only to the mechanisms associated with the cleaning action (Asmus 1978) but also for the quality and efficiency of the intervention (Asmus 1986). However, laser cleaning of precious metals is a very delicate task, because melting occurs at low laser fluences. The shining surface, characteristic of polished precious metals due to its high reflectivity in the visible spectrum, will become dull (Degrigny 2003, Lee 2003) if microscopic melt pools are created in the surface. In order to preserve surface optical properties after laser cleaning, a careful selection of laser beam characteristics must be done, behind the obvious need of selecting the adequate laser fluence. Through an extensive experimental work, using diverse laser sources with specific laser beam characteristics, and different analytical techniques to assess the cleaning state and optical parameters of the cleaned surfaces, the improvement of laser technique was achieved. In this paper a few examples of laser cleaning of precious metal surfaces, illustrative of the influence of the laser beam characteristics on the final cleaning result are presented, although not aiming to present a detailed study on the experimental work done to study the laser cleaning of precious metals.

LASER WAVELENGTH

It has been often referred the relevance of the wavelength of the incident laser radiation in the efficiency of a laser cleaning technique. In a first approach, the wavelength will determine if contamination or underlying substrate will be directly affected by the radiation, depending on the optical absorption length or optical thickness of contamination material(s) to the specific wavelength of the incoming laser radiation. In a deeper insight, the wavelength of the incoming laser radiation will also determine which chemical and physical laser cleaning mechanism will take place, depending if the photon’s energy matches or not the intramolecular energy bonds of the absorbing material. In the first case, mostly with UV radiation, a predominant mechanism of photo-chemical dissociation, also called cold ablation will take place; if the energy of the photon is not enough to break the chemical bonds, as with IR lasers, the energy will be transformed in thermal energy and localized heating. Yet, depending on the absorbance and absorption coefficient of the material, the local temperature increase can be either abrupt, generating processes like explosive vaporization, plasma formation and shock waves within the material or volume distributed and less intense, generating transient heating and stress waves, leading to thermoelastic, also called cold spallation cleaning mechanisms. The cleaning of silver surfaces, contaminated with industrial oil in the production plant, was studied

329

Figure 1. Silver surface SEM and EDX analysis showing contaminated area (arrow), before laser irradiation.

Figure 4. FTIR analysis of the industrial oil contamination.

Figure 2. Silver surface SEM and EDX analysis showing contamination (arrow) with industrial oil, after irradiation with 0.35 J/cm2 at 532 nm.

Figure 3. Silver surface contaminated with industrial oil, after irradiation with 84 mJ/cm2 at 355 nm.

experimentally by scanning a Q-switched Nd:YAG laser beam (7 ns pulse duration) at 1 Hz, quasi perpendicular to the contaminated surface. This contamination was analysed by FTIR (Fourier Transform Infrared Spectroscopy), indicating the presence of an aliphatic oil. The contamination was irregularly spread in the surface of the different samples, with variable thickness (submicron to 5 µm thicknesses) and different shapes such as droplets, isolated areas with irregular contour, or else forming stripes on the surface, but again with irregular thickness across the stripe, as can be observed in Figures 1 and 2. The thin layer of oil was diffusive transparent (Fig. 1), allowing the transmission of the fundamental (1064 nm) and 2nd harmonic radiation (532 nm) of a Q-switched Nd:YAG laser. At these wavelengths, the Ag substrate, not polished, absorbed enough radiation to cause temperature increase, to heat and decompose (carbonization) the oil, but without meaningful removal of the contamination. Figure 2 shows a Scanning Electron Microscope (SEM) image of the silver surface after irradiation

with 0.35 J/cm2 at 532 nm, where the presence of the organic contamination can still bee seen, as it is also indicated by the large peaks of carbon (C) and oxygen (O) present in the Energy Dispersive X-ray Spectroscopy (EDX) analysis performed with a Philips XL30 FEG, equipped with a EDAX X-ray spectrometer. Moreover, several pores on the greasy area can be observed (Fig. 2), indicating the escaping of volatile products of the oil chemical decomposition. This chemical deterioration of the oil was further analysed and corroborated by FTIR. When using the 3rd harmonic of the same laser, emitting in the UV (355 nm) to scan the surface with much lower energy density (84 mJ/cm2 ), the laser removed almost completely the organic contamination layer, as shown in Figure 3 by the EDX spectra obtained in the area irradiated area. The increase of reflectance and colour measurement on the contaminated and laser irradiated areas, with a Avamouse spectrophotometer (Avantes) using the CIE L∗ a∗ b∗ system are in agreement with this conclusion. Figure 5 shows the results of colour measurements performed in the contaminated area and in the laser irradiated area of the silver surface. As can be seen in the a∗ b∗ colour plane (on the left) or on the numerical colour data (on the right side of the images), the decrease in the yellow (b) component of the surface colour before laser irradiation (b = 10.78) and after laser cleaning (b = 3.19) indicates the removal, although not complete, of the yellowish grease contamination.

3

LASER PULSE DURATION

Experimental research work on laser cleaning of tarnished silver coins was also done using several near IR pulsed laser sources with different pulse regimes. Among the systems used are Nd:YAG laser at 1064 nm, short free running (pulse length 20 ms), long Q-switch (70 ns), Q-switched (7 ns) and a Fibre Laser

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Figure 6. Silver coin and SEM image of the surface (up) with elemental EDX analysis of substrate and dark grains (bottom).

Figure 5. Colorimetry of contaminated area (top) and laser irradiated area (bottom), showing a clear decrease in the yellow content after laser cleaning.

(pulse length 200 fs), with a central wavelength at 775 nm and repetition rate PRR = 1 KHz. The silver surfaces were covered by a very thin brownish layer of silver sulphide (Ag2 S), with less than 1 µm thickness; the surface presented also many embedded grains of copper, possibly due to precipitation from the alloy (Fig. 6). Irradiated with the longer Nd:YAG laser pulses no alterations, colour or other modifications were observed in the tarnished surface. There was no observable vaporization and the transient thermal effect was insufficient to cause any effect (such as cold spallation), probably due to the high thermal conductivity of the silver substrate and the long interaction time. When using the IR Nd:YAG Q-switched laser, with few nanosecond pulse duration, the laser radiation was mainly absorbed at the substrate, due to the small thickness of the tarnish layer. The optical radiation absorption, in a short time length, caused a fast and localized temperature increase originating superficial

Figure 7. Analysis of laser irradiated tarnished surface (0.5 J/cm2 at 1064 nm): optical micrograph, SEM and EDX images.

melt pools without removing the tarnished layer, as can be seen in Figure 7. The use of the IR femtosecond laser radiation on the tarnished silver surfaces allowed for the complete removal of the contamination layer with submicron precision, applying laser pulses with energy of few micro joule (2 to 10 µJ for the tested samples) as can be observed in Figure 8. However, laser cleaning within this pulselength regime is not self-limited and successive scanning or overlapping of laser spot on the surface can lead to the removal of substrate material as observed by interferometric surface profilometry (WYCO NT

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a

Figure 8. SEM images of tarnished silver cleaned with femtosecond IR laser pulses of 2 µJ (left) and 5 µJ (right).

b

Figure 10. Energy distribution: a) raw beam; b) laser beam shaped by diffusive homogenizer and positive lens.

a

b

Figure 11. Beam Print: a) on Xeroxed paper; b) on silver.

5

Figure 9. Surface profile of tarnished silver cleaned with femtosecond IR laser pulses of 2 µJ (up) and 10 µJ (bottom).

3300), increasing the surface micro roughness and thus decreasing the reflectance (Fig. 9). 4

ENERGY TRANSVERSE DISTRIBUTION

The geometric and energetic characterization of the laser beam was obtained with a CCD camera and Spiricon software, according the international standards ISO 11146 (1999) and ISO 11554 (1998) respectively. As can be seen in Figure 10a, the energy distribution in the cross section of the laser beam, although very common in solid-state lasers is neither gaussian nor uniform (usually called top hat). This irregular energy distribution or other forms of “hot spots” present in the laser beam can affect or damage partially sensible surfaces. Figure 11 shows the surface of Xeroxed paper and silver, where the beam print is clearly visible over the irradiated area; therefore it was necessary to shape the beam, what was done using a commercial transparent diffusive plate, allowing for a much more regular energy distribution, as shown in Figure 10b.

SPOT SHAPE AND SCANNING OVERLAP

When cleaning by laser large flat areas or uniform contamination layer on a plane substrate, the cleaning process may be controlled by a scanning system moving the incidence point of the laser beam relative to the surface. The sequence of small laser affected areas (irradiated spots) on the surface, originated by successive laser pulses, should completely cover in a regular way the area to be cleaned. Therefore, the laser-irradiated spots should be adjacent or overlapping (Fig. 12). However, most of the used laser beams have a circular or elliptical transversal section that provides different amounts of overlapping in parallel directions to the scanning direction. In order to obtain a more uniform coverage of the surface than the one obtained with a circular laser spot, the beam was shaped approximately rectangular, using a cylindrical lens and a spatial filter. The amount of overlap can be expressed as:

being PRR the pulse repetition rate or frequency, dσ the beam diameter on the irradiated surface and vscan the speed of the relative displacement of the laser spot on the surface. Figure 13a shows the surface of a silver blank coin irradiated with the 2nd harmonic radiation (532 nm) of a Q-switched Nd:YAG laser, using a circular shape laser scanned over the grease contaminated surface with an overlap OL = 8.

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a

However this was only possible for laser irradiation with short wavelength (UV) or short pulse length (femtosecond). Longer wavelengths and pulse duration were inefficient or damaged the surface. Laser beam shaping was also analysed and implemented in order to obtain more uniform, better quality and faster rates for surface cleaning of precious metals.

b

Figure 12. Areas irradiated by laser scanning: a) adjacent spots (OL = 1), b) overlapping spots (OL = 5).

7 ACKNOWLEDGEMENTS

a

b

Figure 13. Areas irradiated by laser scanning (different scales): a) circular spots (OL = 8), b) rectangular spots (OL = 5).

The more uniform effect in Figure 13b) was obtained with the UV radiation (355 nm) of a Q-switched Nd:YAG laser, using a rectangular shape laser spot scanned over a similar surface with an overlap OL = 8. The tree strips on the blank surface, were irradiated with UV laser pulses of few milijoule, producing incomplete cleaning (top), good cleaning (middle) and over cleaned strips. Besides the increased quality of the laser cleaned area due to the uniformity, also an increase of efficiency is obtained due to the faster scanning of the surface. 6

CONCLUSIONS

Laser cleaning of gold and silver surfaces with quasitransparent contamination films was achieved without meaningful alteration of surface characteristics.

The authors would like to thank the support of the British Royal Mint for funding this work. We also acknowledge the kind hosting of Dr. R. Salimbeni and S. Siano (IFAC, Italy), concerning the use of short free running and long Q-switch Nd:YAG lasers, and Dr. Martin Sharp (Lairdside Laser Centre) concerning the use of the femtosecond pulsed laser. The last but not the least, we would like to thank the contribution of A. Geraldes (INETI) in the work related with the Scanning Electron Microscope. REFERENCES Asmus, J. F. 1978. Light Cleaning: Laser Technology for Surface Preparation in the Arts. Technology and Conservation, 14–18. Asmus, J. F. 1986. Light for Art Conservation. IEEE Circuits & Devices Mag., 6–13. Degrigny, C. et al. 2000. Laser Cleaning of Tarnished Silver and Copper Threads on Museum Textiles. Journal of Cultural Heritage, 4: 152s–156s. Lazzarini, L., Asmus, J. F. & Marchesini, M. L. 1972. Lasers for the Cleaning of Statuary-Initial Results and Potentialities, Proc. 1st Int. Symp. Deterioration of Building Stone, La Rochelle, 89–94. Lee, J-M et al. 2000. Experimental Study of the Effect of Wavelength in the Laser Cleaning of Silver Threads. Journal of Cultural Heritage, 4: 157s-161s.

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Laser Cleaning of Documents and Textiles

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Study of laser cleaning of ancient fabric with femtosecond pulses C. Escudero & M. A. Martínez Centre for Conservation and Restoration of the Regional Government of Castilla y León, Valladolid, Spain

P. Moreno, A. García & C. Méndez Laser Facility, University of Salamanca, Spain

C. Prieto & A. Sanz Department of Condensed Matter Physics, Crystallography and Mineralogy, Faculty of Science, University of Valladolid, Spain

ABSTRACT: In the last decade, some works concerning the application of nanosecond pulsed lasers to the cleaning of fabrics have shown that chemical changes as well as a slight decrease in some of the resistance properties of the material take place as a result of photothermal processes. Ultrashort laser pulses are emerging as an alternative tool for cleaning historical artefacts provided the far different mechanism of interaction with matter as compared with nanosecond pulses which allows selective removal and minimizes the collateral effects of laser cleaning on the surrounding material. We have carried out cleaning tests on historical linen fabrics by means of infrared subpicosecond pulses (120 fs at 795 nm) from a Ti:sapphire source at 1 kHz repetition rate. The cleaned areas were analysed by optical and scanning electron microscopy, Fourier Transform Raman spectroscopy and colorimetry. The results show that chemical changes are negligible whereas the window of laser fluences for cleaning is small. Too high fluences give rise to partial fibre ablation which affects mechanical properties and should be avoided.

1

INTRODUCTION

During the last years, pulsed lasers with durations from 6 to 10 ns and wavelengths 1064 and 532 nm have been applied to fabric cleaning, fundamentally silk, linen and cotton (Bloisi et al 2004, Belli et al 2006). For these laser parameters, dirt removal is achieved by means of photothermal mechanisms (Sutcliffe et al. 2000). The effects of cleaning on the fibres have been analysed to shed light on three different features: changes in the fibre structure; chemical modifications and variations in the mechanical resistance properties. The results of former studies establish that nanosecond laser cleaning gives rise to slight chemical modifications and decline of mechanical resistance properties associated with laser energy absorption and subsequent heat diffusion within the material that it is not removed by laser pulses (Kennedy et al 2005). These collateral effects are inadmissible in the field of heritage, given the importance and unique nature of the cultural artifacts. (Martinez & Escudero in press., Lerber 2004, Lerber et al. 2005, Strlic et al. 2003, Sutcliffe et al. 2000) Ultrashort laser pulses (<1 ps) have recently proven to be an outstanding tool for removing thin layers (up to

some tens nanometers) from the surface of bulk materials with great accuracy and minimizing the collateral effects mentioned above (Bäuerle 2000; see contributions in RIKEN 2003). The key fact lies on the different mechanisms that give rise to ablation. For nanosecond and longer pulses, the material absorbs energy from the laser pulses depending linearly on the wavelength and the energy pulse itself. This leads to a temperature increase within a limited area of the material surface that may reach the melting and even vaporization point leading to material removal. Nevertheless, as a result of heat transfer processes, much of the energy is lost by heat conduction across the material what leads to inefficiency as well as to non desired phase transformations and mechanical effects. In contrast, ultrafast pulses are absorbed via non linear processes that depend strongly on the laser intensity. So far, a thin layer on the surface becomes rapidly ionized within the duration of a pulse generating an electronic dense plasma that is able to break down the bonds between ions and therefore remove the layer (Chichkov et al 1997; Gamaly et al 2002). For what it is worth, the process is extremely precise since only the material which is irradiated with intensities beyond the threshold is removed. In addition, the process takes

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place mostly during the pulse duration, so that the amount of energy that will be lost through thermal mechanisms is negligible. This prevents remarkable thermally induced processes like mechanical stress or phase transformations on the remaining material. On the other hand, this ablation process is not very selective regarding the materials. Non linear absorption depends also on the laser wavelength but this dependence is not as strong as intensity dependence. Any material is susceptible to be ablated by ultrashort pulses and the threshold is not that different. This is a potential drawback for the use of ultrashort pulses in heritage cleaning. Our goal is to check the possibilities and disadvantages of ultrashort laser cleaning of historical textiles focusing on the collateral effects on the fibres. 2 2.1

EXPERIMENTAL Samples

Two linen fabrics were selected to carry out the study: LINEN 1 from the lining of the flag of the “Comuneros” from the Cathedral in Salamanca (repaired at the end of 18th century) and LINEN 2, reinforcement of a plan of the tower of the Cathedral in Salamanca (repaired in the second half of 18th century). Scanning electron microscopy together with energy dispersive X-ray analysis shows that the surface stratum is composed of pollution elements and atmospheric dust (Si, Mg, K, Al, Fe). The presence of Cl and Ca (greater in the case of LINEN 2) is linked to the manufacturing process of the fabric. 2.2

Figure 1. Samples LINEN 1 & 2 processed with 5 different fluences (4.27, 2.56, 1.70, 1.33 and 0.86 J/cm2 ) and 3 different scanning speeds (130, 390 and 650 µm/s).

Laser processing of the samples

Laser ablation was carried out in air using a commercial Ti: sapphire oscillator (Tsunami, Spectra Physics) and a regenerative amplifier system (Spitfire, Spectra Physics) based on the chirped pulse amplification (CPA) technique. The system produces linearly polarized 120-fs pulses at 795 nm with a repetition rate of 1 kHz. The pulse energy can reach a maximum of 1.1 mJ and is controlled by means of neutral density filters and measured with a power meter. The transversal mode is Gaussian TEM00. The beam was focused perpendicularly on the target surface which was placed on a motorized XYZ translation stage. We focused the pulses on the surface by means of a cylindrical lens with a focal length of 75 mm, providing a spot size of 6 × 9000 µm2 (1/e2 criterion). In both cases, 15 squares of 1 cm2 were processed, using five different fluences (from 4.27 to 0.86 J/cm2 ) and three scanning speeds (130, 390 and 650 µm/s) (Fig. 1). Some initial tests on the sample gave us the window of fluences that induced apparent

modifications in the fabric appearance preventing textile destruction. In order to achieve homogeneous irradiation of the sample within the square, we used a squared mask and scanned along the direction of the smallest spot dimension. After processing the whole square length we moved the sample in the transverse direction 1500 µm, which means an overlap of 94.6 with regard to the maximum pulse intensity. Motion in the Z axis helped to accurately focus the laser beam on the material surface. 2.3 Analytical techniques To determine the morphology of the fabric and the structure of the linen fibre the processed areas were investigated by means of optical (Zeiss Axio Imager Z1m) and scanning electron microscopy (Zeiss DSM940) + EDX to determine the type of dirt to be eliminated. We carried out colour measurements not only as an indicator of the changes in the fabric but also as an element for evaluating the degree of cleaning and its relation with the quantity of dirt eliminated. The colour was measured using a Minolta CR-200. A minimum of 3 measurements were taken at each sample (8 mm diameter). It was considered important to incorporate the Fourier Transform Raman Spectroscopy (FT Raman) in order to control the effects of the ablation on the fabrics, since it allows us to evaluate the loss of crystallinity of natural polymeric materials. The Raman spectra allow us to correlate the degree of ageing of the material with the evolution of the crystallinity of the natural fibres. The Raman spectra of degraded textile samples generally show a wide fluorescent band which prevents, to a large extent, spectra recording of the molecular dynamic structure of the material. This happens both

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Table 1. Wavenumber, Raman relative intensity and assignement of the experimental bands in linen textiles (Edwards, 2006). s strong, m medium, w weak, sh shoulder. Wavenumber (cm−1 )/Intensity

Attribution

246 w 348 w 381 m 444 w 770 s 1096 ms 1120 ms 1342 w 1381 m 1462 w 1585 m 2735 sh 2900 s

τ(COH) δ(CCC) ring deformation δ(CCC) ring deformation δ(CCC) ring deformation Chlorates, Glasses (Si,Fe,Mg and Ca) νas (COC) of glycosidic bond νs (COC) of glycosidic bond δ(CH2 ), δ(OH) δ(CH2 ) δ(COH), alcoholic groups 1◦ and 2◦ ν(CC) E2g , sp2 ν(CH) methine ν(CH2 )

Figure 2. Raman spectra of linen fibres with different laser excitation lines (325, 514.5 and 1064 nm). Spectra intensity corrected for better comparison.

in the excitations of the infrared and visible range as well as the ultraviolet, leading in the latter case to the decomposition of the textile fibres, as shown in Figure 2. On the other hand, the FT Raman spectroscopy turns out to be an adequate technique for the analysis of linen and natural celluloses given the low excitation energy needed. The FT Raman spectra were carried out in a Bruker RFS-100 equipment, using the 1064 nm from a Nd:YAG laser beam as the excitation line. The spectral range analysed goes from 100 to 3500 cm−1 , with a spectral resolution of 3 cm−1 . The spectral acquisition conditions were 100 mW with a lateral resolution close to 1 mm and 64 accumulations per spectrum. These experimental conditions allow us to obtain the Raman vibrational dynamic spectrum of historical linens with a satisfactory quality and a reasonable signal-noise relation, in order to correlate the crystallinity and ablation conditions. One advantage of the technique is that it allows to obtain in one single spectrum the spectral range needed to record the CH tension modes, the movements of the CC skeleton, CO and CH deformations present in the cellulose. A complete spectral attribution of the bands recorded and their corresponding normal vibration modes can be seen in Table 1. The spectra have been treated for obtaining Raman parameters (wavenumber, relative intensity, FWHM and the Gaussian-Lorenzian character of the profile) of each of the bands by means of the LabSpect program from HORIBA Jobin Yvon.

3

RESULTS AND DISCUSSION

The morphological and colour analyses focused on the LINEN 1 sample which was sewn as a reinforcement onto the flag of the “Comuneros”, presenting a typical problem in the field of textile conservation. The FTRaman analysis was completed with LINEN 2, with a greater presence of fillers due to its application as a reinforcement by means of an adhesive (made of flour paste), which possible interference in Raman results was considered.

Figure 3. Macroscopic images & SEM images for the LINEN1 sample processed with a laser fluence 4.27 J/cm2

3.1 Evaluation of the cleaning: morphology and colour Morphologically, a significant elimination of the layer of dirt in both samples is verified. Macroscopic analysis demonstrates loss of density of the weave threads of the fabric in the samples subjected to higher laser fluences and lower scanning speeds (Fig. 3). Concerning the scanning speeds, the lower the speed, the larger the number of pulses irradiating the samples. We have to bear in mind that one pass along the smallest spot dimension means a total number of approximately 15000 (650 µm/s), 25000 (390 µm/s) and 75000 pulses (130 µm/s) and we have carried out 10 passes per squared area processed. This explains the observations about fibre damage with low speeds.

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Figure 4. Macroscopic images and SEM images for the LINEN1 sample processed with a laser fluence 0.86 J/cm2 .

SEM analysis confirms the macroscopic observations; the structure of the linen fibre is not altered as a whole but the threads lose density due to some ablation of the material, specifically, in the rows of the weft corresponding to the elevated zones (crests). Both for low fluences and high scanning speeds, the loss of fibre due to ablation ceases (Fig. 4). The quantity of dirt particles that remain in the textile is inversely proportional to the fluence and scanning speed (Figs 3, 4). The precise determination of the ablation threshold fluence both for the dirt and the linen substrate had been useful to assess the processing parameters that could lead to fibre ablation but this is a extremely complex task since, on one hand, the dirt is a very heterogeneous layer in composition and structure, so that the evaluated threshold could be dependent of the area evaluated while, on the other hand, the structure of linen fabric make the determination of the threshold extremely difficult as a result of the non uniform height of the fibres. The colour measurements obtained, even when evaluating the normal heterogeneities of the fabric, are relative and should be placed in relation to the macroscopic data and SEM, since the loss of material in the threads could be due to the interference of the underlying surface (white paper) (Dignard et al. 2005) recording data: L∗ increases in terms of the quantity of dirt eliminated while a∗ and b∗ do not undergo significant changes for the lowest fluences. Changes for the highest fluences are due to the greater participation of the underlying substratum in relation to the loss of fibre in the textile fabric (Fig. 5). 3.2

Chemical and mechanical properties changes: the crystallinity

The Raman spectrum of the cellulose has been previously obtained and analysed in the bibliography (Jahn et al. 2002; Edwards et al. 1994). The assignment of

Figure 5. Colour graphics: L∗ increases in terms of the quantity of dirt eliminated, a∗ and b∗ don’t undergo significant changes in lower fluences.

the Raman vibrational bands is shown in Table 1. The Raman spectra of original sample LINEN 2 (dirty) and after laser processing with different scanning speeds (130, 390 and 650 µm/s), and the maximum fluence 4.27 J/cm2 are shown in Figure 6. It can be observed, together with the characteristic bands of linen, the presence of two broad and intense bands centered around 770 and 1575 cm−1 , that can be attributed to traces of chlorates with silicated material of low crystallinity, associated with the thickness of the dirty layer of the fabric. The band at 1575 cm−1 disappears in the spectra from laser processed areas, independently of the scanning speed. The band at 770 cm−1 decreases its relative intensity for low speeds, maintaining its intensity for high speeds. This appears to point out that cleaning is more effective for low speeds. On the other hand, the Raman spectra are very similar to those of commercial linen samples. The latter present additional bands which may be due to the presence of components which are mixed with the linen, such as lignin and mineral fillers. The absence of the band at 1605 cm−1 , attributable to the C = C stretching of the phenolic ring of the lignin, which is present in almost all modern linens, indicates that we are dealing with ancient historical fabrics (Edwards et al. 1994), since lignin is vulnerable to oxidation caused by exposure to light, which accelerates its degradation (Edwards et al. 2006).

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Figure 6. Raman spectra of dirty cellulose (2Lino = LINEN2) and laser cleaned sample for scanning speeds 130 (2V1E1), 390 (2V2E1) and 650 µm/s (2V3E1) and fluence 4.27 J/cm2 . Spectra normalized by height to highlight differences.

Figure 8. Raman spectra of dirty cellulose (1Lino = LINEN1) and laser cleaned sample for scanning speeds 130 (1V1E1), 390 (1V2E1) and 650 µm/s (1V3E1) and fluence 4.27 Jcm−2 . Spectra normalized by height to highlight differences.

Figure 7. Raman spectra of dirty cellulose (1Lino = LINEN1) and laser cleaned sample for scanning speed 390 µm/s and fluences 0.86 (V2E5), 1.33 (V2E4), 1.70 (V2E3), 2.56 (V2E2) and 4.27 (V2E1) Jcm−2 . Spectra normalized by height to highlight differences.

Figure 9. Raman spectra of LINEN 2 sample processed at 130 µm/s and a fluence of 4.27 Jcm−2 : experimental, filtered and component bands.

A comparison of the Raman spectra of the non processed sample and the areas treated with different fluences for the intermediate speed (390 µm/s) is shown in Figure 7, corresponding to the LINEN 1 sample. It can be observed how the bands attributed to the dirt at 770 and 1575 cm−1 decrease their relative intensities as the fluence increases and the bands attributed to the linen appear with more clarity. This is evident in all the tests performed independently of the scanning speed, as can be observed in Figure 8, where one can see a comparative analysis of cleaning of LINEN 1 sample for different scanning speeds with the maximum laser fluence. It is clearly shown that the molecular structure and vibrational dynamic of the fabric is respected independently of the laser fluence, and therefore laser pulse energy. The possible laser-induced photodegradation can be monitored through the variations undergone by the Raman spectra of the fabric, dirty fabric and laser cleaned fabric. Recent studies show that analysis of the relative intensities of the bands corresponding to the anti-symmetric and symmetric ν(COC) stretching modes of the glycosidic bond in cellulose chains, at 1096 cm−1 and 1120 cm−1 respectively, can be used

as an indicator of the relative ageing of the sample (Edwards et al. 1994). The intensity ratio of the (I1120 /I1096 ) bands appears to be related to the hydrolytic rupture of the cellulose chains by the ester bonds of the glycosidic COC and their monitoring may give an idea of the optimum conditions and potential damage associated with laser cleaning. In order to obtain the Raman parameters of the whole set of Raman bands, all the spectra have been processed and resolved into component bands by means of the LabSpect program from HORIBA Jobin Yvon. In Figure 9, the spectrum resolved into component bands is shown, corresponding to low speed and high fluence (high pulse energy) test (130 µm/s and 4.27 J/cm2 , 0.9 mJ). The evolution of the intensity ratio with the laser pulse energy (I1120 /I1096 vs mJ) for the three speeds in the LINEN 1 sample is shown in Figure 10. A small relative increase in the intensity of the νas COC band at 1096 cm−1 , with respect to the νs COC band of 1120 cm−1 can be observed in all the energy range for the three scanning speeds. This indicates that there are very small variations in the glycosidic ring with laser processing, although a slight surface crystallization of

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Figure 12. Evolution of Raman intensity ratio of bands at 1342 cm−1 [d(OH) + d(CH2)] and 1381 cm−1 [d(CH2)] (I1342/I1381 vs pulse energy) after laser processing with scanning speeds 130, 390 and 650 µm/s.

Figure 10. Raman intensity ratio of the glycosidic stretching (I1120 /I1096 vs pulse energy) after laser processing with scanning speeds 130, 390 and 650 µm/s.

observed that the variation is minimal and the intensity ratio keeps constant in all the range of energies and speeds analysed. Thus, laser processing of linen fabrics appears to respect the molecular structure of the historical material, without modifying the degree of alteration and degradation that the dirty material displays.

4 Figure 11. Evolution of the width at medium height of 2900 cm−1 CH2 stretching (FWHMν(CH2 ) vs pulse energy) after laser processing with scanning speeds 130, 390 and 650 µm/s.

the cellulose may occur, with the consequent increase in the rigidity of the glycosidic ring. The degree of alteration of the fabric produced by the photonic interaction may be evaluated by means of the ν(CH2 ) at 2900 cm−1 , which can give a hint about the possible modifications induced in the biocomposite chains. Figure 11 shows the evolution of FWHM of the CH2 stretching in terms of the pulse energy for the three scanning speeds used. The lineal consistency of this Raman parameter is, to a large extent, clearly shown in the range of pulse energies analysed. This is indicative of the zero effect that the laser cleaning process has on the crystallinity of the polymeric chains of the cellulose for the energies and speeds studied in this paper. Another Raman parametric relationship to be considered, related to the degradation of the cellulose chains, is the increase in the intensity of the band at 1342 cm−1 as compared with the most intense band at 1381 cm−1 . This results from the greater contribution of the δ(OH) modes with respect to the δ(CH2 ) modes in the degraded samples, due to the effects of hydrolytic rupture of the cellulose chains. In Figure 12, we can observe the intensity ratio for both bands, in the range of pulse energies used in the study. It can be

CONCLUSIONS

The different analytical methods applied in this study in order to evaluate whether changes take place in historic linen fabrics which undergo femtosecond laser cleaning indicate that no significant chemical variations occur which might have a negative repercussion on their conservation. The morphological study shows that no remarkable change take place in the fibres, although excessive fluences and/or low scanning speeds -large number of pulses- may induce partial ablation of the fibres. This affects the density of the thread, in particular, and of the weave in general. It is therefore necessary to establish a precise window for both parameters and a thorough determination of the ablation thresholds for both the linen and the dirt. Following the line of research presented in this article, it would be necessary to carry out comparative studies between laser cleaning and conventional cleaning techniques within the field of textile restoration in order to evaluate the advantages and disadvantages of the methods in terms of the priority objective: to guarantee the conservation of our textile heritage.

ACKNOWLEDGEMENTS We thank Juan González Julián, Servicio de Microscopía Electrónica of the Universidad de Salamanca, for SEM images and assessment from Mercedes Barrera. C. M., A.G. and P.M. acknowledge financial

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support from the Ministry of Education and Science (project FIS2006-04151). REFERENCES Bäuerle, D. 2000. Laser Processing and Chemistry. Berlin: Springer Verlag. Belli, R., Miotello, A., Mosaner, P, Toniutti, L. 2006. Laser cleaning of artificially aged textiles. Appl. Phys. A 83: 651. Bloisi, F., Vicari, L., Barone, A.C., Martuscelli, C., Gentile, G., Polcaro, C. 2004. Effects of Nd YAG (532 nm) laser radiation on “clean” cotton. Appl. Phys. A 79: 331. Dignard et al. 2005 Cleaning of Soiled White Feathersn Using the Nd:YAG Laser and Traditional Methods: Lasers in the Conservation of Artworks 227–235. Chichkov, B.N., Momma, C., Nolte, S., von Alvensleben, F., Tünnermann, A. 1997. Precise laser ablation with ultrashort pulses. Appl. Phys. A 109–110: 15. Edwards, H.G.M., Farwell, D.W., Williams, A.C. 1994. FT-Raman spectrum of cotton: a polymeric biomolecular análisis. Spectrochim. Acta, Part A; 50: 807. Edwards, H.G.M., Nikhassan, N.F., Farwell, D.W., Garside, P., Wyeth, P. 2006. Raman spectroscopic analysis of a unique linen artefact: the HMS Victory Trafalgar sail. Journal of Raman Spectroscopy. 37: 1193. Gamaly, E.G., Rode,A.V., Luther-Davies, B.,Tikhonchuk,V.T. 2002. Ablation of solids by femtosecond lasers: Ablation

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mechanism and ablation thresholds for metals and dielectrics. Phys. Plasmas 9: 949. Jahn, A., Schroder, M.W., Futing, M., Schenzel, K., Diepenbrock, W. 2002. Characterization of alkali treated flax fibres by means of FT Raman spectroscopy and environmental scanning electron microscopy. Spectrochim. Acta, Part A, 58: 2271. Kennedy, C.J., Lerber, K., Wess, T.J. 2005. Measuring crystallinity of laser cleaned silk by X-ray diffraction. Preservation Science 2: 31. Lerber K.V 2004. Untersuchung zur Reinigung ungefärbter Seide mit Laser Diplomarbeit thesis. Lerber, Karin; Pentzien, Simone; Strlic, Matija; Kautek, Wolfgang 2005. Laser cleaning of silk: a first evaluation: ICOM Committee for conservation triennial meeting (14th) 978–988. Martinez, A. Escudero, C in press. Study of the Effects of Laser Cleaning on Historic Fabrics: Review and Results Eight Years after Applications. In this Volume. RIKEN Review, K. Sugioka (Ed.) 2003. Laser Precision Microfabrication LPM2002. Strlic, M. et al. 2003. Surface modification during Nd:YAG (1064 nm) pulsed laser cleaning of organic fibrous materials. Applied Surface Science 207: 236–245. Sutcliffe, H., Cooper, M., Farnsworth J. 2000. An initial investigation into the cleaning of new and naturally aged cotton textiles using laser radiation. J. Cult. Heritage 1, Supplement 1: 241.

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Monitoring of the laser cleaning process of artificially soiled paper S. Pentzien, A. Conradi & J. Krüger Division VI.4 Surface Technologies, BAM Federal Institute for Materials Research and Testing, Berlin, Germany

R. Wurster Institute for Physics and Meteorology, Hohenheim University, Stuttgart, Germany

ABSTRACT: Laser cleaning of soiled paper is a challenging task due to the fact that a contamination has to be removed and a fragile organic material has to be preserved. The ejection of particles forms a significant channel for the removal of unwanted surface contaminations and can be exploited for an in situ monitoring of the cleaning procedure. 532 nm nanosecond single and multi pulse laser cleaning of artificially soiled Whatman© paper was performed. Particles were registered with a dust monitor. These in situ experiments were combined with ex situ investigations of cleaning and substrate damage thresholds by means of light and scanning electron microscopic techniques. The cleaning efficiency measured by a multi-spectral imaging system was compared to the in situ particle monitoring. Additionally, possible colour changes of the paper substrate were evaluated.

1

INTRODUCTION

Cleaning of fragile organic materials like paper is a crucial part of the conservation process (Kautek et al. 1998). In principle, laser cleaning as a non-contact method offers bright prospects. For an effective cleaning of paper, a working range for the laser has to be found. A cleaning threshold of the soiling and a damage threshold of the matrix material have to be considered. Therefore, it is desirable to monitor the cleaning process. Laser induced breakdown spectroscopy (Anglos et al. 1997) and acoustic techniques (Strliˇc et al. 2005) were established. The removal of particles during the cleaning process can also be used as an in situ monitoring approach (Wurster et al. 2006). The aim of this paper is the combination of in situ and ex situ techniques for the determination of the apparent fluence thresholds and the cleaning efficiency. Single and multi pulse cleaning experiments with 532 nm nanosecond laser radiation on artificially soiled Whatman© filter samples are reported.

2 2.1

Figure 1. Scanning electron microscope (SEM) picture of a Whatman© filter No. 1506 as received. Pure cellulose fibres can be seen.

EXPERIMENTAL Sample preparation

Paper samples were prepared employing Whatman© filter paper No. 1506 (Fig. 1) according to the following procedure. Charcoal was mechanically rubbed and

distributed with a brush on the filter. The pulverized soiling was sucked in with a vacuum cleaner for five minutes. Thereafter, the soiled paper was treated again with a brush. Finally, a combined action of brush and

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Figure 3. Laser cleaning system at the BAM Federal Institute for Materials Research and Testing.

Figure 2. Scanning electron micrograph of a Whatman© filter No. 1506 mechanically soiled with charcoal dust.

vacuum cleaner was used for two minutes to achieve a strong adherence between artificial pollution and paper. Figure 2 depicts a sample after artificial soiling. Clearly, the soiling particles can be seen. An image analysis of some SEM pictures yielded a rough estimation of the size of the deposited particles. About 70% of them featured a size <1 µm while 30% showed a diameter >1 µm. 2.2

Sample characterization

Prior to the laser experiments, the homogeneity of the soiling was controlled down to a lateral scale of <0.5 mm on each sample utilizing a multi-spectral imaging system (MUSIS 2007, Model D-HFA-12, Art Innovation). Differences of lightness measurements in the visible spectral region (visible reflection mode) of maximal ± 10% were detected over the area of each sample. Major (C, O) and minor (K, Ca, Cu, Zn) components of the elemental composition of the soiling were analyzed by means of energy dispersive X-ray spectrometry (Wurster et al. 2006). 2.3 Laser treatment A prototype laser cleaning station (Kautek & Pentzien 2005) was applied for the laser treatment (Fig. 3). A Q-switched Nd:YAG laser (DINY pQ, IB Laser) was operated at 532 nm wavelength at a pulse duration of 8 ns and a repetition rate of 500 Hz. Pulse energies <1 mJ were measured by means of an energy meter (Nova, Ophir). The spatial beam profile was Gaussian. Beam radii (1/e2 ) of 42 µm, 51 µm, and 304 µm on the sample surface were set. The maximum laser

fluence in the focal spot F0 was changed between <0.007 J/cm2 and 14.9 J/cm2 . The laser beam was scanned over the sample through a remote computer control system. With a camera, the cleaning action was monitored on a computer screen. Depending on the number of laser pulses per spot N and the Gaussian beam radius w, a 3 mm square was scanned in 9 s (N = 1, w = 51 µm), 7 min (N = 36, w = 51 µm), <1 s (N = 1, w = 304 µm) and 3 s (N = 36, w = 304 µm), respectively. The laser-processing compartment fulfils laser class I conditions. An exhaust system is integrated to remove gaseous and solid laser ablation products. 2.4 Monitoring of the laser cleaning process During the cleaning procedure, a dust monitor (Portable Dust Monitor Mod. 1.108, GRIMM-Aerosol Technik) was used for the measurement of particle size distributions in airborne state. The aerosol inlet of the dust monitor is positioned next to the laser impact area (1 mm distance). Particle size distributions (eight size classes) were recorded at a time resolution of one second. The single particle detection bases on light scattering (using a laser diode) with a minimal detectable particle size of 0.3 µm. Aerosol flow control was adjusted and stabilized to 1.2 l/min. Additionally, particles released from the treated area were collected actively by means of a five stage mini cascade impactor (MCI) on ultrathin substrate films (polyethylene and polyimide) for off-line microscopic inspection. A scheme of the in situ and ex situ multi-method diagnostics is drawn in Figure 4.

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Ex situ evaluation of the laser cleaning results

The laser cleaning workstation incorporates a diagnostic tool (MUSIS 2007) for ultraviolet, visible and fluorescence imaging. The multi-spectral imaging system can operate in a wavelength range from 320 nm to 1550 nm. Relative colour measurements can be performed. In the visible reflection mode, a qualitatively similar spectral illumination characteristics as standard illuminant D65 (medium daylight with ultraviolet component) is provided and CIE L∗ a∗ b∗ (CIELAB) colour coordinates can formally be applied. Additionally, the results of the laser processing were inspected by means of optical (Eclipse L200, Nikon) and scanning electron microscopy (high resolution SEM ABT DS 150F, Topcon). All diagnostic methods are summarized in Figure 4.

3 3.1

RESULTS AND DISCUSSION Laser cleaning threshold

The determination of a working range for the laser cleaning procedure is a crucial factor for the success of the treatment. In this chapter, the evaluation of the cleaning threshold of the charcoal dust is described. Figures 5–7 display the temporal course of size distributions (eight size classes, time resolution 1 s) of ejected particles recorded during laser illumination of soiled samples. Different laser fluences were utilized. Figure 5 depicts a particle spectrum which corresponds to the laboratory aerosol background signal. Laser fluences <0.007 J/cm2 did not lead to a removal of dust from the surface of the paper. This observation was confirmed by microscopic investigations and colorimetric measurements.Therefore, the particle spectrum of Figure 5 was used as reference. Figure 6 shows the temporal course of the particle number concentration at the threshold of the cleaning procedure. For each size class, the number of particles increased by a factor of 3–4 in comparison to Figure 5.

Figure 5. Size distribution of particles for the application of 36 laser pulses with a fluence <0.007 J/cm2 . Background signal. Each data point represents the average over an integration time of 1 s. Size class 1 = 0.3–0.4 µm, size class 2 = 0.4–0.5 µm, size class 3 = 0.5–0.65 µm, size class 4 = 0.65–0.8 µm, size class 5 = 0.8–1.0 µm, size class 6 = 1.0–1.6 µm, size class 7 = 1.6–2.0 µm, size class 8 > 2.0 µm. 100 1 Particles/Litre
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Figure 4. Multi-method diagnostics. Combination of in situ and ex situ instruments and procedures.

80

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Figure 6. Size distribution of laser ablated particles for an illumination with a fluence of 0.007 J/cm2 . N = 36. Cleaning threshold. Each data point represents the average over an integration time of 1 s. Size class 1 = 0.3–0.4 µm, size class 2 = 0.4–0.5 µm, size class 3 = 0.5–0.65 µm, size class 4 = 0.65–0.8 µm, size class 5 = 0.8–1.0 µm, size class 6 = 1.0–1.6 µm, size class 7 = 1.6–2.0 µm, size class 8 > 2.0 µm.

The peak of the signal coincides with the laser exposition of the sample. The monitoring of the process was started prior to the laser illumination which results in a time delay of the maximum signal of about 3 s. The result obtained by means of the dust monitor was confirmed with an ex situ experiment. The removed particles were collected with the MCI. Figure 8 shows a significant number of particles on the polyethylene foil. Undoubtedly, a loss of soiling particles from the sample was detected at the cleaning threshold. It should be noted that a reliable detection of the cleaning threshold with ex situ methods like the naked eye, light microscopic techniques or even colorimetric measurements is more difficult (compare Table 1).

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Table 1. Yellow shift b∗ and lightness change L∗ of the samples after laser processing utilizing various maximum laser fluences (N = 36).

16000 Particles/Litre
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Figure 7. Size distribution of laser ablated particles for an illumination with a fluence of 0.035 J/cm2 . N = 36. Monitoring above cleaning threshold. Each data point represents the average over an integration time of 1 s. Size class 1 = 0.3–0.4 µm, size class 2 = 0.4–0.5 µm, size class 3 = 0.5–0.65 µm, size class 4 = 0.65–0.8 µm, size class 5 = 0.8–1.0 µm, size class 6 = 1.0–1.6 µm, size class 7 = 1.6–2.0 µm, size class 8 > 2.0 µm.

Figure 8. LM-micrograph of laser ablated particles deposited on a polyethylene foil of the mini cascade impactor. A laser fluence of 0.007 J/cm2 and multi pulse conditions (N = 36) were chosen. Particle collection at the cleaning threshold.

Using the dust monitor, an enhancement of the laser fluence by a factor of five yielded significantly higher numbers of ejected particles for all size classes (Fig. 7). The cleaning of the samples was much more effective at that fluence in comparison to fluence values close to the threshold (Fig. 6). As a major indicator for the final outcome of the cleaning, colour changes and lightness differences on the laser-treated areas were measured with a multispectral imaging system (MUSIS). The CIE L∗ a∗ b∗ (CIELAB) colour coordinates were used. The CIELAB model includes the parameters lightness of the colour (L∗ ) and two chromaticity coordinates (a∗ , b∗ ). In an ideal case, L∗ = 0 yields black

and L∗ = 100 indicates white. a∗ denotes the position between red and green, negative values indicate green. b∗ marks the position between yellow and blue, a negative b∗ describes blue. The overall colour difference E ∗ between a (white) reference and a sample can be calculated according to (DIN 6174:1979–01)

with the lightness difference L∗ and the variation of the colour positions a∗ and b∗ , respectively. For the experiments presented here, a decreasing lightness difference L∗ can be correlated with an improved cleaning quality (reference white Whatman© filter No. 1506 as received: L∗ = 0) and negative b∗ values would indicate a yellowing of the sample. Table 1 lists the yellow shift b∗ and the lightness difference L∗ for the processing of artificially soiled paper samples at laser fluences close to the cleaning threshold. The in situ experiments with the dust monitor (Figs. 5–7) are compared with ex situ colorimetric data. Obviously, a remarkable cleaning effect was observed for a laser fluence of 0.035 J/cm2 . For all fluences <0.035 J/cm2 , no considerable change of the lightness differences was detected. On one hand a microscopic in situ cleaning threshold of ≈0.007 J/cm2 was found for a multi pulse laser treatment (Fig. 6). On the other hand, a practically relevant macroscopic machining threshold of ≈0.03 J/cm2 was observed (Fig. 7, Table 1). Taking into account that a b∗ deviation of 2 counts can be considered as experimental uncertainty, a yellowing of the samples after laser illumination was not found. 3.2 Pulse-to-pulse monitoring So far, multi pulse experiments close to the (microscopic and macroscopic) cleaning thresholds of about 0.007 J/cm2 and 0.03 J/cm2 were discussed. Now, an in situ monitoring with the dust monitor of a laser processing consisting of three runs is shown. Figure 9 displays a size distribution of the ejected particles after

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Figure 9. Size distribution of laser ablated particles for a laser treatment with a fluence of 3.1 J/cm2 . First, second and third run. Each column represents the average over an integration time of 7 s. Size class 1 = 0.3–0.4 µm, size class 2 = 0.4–0.5 µm, size class 3 = 0.5–0.65 µm, size class 4 = 0.65–0.8 µm, size class 5 = 0.8–1.0 µm, size class 6 = 1.0–1.6 µm, size class 7 = 1.6–2.0 µm, size class 8 > 2.0 µm.

single pulse machining. The data were averaged over an area of 3×3 mm2 which corresponds to 1028 positions per run for a focus radius of 42 µm. Each sample position of the treated area was irradiated with a single laser pulse within one run and the scanning routine was performed 3 times consecutively (3 × N = 1). Obviously, the first pulse removed the bigger part of the soiling. The number of particles decreased considerably for the second and the third run, respectively. The online monitoring clearly demonstrates the differences of effectiveness for different scanning procedures.

3.3

Figure 10. Size distribution of laser ablated particles for different fluences applied and multi pulse illumination conditions (N = 36). Each data point represents the average over an integration time of 30 s. Size class 1 = 0.3–0.4 µm, size class 2 = 0.4–0.5 µm, size class 3 = 0.5–0.65 µm, size class 4 = 0.65–0.8 µm, size class 5 = 0.8–1.0 µm, size class 6 = 1.0–1.6 µm, size class 7 = 1.6–2.0 µm, size class 8 > 2.0 µm.

Damage threshold of the paper substrate

Figure 10 shows particle size distributions measured by means of the dust monitor during laser treatment of the samples with various laser fluences. For all fluences ≤ 2.4 J/cm2 , similar dependencies of the number of particles vs. size class were observed. For a high fluence of 14.9 J/cm2 , a considerable increase of the number of particles was detected. Especially, the number of particles of size class 8 (>2.0 µm) rised disproportionately. This growth can be attributed to a damage of the paper substrate. In addition to the removal of the soiling, paper fibres were detected. Even for the processing of a pure Whatman© filter, a remarkable particle signal was found. Additionally, a detailed evaluation of the paper damage threshold was performed by means of the mini cascade impactor and scanning electron microscope investigations. Figure 11 displays laser-ablated parts of fibres collected during single pulse laser illumination of a pure Whatman© filter. The collected particles are much larger than those observed after laser cleaning of soiled Whatman© at a small laser fluence (Fig. 8).

Figure 11. LM-micrograph of laser ablated particles from pure Whatman© filter No. 1506 deposited on a polyimide foil of the MCI. Laser fluence 10 J/cm2 , single pulse illumination.

A laser fluence of 10 J/cm2 marks the single pulse damage threshold of the paper substrate. Figures 12 and 13 depict scanning electron micrographs of laser-treated Whatman© filters artificially soiled with charcoal dust after single and multi pulse laser processing, respectively. In both cases, the laser fluence was chosen above damage threshold of the paper substrate. Clearly, a disintegration of the cellulose fibres is visible. In the multi pulse case, a higher degree of damage can be seen for a constant laser fluence. Figure 14 shows the deterioration of cellulose fibres as a result of multi pulse laser processing at

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Figure 12. Scanning electron microscope picture of Whatman© filter No. 1506 mechanically soiled with charcoal dust after single pulse laser processing with 14.9 J/cm2 .

Figure 14. Scanning electron microscope picture of pure Whatman© filter No. 1506 after multi pulse (N = 36) laser processing with 14.9 J/cm2 .

4

Figure 13. Scanning electron microscope picture of Whatman© filter No. 1506 mechanically soiled with charcoal dust after multi pulse (N = 36) laser processing with 14.9 J/cm2 . 2

©

14.9 J/cm of a pure Whatman filter. Though laser target interaction will surely be influenced by the soiling particle layer, the apparent damage symptoms of the filter substrates exhibit no significant differences (compare Figs 13 and 14).

CONCLUSIONS

Nanosecond single and multi pulse laser cleaning of mechanically soiled Whatman© filter paper was performed at 532 nm wavelength. The ejection of particles during the laser treatment of the samples can be employed for an in situ monitoring of the cleaning process. A multi pulse in situ cleaning threshold of ≈0.007 J/cm2 representing a microscopic removal of a small amount of particles was determined. Laser fluences ranging from about 0.03 J/cm2 up to 3 J/cm2 can practically be utilized for the processing. A yellowing of the paper was not observed. With an ex-situ multi-spectral imaging system, the cleaning efficiency was correlated precisely with the in-situ particle monitoring. A wavelength of 532 nm seems to be an appropriate choice for the cleaning task in agreement with previous investigations (Kolar et al. 2000a, Kolar et al. 2000b, Kaminska et al. 2004). A disintegration of the cellulose fibres of the paper was found for single pulse illumination at a laser fluence of approximately 10 J/cm2 . REFERENCES Anglos, D., Couris, S., & Fotakis, C. 1997. Laser diagnostics of painted artworks: Laser-induced breakdown spectroscopy in pigment identification. Applied Spectroscopy 51: 1025–1030. DIN 6174:1979-01. 1979. Colorimetric evaluation of colour differences of surface colours according to the CIELAB formula. Berlin: Beuth.

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Kaminska, A., Sawczak, M., Cieplinski, M., Sliwinski, G., & Kosmowski, B. 2004. Colorimetric study of the postprocessing effect due to pulsed laser cleaning of paper. Optica Applicata 34: 121–134. Kautek, W., Pentzien, S., Rudolph, P., Krüger, J. & König, E. 1998. Laser interaction with coated collagen and cellulose fibre composites: fundamentals of laser cleaning of ancient parchment manuscripts and paper.Applied Surface Science 127–129: 746–754. Kautek, W. & Pentzien, S. 2005. Laser cleaning system for automated paper and parchment cleaning. Springer Proceedings in Physics 100: 403–410. Kolar, J., Strliˇc, M., Pentzien, S. & Kautek, W. 2000a. NearUV, visible and IR pulsed laser light interaction with cellulose. Applied Physics A 71: 87–90.

Kolar, J., Strliˇc, M., Müller-Hess, D., Gruber, A., Troschke, K., Pentzien, S. & Kautek, W. 2000b Near-UV and visible pulsed laser interaction with paper. Journal of Cultural Heritage 1: S221–S224. Strliˇc, M., Selih, V.S., Kolar, J., Koèar, D., Pihlar, B., Ostrowski, R., Marczak, J., Strzelec, M., Marinèek, M., Vuorinen, T. & Johansson, L.S. 2005. Optimisation and on-line acoustic monitoring of laser cleaning of soiled paper. Applied Physics A 81: 943–951. Wurster, R., Pentzien, S., Conradi, A. & Krüger, J. 2006. Characterization of laser-generated microparticles by means of a dust monitor and SEM imaging. Laser Chemistry 2006: Article ID 31862.

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Systematic case study on common cleaning problems on paper and parchment by using Nd:YAG laser (ω, 2ω, 3ω) J. Hildenhagen, M. Lentjes & K. Dickmann Laser Center (LFM), Münster University of Applied Sciences, Steinfurt, Germany

B. Geller LWL-Archivamt für Westfalen, Münster, Germany

ABSTRACT: Not only cleaning problems, at which conventional techniques reach their limits, present a potential for laser application in conservation of artworks. Various unwanted contaminations on paper and parchment can be removed by traditional techniques. However, in many cases laser cleaning produces a better result. For a successful practical laser treatment, the reactions and thresholds of all involved materials have to be studied and should be considered before a laser application. In this work, a variety of paper and parchment objects with original ink scripts, printed fonts and typical pollutions were treated by different Nd:YAG laser wavelengths to investigate their reaction on laser radiation by visual control via optical microscopy and scanning electron microscopy (SEM).

1

2

INTRODUCTION

Sticky surface dirt, old archive markings or insect excrements are some of the problems old documents often deal with. These are the every day cleaning problems faced by document restorers. These problems can be solved conventionally. However, a conventional treatment is often laboriously, takes a lot of working time and the obtained results are not always satisfying. For various cases, laser cleaning leads to better results either in quality or quantity. The special structure of paper and parchment should be considered, whereby many contaminations can penetrate in different extents inside the object. In the case of paper objects, the pollution can deposit between or infiltrate into the paper fibers. The laser cleaning effect is particularly limited by its depth of penetration, which depends on the irradiated substance and the used laser wavelength. Another important (adverse) feature is the fact that visible and IR wavelengths can also pass through sheets of paper and ablate substances on the backside. For a successful laser treatment, the response and ablation thresholds of all involved materials (carrier, elements to be preserved and defilements) should be considered. If contaminations can be removed by ablation with a lower fluence than the ablation threshold of all other concerned substances, there could be an option for a successful cleaning operation.

STUDIES

In this study the mechanical destruction and discoloration of carriers, of elements to be preserved and defilements were investigated at different fluences and wavelengths (chemical modifications remain out of consideration). The experiments were carried out with a SAGA 220/10 Nd:YAG laser (10 ns, 1–10 Hz, articulated arm) at the first three harmonics (1064 nm, 532 nm and 355 nm). The polluted original documents researched within this study are a variety of papers and parchments, provided by the Westfälisches Archivamt (Germany). The authentic information on the carriers consists of original ink, printed fonts and carrier specific attributes, like writing lines. Since the degree of ageing is an important factor for the behaviour against laser irradiation, only original and aged materials were used for this study. First of all, the different types of paper and parchment were irradiated and examined by Scanning Electron Microscopy (SEM) (Fig. 1). For each sample an irradiation fluence was found at which a mechanical modification starts, rib of paper fibres or surface cracking on parchment. Some of these obtained carrier destruction thresholds are shown in Figure 5. All kinds of elements to be preserved, e.g. original inks and printed fonts, formed the essential part of this study. Unwanted possible reactions can be discoloration and ablation (removal).

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Figure 1. Paper (top) and parchment (bottom) with mechanical destruction after intensive laser irradiation with λ = 1064 nm.

Figure 2. Studies on iron gall ink (top, λ = 532 nm, 240 mJ/cm2 ) and vegetable ink (bottom, λ = 355 nm, 220–500 mJ/cm2 ).

Since there is a wide variety of possible pollutions, a small selection of daily cleaning problems was taken for this study. The investigated defilements are: regular surface dirt, insect excrements and old archive markings written with crayon. At first, the thresholds for these defilements were obtained. Surface dirt (see Figs. 2 and 3) and crayon could be removed without visible residues, only the mechanical deformation from the previous writing process could not be reversed. The removal of excrements from insects, like flies, is more complicated, since a part of this pollution penetrated into the paper/parchment. This pollution could not be removed by laser irradiation without ablation of the carrier material. Hence, only a reduction is practical in this case. Comparing the obtained results of laser cleaning with results of regularly used techniques (scalpel, eraser, etc.) demonstrate how a saving of time can be obtained by using laser technology, e.g. a typical parchment book cover of 20 × 30 cm2 can be cleaned

Figure 3. Parameter matrix of removal of regular surface dirt on calf parchment; the applied laser fluence rises from the left to the right (120–400 mJ/cm2 ,
= 1064 nm).

from sticky surface dirt in 5 minutes which is more than ten times faster than mechanical cleaning. Real challenges are these areas where pollution overlays the original elements which should be preserved, like ink or printed lines. The collected data give an overview about the possibility of removing a specific defilement from a specific carrier (with and without authentic information). However, it should be considered that the storage conditions and the variations in chemical composition influence the ablation

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and modification thresholds. The summarized results in Figure 5 show exemplarily how the behavior of involved materials can be visualized to derive the possible chances for laser cleaning. These results can only

limitedly transferred to other objects and laser systems and must be investigated case by case. In a positive case, the defilements could be removed with a lower laser fluence than the mechanical destruction threshold of the substances underneath. Two simple results may clarify the informational value of a threshold comparing diagram. Figure 4 shows the successful removal of blue crayon from a violet matrix print (lined vellum from 1908, 1064 nm, 80 mJ/cm2 ). Figure 6 shows a Dutch invoice from 1903 of the same kind of paper, with a red crayon marking which covers a lithographic printed dotted line and an iron gall ink font. The laser fluence which was just high enough to reduce the crayon, partially removed the dotted line but did not influence the handwritten font (λ = 532 nm, 80 mJ/cm2 ).

3

Figure 4. Lined vellum document from 1908, before (left) and after removal of blue crayon archive marking (right), λ = 1064 nm, 80 mJ/cm2 .

CONCLUSIONS

The application of laser cleaning on paper and parchments is not limited to problems which are insolvable for conventional techniques. This practical study demonstrates by some special examples that using laser irradiation can have advantages concerning the

Figure 5. Thresholds (determined by optical inspection) of different studied materials for λ = 1064, 532 and 355 nm. The fluence for removal of defilements has to be lower than the mechanical modification threshold of the elements to be preserved (ink, paper, etc.)

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Possible chemical modifications or longtime reactions were not the emphasis of this study. These studies are the subject of previous work by other authors (Kautek et al. 2001, Pilch et al. 2005, Scholten et al. 2005, Ochocinska-Komar et al. 2005, Kaminska et al. 2005).

REFERENCES

Figure 6. Dutch invoice from 1903, before (left) and after removal of red crayon archive marking (right), unfortunately the printed dotted line has been reduced, the iron gall ink font is unaffected. λ = 532 nm, 80 mJ/cm2 )

velocity and/or quality in comparison to conventional cleaning methods. However, the specific characteristics of paper and parchment have to be considered and limit the usability of laser light. Penetrated substances can only be reduced and substances on the opposite side can be influenced by VIS and IR laser irradiation. Cleaning results and detected thresholds were only defined with optical inspection via optical microscopy and SEM. For important paper and parchment objects the use of further analysis methods has to be recommended. All three studied Nd:YAG wavelengths offer possibilities for laser cleaning of documents. For the mentioned cleaning problems the green line, 532 nm, in general seems to be the best compromise.

Kautek, W., Pentzien; S., Mueller-Hess, D., Troschke, K., & Teule, R. 2001. Probing the limits of paper and parchment laser cleaning by multispectral imaging. In Renzo Salembeni (ed.), Laser Techniques and Systems in Art Conservation, SPIE 4402, 130. Pilch, E., Pentzien, S., Mädebach, H. & Kautek, W. 2005. Anti-Fungal Laser Treatment of Paper: A Model Study with a Laser Wavelength of 532 nm, LACONA V Proceedings, Springer Verlag. Scholten, J.H., Schipper, D., Ligterink F.J., Pedersoli, J.L. Jr., Rudolph, P., Kautek, W., Havermans, H.A., Aziz, B., Bekk, B. van, Kraan, M., Dalen, P. van, Quillet, V., Corr, S. & Hua-Sröfer, H.Y. 2005. Laser Cleaning Investigations of Paper Models and Original Objects with Nd:YAG and KrF Laser System, LACONA V Proceedings, Springer Verlag. Ochocinska-Komarl, K., Kaminska, A., Martin, M. & Sliwinski, G. 2005. Observation of the Post-Processing Effects due to Laser Cleaning of Paper, LACONA V Proceedings, Springer Verlag. Kaminska, A., Sawczak, M., Cieplnski, M. & Sliwinski, G. 2005. The Post-Processing Effects due to Pulsed Laser Ablation of Paper, LACONA V Proceedings, Springer Verlag.

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Use of laser and optical diagnostic techniques on paper: The Pomelnic from Sucevita Monastery (Romania) M. Strlic University of Ljubljana, Faculty of Chemistry and Chemical Technology, Ljubljana, Slovenia

J. Kolar National and University Library, Laboratory for Cultural Heritage, Ljubljana, Slovenia

G. Pajagic Bregar National Museum of Slovenia, Ljubljana, Slovenia

V. Ljubic Technical Museum Vienna, Vienna, Austria

R. Radvan, M. Simileanu & W. Maracineanu National Institute of Research and Development for Optoelectronics INOE 2000 – Centre for Restoration by Optoelectronical Techniques CERTO, Bucharest, Romania

J. Hildenhagen Lasercenter Fachhochschule Muenster (LFM), Department of Engineering Physics, Steinfurt, Germany

M. Castillejo & M. Oujja Institute of Physical Chemistry Rocasolano, CSIC – Consejo Superior de Investigaciones Científicas, Madrid, Spain

W. Kautek University of Vienna, Department of Physical Chemistry, Vienna, Austria

V. Zafiropulos Technological Educational Institute of Crete (TEI), Department of Human Nutrition & Dietetics, Crete, Greece

T. Sinigalia National Institute for Historical Monuments, Bucharest, Romania

O. Boldura National University of Art, Bucharest, Romania

N. Melniciuc Faculty of Theology, Department of Chemistry, Iaci, Romania

ABSTRACT: During the recent Culture 2000 workshop taking place at the Sucevita monastery in Eastern Romania, a group of conservation and research experts was entrusted a document from the monastery’s rich collection – to examine its material properties, state of preservation, and eventually to propose a suitable conservation strategy. The document, Pomelnic, showed signs of extensive use. Its age was unknown. Signs of past reparation treatments were obvious: several layers of paper were glued under the particularly damaged areas and the whole document was supported by a piece of cardboard. Microscopic investigation and especially fluorescence imaging showed evidence of fungal activity. Inks were examined using colourimetry and laser-induced breakdown spectroscopy. The latter technique indicated the presence of iron in the ink; however, the quantity was estimated to be low, so that the ink can probably not be classified as iron-gall ink. Microdestructive measurements

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of pH have shown that the paper is acidic (pH 3.6), based on this fact, we can model the remaining lifetime of the paper material. Additionally, the support and the papers used for lining were also acidic (3.4 and 5.4, respectively). The mode of production and historical evidence indicate that the document was probably produced between 1850 and 1875.

1

INTRODUCTION

The Culture 2000 Project “Saving Sacred Relics of European Medieval Cultural Heritage” – Romania 2006 – envisaged creation of the professional approach for a pertinent solving of an acute aspect in restoration process for rare pieces with artistic and historic value at the same time, being dedicated to the proper restoration and conservation strategy of an ensemble of three religious sites from North of Moldavia – important marks of the historical events that are strongly correlated with European history – namely Sucevita Monastery, Balinesti Church and Popauti Church. Because of the inadequate environmental conditions, the fabulous paintings and important collections (books, documents and textiles) are in an advanced degradation process or in just good restoration condition, but with poor conservation strategy. During this project, at Sucevita Monastery’s museum, an in situ restoration/conservation workshop using optoelectronical techniques took place, corroborating numerous parallel activities, strongly entrusting all categories of participants in the high quality of this multidisciplinary work. The document (Fig. 1) was Pomelnic, inv. no. 25, and was particularly fragile, which was obviously due to very intensive use. The age and composition was difficult to determine visually. The object itself was

Figure 1. The Pomelnic Document.

glued onto a support, which was also very badly preserved. Both were loosely glued onto a cardboard support. Immediately after the first examination, a protective envelope was made to prevent information and material loss due to handling during the workshop. 2 ANALYSIS AND TESTS Upon visual inspection it was immediately evident that several different inks were used. They could be examined using colorimetry – it was established that at least four inks were on the sheet of paper. The composition of the black ink was of particular interest, as it could be composed of iron ions and extracts of galls. The iron gall inks are infamous for their corrosivity, so that it needed to be established whether the ink in question in fact contains a large amount of iron. The analyses done using laser induced breakdown spectroscopy showed that the peaks for iron were only slightly higher when inks were examined than when paper was examined (Fig. 3). The LIBS technique was characterized as microdestructive by the conservators, as there is no immediately noticeable damage on the artifact. Additional analyses using iron-indicator paper strips indicated no presence of Fe(II), which means that it is very likely that the ink used was a carbon-based ink, usually safe for paper. Analyses using the imaging camera indicated that at particular places (especially where loss of material

Figure 2. Colorimetric investigation of different types of inks.

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took place) in the manuscript, there is evidence of biological attack. The presence of hyphae was indicated by very thin threads, which only became visible as they fluoresce (Fig. 4). In this way, however, it is not possible to determine whether the microorganisms are active or not. In fact, their presence could be due to a past mistreatment. In any case, thorough cleaning of the document could be recommended. Using the pH-indicator pen, it is possible to determine whether a sample of paper (only a few fibers are necessary) is very, or moderately acidic and whether it is alkaline (Fig. 5). We determined that the pH of the sample was less than 5.5. In such cases, deacidification is recommendable, provided that the inks do not migrate during a treatment. In any case, a preliminary test on moisture sensitivity would be needed. It turns out (Fig. 6) that the original document is very acidic (pH 3.6), however, the material with which it is lined, is also acidic (pH 3.4). Such high acidity of

the paper leads to rapid deterioration of the cellulosic support, so that the conservators proposed deacidification, provided that the inks are not damaged in the process. The low-quality cardboard support is also acidic (pH 5.4). More exact knowledge of the pH of the artifact could only be obtained using microdestructive determination of pH (Fig. 7). This was done by sampling a few fibres (few micrograms) from different parts of the item. Microscopical examination of the Pomelnic was also made using an x-y laser OM developed by INOE

Figure 5. The marks left by the pH-pen on papers of different pH (from left to right: paper of pH 3, pH 5, pH 7 and pH 9).

Ca II, Al I

Intensity/ a.u.

5.4

Ca I Fe I Fe I Fe I

INK

PAPER 300

350

Na I Ca I

Cr I Ca I

KI

400

450

500

550

600

3.4

Wavelength/nm

3.6 Figure 3. LIBS spectrum. Figure 6. pH of different paper materials constituting the artifact.

Figure 4. Examination of a part of the document using visible light (upper image) and fluorescence imaging.

Figure 7. The pH electrode setup and the pH meter.

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3

CONCLUSIONS

The document which was examined during the Culture 2000 workshop in Sucevi¸ta was a late 19th century manuscript, written in a variety of inks, which were analysed using colourimetry. The inks were not iron gall inks, as demonstrated by LIBS and microchemical analysis. Using a micro-pH electrode, it was shown that the paper is acidic and machine-made and is lined with another acidic paper and additionally with an acidic cardboard. Signs of biological attack were evident from fluorescence images. Using technological and historical evidence, the document was dated from 1850–1875.

Figure 8. Microscopic images of the Pomelnic.

REFERENCES

Figure 9. Names of Austrian emperors on the Pomelnic.

for accurate and high precision investigations and interventions. Approximate dating of the document was possible based on technological and historical evidence. The paper itself is machine-made and acidic, which means that it was probably produced after 1850, when the production technology changed in the way that such paper became widespread. Historical analysis of the document made it evident that the Pomelnic was used during ceremonies and that it contained names of dignitaries, who were already deceased at the time of its making. Close inspection of the text revealed that the names of three Austrian emperors appeared, i.e. Josif, Leopold and Franc (Fig. 9). As they were probably successors, they were Joseph II (1741–1790), Leopold II (1747– 1792), Francis II (1768–1835), while the successor Ferdinand I (1793–1875) was not mentioned.

Kolar, J., Strlic, M., Pentzien, S., Kautek,W. 2000. Near-UV, visible and IR pulsed laser light interaction with cellulose. Appl Phys A, 71: 87–90. Salimbeni, R., Zafiropulos,V., Radvan, R.,Verges-Belmin,V., Kautek, W., Andreoni, A., Sliwinski, G., Castillejo, M., Ahmad, S.R. 2005. Lasers in conservation of Artworks: the European Community research-Proceedings SPIE Int. Soc. Opt. Eng. 5850(33). Simileanu, M., Striber, J., Radvan, R. 2006. Investigations into the effect of wavelength in laser cleaning of paper. Acta Technica Napocensis, Series: Applied Mathematics and Mechanics, 49: 923–928. Simileanu, M., Maracineanu, W., Deciu, C., Striber, J., Radvan, R. 2006. A complex portable optoelectronic setup for on site interventions. Case studies. Proc. SPIE: Seventh International Conference onVibration Measurements by Laser Techniques: Advances and Applications 6345: 63450U. Striber, J., Maracineanu, W., Simileanu, M., Savastru, R., Mohanu, D. 2005. Laser cleaning application on Lady’s Church Atrium from Bucharest City Centre. Nonconventional Technologies Review 4: 89–92. Strlic, M., Kolar, J., Selih, V.-S., Marincek,M. 2003. Surface modification during Nd:YAG (1064 nm) pulsed laser cleaning of organic fibrous materials.Appl. Surf. Sci., 207: 236–245. von Lerber, K., Pentzien, S., Strlic, M., Kautek, W. 2005. In A. Boccia Paterakis, M. Cassar, D. Thickett, C. Villers, J. Wouters (eds.), ICOM Committee for Conservation, 14th triennial meeting the Hague preprints, 2: 978.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Laser cleaning of 19th century papers and manuscripts assisted by digital image processing G.M. Bilmes, C.M. Freisztav, N. Cap & H. Rabal Centro de Investigaciones Opticas (CONICET-CIC) and Universidad Nacional de La Plata, La Plata, Argentina

A. Orsetti Area de Conservación y Restauración, Ministerio de Cultura, Gobierno de la Ciudad de Bs. As, Buenos Aires, Argentina

ABSTRACT: One of the main problems in the restoration of documents and antique manuscripts is the cleaning of soil from handling as well as soot and grease stains. Many documents also have ink or graphite pencil marks that must be eliminated. In the case of manuscripts dated before the 20th century, most of them were written on paper fabricated by processing textile remnants (rag paper). Standard procedures for cleaning are usually strongly abrasive or involve the use of chemicals that in certain cases could be problematic. In this work, we applied laser ablation with nanosecond laser pulses to the cleaning of 18-19th century documents and manuscripts belonging to the National General Archive of Argentina. We used digital image processing tools to assess the laser cleaning treatments and compared this procedure with conventional cleaning methods.

1

INTRODUCTION

In the restoration of documents and antique papers, a common issue is that of optimal cleaning. It must be done under careful conditions in order to avoid damaging the cultural value of the sample and to preserve fine details and the effects of natural ageing. In most cases this entails the removal of surface dirt from handling, wear, moisture, grease stains, and candle soot. Many documents also have ink or graphite pencil notations and marks that must be eliminated. In the case of manuscripts dated before the 20th century, most documents were elaborated with papers fabricated by processing textile remnants (rag paper).The traditional cleaning procedures are dry cleaning with soft rubber, hard rubber, glass fiber pencils, and (when possible) wet cleaning with water or diluted chemical solvents (especially for stain elimination). These standard cleaning procedures, and especially wet cleaning, usually produce abrasion with subsequent wear of the paper and loss of chemical and structural properties. When chemicals are used, residual effects may be problematic as well. Finally, such treatments are not recommended for application more than one time and often involve time consuming and difficult manipulations. Recently, the advantages and novel possibilities offered by means of laser cleaning procedures where successfully demonstrated in treatments applied to

papers and parchments (Asmus 1997, Kautek et al. 1998, Kolar et al. 2000, 2001, 2002, 2003, Ochocinska et al. 2001, Pérez et al. 2001, Andreotti et al. 2005, Komar et al. 2005, Strliˇc et al. 2005). The physical phenomenon that underlies the cleaning process is dirt ablation produced by the action of the laser on the surface, giving rise to de-bonding of the outermost layer without deleterious effects on the substrate (Zafiropulos 2002). One important advantage of cleaning using laser irradiation is that abrasion and wear processes, that are usual in conventional methods, are avoided. Both standard and laser cleaning methods require tools to measure the amount and distribution of superficial dirt for the assessment of the treatment quality. Procedures to compare and determine the advantages or shortcomings of the different methods are also needed (Freisztav et al. 2000). In this work, we applied dry cleaning and laser cleaning to the elimination of candle soot deposits on 18th and 19th century documents and manuscripts belonging to the National General Archive of Argentina. We compared dry cleaning with soft rubber, hard rubber and laser cleaning treatments by using digital image processing tools. Through this procedure we evaluated parameters such as dirt concentration (before and after cleaning), its homogeneity, and the quality of the cleaning processes.

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2

EXPERIMENTAL

The samples studied were a collection of 18th and 19th century documents. Some of them had writings produced with iron gall inks. Some of the samples were well preserved while others had superficial soils caused by handling as well as candle soot stains. The oldest papers, called rag papers, were made by processing textile remnants (c.a 70% cotton and 30% linen or hemp) to which a load of kaolin or calcium carbonate and arabic or rabbit glue was added. The newer papers were made by mechanical treatment of vegetable fibers with forced whitening by celulosic paste. These papers are more fragile than the rag papers. To evaluate the quality of the cleaning procedures we treated some of the papers and manuscripts that have no patrimonial value, by soiling their surfaces with candle soot (a very dark, greasy substance). To these samples we applied both conventional dry cleaning (rubber) and laser cleaning at two wavelenghts. For the rest of the documents laser cleaning was used after first evaluating the quality of the initial procedures. Laser tratments were performed using a Q-switched Nd-YAG laser (Surelite II-Continuum with 7 ns pulse duration) operating at 1064, 532 and 355 nm, and at repetition rates from 1 to 10 Hz. Fluence was estimated (±30%) by dividing the pulse energy by spot area. The excitation fluence was changed by focusing with appropriate lenses and by using a neutral density wedge filter to change the energy of the laser pulse. A circular pinhole was used, to have a well controlled irradiation area. Laser fluences (F) ranging from 0 < F < 4.5 J/cm2 were used. The energy was controlled by splitting off a portion of the laser beam to a pyroelectric detector (Rj-7100/Rj-735, Laser Precision Corp). In order to change the impinging location of the beam; the samples were mounted on a XYZ controlled positioning step motor system. For the different types of papers, threshold damage fluences as a function of wavelength were determined before cleaning. Damage assessments were made by visual inspection with a microscope (20x) and a CCD camera with optical zoom (3x). Laser cleaning was performed by scanning the laser beam across the samples. In each scan the same place of the surface received 15 consecutive shots. The cleaning with rubber was done using soft rubber (pencil type) and hard rubber (ink type) and it was performed in the usual way. To evaluate the cleaning process and the quality of the results, digital image processing was employed. In all cases, color as well as black and white images were registered using either a Sony CCD color camera and a Pulnix (black and white) camera. A frame grabber (Imaging Technology series 151) was used to digitize to 8 bits (every color channel in the color images case) and to store and process the images. Images were

registered: before soiling the samples; after soiling; during the cleaning process and after the final (clean) state had been reached. To reduce possible effects of changes in the illumination and other error factors, in each situation, four images were taken and the most dissimilar discarded, whereas those remaining were averaged. 3

RESULTS AND DISCUSSION

3.1 Damage threshold fluences Laser cleaning must be conducted by using excitation fluences high enough to eliminate dirt, without affecting the substrate. For that purpose the damage threshold fluence for each substrate must be known as well as the threshold fluences for dirt and iron ink ablation. Then, before laser cleaning treatments, we determined threshold fluences as a function of wavelength for all the papers. The samples were treated with increasing values of fluence in different regions of the papers. Each test area received an average of 15 pulses per unit area. Damage was determined by microscopic inspection (visible fibre damage or disruption). The results obtained show that the studied papers are more sensitive to UV radiation than to the other wavelengths. Damage threshold fluences of both rag and mechanical paste papers were roughly the same with values of 1.0, 2.2 and 2.8 J/cm2 for excitation wavelengths of 355, 532, and 1064 nm, respectively. Threshold levels for dirt ablation (candle soot, pencil writing, and handling dirt are ca. one order of magnitude lower than the damage threshold fluences of the substrates. Tests at 1064 and 532 nm show that partial removal of the iron ink occurred at fluence levels above 1.3 J/cm2 and 1.0 J/cm2 , respectively. Figure 1 shows an example of irradiation of a rag paper with ten 1064 nm laser pulses at two different fluences. Figure 1b corresponds to a fluence value above the damage threshold. Figure 1c is the result obtained with 0.6 J/cm2 , a fluence value above dirt thresholds but ca. five times lower than the damage threshold of the paper. Figure 1a shows the paper before laser irradiation. Taking into account these results we determined that fluences between 0.3 and 0.6 J/cm2 are optimum for 532 or 1064 nm laser cleaning of these types of papers. At these fluences no damage to the substrate and iron ink writing is produced. Figure 2 and Figure 3 are examples of 532 nm laser cleaning of rag paper that has surface dirt from handling and dust. 3.2 Cleaning assisted by digital image processing A first procedure to evaluate the relative degree of dirt on the sample and its homogeneity after cleaning is as

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Figure 2. Rag paper with surface dirt partially cleaned with laser radiation: 532 nm and a fluence of 0.3 J/cm2 .

Figure 3. Manuscript from 1830 with surface dirt produced by handling cleaned by laser at 532 nm and at a fluence of 0.3 J/cm2 . The paper was fabricated by mechanical treatment of vegetal fibers.

Figure 1. Rag paper treated with ten 1064 nm Nd: YAG laser pulses, a) before irradiation, b) using fluences above the damage threshold (2.8 J/ cm2 ), c) at 0,6 J/cm2 , a fluence above dirt thresholds and ca. five times below the damage threshold of the paper.

follows. First, an image of a manuscript in its original state was taken. Then, the manuscript was soiled with candle soot and after that, cleaned by the different procedures. For each cleaning procedure, an independent region of the manuscript was selected. Images of the different stages of the cleaning were taken. Digital subtraction between the image of the treated sample and the original were performed, and the mean value S, of the intensity of the resulting image and its standard deviation were calculated. S represents the amount of

dirt remaining on the surface. It is important to mention that the procedure requires a reference state (i.e. an image of the sample without dirt, or on a previous state considered a reference). Figure 4 shows an example of laser cleaning of a non patrimonial value manuscript of 1818. The manuscript was made of rag paper and written with iron gall inks. Figure 4a shows the original manuscript. Figure 4b corresponds to the manuscript soiled with candle soot, and Figure 4c shows the result after laser cleaning Figure 5 shows how this procedure allowed us to obtain the comparison between results obtained by laser cleaning of a manuscript with 1064 nm and 532 nm. It can be seen that cleaning with 532 nm is more effective than that performed with infrared light, for which a pale yellow patina is observed after cleaning.

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Surface dirt (S) [a.u]

60

Laser at 532 nm Láser at 1064 nm Soft rubber Hard rubber

45

30

15

0 Dirt before cleaning Dirtafter cleaning

Figure 6. Cleaning quality of the different procedures was determined by using digital image processing.

Figure 4. Manuscript from the Buenos Aires Customs House (ca. 1818), made of rag paper and written with iron gall ink, a) original, b) covered by a dirt film of candle soot, c) partially cleaned with laser at 1064 nm and fluence of 0.3 J/cm2 .

3.3 Quality cleaning factor

60 λ = 532 nm

To assess the quality of the cleaning procedures we define a quality cleaning factor L as:

λ = 1064 nm

50

Surface dirt (S)[u.a]

damage and bleaching of the paper due to thermal effects. In this way, when this procedure is applied, the sign reversal could be used as a control parameter of damage in the sample. Figure 6 shows a comparison between laser cleaning and dry cleaning with rubbers.

40 30 20 10 0 -10 0

2

4

Number of scans

Figure 5. Comparison between laser cleaning of a manuscript with 1064 nm and 532 nm as a function of the number of scans, performed by using digital image processing. S represents the dirt that remains on the surface.

As it can be seen with 532 nm, two scans are sufficient to recover the original state. With multiple scans, negative values can be obtained indicating degrees of dirt lower than the initial value. This effect is more evident by increasing laser repetition rate, indicating

Where Ic is the intensity in each pixel of the image of the sample after cleaning, Io is the intensity in the image in its original state, and Id is the intensity in the image of the sample in the soiled state. For each image the value of L was averaged over the cleaned region. The cleaning factor L gives a measure of the cleaning effectiveness per unit of dirt. L gives the same qualitative information as that obtained with the parameter S, and also affords the possibility of a comparison between different cleaning procedures which is independent of the concentration of dirt on each sample. Table 1 shows calculated values of the parameter L for the different cleaning procedures. Results of Figure 6 and of Table 1 show that the cleaning with soft rubber is of lesser quality than that was obtained with laser at 532 nm (L = 0.070 ± 0.070) both in mean value as in uniformity. The use of hard rubber allows to improve the quality of the cleaning approaching the result obtained using laser

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support from the Faculty of Engineering of Universidad Nacional de La Plata, CONICET, and CIC of Argentina. GMB is a research member of CIC and UNLP. H.R. is a research member of CONICET and UID Optimo, DCB-FI-UNLP.

Table 1. Calculated values of parameter L for the different cleaning procedures. Cleaning method Laser λ = 532 nm

Laser λ = 1064 nm

Soft rubber Hard rubber

State

L

σ

0 scans 1 scans 2 scans 3 scans 4 scans 0 scans 1 scans 2 scans 3 scans 4 scans Initial (dirty) After cleaning Initial (dirty) Intermediate cleaning Final cleaning

1 0.175 0.076 0.070 0.068 1 0.413 0.273 0.230 0.230 1 0.182 1 0.079 0.085

– 0.130 0.080 0.070 0.080 – 0.270 0.105 0.090 0.090 – 0.090 – 0.070 0.070

(L = 0.079 ± 0.080). Nevertheless, the abrasion produced by the rubbing partially destroys the fibers, the paper (that showed a corrugated structure before treatment) is leveled and the thickness in the processed region is diminished as can be observed at the naked eye.

4

CONCLUSIONS

We determined optimum fluence values for laser cleaning of 18th and 19th century documents and manuscripts belonging to the National General Archive of Argentina. By using digital image processing tools it was possible to compare the quality of different cleaning procedures. We developed a method that can be applied to samples in which a reference image (original state or reference state of the sample) was previously taken. Conventional methods such as dry cleaning with soft and hard rubbers in comparison to laser cleaning treatments were analyzed. We demonstrated that 532 nm pulsed nanosecond laser cleaning is the best procedure to eliminate surface dirt produced by candle soot. We conclude that digital image processing is a useful tool to evaluate such parameters as the relative amount of dirt, its homogeneity, and the quality of the cleaning processes.

ADKNOWLEDGEMENTS We would like to thank Professor Dr. John Asmus and Dr. Nestor Bolognini for useful comments on this manuscript. This work was partially financed by

REFERENCES Andreotti, A., Colombini, M., Conti, S., de Cruz, A. Lanterna, G., Nussio, M. L., Ankara, K. & Penaglia, F. 2005. Preliminary results of the Er:YAG laser cleaning of textiles, paper and parchment. Proceedings of Lasers in the Conservation of Artworks, LACONA VI Vienna, Austria. Asmus, J. 1997. Radiation-Divestment Paleontology. LACONA II Liverpool, United Kingdom (Reprinted in SPIE 4402: 72–85.) Freisztav, C., Cap, N., Rabal, H., Orsetti, A. & Bilmes, G. M. 2000. Control de calidad de la limpieza de superficies con láser, utilizando procesamiento digital de imágenes. Taller Iberoamericano de Física Aplicada en la Ingeniería -EFING 2000. La Habana-Cuba. Kautek, W., Pentzien, S., Rudolph, P., Krüger, J. & König, E. 1998. Laser interaction with coated collagen and cellulose fibre composites: fundamentals of laser cleaning of ancient parchment manuscripts and paper, Appl. Surf. Sci., 746–754. Kolar, J., Strlic, M., Müller-Hess, D., Gruber, A., Troschke, K., Pentzien, S., Kautek, W. 2000. Near-UV Pulsed Laser Interaction with Paper.Journal of Cultural Heritage, 1: S221–S224. Kolar, J. Strlic, M., Müller-Hess, D., Gruber, A., Troschke, K., Pentzien, S. & Kautek, W. 2001. Interaction of laser light with soiled paper. Proceedings of Lasers in the Conservation of Artworks, LACONA IV–ICOMOS, Paris, France. Kolar, J. Strliˇc, M. & Marinˇcek, M. 2002. IR pulsed laser light interaction with soiled cellulose and paper. Appl. Phys A: 75: 673–676. Kolar, J., Strlic, M., Müller-Hess, D., Gruber, A., Troschke, K. &Pentzien, S.2003. Laser cleaning of paper using Nd:YAG laser running at 532 nm. Journal of Cultural Heritage. 4: 185–187. Komar, K., Sliwinski, G. 2005. Non-destructive observation of the ablative cleaning effect on historical paper via the laser-induced fluorescence spectra. Proceedings of Lasers in the Conservation of Artworks, LACONA VI Vienna, Austria. Ochocinska, K., Kaminska, A., Sliwinski, G. 2001. Experimental investigations of stained paper documents cleaned by Nd: YAG laser pulses,Lasers in the conservation of artworks Lacona IV–ICOMOS, Paris. Pérez, C., Barrera, M. & Diez, M. L. 2001.Positive findings for the use of laser in the cleaning of cellulose based supports, Proceedings of Lasers in the Conservation of Artworks, LACONA IV–ICOMOS, Paris, France. Strliˇc, M. & Kolar. J. 2005.Laser cleaning of paper - an extensive optimisation study. Proceedings of Lasers in the Conservation of Artworks, LACONA VI Vienna, Austria. Zafiropulos, V. 2002. in Laser Cleaning. Boris Luk¨ yanchuk ed. World Scientific.

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Laser reduction of stamps from paper to avoid migration to the recto side: Case study based on illustrations from Jan Heesters M. Lentjes & K. Dickmann Laser Center (LFM), Münster University of Applied Sciences, Germany

P. van Dalen Art Conservation, Kon.Wilhelminahaven ZZ19, The Netherlands

ABSTRACT: In the past a fraction of the Jan Heestershuis museum’s collection was marked with stamps on the verso side of the artworks. The stamps ink migrated over several years into the paper with the result that the stamps become visible on the recto side. A part of these artworks shows the presence of the stamps on the recto side. The company Art Conservation was ordered to treat about 300 flat paper objects stamped on the verso side with in water soluble blue ink by a dry method. Part of these artworks (170 of the 300) was treated by laser radiation. The remaining objects did not come through the selection procedure. The results obtained using laser irradiation were positive, the stamps could be reduced within 8 minutes.

1

INTRODUCTION

Migration of stamp ink from the verso to the recto side is a well known problem within different art collections (Fig. 1). This problem concerns also the 300 drawings of the illustrator Jan Heesters (1893–1982) within the collection of the Jan Heestershuis museum (Schijndel, Netherlands). These drawings are made on wood-pulp vellum paper, rag paper and wood-pulp woven paper. In the past, these stamps (6 × 2 cm2 ) consisted of water

Figure 1. Stamp ink migrated from the verso to the recto side.

soluble blue ink were placed as an ownership mark on the verso side of the drawings. In recent years, there were some trials aimed at the elimination of the stamp ink from these drawings by the use of different solvents. The result was the leaking of the stamp ink whereupon the treatment was stopped. In general, it is possible to locally remove or reduce stamp ink by means of solvents and low-pressure table. However, this method may subsequently induce tidemarks (Fig. 2). An outcome of the European laser project PaReLa (Paper Restoration using Laser Technology, Havermans et al. 2003) shows that stamp ink can be reduced by green (532 nm) laser radiation without affecting the

Figure 2. Water soluble ink leaked after treatment with a solvent. After artificial ageing, tide-marks became visible at the positions treated with solvents.

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paper fibres. Complete removal of the stamps without damaging the paper is not possible, since the ink is soaked deeply into the paper fibres. Nevertheless, a reduction of the stamp ink results in a decrease of the translucence of the stamp and stops the migration into the paper. In individual cases the stamp ink had even disappeared completely at the recto side after laser treatment. Due to the results obtained in the frame of the PaReLa project and internal research, the conservation company (Art Conservation) and the Laser Center (Münster University of Applied Sciences) decided to treat the drawings from Jan Heesters museum with laser radiation. This paper presents the results obtained in the frame of the cooperation between restorers and scientists working on a real case study. 2

CONVENTIONAL TECHNIQUES

Different conventional techniques have already been developed to reduce or to remove contamination (e.g. ink) from paper objects. The use of conventional techniques based on solvents can increase the probability that the stamp ink will leak. Therefore, the stamp ink is usually removed in advance in order to avoid irreparable damage to the artwork. A method frequently used consists in the use of a low-pressure table in combination with solvents and filtration paper which serves as an absorber. Other methods are based on spreading gel or pulp (composed of methylcellulose or fuller’s earth) on the ink in order to extract it from the paper, and the application of bleaching agents to remove or reduce this stamp ink. The disadvantage of the above mentioned methods is that they produce often more damage to the object than restore it. During these treatments the ink can leak (Fig. 2) or migrate to the recto side. To avoid this problem the stamp ink is fixed using a fixing agents with the inconvenient of fixing also the contamination between and on the paper fibres. This procedure can induce discoloration and negative effects on the paper in the future. 3

Research investigations within the PaReLa framework (Havermans et al. 2003, Scholten et al. 2005a, b, Rudolph et al. 2004a, b) using different wavelengths (248 nm, 532 nm, 1064 nm) confirmed that 532 nm is the best wavelength to remove stamp ink from paper substrates.The wavelength of 532 nm showed the highest cleaning efficiency without discoloration effects and chemical modifications of the paper carriers (pulse energy density used below paper modification threshold). Also, after artificial ageing, no undesired modifications of the paper carriers were detectable. Paper reflects, transmits and absorbs laser radiation at the wavelength 532 nm. Hence, it is not only possible to remove stamp ink from the paper surface but also to remove it from between the paper fibres. When the applied pulse energy density is lower than the paper modification threshold, the paper fibres are not affected. The amount of stamp ink removed per laser pulse increases with intensity. Nevertheless, stamp ink soaked into the paper fibres can not be removed without affecting the fibres. In this case a complete removal of the stamp ink by laser radiation is impossible. However, the amount of stamp ink can be reduced. A disadvantage of the transmitted laser radiation is the removal of media and contamination at the recto side during irradiation of the stamp ink at the verso side (erasing). Especially for charcoal drawings with a strong surface contamination, this contamination at the recto side will also be removed. This originates an elucidation of the surface (Fig. 3). This effect is often less serious than the migration of the stamp ink itself and can be retouched. Stamps partially placed behind the drawings are partially treated since complete treatment would result in removing the media. Both effects decrease with increasing paper thickness. Some side effects that can occur during incorrect laser cleaning of paper objects are (Scholten et al. 2005a): a) Fluffing: standing up of paper fibres due to an energetic expulsion of small particles located between the fibres,

LASER TREATMENT

Laser cleaning is a good alternative to conventional methods for the reduction of the stamp ink from paper substrates. General advantages of laser cleaning are cited bellow: a) b) c) d) e)

Free contact method, High lateral resolution, Results directly visible at the naked eye, Easy control of the laser radiation parameters, No other waste material generated than the removed material.

Figure 3. Elucidation of the surface as a result of removed surface contamination by transmitted laser radiation.

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b) Thinning: unwanted removal of carrier material, the paper becomes thinner, c) Charring or carbonization: a too high energy density can result in local burning of the paper, d) Yellowing: discoloration of the paper, that can especially occur in wood pulp papers, e) Erasing: unwanted removal of media or contamination by transmitted laser radiation. During the trials if one of the above effects appeared during laser cleaning, the laser energy density was directly reduced or the treatment was stopped immediately. 4

LASER CHARACTERISTICS

The drawings were laser treated at the Laser Centre of the University of Applied Sciences Muenster (Germany) by a frequency doubled Q-switched Nd: YAG laser with articulated arm (Thales, SAGA 220/10, 532 nm, 10 ns, 10 Hz) (Fig. 4). By adjusting the telescope at the hand piece the working spot diameter can be varied and consequently the laser density.The stamp ink treatment was accomplished using 532 nm and energy densities ranging between 200 and 800 mJ/cm2 . For precise positioning of the laser radiation on the

Figure 4. Photograph of the applied Nd:YAG laser cleaning system with articulated arm.

objects a He-Ne laser was coaxially implemented with respect to the Nd:YAG beam. 5 TREATMENT METHODOLOGY The concerned drawings of the Jan Heestershuis vary in size, thickness, medium, type of paper, stamp placement, etc. Table 1 shows the treated paper-media combinations. All drawings were handled with the same procedure: a) Selection, b) Documentation, c) Testing and if necessary adjustment of the laser parameters, d) Laser treatment, e) Documentation. 5.1 Selection of the drawings to be treated It is known that paper can be transparent at 532 nm. This means that radiating a stamp positioned opposite to the drawing on the reverse side can result in partially erasing a part of this drawing. On the basis of this experience only drawings where the stamps are placed completely or partially outside the drawing boundaries were selected to be treated. In case of fractional treatment, the stamps were partially covered with metal sheets to protect the medium at the reverse side during laser irradiation (Fig. 5). Previous investigations demonstrated that the applied laser radiation had a negative effect on the colored paper. This paper becomes darker in the irradiated

Figure 5. Locally treated stamp ink, the not to be treated parts were covered with metal sheets.

Table 1. Treated paper types and medium combination.

Charcoal Aquarelle Pencil Chalk

Wood-pulp vellum paper (bleached cellulose)

Wood-pulp woven paper (bleached cellulose)

80 g/m2

100 g/m2

115 g/m2

X X

X

125 g/m2 X X

X X

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Rag paper 135 g/m2 X

Figure 6. Stamp ink before and after laser treatment, reduction of the ink is clearly visible.

positions. On the basis of the above described criteria, 130 drawings were excluded from the treatment. Nevertheless, 170 drawings were laser treated. 5.2

Documentation

The characteristics and the results of the treated drawings have been saved in a database with the following entries: inventory number, paper type, paper thickness, medium type, laser parameters, reduction/removal of the ink, processing time in minutes and additional notes. Each object was photographed before and after the laser treatment.

negatively to laser radiation were rejected during the test procedure. Since every object is special on its own, no table can be presented showing the optimal parameters for the treated carrier-medium combinations. In general, the smooth and thinner paper types (≤80 g/m2 ) were treated with a lower energy density than the thicker paper types (≥120 g/m2 ) and the surface contamination degree had a high influence on the applied energy density. Objects with a lower surface contamination degree could be treated with a higher energy density than objects with a high surface contamination degree without originating an elucidation of the surface at the recto side. 7

CONCLUSIONS

The stamp ink on the verso side of 170 artworks on paper from the illustrator Jan Heesters was successfully reduced. After the laser treatment, the ink was barely visible or even invisible at the recto side. It was intended to treat a larger number of objects (300). However, the remaining objects did not come through the selecting procedure and were rejected from the treatment. REFERENCES

5.3 Tests at the corner of the artwork Since the treated objects are original artworks (with differences in paper type, thickness and surface pollution) the laser energy density was tested on the corner of each paper document and optimized for the object. The tests were done on the carrier surface in order to verify that the paper was not affected during the adjusted laser irradiation density. If unpredictable reactions appeared during the test procedure (e.g. local discoloration) which could not be avoided by adjusting the irradiation density, the object was excluded from the treatment. 6

RESULTS OF THE LASER TREATMENT

From the restorer’s point of view the obtained results are satisfying. The stamp ink (verso side) on the laser treated objects was reduced so far that the ink was barely visible or even invisible at the recto side. Figure 6 shows a photo of the typical stamp placed on the verso side of the drawings, the right half is treated with laser radiation, a clear reduction of the stamp ink is observed. The treatment time per stamp was about 15 minutes, this includes dismounting, photographing and documenting. The effective laser treatment time was about 8 minutes per stamp. During the treatments no inconvenient effects were observed. The objects responding

Havermans, J.B.G.A., Aziz, H.A., Rudolph, P., Kautek, W., Ligterink, F.J., Pedersoli, J.L., Scholten, H., Schipper, D., Quillet, V., Kraan, M., van Beek, B., Hua-Ströfer, H.Y. & van Dalen, P. 2003. PARELA – PAPER RESTORATION OF PAPER OBJECTS USING LASER TECHNOLOGY. In Conference Proceedings of the International Conference Chemical Technology of Wood, Pulp and Paper, Bratyslawa, 402–407. Scholten, H., Schipper, D., Ligterink, F.J., Pedersoli, J.L., Rudolph, P., Kautek, W., Havermans, J.B.G.A.,Aziz, H.A., van Beek, B., Kraan, M., van Dalen, P., Quillet, V., Corr, S. & Hua-Ströfer, H.Y. 2005. Laser Cleaning Investigations of Paper Models and Original Objects with Nd:YAG and KrF Laser Systems. In Dickmann, K. & Fotakis, C. & Asmus, J.F. (eds), Springer Proceedings in Physics 100, Lasers in the Conservation of Artworks, 11–18. Scholten, H., van Dalen, P., Corr, S., Rudolph, P., Havermans, J.B.G.A., Aziz, H.A. & Ligterink, F.J. 2005. Laser Cleaning of Pressure Sensitive Tapes on Paper. In Dickmann, K. & Fotakis, C. & Asmus, J.F. (ed), Springer Proceedings in Physics 100, Lasers in the Conservation of Artworks, 43–49. Rudolph, P., Ligterink, F.J., Pedersoli Jr. J.L., van Bommel, M., Bos, J., Aziz, H.A., Havermans, J.B.G.A., Scholten, H., Schipper, D. & Kautek, W. 2004. Characterization of laser-treated paper. Applied Physics A, 79: 181–186. Rudolph, P., Ligterink, F.J., Pedersoli Jr. J.L., Scholten, H., Schipper, D., Havermans, J.B.G.A., Aziz, H.A., Quillet, V., Kraan, M., van Beek, B., Corr, S., Hua-Ströfer, H.-Y., Stokmans, J., van Dalen, P. & Kautek, W. 2004. Laser-induced alteration of contaminated papers. Applied Physics A, 79: 941–944.

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Laser cleaning and multi-method diagnostics of textile pieces of art W. Kautek University of Vienna, Department of Physical Chemistry, Austria

M. Oujja & M. Castillejo Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain

J. Hildenhagen Fachhochschule Münster, Steinfurt, Germany

V. Ljubic Technisches Museum Wien, Vienna, Austria

M. Simileanu, W. Maracineanu & R. Radvan National Institute of Research and Development for Optoelectronics, Bucharest, Romania

V. Zafiropulos Technological Educational Institute of Crete, Sitia, Greece

N. Melniciuc Alexandru Ioan Cuza – University of Ia¸si, Romania

G. Pajagic Bregar National Museum of Slovenia, Ljubljana, Slovenia

M. Strlic University College London, Centre for Sustainable Heritage, London, UK

ABSTRACT: A cleaning and multi-method diagnostic study of original ancient textiles of the Sucevita Monastery, Bukovina, Romania, is reported. Laser cleaning was performed with a Q-switched Nd:YAG laser. Maximum contrast between contaminant and substrate was realized in employing green laser light at 532 nm. Laser-Induced Breakdown spectroscopy (LIBS) was complemented by colorimetry, multi-spectral imaging, and optical microscopy. These diagnostics served to monitor the laser process in order to define strategies to preserve the textile substrates with optimum cleaning results.

1

INTRODUCTION

Pulsed lasers are becoming new tools in arts conservation (Cooper 1998, Fotakis et al. 2006, Kane 2006, Nimmrichter et al. 2007, Schreiner & Strlic 2008). The treatment of biopolymeric material, such as varnishes on paintings, biogenetic surfaces such as paper, cellulose, collagen, parchment (Cooper 1998, Fotakis et al. 2006, Kane 2006, Nimmrichter et al. 2007, Schreiner & Strlic 2008, Kautek et al. 1997, Kautek et al. 1998, Kautek 2008, Kolar et al. 2000, Strlic et al. 2003, Rudolph et al. 2004), and of textiles such as silk (von

Lerber et al. 2005, von Lerber et al. 2007) has been approached only in recent years whereas stone and façade cleaning has reached a mature commercial level conservation (Cooper 1998, Fotakis et al. 2006, Kane 2006, Nimmrichter et al. 2007, Schreiner & Strlic 2008). Treatment of biogenetic fibrous materials at a wavelength of 1064 nm often causes yellowing and polymeric crosslinking (Kolar et al. 2000, Strlic et al. 2003). UV laser irradiation, on the other hand, leads to bond breaking and polymer scission and a decrease of the degree of polymerization (Kolar et al. 2000). Maximum contrast between contaminant and substrate

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can be exploited by green laser light at 532 nm were the substrate exhibits minimum interaction guaranteeing minimum deterioration (Kautek et al. 1997, Kautek et al. 1998, Kolar et al. 2000). The present cleaning and multi-method diagnostic study of original ancient textiles was performed during the Culture 2000 Project CLT “Saving sacred relics of European medieval cultural heritage” (2005/A1/CHLAB/RO-488) at the Sucevita Monastery, Province of Bukovina, Suceava County, Romania. In this investigation, a particular challenge represented the cleaning of composite textiles such as metal-threads with silk and cellulose-based cores.

2 2.1

Figure 1. Stole (epitrachion). Sucevita Monastery Inv. Nr 353/3.

EXPERIMENTAL Instrumentation

Laser cleaning was performed with a Q-switched Nd:YAG laser (Quanta Systems, pulses of 6 ns, repetition rate 10 Hz). The samples were irradiated through a focussing lens (f = 100 cm) achieving fluences up to 9 Jcm−2 . Maximum contrast between contaminant and substrate was achieved in employing the second harmonic green laser line at 532 nm. Laser-induced breakdown (LIBS) spectra were recorded in the 300–700 nm wavelength range with a Michelle spectrograph coupled to a time gated ICCD camera (Andor Technologies). The temporal gate was operated at 500 ns time delay and a width of 3 µs. This was complemented by colorimetry (Spectrolino, GretagMacbeth), multispectral imaging (Artist, Art Innovation), and optical microscopy.

2.2

Figure 2. Maniple (minecute). Sucevita Monastery Inv. Nr. 586, 28 × 13 cm. 18th c., Parohia Beresti, St. Nicolae, Suceava.

Objects

The objects chosen for this study were a stole (epitrachion, Sucevita Monastery; Fig. 1) and a maniple (minecute, 18th, from Parohia Beresti, St. Nicolae, Suceava; Fig. 2). The stole and the maniple exhibited metal-wrapped fibres and textile bordures.

3

RESULTS AND DISCUSSION

The general laser cleaning strategy was based on the fundamental discovery that cellulose and collagenbased materials degrade by UV-laser treatment due to depolymerization, but show no detrimental effects at 532 nm (Kautek et al. 1998, Kolar et al. 2000). This can be ascribed to the fact that these biogenetical materials are more or less transparent in the visible region, however strongly absorb UV radiation, e.g. by the CC-bond at 347 nm, the C-O-bond at 333 nm, and the C-H-bond at 289 nm, followed by depolymerization and/or photo-oxidation (Kautek 2008).

Figure 3. Laser-induced breakdown (LIBS) spectrum of stole (epitrachion, Fig. 1). Sucevita Monastery.

LIBS unveiled the chemical composition of the metal threads and the contaminants. The bordure of the maniple exhibited mainly Cu (Fig. 3) indicating that the metal threaded ornaments consisted of Cuwire covered textile threads. The contaminants were mainly Mn, Si, Fe, Al, Ca, Na, which can be representative for typical dust (various oxides or salts). The analogous experiment on the stole (epitrachion, comp.

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5 9

7

8

6

Figure 4. Laser cleaning spot series for colorimetric measurements of backside of stole (epitrachion, Fig. 1). 0.05 J. 5: reference; 6: 5 pulses, N = 5; 7: N = 10; 8: N = 20; 9: N = 50. 60,00

Figure 6. Laser cleaning spot series for colorimetric measurements of front side of stole (epitrachion, Fig. 1). 0.05 J. 1: reference; 2: N = 5; 3: N = 10.

40,00 30,00

40,00

20,00

35,00

10,00

30,00 L*a*b Values

L*a*b* Values

50,00

0,00 0

10

20

30

40

50

No. of Pulses

25,00 20,00 15,00 10,00

Figure 5. Colorimetric measurements of backside of stole (epitrachion, Fig. 1). L∗ a∗ b∗ standard. a: , b: , L: .

5,00 0,00 0

Fig. 1) showed that the centrally located ornament loops contained mainly Ag, instead, with the contaminants Ca, Na, Al, and Mn. The bordure exhibited no metals indicating the usage of only organic textile material contaminated with material (dust) containing Fe, Mn, Ca, and Na. Silk threads showed no metals, as expected, with Na, Ca, and Al contamination. Colorimetry according to CIE-L∗ a∗ b∗ colour coordinates yielded valuable data in respect to both lightness (L) and saturation and hue given by the chromaticity coordinates a∗ and b∗ . The linen backside of the epitrachion was treated with various numbers of pulses (Fig. 4). The colorimetric evaluation shows with even better resolution than the naked eye that the lightness and the coordinate b∗ reaches the final cleaning state after somewhat more than 10 pulses (Fig. 5). The coordinate a∗ is not changed at all. Thus an optimum and minimal treatment can be defined with ultimate cleaning effect and minimum doses. The same diagnostics is demonstrated for loop ornaments (identified by LIBS as silver-rapped) on the front side of the epitrachion (Fig. 6). In this case the final cleaning state could be reached after N < 5 (Fig. 7). The multi-spectral imaging after laser treatment documents the cleaning effect in the visible (Fig. 8). The UV-reflectivity (Fig. 9) indicates either removal of absorptive material (cleaning) or chemical changes

2

4

6

8

10

No. of Pulses

Figure 7. Colorimetric measurements of front side of stole (epitrachion, Fig. 1). L∗ a∗ b∗ standard. a: , b: , L: .

Figure 8. Multi-spectral image of backside of stole (epitrachion, Fig. 1) after local laser cleaning (centre). Visible image.

according to the UV-absorptivity as a prerequisite for fluorescence emission. The Fluorescence image is a sensitive diagnostics for chemical changes of the substrate (Fig. 10). One can see that both UV- and fluorescence changes correlate with the removal of contaminants rather than with the alteration of the textile. The IR-image (Fig. 11) supports this evaluation

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imaging, and optical microscopy can serve as dependable diagnostics even under sever field conditions. ACKNOWLEDGEMENTS

Figure 9. Multi-spectral image of backside of stole (epitrachion, Fig. 1) after local laser cleaning (centre). UV reflection image.

This study was partially supported by the Culture 2000 Project CLT 2005/A1/CHLAB/RO-488 “Saving sacred relics of European medieval cultural heritage”, which was initiated and supervised by one of the authors (R. Radvan). The invaluable discussions with and suggestions by Octaviana Marincas (University of Arts – George Enescu – Ia¸si) are acknowledged. REFERENCES

Figure 10. Multi-spectral image of backside of stole (epitrachion, Fig. 1) after local laser cleaning (centre). Fluorescence image.

Figure 11. Multi-spectral image of backside of stole (epitrachion, Fig. 1) after local laser cleaning (centre). IR reflection image.

by indicating absorption changes from deeper layers. As in the UV case, the IR-reflectivity of the cleaning area resembles that of areas practically not covered by contaminant layers. This multi-methodical study demonstrated that the application of the second harmonic green laser line at 532 nm together with laser-induced breakdown spectroscopy (LIBS), colorimetry and multispectral

Cooper, M. 1998. Laser Cleaning in Conservation. Butterworth-Heinemann. Fotakis, C., Kautek, W. & Castillejo, M. (eds) 2006. Laser Chemistry, Special Issue, Lasers in the Preservation of Cultural Heritage. Hindawi Publishing Corporation New York. Kane, D.M. (ed) 2006. Laser Cleaning II. World Scientific, New Jersey, London, Singapore. Kautek, W. & König, E. (eds) 1997. Restauratorenblätter, Special Issue, Lasers in the Conservation of Artworks I. Verlag Mayer & Comp. Wien. Kautek, W., Pentzien, S., Krüger, J. & König. E. 1997. Restauratorenblätter, Special Issue, Lasers in the Conservation of Artworks I, 69. Verlag Mayer & Comp. Wien. Kautek, W., Pentzien, S., Rudolph, P., Krüger, J. & König, E. 1998. Appl. Surf. Sci. 127–129: 746. Kautek, W. 2008. Laser for Cleaning: Paper, Parchment, Textiles. In Handbook on the Use of Lasers in Conservation and Conservation Science, M. Schreiner & M. Strlic (eds), COST G7. In press. Kolar, J., Strlic, M., Pentzien, S. & Kautek, W. 2000. Appl. Phys. A 71: 87. von Lerber, K., Pentzien, S., Strlic M. & Kautek W. 2005. In 14th Triennial Meeting the Hague, (ed.) I. Verger, ICOM Committee for Conservation, Preprints Vol. 2, p. 978. James and James London. von Lerber, K., Strlic, M., Sokhan, M., Krueger, J., Pentzien, S., Kennedy, C., Wess, T. & Kautek, W. 2007. In Springer Proceedings in Physics, Vol. 116, Lasers in the Conservation of Artworks VI, p. 313. Springer-Verlag Heidelberg. Nimmrichter, J., Kautek,W. & Schreiner, M. (eds) 2007. Springer Proceedings in Physics, Vol. 116, Lasers in the Conservation of Artworks VI. Springer-Verlag Heidelberg. Rudolph, P., Ligterink, F.J., Pedersoli, Jr. J.L., van Bommel, M., Bos, J., Aziz, H.A., Havermans, J.B.G.A., Scholten, H., Schipper, D. & Kautek, W. 2004. Appl. Phys. A 79: 181. Schreiner, M. & Strlic, M. (eds) 2008. Handbook on the Use of Lasers in Conservation and Conservation Science. COST G7. In press. Strlic, M., Kolar, J., Selih, V.S. & Marincek, M. 2003. Appl. Surf. Sci. 207: 236.

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Study of the effects of laser cleaning on historic fabrics: Review and results eight years after applications A. Martínez & C. Escudero Centro de Conservación y Restauración de Bienes Culturales de Castilla y León (CCRBC). Valladolid, Spain

ABSTRACT: The CCRBC, after a phase of experimental tests on modern manufactured fabrics in order to evaluate the effects of laser cleaning on the fibres and delimit the type of interaction which takes place between the laser and the fabric, introduced in 1997, as work samples, historic textile remains of scarce importance. These samples with a real ageing and alteration made possible to study and analyse the effects of laser radiation with greater reliability than on artificially prepared samples. This article presents the results obtained immediately after the application of laser on the samples and an analytical review of the treated fabrics eight years later, investigating the evolution, in the mid-term, of the experimental treatment.

1

INTRODUCTION

The application of lasers in the field of cultural assets began more than three decades ago as part of the cleaning processes of stone materials. In recent years, we have seen the development of new equipment, with the introduction of various wavelengths in the cleaning of cultural assets. At the same time, research has progressed with regard to the minimization of the unwanted alterations that result from this system and its validation. Efforts are being made to extend the application of this technology to new materials: metal, ivory, wood, paper and parchment, among others, with textiles as the latest material to be included. The publications on the use of laser with fabrics are few and when we review them we can see that they focus on two problems in particular: 1) The cleaning of so-called ‘braids’ (textile decorations that consist of coating fibre with gold, silver, gilt-silver or gilt-copper leafing) to remove the alteration products that cover up the decorative features of the metal (Campos & Hermitte 2000, Degrigny et al. 2001, Lee et al. 2001, Sokhan et al. 2003). 2) Cleaning of actual textile fibres. Most research work focuses on silk and cotton (Kolar et al. 2003, Sutcliffe et al. 1999), with few examples of the application of laser radiation on the cleaning of historic fabrics, i.e. concerning real problem dealing with ageing and alteration (Brunetto et al. 2004, Polonovki & Oger 1994, Several Authors 2002).

Since the use of laser as one of the tools applied in conservation and restoration work, the CCRBC has shown its concern for the rigorous application of lasers on historic materials (Escudero et al. 2002, Pérez et al. 2001). These materials possess inherent features that are not easily assessed in new ones, even though they may be taken as an interesting initial reference. Consequently, in 1997, and after initial experiments with modern fabrics which produced encouraging results, a laser-cleaning process was applied to relatively insignificant textile remains from the Middle Ages that were subject to real ageing and alteration. This enabled a more reliable study and analysis of the effects of laser radiation than the results on an artificially prepared sample. The laser was used to remove particles of dirt and organic remains that were stuck to the structure of the fabric. The particles came loose apparently thanks to the mechanical phenomena induced by the laser energy. Consequently, the photothermal phenomena will be a ‘secondary’ feature of the cleaning process, but significant when assessing the changes that take place in the fibres. Indeed, given their organic nature, alteration processes introduced during the laser cleaning process and susceptible to a negative evolution over time, cannot be discarded (thermal oxidation and/or photo-deterioration of the fibres). As a result, after the intervention, it is considered necessary to perform successive inspections to discover the effectiveness of the treatment and observe the conservation of the work, which is the aim of any intervention. As a result, this work offers a review of the fabrics which were tested with laser eight years ago. This

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one single pulse at a greater energy density was more effective than accumulated pulses at lower density. 2.3 Instrumental laboratory methods

Figure 1. a) Fragment of silk, right zone shows laser cleaning, b) wool, centre zone with a laser cleaning test, c) fragment of linen, right zone, with a laser cleaning test.

involves a more complete analysis of the study of the evolution of the experimental treatment. 2 2.1

EXPERIMENTAL Samples

The usual water-based cleaning process of historical fabrics throws up different results depending on the material nature of the fibre, regardless of the usual variables of the state of conservation and treatment chosen. Therefore, in 1997, different types of fabric samples were selected for this experimental work with a view to establishing possible differences between proteinaceous and cellulosic media. For the protein fibres, two fragments were chosen from the Royal Pantheon of San Isidoro in León, dating from between the 11th and 13th centuries: a piece of silk (Fig. 1a) and a piece of wool (Fig. 1b), both showing the signs of deterioration expected of funerary materials. The cellulosic fabric was linen (Fig. 1c) used as a flat cloth covering. It was an interesting example owing to the fact that it was a textile showing natural degradation and an abundance of dirt and remains of the paste adhesive, as well as a significant rigidity caused by said adhesive. 2.2

Laser specifications

The laser used in the cleaning of the textile samples was a Nd:YAG laser with a wavelength of 1064 nm, frequency of 20 Hz, pulse duration of 6 ns and maximum energy density of 420 mJ. The tests were performed using a converging and a collimated energy beam. The converging beam is obtained by including a lens in the hand-piece. In this case, the energy applied depends on the diameter and the application distance, using fluences between 276 mJ/cm2 and 466 mJ/cm2 . In the case of the collimated beam, the energy does not depend on the distance, with a spot of 6 mm, working with fluences of between 19 and 369 mJ/cm2 . Pulses are not repeated in the same area, since in the preliminary tests on new materials, we saw that

In 1997 the analytical techniques focused on the study of the structure of the fibres using a scanning electron microscope (SEM JEOL JSM-840) at 1000x and 100x and an optical microscope at 40x before and after laser cleaning. In 2005 eight years later, the same fragments were subjected to various analytical tests and the study was extended to other specific analyses that could provide greater information about the state of the fibres. In order to evaluate whether morphological changes took place in this eight years period, both in the laser and non-laser treated zones, they were studied by a microscope on the longitudinal and cross-section modes with magnification 20x and 50x (Olympus BX 41 optical microscope) and a 600x & 2400x scanning electron microscope (Philips-Fei. Quanta). The effect of the laser energy on the chemical bonds of the materials treated was studied by using Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer-1600). Finally, it was considered interesting to evaluate whether the resistance properties were seen to be affected after the cleaning treatment. Thus, viscosymmetry was incorporated as a useful tool for establishing the degradation processes. Given the quantity of samples required for this type of analysis, the process was performed on silk and linen. The process is different depending on the sample since it concerns cellulosic and proteinaceous fibres: – Silk: Cannon-Fenske capillary viscosymeter, ref. K001, model 52001 Schott. Procedure, according to Swiss standard SNV 95595-1963. Sample preparation: washed by immersion in 50 ml of a diluted dissolution of neutral soap (1 g/l) at 30◦ C until all the particles of dirt were removed, a final rinse with distilled water and dried at 23◦ C and 50% HR for 48 hours. Dissolution of the silk samples in a concentrated dissolution (9 mol/l) of pure lithium bromide in closed retorts, leaving them in an oven at 60◦ C for 2 hours. Once the final dissolutions have been prepared, the effusion times were measured and a minimum of five readings were recorded for each liquid sample. The effusion time temperature was 25◦ C. – Linen: Cannon-Fenske viscosymeter (K002, type 52013/150), on an AVS/S-CF support with automatic measurement of the effusion time using Schott AVS350 equipment and programme. Sample preparation: in order to remove the starch from the paste, it was washed vigorously with 1 g/l of a detergent with enzymes (amylases) in a bath at 60◦ C and given a final rinse in distilled water. The absence of starch was verified by means of a test

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Figure 3. a) Silk fibres before laser cleaning with an important quantity of dirt particles, b) the silk fibres after cleaning. Figure 2. Left: untreated silk. On the right: microscopic aspect of the silk fabric subject to laser cleaning.

with an iodide/iodate reagent in a slightly acidic medium. Dissolution of the fabric was performed with a copper-ethylene diamine reagent.

3

RESULTS AND DISCUSSION

At first sight, the laser cleaning tests were highly satisfactory in the case of the silk fabric, where a large amount of dirt was removed (more than 87% of the adhered particles according to our calculations), showing a significant recovery of the inherent characteristics of brilliance and flexibility of this type of fibre (Fig. 1a). Particular mention must be made of the fragment of wool (Fig. 1b, Fig. 4) which was very deteriorated and which, before laser treatment, had already undergone a conventional cleaning typical of the restoration processes, consisting of aqueous cleaning on a suction table. This procedure reduced the intervention time helping to safeguard the fragile fibres and obtaining results that were somewhat limited. Following this treatment, further cleaning was made with the laser and the results revealed a substantial improvement. Analysis under an optical microscope (20x and 50x) established effective, uniform cleaning with no damage observed, even at the highest fluences. Furthermore, no negative evolution was seen over time (Fig. 2). The SEM study enabled a comparison between the linen, silk (Fig. 3) and wool (Fig. 4) fragments with and without laser treatment, comparing the results of 1997 and 2005. No significant differences were seen between the state of the cleaned and uncleaned samples. The spectra obtained by FTIR did not reveal any outstanding changes in the cellulosic material (linen), whereas in the proteinaceous material, there was a tendency to change in the intensity of the CH2 -link tension band (Fig. 5).

Figure 4. Wool fibres: a) and b) SEM analysis from 1997. a) Wool before laser treatment. b) After laser cleaning. c) and d) SEM analysis performed in 2005. c) Before the cleaning. d) After laser cleaning. The differences in the degree of conservation are due to the typical heterogeneities of the ancient fabric and correspond to the sample-taking zone.

The viscosymmetry tests did not detect a reduction in the intrinsic viscosity of the laser-treated silk samples in comparison with the untreated samples. A reduction was observed, however, for the treated linen samples (DPv value for untreated linen is 468.6; value for the laser-cleaned linen is 390.5).

4

CONCLUSIONS

The application of laser in the cleaning of fabrics is viable, but it must be performed rigorously in order to minimize the risks involved in the treatment. This study shows small changes that can be used to design future research to determine whether or not the reported minimum modifications are due to the tool or to the casuistic of the material under study. It must be remembered that all the analyses were performed

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REFERENCES

Figure 5. FTIR spectra performed on the proteic materials. a) Untreated silk, b) silk after laser cleaning, c) untreated wool, d) wool after laser cleaning.

on minimum sections of yarn and the historic fabrics showed consistent degradation. Results obtained encourage us to continue our research, increasing the number of samples in order to establish the cause of the aforementioned changes to define their origin. This would make possible to propose modifications to the laser settings. The laser cleans the fabrics in a way that requires more detailed research for use in the treatment of textile works or as a complement to other techniques. ACKNOWLEDGEMENTS We are grateful for the collaboration of the CCRBC laboratory of the Junta de Castilla y León and the interest shown by the laboratories that have taken part in the support analysis of this work: the University of Valladolid, the Centre for the Study of Cultural Assets ARTELAB and the ETSII of the Polytechnic of Catalonia.

Brunetto, A., Lucchini, G., & Rava, A. 2004. Un caso conservativo di una tela contemporanera con il LASER. Il restauro dell’opera di Marco Gastini. Rev. Progetto restauro no 29. Florencia 2004 Ed. Il prato p. 4–11. Campos & Hermitte 2000. Determination des conditions de restauration par faisceau láser de filés metalliques inséres dans les matériaux textiles. International Report, Arc’Antique, Nantes, 2000. Degrigny, C. et al. 2001. Laser cleaning of tarnished solver and copper threads in museum textiles. The 4th International Conference on Lasers in the Conservation of Artworks, Paris, p. 89–92. Escudero, C., Barrera, M. & Pérez, C. 2002. Estudio y preservación de pátinas cromáticas en monumentos. El láser como instrumento de limpieza. V International Symposium on the Conservation of Monuments in the Mediterranean Basin, Sevilla, 2002. Kolar, J et al. 2003. Surface modification during Nd:YAG (1064 nm) pulsed laser cleaning of organic fibrous materials. The 5th International Conference on Lasers in the Conservation of Artworks, Osnabrueck, p. 55–58. Lee, J.M.,Yu, J.E. & Koh.Y. 2001. Effect of wavelength in the laser conservation of silver textile. The 4th International Conference on Lasers in the Conservation of Artworks, Paris, 2001 p. 157–161. Pérez, C. et al. 2001. Positive findings for laser use in cleaning cellulosic supports. The 4th International Conference on Lasers in the Conservation of Artworks, Paris, p.194–200. Polonovki, P. & Oger, B. 1994 L’utilisation de la soie dans les plans-reliefs: faisabilité du nettoyage au laser. La conservation Des Textiles Anciens, Journées d’Etudes de la SFICC, Angers 1994 p. 20–22. Several Authors 2002. La veste funebre di sigismondo Pandolfo Malatesta nel tempio Malatesiano di Rimini: Il laser nel restauro dei tessili. Rev. Kermes, Florencia 2002, Dossier no 42 p. 29–49. Sokhan, M., Hartog, F. & Mcphail 2003. Surface Analysis of the Laser Cleaned Metal Threads. The 5th International Conference on Lasers in the Conservation of Artworks, Osnabrueck, p. 238–244. Sutcliffe. H., Cooper & Farnsworth 1999. An initial investigation into the cleaning of new and naturally aged cotton textiles using laser radiation. The 3th International Conference on Lasers in the Conservation of Artworks, Florencia, p. 241–247.

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Structural Diagnosis and Monitoring

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Multifunctional encoding system for assessment of movable cultural heritage V. Tornari & E. Bernikola Foundation for Research and Technology-Hellas (FORTH), Institute of Electronic Structure and Laser (IESL), Heraklion, Crete, Greece

W. Osten & R.M. Groves ITO Institut für Technische Optik, Universität Stuttgart, Stuttgart, Germany

M. George & T. Cedric Centre Spatial de Liège, Angleur Liege, Belgium

G.M. Hustinx OPTRION, Liege, Belgium

E. Kouloumpi, A. Moutsatsou & M. Doulgeridis Conservation Department, National Gallery – Alexandros Soutzos Museum, Athens, Greece

S. Hackney & T. Green Conservation Department, Tate, London, UK

ABSTRACT: The proposed multiencode methodology utilizes field interference to perform assessment of movable artworks. In this paper the procedure under which the artwork is forced to generate self-encoded interference fields is described, indicating both authenticity and impact versus time. Records of surface displacement in the encoded form of fringe patterns as governed by interference-based principles are being exploited. The manipulation of recording parameters to achieve secondary fringe formation indicative of artwork descriptor elements is conceived. If these “signatures” of artwork are monitored over time they become authenticity or impact indicators. The self-encoded fringe representation is the secure and major advantage for the development of an anti-fraud inspection methodology. The concept is addressed through reference to a current 6th EU Framework program funded research project in the frame of a call whose special task is the impact assessment and movable Cultural Heritage.

1 1.1

INTRODUCTION Holography applications in art conservation

Holography related techniques have found a great field of application in cultural heritage ranging from three-dimensional representation of museum objects, to applications for structural diagnosis and authentication control (Hecht 1974, Vest 1979, Hariharan 1984, Saxby 1994). The property of three-dimensional representation is based on the unique properties of holographic recording and reconstruction which records the whole information of a wave field from the artwork with a reference wave field. In reconstruction,

the object wave field is reproduced by illumination of the reference beam. Through reference beam reconstruction an exact replication of the object wave field is achieved strictly following the physical principles of grating diffraction. The reconstructed first order diffracted beam is a three-dimensional representation of the object at the moment of recording with full horizontal and vertical parallax providing information on depth and angle perspective. The phenomenon of holography based on wave principles and on two fundamental wave phenomena of coherent optics, namely interference and diffraction, should not be compared with pseudo-three-dimensional techniques

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based on stereoscopic imaging. Therefore holography is the only available technique based on the above mentioned physical phenomena to record and reconstruct wave fields (Leith 1964, Powell 1965, Okoshi 1976). The structural diagnosis provided by holography for the sake art conservation was introduced during recent years with great expectations since it allows highly accurate non contact and non destructive fullfield inspection in the structural condition assessment of artworks. It may be applied in visualising hidden defects in the bulk and subsurface as cracks and detachments, worm tunnelling, stressed areas, and inhomogeneous materials (Amadesi 1974, Schirripa 1998). Additionally, holography properties provide a highly flexible and quantitative tool to perform simulation experiments under varying environmental conditions or microclimate variations inside displaycases, as well as conditions simulating handling and transportation effects (Gulker 1990, Boone 1995, Tornari 1995, Boone 1996). Research by holography and specially holographic interferometry related techniques and related developed instrumentation have thus proved among the most powerful candidates as alternative modern technological solution compared to practical finger-knocking conservation practice or to X-ray imaging and thermography, which are considered qualitative techniques (Tornari 2007). They also exhibit a number of advantageous properties as the use of fully expanded laser beams covering extended targets, independence of the surface texture, shape and height differences, being totally non invasive and non interacting with artwork sensitive surface, as well as providing direct visual and quantitative results (Hinsch 2000, Osten 2000). Recently, systems for applications on artworks have been specifically developed which are highly compact, portable, lightweight and user friendly (Tornari 2000, Holoauthentic 2002, LaserACT 2006). Another very important holography application for artwork conservation is the authenticity (mastering) principle implemented in the properties and performance of the technique for authentication. Holograms are known to be widely used for identification of many different authenticated products, from commercial goods, cards and bank notes to long duration metro tickets in EU countries. For artwork authentication, advanced holographic techniques based on Computer Generated Hologram (CGH) have been recently implemented. Whatever small (≈ 200 µm) a CGH can become, it remains an added feature that can be localized and removed. The feature encoding of holographic fringes is another important property of holographic interferometry which may prove a unique method to authenticate not only the artwork but its structural condition. This method is the one exploited here (Fotakis 2006, Multiencode 2008).

1.2 State of the art in authenticity/impact assessment Compared to the optically generated master holograms glued on products for commercial authentication purposes, a CGH provides more discrete features suitable for artwork authenticity control. It involves a direct laser-etching of the computer generated diffraction grating, usually produced by numerical calculation of a logo identifying the owner on a hidden area of the back or front surface of the artwork. The authentication is performed by reading back the grating which reproduces the expected logo with a special device. However, the highly secure CGH writing-reading process involves severe drawbacks like the difficulty of the method to etch a functionable grating on most of artwork exposed surfaces or the risk to be found and removed for a malevolent fraud. The proposed feature encoding method foresees providing authentication eluding surface etching or gluing, etc. The feature encoding method implements the generation of fringe patterns directly from the artwork surface under concern. These fringe patterns can act as multiple sources of information including both identification of authentication and structural analysis. Thus, the method involves for the first time ever the exploitation of interference fringe patterns as self-secured encoded features of the target which generates them. Any alteration when strictly controlled and archived is used to assess impact over time. Hence, the presented project is highly innovative, extrapolating existing knowledge and technical advances to converge with existing conservation practices, making the state-of-the-art in conservation technology field to step forward and opening up new metrology applications. 2

METHODOLOGY

2.1 Operation principle In order to use metrology techniques to produce interference fringe patterns suitable for feature-encoded applications, the fundamental interference principles should be followed. The operation principle of coherent metrology presupposes mutual displacement of object points at two different instants in time domain. The initial surface is in position L and is illuminated by an expanded coherent laser beam whose reflectance modulates a photosensitive recording medium. Then, the surface undergoes a displacement to position L . These two positions represent the object before and after displacement, see Figure 1. The overlapping of the two mutually coherent wave fields produces an interference pattern consisting of constructive and destructive intensity peaks. In visual terms, the pattern consists of bright and dark contour planes accordingly.

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Table 1. Typical procedure for IAP. Object

Encoding : Sequential exposure superposition

P

Step 1 P´

Step 2

M M´

Step 3 before after displacement

Irradiance in observation plane

Figure 1. The surface of the object is displaced in order to generate an optical path difference to allow interference. Fringe separation equals to half the laser wavelength. Intensity Profile 160 140

Intensity

120

Non-Homogeneous

100 80

Homogeneous

60 40 20 0 -20 0

FEATURE GENERATION: Artwork holographic images superimposed for Hν0 − ν DATABASE: Digitized Raw data is archived MANIPULATION: Post-Processing – Comparison Decoding : Statistical correlation of events

100

200

300

400

500

600

700

position

Figure 2. Intensity profile across an interference fringe intensity distribution. Two areas are well distinguished describing homogeneous and non homogeneous distribution.

If and only if the displacement is of the order of few multiples of the laser wavelength, a visible fringe pattern appear. The exact geometrical configuration depends on the aim, which means that various designs can be introduced operating with the same principles. Most commonly a laser beam is split so that one act as reference beam and generates interference with the object beam at the recording plane. The number of fringes N provides the amount of displacement through Nλ/2. The exemplary fringe pattern in the scheme of operation of Figure 1 has been generated from a void under the artwork surface using holographic principle of recording. An equidistant fringe distribution indicates homogeneous displacement of neighbouring object points. However, it is more common, in artwork applications, to witness non-homogeneous fringe distribution due to subsurface defects which interrupt the overall homogeneous surface displacement, as it is shown in Figure 2.

Fluctuation of fringe intensity and width contributes to determine the neighborhood of object point’s behavior which can be described as homogeneous or non homogeneous displacement. The discontinuous signals indicate object points displaced differently than the rest of neighbouring objects points. The differences in fringe distribution have to do with the structural integrity reactions allowing localization and sizing of anomalous invisible effects such as defects and stressed locations. Areas which are covered with symmetric-like pattern of homogeneous displacement are correlated to homogeneous displacement due to induced alteration. Thus, one can distinguish two clearly defined fringe pattern distributions as keyfeatures which contribute to encoding of the target responses. It is not the intensity of the signal that matters since normalization and smoothing are common post processing tools. It is the interruption of a continuous distribution that indicates the modified area. Correlating the localized non-homogeneous features with subsurface effects and the homogeneous features with external events constitutes the decoding of the generated fringe patterns which provides the “signatures” for authentication and structural evaluation. 2.2

Operation for Impact Assessment Procedure (IAP)

The standardized procedure depends on a priori selection of fixed experimental parameters to produce a sequential recording superposition and consists of three distinct steps (Table 1). At first step, the generation of useful features from the examined artwork is considered. The fringe patterns are generated according to the experimental constraints of sequential holographic superposition (H01 , H02 , H03 , . . .) where H0 is the first exposure before displacement in order to produce records of object points displaced at a variety of positions. The variety of displaced positions warrants observation of different hidden defects forming useful features. The protocols under which the artwork is loaded for achieving a range of displaced positions are elaborated by

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an experienced conservator specialized in the artwork type and construction. After the features have been generated and selected, the data are termed: signature: data. The entries are processed in a specially developed database in which the present data as well as any future data of the artwork and experimental investigation are archived. The completion of database entries allows post processing routines for statistical analysis of fringe density distribution to take place. Results are to be compared locally with previous entries of the artwork signatures to allow impact assessment. Therefore, the standardized inspection procedure delivers datasets of retrieved artwork features correlated to known or unknown events and can statistically be used to prove authentication and/or negative or positive impact, e.g. deterioration vs. restoration. In this context, a signature categorization with the elements which can allow Impact Assessment Procedure is derived being 1) symmetry and 2) nonsymmetry in fringe density. The former is used to assess 1a) artwork sensitivity, 1b) ageing and 1c) storage conditions, handling and transportation impact while the later allows structural defect assessment. 2.3

Principles for feature extraction: the topology approach

An interferogram is a visible cosine intensity distribution of bright and dark peaks corresponding to constructive and destructive interference of surface displaced points. A surface which has undergone an externally induced excitation is forced to change its initial state proportionally to the force applied. When a solid material is homogeneous this results in equidistant fringe formation satisfying the identity function id(x), id(x) = x, id{id(x)} = x, which indicates one-to-one correspondence between points, as in Figure 3. A symmetrical displacement characterises a continuous surface and cannot be generated if discontinuities in the subsurface are excited by the induced load in such amplitude that can affect the surface, as it is shown in Figure 4. In the latter case a non-symmetric distribution is expected due to defects characterising a non continuous object of unique non continuous topological space. For the purpose of feature extraction both density signature and defect signature are used as impact assessment and authenticity elements. 3

EXPERIMENT

The experimental methodology is designed in order to verify use of the elements extracted of raw interferometric data, thus the density and defect data, as signatures which can report alterations. Given that movable artworks undergo controlled or random condition changes according to their display, storage,

(a)

(b)

(c)

Figure 3. a) Example of symmetric displacement of surface, b) homogeneous 3D plot, c) the identity graph visualizing the distinct point symmetry of uniform density across axis.

(a)

(b)

(c)

Figure 4. a) Example of non-symmetric displacement of surface with a defect, b) skeletonization to extract the defect and c) the non homogeneous 3D plot.

handling and transport, it is assumed that both density and defect signature may show alterations over time. The experimental objective is a feasibility study which clearly shows that the selected signatures remain signatures over time. Signatures measurable and comparable to initial entries of raw and analysed data are collected under various experimental settings, namely excitation time, interval between consequent exposures, load type, value and direction. To achieve the experimental goal, the methodology implies stimulated conditions corresponding to a) environmental cycling and/or b) ageing with specific conservation protocols. Samples were constructed by specialized conservators and include known defects accompanied by detail topographic maps and cross sectional composition. The defects are distinguished as mechanically stable or unstable, according to the expected alteration dynamics. A Byzantine icon sample is shown in Figure 5. The sample is first investigated under controlled parameters to acquire the first set of data and then ageing is induced for the second set. The ageing protocol of this series of samples was “Continuous heating at 60◦ C, for a total of 66.5 hrs”. The aim of this load by heat was to provoke deterioration of the wooden structure, avoiding to cause excessive deterioration of structure but affecting the size of defects. Figure 6 shows raw interferometric results before and after the ageing. Therefore, the experimental procedure aims to generate interference data which can be compared to previous or later entries. Technical parameters of the

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(a)

(b)

Figure 5. a) Sample of Byzantine icon and b) topographic visualization of known defects.

Figure 7. Intensity profiles scanning of the sample shown in Figure 6. The profiles are obtained across X, Y and diagonal axes. 2.2 2.0

Fringes/cm on defect

Fringe Density/cm

1.8

Fringes/cm on the rest sample

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2

Figure 6. The sample before (left) and after (right) ageing. Defects act as signatures of the sample and report the impact of cycling.

-20

20 40 60 80 100 120 140 160 180 200 220 Time, sec (9 s interval)

system, detailed description of the artwork under investigation, and raw data are inserted in special windows of the database to support the Impact Assessment Procedure through time. Next time that the same sample is inspected, the experimental parameters are reproduced.Any changes of the key-elements constituing the artwork signatures are isolated and compared. In general, the chosen experimental parameters are not critical for the experimental success but for the aim of future comparison establish strict boundary conditions. 4

0

RESULTS

In Figure 6, we can observe two dominant defects. Both are traced before and after conditioning. Although the defects are present and their structure remains unchanged, the expansion of the defect dimensions is outstanding. Similar behaviour was exhibited by most samples under investigation confirming the usefulness of fringe patterns to act as multiple information sources. The defect is thus detected as signature of the artwork allowing its characterisation. Identification

Figure 8. A representative graph indicating the difference in fringe density values between areas with and without defects.

of authenticity of the sample under concern can be issued and is also quantitatively analysed to identify impact. Defect coordinates and scanned intensity profiles are PC derived and held in the database. An example of intensity profiles before and after ageing is shown in Figure 7 and some locations are selected for comparison. The values of the intensity profiles are processed in order to represent graphically the dynamics of sample reaction inside and outside the region where defects are present. Figure 8 shows a graph resulting from measurements of areas with and without defects. The areas with defects show higher response reported by higher fringe density values. However, in this example, the area with defects exhibits higher density values but this is not a general rule. Defects deep in the bulk can have only minor influence on the surface. It is not the value but the different displacement response what is interesting. Fringe density is a safe method to derive the different response between defects and original surfaces

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out of the high number of intensity profiles drawn across the axes. However, it should be remembered that they do not indicate directly the area condition concerning the presence of defects because we cannot assume that defects generate higher amplitudes. If a defect deep inside the bulk or due to its construction is displaced less than the rest of the sample less fringe density is expected. Hence the density value alone is not indicative of defect signatures throughout the artwork. It is the comparison which illustrates that there are areas with different response as seen in Figure 8. The research for establishing direct indication of defect existence and deterioration growth is another important issue of the structural studies. IAP as developed in this project aims to deliver signals of interest with comparable values for feature encoding and is not concerned directly with defect interpretation. 5

CONCLUSIONS

The feature encoding project or MultiEncode aims to prove the satisfactory use of fringe patterns as multi-encoding elements reporting distinct reactions of artwork complex constructions under pre-specified investigation procedure. Many hidden defects may affect the surface and generate a discontinuity in an otherwise homogeneous response. Despite such complex structure, the distinct response of each artwork can be retrieved and used as a unique identification to report either structural impact or authentication. The fringe density and the fringe pattern formation used as direct signatures can be compared at any time to give an indication of impact or fraud. The developed methodology allows accurate measurements from the raw data. Portable instrumentation, post-processing software, dedicated database and user-friendly software are developed for this unique metrological application as well. For the first time, it is elaborated the possibility for a fringe pattern based method capable to allow conservators to study changes in artworks for conservation, restoration and authentication purposes. ACKNOWLEDGEMENTS Authors wish to acknowledge the FP6 project MultiEncode SSPI-006427 for the opportunity to study the problems involved in the presented paper. The project is still in progress. REFERENCES Amadesi S., Gori F., Grella R. & Guattari G. 1974. Holographic methods for painting diagnostics. Applied Optics 13: 2009–13.

Boone, P.M. & Markov, V.B. 1995. Examination of museum objects by means of video holography. Studies in Conservation 40: 103–109. Boone, P.M. et al. 1996. Coherent-optical localization and assessment of importance of damage and defects of cultural heritage. Optics and Lasers in Engineering 24: 215–229. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S & Tornari, V. 2006. In R. G. W. Brown & E. R. Pike (eds.), Lasers in the Preservation of Cultural Heritage; Principles and applications: Chapter 5. New York: Taylor and Francis. Gulker G., Hinsch K., Holscher C., Kramer A. & Neunaber H., 1990. In-situ application of electronic speckle pattern interferometry (ESPI) in the investigation of stone decay. Proc. SPIE /Laser Interferometry: Quantitative Analysis of Interferograms 1162: 156–167. Hariharan, P. 1984. Optical holography. Cambridge: Cambridge University Press. Hecht, E. & Zajac, A. 1974. Optics. Reading: AddisonWesley. Hinsch, K.D., Frice-Begermann, T., Gulker, G. & Wolf, K. 2000. Speckle correlation for the analysis of random processes at rough surfaces. Optics Lasers Eng. 33: 87–105. HOLOAUTHENTIC, EC GROWTH, 1999–2002. Leith, E.N. & Upatnieks, J. 1964.Wavefront reconstruction with diffused illumination and three-dimensional objects. J. Opt. Soc. Am. 54: 1295–1301. MULTIENCODE, EC SSPI, 2005–2008. Okoshi, T., 1976. Three-dimensional imaging techniques. London: Academic Press. Osten, W., Kalms, M., Juptner, W., Tober, G., Bisle, D. & Sherling, D., 2000. A shearographic system for the testing of large scale aircraft components taking into account non cooperative surfaces. Proc. SPIE 4101B: 432–438. Powell, R.L. & Stetson, K.A. 1965. Interferometric analysis by wavefront reconstruction. J. Opt. Soc. Am. 55: 1593– 1598. Saxby, G. 1994. Practical Holography. Hemdt Hempstead: Prentice-Hall International. Schirripa S. G., Ambrosini D. & Paoletti D. 1998. Optical methods for mosaic diagnostics, Journal of Optics 29: 394–400. Tornari, V. & Papadaki, K. 1995. Continuous wave and pulse holographic interferometry used for monitoring environmental effects on materials used in artifacts. Proc. of the 1st International Congress on Science and Technology for the safeguard of Cultural Heritage in the Mediterranean Basin. Catania – Siracusa. Tornari, V., Georgiou, S., Zafiropulos, V. & Fotakis, C. 2000. Modern technology in artwork conservation:A laser based approach for process control and evaluation. Journal of Optics and Lasers in Engineering 34: 309–326. LASERACT. Laser Multitask Non Destructive Technology In Conservation Diagnostic Procedures. EC LASERACT EVK4-CT-2002-00096, 2003–2006. Tornari V. 2007. Laser interference-based techniques and applications in structural inspection of works of art. Anal. Bioanal. Chem. 387: 761–780. Vest, C.M. 1979. Holographic interferometry. USA: John Wiley & Sons.

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Digital preservation, documentation and analysis of heritage with active and passive sensors F. Remondino Institute of Geodesy and Photogrammetry, ETH Zurich, Switzerland

ABSTRACT: Attention to digital documentation and preservation of heritage is always increasing and fast, reliable, low cost, portable and practical solutions are of great interest for archaeologists, restorators and the entire heritage community. Many discussions are opened on which approach is better. Image-based modelling needs some experience in the acquisition and data processing and still some manual interaction to obtain precise and reliable results. Range-based modelling needs a lot of editing time and is unpractical in some field campaigns. The goal of this work is to give an overview of the available digital technologies for the documentation and virtual preservation of artworks. We also present some results of a developed image-based technique able to capture the fine 3D geometric details of objects or sites. With our methodology, we can provide geometrical details in an automatic and reliable manner to achieve photo-realistic visualization for heritage and archaeological objects.

1

INTRODUCTION

Three-dimensional documentation from images is a great topic of investigation in the research community, even if range sensors (e.g. laser scanners) are becoming more and more a common source and a good alternative for generating 3D information quickly and precisely. 3D modelling of a scene should be understood as the complete process that starts with data acquisition and finishes with a virtual model in three dimensions visible interactively on a computer. The interest in 3D modelling is motivated by a wide spectrum of applications, such as animation, navigation of autonomous vehicles, object recognition, surveillance, visualization and recently digital documentation, preservation, conservation and restoration of cultural heritages as well. In the last years, the use of computer technologies for the production of digital archives of cultural and artistic objects with different characteristics and dimensions (monuments, archeological sites or finds, architectural features, etc.) has received a considerable interest. This is due from one side to the availability of new technologies for digitalization and investigation purposes and, from the other side, to the interest of archaeologists and restorers in instruments able to provide innovative investigations.Although the increasing interest for virtual restoration, diagnostic and noninvasive documentation at multiple scales is pretty evident, it is important to remark that: (i) many 3D surveys performed with range sensors require high

budgets, a lot of time for data processing and dedicated software and hardware; (ii) the surveys executed with digital cameras, even if cheaper, require experience in the acquisition step, dedicated software and often manual interaction; (iii) new sensors able to acquire data in a particular range of the electromagnetic spectrum (IR, UV, etc.) can provide specific information on the conservation status or detect details not visible by the human eye. Considering this, the selection of the right approach and technique requires great experience and the consideration of different project parameters. In this contribution we report the available digital techniques for the documentation of cultural heritage. The objective is to explain the different non-destructive technologies and the methodologies used for digital recording of heritage and to demonstrate how modern techniques can be very useful and easily adopted by conservators, archaeologists and decision-making managers. Furthermore, we report some results coming from our image-based surface modeller, able to accurately reconstruct detailed objects with results similar to active sensors but much less expensive. 2

CULTURAL HERITAGE DOCUMENTATION TECHNIQUES

The realization of an accurate and photo-realistic survey and documentation of an artwork is one of the most complete, adequate and flexible way to show it all over the world, to study and digitally document it in

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detail as well as to conserve and preserve it all along. Therefore, a 3D model can be exploited for numerous purposes. Depending on several factors, different techniques can be used to model cultural heritage. Digital cameras (consumer or SRL) and range sensors, often coupled with traditional topographic surveys, are the common techniques used to digitize historical buildings, landscapes, archaeological finds, statues and other important ancient or art objects. The typical 3D documentation process generally consists of the following steps: (i) project planning, (ii) surveying, (iii) imagebased modelling, (iv) range-based modelling and (v) integration. The integration of images and range data is not always necessary, as well as the surveying. It is generally complicate to select which technique is better to use. As it was already demonstrated in the research community (El-Hakim et al. 2007, Rizzi et al. 2007), image- and range-based techniques can reach very similar results in many terrestrial applications. Typical factors still involved in the choice of the technology are the costs, ease of use, portability and usability, experience of the operator, location constraints, object dimensions and final goal of the documentation. Nevertheless, to achieve a good result that respects the required level of detail, the better way is so far the combination of different techniques (El-Hakim et al. 2004, Voltolini et al. 2007). In fact a single technique is not able to give satisfactory results in all situations, concerning high geometric accuracy, portability, automation, photo-realism and low cost as well as flexibility and efficiency.

2.1

Image-based techniques

Image-based documentation techniques (mainly photogrammetry and computer vision) (Remondino & El-Hakim 2006) are generally preferred in the case of lost objects (Fig. 1), architectures with regular geometric shapes, low budgets, good experience of the working team, time or location constraints for the data acquisition and processing. Image data are generally always available and are probably the most complete source for digital documentation. Indeed, geometry and texture information can be derived at the same time, using cheap sensors and fast procedures. Generally speaking, the image phase measurement is regarded as the most tedious part leading non-experts to consider this documentation approach as inappropriate, time consuming and, with the advent of range sensors, as obsolete. Among image-based methods, photogrammetry is a highly accurate and flexible approach, suited for different scales and used to derive metric and reliable information of a scene from a set of images. Images acquired in the visible domain are generally used, even if IR or UV

Figure 1. 3D model of the Great Buddha of Bamiyan, Afghanistan and its actual empty niche. The digital model of the statue has been realized using old images. The results were used to generate a 1 : 25 scale physical replica of the Buddha and also to prove that a physical reconstruction using the leftovers of the explosion is not feasible (Gruen et al. 2004, 2005).

data could also be used. Generally, at least two images are required, even if it is possible to calculate 3D data using a single image using geometrical constraints and image invariants. Semi-automatic image-based modelling techniques are particularly suitable for 3D reconstruction of buildings and man-made objects that contain generic geometrical shapes as for example parallel lines, planes and right angles. But the latest developments, in particular related to surface measurement and reconstruction, are increasing the automation (El-Hakim 2006, Remondino & Zhang 2006) and the trend is to create automatic and user friendly tools that site managers, archaeologists, restorers, conservators and the whole heritage community could use instead of expensive and cumbersome range sensors. Some user interaction is still demanded as fully automated imagebased approaches (Nister 2001, Pollefeys et al. 2004) demonstrated that are not yet suitable for reliable and precise documentation of heritage. 2.1.1 IR imaging technique Infrared (IR) images have been extensively used for the documentation of cultural heritage, in particular for artwork studies (Pelagotti et al. 2007) and for monitoring or conservation of historical buildings (Moropoulo et al. 2001). The infrared spectrum has a number of technological uses including target acquisition and tracking by the military, remote temperature sensing, short-ranged wireless communication, spectroscopy, weather forecasting and astronomy. The sensors able to record infrared radiations are usually designed to collect radiations only within a specific bandwidth. In fact, the infrared band is pretty wide (from ca 0.78 µm to

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ca 1000 µm) and it is often subdivided into smaller sections that reveal different features. Short IR wavelength, also referred as near-IR, contains wavelengths near the visible range (0.78 µm to ca 1.5 µm). Some consumer digital cameras allow to acquire images in this domain, but dedicated hardware (sensors and filters) is generally required. IR reflectography, in the range between 0.8 and 2 µm, allows the analysis of many materials which compose the pictorial layers (Fontana et al. 2005). Most of the materials within a painting layer are indeed at least partially transparent to the IR radiation, therefore IR imaging generally allows to see features underneath the visible layers and to visualize under-drawings or changes made by the artist. This is a very useful information e.g. to decide whether a painting is the prime version or a copy and whether it has been altered by restoration work or not. Longer wavelengths (>8 µm) of the IR band are instead referred as far-IR. In this part of the spectrum it is possible to see the environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows to see variations in temperature. IR thermography transforms the thermal energy, emitted by an object in the infrared band (Fig. 2). All objects above absolute zero temperature emit thermal infrared energy, so that thermal imagers can passively see all object temperatures. Infrared thermography cameras produce images of invisible infrared or heat radiation and provide precise non-contact temperature measurement capabilities (nowadays up to 0.1 degree resolution). By utilizing a thermal imaging system, it is possible to detect and display the normally invisible infrared radiation emitted by an object or analyze how the different components of an object react to radiation. Generally, the infrared energy is converted to a real-time visible light image, which is displayed on a monitor. Depending upon the type of thermal imager, measurement of object temperature is also possible. Thermography can be used for the assessment of various traditional–historical materials and structures before and after they have been conserved, restored or repaired. Non-destructive testing and evaluation are performed on the object’s materials and structures in order to assess the physical-chemical behaviour of the materials under conservation treatments (such as stone cleaning or stone consolidation), as well as to disclose any substrate features (such as tesserae on plastered mosaic surfaces), to detect buried defects, inhomogeneity, stoppings and paddings, to monitor hidden structures and moisture content, to analyze corrosion and to check the quality of welding and reinforcements elements. Hidden structure identification, adhesion of frescoes checking, cracks mapping and air flows studies are among the most important applications for conservators and restorers.

Figure 2. A visible (above) and thermography (below) image of frescoed vaults in the Buonconsiglio castle in Trento, Italy. The IR analysis shows a very hot area (probably a pipe, here highlighted by the contour) passing over the frescos and producing damages.

Infrared thermography can be used both as a qualitative and a quantitative tool. Some applications do not require obtaining exact surface temperatures. For example, to identify hidden structure or adhesion of frescoes, it is enough to acquire thermal signatures. This method of qualitative visual inspection is based on image interpretation. On the other hand, quantitative analysis requires a rectification of the thermal images to provide correct length or surface measurements or the use of 3D digital models textured with the IR information (Rizzi et al. 2007). Even if it is a simple technique, IR thermography is not yet well diffused because of the lack of adequate knowledge and due to the very expensive equipments. Moreover, performing correct measurements in long wave IR images presents some difficulties because the value of energy captured by the camera is affected by material’s characteristics (like emissivity) and ambient conditions (presence of sunlight, ambient temperature, moisture, distance of object, etc).

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2.1.2 UV imaging technique UV radiation is characterized by a wavelength shorter than 0.4 µm but longer than soft X-rays (10−2 µm). It is possible to subdivide the UV range in near UV (0.4–0.32 µm), far UV (0.32–0.1 µm) and deep UV (<0.1 µm). In artwork studies, near UV radiation is often used to identify different varnishes and over-paintings, in particular with induced visible fluorescence imaging systems. When UV light is shined onto an object, the result is the emission of photons in the visible region of the spectrum. This phenomenon, called fluorescence, occurs when light photons of high energy are absorbed and re-emitted in the visible wavelength regions. Indeed, photons in the UV region of the spectrum are absorbed by the painting varnish and this effect, imaged with dedicated image systems, allows to characterize the material, evaluate the artwork conservation state or localize retouches and previous restorations (Fontana et al. 2005, Pelagotti et al. 2006). In a painting, the aged varnishes have different fluorescence properties than new varnishes and some modern pigments have a different fluorescence emission than traditional pigments, even if they look alike under visible light. Not only the paintings can be analyzed with UV but also other objects like porcelain, ceramic and paper art showing their hypothetical repairs and cracks. 2.2 Range-based techniques Active systems do not need a mathematical model to infer 3D information from the observation but provide directly and quickly 3D data to the user. They can be divided into those based on time of flight and those working with the triangulation principle (Blais 2004). Time of flight systems (pulse principle and phase shift) work on distances between 1 and 1000 m and are generally used for large objects as for example architectures or sites. On the other hand, triangulation-based instruments (laser light or pattern projection systems) work at shorter distances (0.1 to 500 cm), are more accurate and are generally used to model statues, sculptures and low reliefs. Also conoscopic systems are often used in cultural heritage field but in particular for planar objects, as for example paintings and small coins. They have better precision than triangulation-based systems but a smaller field of view. Fontana et al. (2005) developed a system specifically for scanning paintings using a conoscopic micro-profilometer, with a depth resolution <6 µm and a maximum acquisition speed of 500 points per second. An actual deficit of most of the range sensors is the lack of high quality texture information, which is generally acquired with a separate digital camera and afterwards registered onto the range data for texture mapping purposes and photo-realistic results.

Figure 3. Three (out of 5) images of a relief in Dresden, Germany. 3D model results displayed as shaded, color-shaded and textured mode.

2.3 Data integration When multiple data sources are used and an unique final product must be delivered, all the information should be integrated to provide the better interpretation. The combination of the different models, generated with different data sources and techniques, is a key step in the 3D documentation process. A reliable integration of various data sources is generally obtained through a precise spatial registration and superimposing the different datasets (Marras et al. 2003, El-Hakim et al. 2004). Multi-spectral imaging techniques become a powerful tool in the scientific analysis and documentation of objects, since images in different wavelengths provide information that human eye cannot see. Applied in the spectral range from UV, visible up to IR, these techniques can provide documentation with both spatial and spectral information. Merging these data gives an additional dimension to the results and spectral information can be presented together with 3D data (Rizzi et al. 2007). When images and range data are used, the different 3D models are assembled and registered together into the same reference system either with common points or with surveyed control points, or both (El-Hakim et al. 2004, Voltolini et al. 2007). Afterwards, 3D data can be textured with any registered image data. 3 APPLICATIONS In this section we report different applications of cultural heritage documentation, for 3D modelling purposes, analysis, restoration as well as digital preservation, conservation or simple visualization. The 3D models are all produced using images acquired with consumer digital cameras. The images are first oriented with a photogrammetric bundle-adjustment, using common points between them. Then, using the recovered orientation parameters, a matcher algorithm

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Figure 5. 3D documentation of a low-relief (approx. 1 m long and 4 cm depth) performed using 4 images. Here, it is displayed as wireframe, shaded, color-shaded and textured model.

Figure 4. A detailed relief (ca 50 cm width) modelled with 6 images acquired with a 6 Megapixels consumer camera.

(Remondino & Zhang 2006) extracts a 3D point cloud of the area of interest. Finally, the 3D data are converted into a mesh (polygonal surface) and textured for photorealistic presentation. A case like Figure 3, consisting of 5 images (6 Megapixels) required about 3 hours of processing to generate the final textured 3D model. The method that we have developed to document heritage using images is able to extract reliable and precise 3D point cloud using both points and edges and performing sub-pixel measurements. The method can be employed to survey objects at different scales (Figs. 4, 6) as well as almost planar or complete round objects (Figs. 5, 7).

4

CONCLUSIONS

The digital documentation, conservation and analysis of cultural heritage can nowadays be performed with different techniques. In this short contribution we have reported most of them and presented more details and results about the image-based approach. Images are always available and the entire procedure is quite cheap, even thus it requires some experience and dedicated software. For these reasons range sensors are often preferred by non experts. Even if very expensive, they are becoming more and more a common source and a good alternative for generating 3D information quickly and precisely. Therefore commercial software for detailed 3D restitution of objects from images is really required. In this way the image-based pipeline could be more easily adopted by conservators,

Figure 6. Original image (out of 5) and generated 3D model of the statue (ca 2 m high) located in the fountain of Piazza del Campo in Siena, Italy.

restorators, archaeologists and decision-making managers. Many discussions are opened on which technology should be adopted in a specific case: indeed image-based modelling needs some experience in the acquisition and data processing and still some manual interaction for precise and reliable results; range-based modelling needs a lot of editing time and is unpractical in some field campaigns. We cannot say which approach is better than the other. Many comparisons have been done in the research community, but there are too many parameters involved. For sure the accuracy and the achievable detail are not playing anymore an important role in the decision, at least in most applications at the object scale. Nice-looking 3D models (e.g. derived by ‘structure-from-motion’ approaches) are of limited interest for precise and detailed heritage documentation, but are mainly usable in quick visualisation applications. Therefore the documentation and 3D modelling technique should also be selected

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Figure 7. A small statue (ca 12 cm high and 9 cm width) modelled with 25 images (12 Megapixels) and a 28 mm objective. A closer view of the reconstructed upper details is also presented.

considering many variables in order to achieve satisfactory results that provide an added value for the documentation and conservation purpose. REFERENCES Blais, F. 2004. A review of 20 years of range sensors development. Journal of Electronic Imaging, 13: 231–240. El-Hakim, S.F., Beraldin, J.A., Picard, M. & Godin, G. 2004: Detailed 3D reconstruction of large-scale heritage sites with integrated techniques. IEEE Computer Graphics and Application, 24: 21–29. El-Hakim, S. 2006. A sequential approach to capture fine geometric details from images. IAPRS&SIS, 36: 97–102. Dresden. El-Hakim, S., Gonzo, L., Voltolini, F., Girardi, S., Rizzi, A., Remondino, F. & Whiting, E. 2007. Detailed 3D modelling of castles. Int. Journal of Architectural Computing, 5: 199–200.

Fontana, R., Gambino, M.C., Greco M., Marras, L., Pampaloni, E., Pelagotti, A., Pezzati, L. & Poggi, P. 2005. 2D imaging and 3D sensing data acquisition and mutual registration for painting conservation. Videometrics VIII, Proc. SPIE, 5665: 51–58. Gruen, A., Remondino, F. & Zhang, L. 2004: Photogrammetric Reconstruction of the Great Buddha of Bamiyan, Afghanistan. Photogrammetric Record, 19: 177–199. Gruen, A., Remondino, F., Zhang, L. 2005. The Bamiyan project: multi-resolution image-based modelling. In Proc. of the International Workshop “Recording, Modelling and Visualization of Cultural Heritage”, Ascona, Switzerland, 22–27 May, Balkema. Marras, L., Fontana, R., Gambino, M. C., Greco, M., Materazzi, M., Pampaloni, E., Pelagotti, A, Pezzati, L. & Poggi, P. 2003. Integration of Imaging Analysis and 3D Laser Relief of Artworks: A Powerful Diagnostic Tool. Proc. of Lacona V, Osnabrück, Germany, Sept. 15–18. Moropoulo, A., Avdelidis, N., Koui, M., Delegou, E. & Tsiourva, T. 2001. Infrared thermographic assessment of materials and techniques for the protection of cultural heritage. In Multispectral and Hyperspectral Image Acquisition and Processing, SPIE 4548. Nister, D. 2001: Automatic dense reconstruction from uncalibrated video sequences. PhD Thesis, Computational Vision and Active Perception Lab, NADA-KHT, Stockholm. Pollefeys, M., Van Gool, L., Vergauwen, M., Verbiest, F., Cornelis, K., Tops, J. & Koch, R. 2004: Visual modelling with a hand-held camera. IJCV, 59: 207–232. Pelagotti, A., Pezzati, L., Piva, A. & Del Mastio, A. 2006. Multispectral UV Fluorescence Analysis of Painted Surfaces. In Proceedings of 14th European Signal Processing Conference EUSIPCO 2006, Firenze, Italy. Pelagotti,A., Del Mastio,A. & Razionale,A. 2007.Active and passive sensors for art works analysis and investigations. Videometrics IX, Proc. SPIE-IS&T Electronic Imaging, 6491. Remondino, F. & El-Hakim, S. 2006. Image-based 3D modelling: a review. Photogrammetric Record, 21: 269–291. Remondino, F. & Zhang, L., 2006. Surface reconstruction algorithms for detailed close-range object modelling. IAPRS&SIS, 36: 117–121. Rizzi, A., Voltolini, F., Girardi, S. Gonzo, L. & Remondino, F. 2007. Digital preservation, documentation and analysis of painting, monuments and large cultural heritage with infrared technology, digital cameras and range sensors. In Proc. of XX CIPA Symposium, Athens, Greece (in press). Voltolini, F., El-Hakim, S., Remondino, F., Girardi, S., Rizzi, A., Pontin, M. & Gonzo, L. 2007. Digital Documentation of complex architectures by integration of multiple techniques – The case study of Valer Castle. Videometrics IX, Proc. SPIE-IS&T Electronic Imaging, 6491, San Jose (CA), USA.

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Monitoring of changes in the surface movement of model panel paintings following fluctuations in relative humidity: Preliminary results using Digital Holographic Speckle Pattern Interferometry E. Bernikola & V. Tornari Institute of Electronic Structure and Laser (IESL), FORTH, Heraklion, Crete, Greece

A. Nevin Courtauld Institute of Art, University of London, London, UK and Dipartimento di Fisica, Politecnico di Milano, Milano, Italy

E. Kouloumpi Conservation Department, National Gallery – Alexandros Soutzos Museum, Athens, Greece

ABSTRACT: The deterioration of wooden panel paintings is of major concern. Sudden environmental and climate changes can rapidly cause the deterioration of an artwork, either during exhibition in museums, while in transit, or even due to changes in weather or heating within a building, as it is often the case for unmovable cultural heritage. However, for movable cultural heritage, art exhibition and transport may result in deterioration and damage. While non-invasive and non-contact methods are useful in order to monitor the continuous changes in the artwork, very few are able to measure the relatively rapid and microscopic surface movement in paintings. Research is presented on the monitoring of changes in the surface movement of paintings on a wooden support or panel provoked by induced and natural fluctuations in relative humidity using Digital Holographic Speckle Pattern Interferometry. This technique was used in order to obtain a complete surface analysis of model panel painting samples with typical presence of inhomogeneities within the structure. Real-time movement of the objects was mapped as a function of simulated changes in relative humidity.

1

INTRODUCTION

The deterioration of panel paintings can be due to physical processes which take place during exhibition or transit, or as a result of fluctuations in heating (temperature) and humidity within a building, church or museum. In response to environmental alterations, a panel painting can expand or contract with both temperature and humidity. A new equilibrium state is achieved though changing shape in order to accommodate to the new conditions. Hence, the dynamical equilibrium of the painting structure may be continually changing as the panel adjusts to the environment. Paintings on wooden surfaces and supports are extremely widespread, both historically and geographically, and vary in morphology, size, composition and condition. Wood was chosen as a support for many reasons, namely its strength, stability and availability, which explain the variety of types of wood used in the construction of a support for paintings. A typical icon comprises a number of layers, which include

the substrate (wood), preparation layers, paint layers, surface treatments (varnishes) and added materials. The substrate of a panel painting is almost always some kind of softwood. The preparation layer, applied onto a roughened surface of the wood, often consists of reflective white minerals including calcite (powdered calcium carbonate) or gypsum (hydrated calcium sulphate) mixed with animal glue and may include a canvas. Paint layers would have been applied using inorganic and organic pigments and colorants dispersed in a medium, which was often based on proteinaceious materials including animal glues, eggs and milk, or oil-based media (D’Andrea Cennini 1960, Theophilus 1963). The varnish is the last constituent of an icon and was often applied in multiple layers to change the appearance of the painting and to protect the paint layers. Thus, painted wooden artworks can be considered as a class of composite structures of multiple layers. Hence, they react structurally as composite objects, with contributions to the structural behaviour that

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result from the combined responses of their components. Changes in shape which occur as the panels adjust to climate changes are in part due to movement of vapour and liquid water within the porous structure of the wood support. Dimensional alterations may manifest themselves in several ways and provoke serious problems in painted wooden panels. These include warping and buckling of the structure, as well as delamination of the brittler ground. Thus, sudden fluctuations in relative humidity (RH) which may cause damage are of major concern within museums, churches and during transport. In the case of panel paintings, even small changes of the RH may cause major problems, which makes environmental control particularly important (albeit potentially almost impossible) in the open structure of many museums and buildings which have significant air exchange and hence large variations in temperature and relative humidity. Holographic metrology provides one means of measuring surface movement in objects and has the advantage of being non-contact and remotely assessed, which are two potentially important criteria for analysis of cultural heritage and represents a separate branch of coherent interferometry. In this paper, the specific advantages provided by the technique are exploited for direct real-time monitoring of dimensional changes from model panel paintings as a result of an induced change in RH. The technique used in the measurements provides detailed information about microdisplacements. Digital Holographic Speckle Pattern Interferometry (DHSPI) has been developed to provide information in real time about the microsocopic movement of the surface of paintings subjected to several climatic changes (Vest 1979, Tornari et al. 2001, Tornari et al. 2003, Fotakis et al. 2006). In addition, the technique allows the detection of deformations with sensitivity smaller than the wavelength of employed laser source. 2 2.1

EXPERIMENTAL METHOD AND PROCEDURES Experimental setup

Coherent interferometry techniques are based on holographic principles (Vest 1979), which measure phase variations of mutually coherent laser beams recorded at two different stages, and have been used to monitor changes in surface movement. The recorded phase difference provides a measurement correlated to the magnitude of displacement expressed in fractions of micrometers. In the first stage, an object is examined in its reference state while in the second stage a dynamic or altered state is recorded. Following superposition, the two recorded states generate interference fringes.

The total displacement of the object Z(x) is equal to the number of fringes multiplied by half of the wavelength used for the observation (Vest 1979):

where N = 1,2,3. . . represents the number of dark or bright fringes and λ represents the wavelength of the light source which is 532 nm in our case. A conventional holographic interferogram is a superposition of two waves that are scattered from an object in two different states. This interferogram carries information about the phase change between the waves in the form of dark and bright fringes. In optical holographic interferometry, the recording medium is conventionally a photosensitive film whereas in DHSPI a CCD camera is used to record the information. Laser light is used for the object illumination. The back-scattered light is collected with a lens and imaged on the light sensitive chip of a CCD. In addition, a reference beam originated from the same laser source is superimposed on the camera via a beam splitter. Both wavefronts, back-scattered from the object and the reference beam, interfere and form a speckle pattern, which can be detected using the CCD camera. This speckle pattern of the non-deformed object is digitized and stored by a computer. The intensity at each point of the CCD can be described by the following equation (Burke et al. 1998):

where x, y are the coordinates on the chip, I0 is the intensity of the laser light, γ represents the contrast function and φ the phase of the wavefront. After the deformation of the surface, the wavefront originated from the object is slightly deformed while the wavefront of the reference beam remains constant. The new resulting speckle pattern on the CCD is then digitized and stored in the computer.The subtraction of the two images corresponds to a fringe pattern which represents the deformation of the object between the original and the deformed state. The experimental setup is represented in Figure 1. The experimental setup used for the measurements is a DHSPI portable system with an Nd:YAG Elforlight 64 laser used as a light source with the following characteristics: 250 mW, 532 nm, DP, TEM00 SLM, with a coherent length of 30 m. The CCD detector used is a Basler A102f (resolution 1392 H × 1040 V; pixel size 6.45 µm × 6.45 µm). The captured images are transferred to a PC using the Firewire 1394 protocol. Its state is recorded using the 5-frame technique which is given by the algorithm described below. This algorithm uses two sets of five captured images. The first set of images is captured using a π/2 phase difference

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in the object by the changes in relative humidity and ambient temperature. Thus, in these measurements the changes in ambient conditions are responsible for the induced microdisplacements of the panel paintings. 2.2 Experimental methodology

Figure 1. DHSPI Experimental setup.

in a relaxed state of the sample. The sample’s phase is extracted using the five image algorithm (Coggrave):

The second image is captured using also a phase difference in an excited stated. In our case, the excited state corresponds to the state of the object whereas it adjusts to the new environmental conditions in real time. The sample’s phase is then extracted using the five image algorithm given in Equation 3. The two images are subtracted to give the fringe pattern of the phase change. The image produced from subtraction is then normalized to use the full scale of the dynamic range and filtered to increase the fringe visibility. For the measurements described in this paper DHSPI has been used. With this non-invasive method it is possible to observe the microdisplacements or movements in the structure of the painting, before and after the application of some kind of load (traditionally a thermal load, or heat) (Albrecht et al. 2000). The existence of ambient drifts and changes in relative humidity and temperature suggests that this method can be used without external or further application of some kind of stress; in this case, no change of the temperature of the object is necessary. Furthermore, the synergistic effect of temperature and humidity changes must also be considered since panels are rarely subjected to variation of just one of them. Because of differential absorption of humidity within a panel, different zones of the support experience different expansions and contractions following ambient humidity changes. These anisotropic deformations can produce large strains and eventually lead to the formation of cracks in the priming layers, as they become less flexible with age (Spagnolo et al. 1997). Instead of using some type of thermal loading such as lamps of infrared radiation, it was decided to consider instead a “loading” from the stresses caused

A model painting was placed in an air-tight chamber with different environmental conditions created using selected saturated salt solutions, for different time periods and recorded with DHSPI after their removal from the climate box. So far, the movement of the painting surface was monitored as the object adjusted to the environmental conditions of the laboratory. The sample was exposed to RH = 10% (25◦ C) and to RH = 75% (25◦ C). Although it is recognized that a RH of approximately 10% is rare in northern european climates, this is commonly found in desert climates and within non air conditioned airplanes. Double exposure holographic interferograms were recorded immediately after the sample came out of the climate box and the changes were recorded as the sample relaxed to equilibrium conditions (RH = 45%). The humidity or water content of the painting was not directly measured during the experiment being out of the scope of this work. Rather, monitoring of the real-time movement as a result of climate change was addressed. The way the changes in RH may affect the artwork and the way the inhomogeneities present within the object may react and hence affect the movement of the object are of key interest. A climate box was designed and constructed in order to create and control the variation in RH. The fluctuations in RH have been produced with a NaCl saturated salt solution and dessecated silica gel (SiO2 ). Saturated salt solutions are fast and economical means of achieving constant RH within an isolated chamber. In the case of silica gel, the lowest value of RH achieved was 10% with a mean value of temperature equal to 25◦ C. With the saturated solution of NaCl a value of 75% RH was achieved while the mean value of temperature was 25◦ C. 3

RESULTS

Interferograms of objects have been recorded every 6 seconds and different aspects of surface movement have been revealed. Several interferograms are shown below in order to demonstrate the potential of the technique to record the relaxation of wooden panel paintings following changes in ambient conditions. Quantified surface deformation during relaxation to ambient conditions has been calculated as well: a) Analysis of the model sample with a change in RH from 75% (obtained with NaCl saturated solution) to 44%.

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Figure 2. Interferograms recorded at (a) 60 s, (b) 120 s and (c) 180 s.

Figure 4. Displacement vs time graphs for RH changes (a) 75% to 45% and (c) 10% to 45%, velocity vs time graphs for RH changes (b) 75% to 45% and (d) 10% to 45%.

Figure 3. (a) Unwrapped picture and (b) 3D image of net displacement of the entire object.

b) Analysis of the model sample with a change in RH from 10.5% (obtained with silica gel) to 44%. The number of fringes (and therefore the absolute displacement) has been automatically counted for a total duration of 200 s. For both cases, the velocity of changes has been calculated in the direction which is indicated by the white line (Fig. 2a). The interferograms presented in Figures 2a, b and c represent measurements taken as the model sample adjusted from 10.5% to 44 % at different time delays from the induced change. Since the fringes are symmetrical for both directions (from the right to the left) it is possible to measure the dark fringes in the direction indicated with the white arrow in order to create the diagrams which represent the total displacement in time. The images in Figure 3 represent the unwrapped and 3D image for the case of the interferograms in Figure 2. In the 3D image (Fig. 3b), the Y axis corresponds to the sides of the panel, X axis to the top and bottom and Z axis to relative displacement, 60 s after climate change. The scale has been normalized to represent the total displacement with 10 different grey levels where dark grey indicates minimum movement and light grey indicates maximum. In this case, movement is greater at the edges of the panel and less in the centre, but the direction of the movement cannot be determined using this image

processing and DHSPI, and only relative displacement is indicated in Figure 3a. Hence, it is not possible to determine directly if the wood is under compression or traction and this represents a certain limitation in the application of DHSPI. Due to the resolution of the CCD camera it is not possible to detect the number of fringes after the lapse of 200 s because of the significant difference between the object’s original position and the position of the moving object following the change in RH. As it is observed in Figure 4, where displacement vs. time is presented, in the case where the sample adjusts from dry conditions to increasing RH, the absolute (total) displacement is much smoother and smaller than in the case for which the sample adjusts from high RH to a lower RH. Even after only 50 seconds, a change of 1.5 micrometers is induced. It is also interesting that the velocity of change is stable in the first case, but high at 0.035 µm/s. In contrast, in the second case a smaller velocity of 0.028 µm/s deccelerates at approximately 5.7 × 10−5 µm/s2 to 0.015 µm/s after 200 s. 4

CONCLUSIONS

The effect that changes in RH have on the structural movement of model paintings has been assessed with DHSPI, and 3D representations of displacement of the objects indicate differential movement of the surface. Especially important for conservation is the immediate response of wooden objects to even small changes in RH which can be quantified and monitored using HI and, in this case, with DSHI, real-time monitoring can be achieved. More research on climate changes and structural movement of cultural heritage is required to explain movement variation and determine thresholds of risk as climate fluctuations in museums and during transport of objects may be inevitable. Hopefully, with

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higher resolution CCD cameras, larger objects could be examined. Finally, it is likely that the application of the ESPI for routine monitoring will become particularly useful for conservation of cultural heritage.

ACKNOWLEDGEMENTS The work presented has been funded by the European Project MULTIENCODE SSPI 006427, as part of the 6th EU framework programme.

REFERENCES Albrecht, D., Franchi, M., Lucia, A. C., Zanetta, P. M., Aldrovandi, A., Cianfanelli, T., Riitano, P., Sartiani, O. & Emmony, D. C. 2000. Diagnostic of the conservation state of antique Italian paintings on panel carried out at the Laboratorio di Restauro dell’Opificio delle Pietre Dure in Florence, Italy with ESPI-based portable instrumentation. Journal of Cultural Heritage 1: S331–S335. Burke, J., Helmers, H. et al. 1998. Messung schnell veränderlicher Verformungen mit räumlich phasenschiebender elektronischer Specklemuster-Interferometrie (ESPI). Z. Angew. Math. Mech. 78: 321–322.

Coggrave, C. R. Quantitative Interferogram Analysis, http://www.phasevision.com/technology_phase_analysis. htm. D’Andrea Cennini, C. (1960, trans. by D. Thompson). The craftsman’s handbook: The Italian “Il libro dell’Arte”. New York: Dover Publications. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2006. In R. G. W. Brown & E. R. Pike (eds.) Lasers in the Preservation of Cultural Heritage; Principles and applications. Chapter 5. NewYork: Taylor and Francis. Spagnolo, G. S., Ambrosini, D. & Guattari, G. 1997. Electrooptic holography system and digital image processing for in situ analysis of microclimate variation on artworks. J. Opt. 28: 99–106. Theophilus (1963, trans. by J. Hawthorne and C. Smith) On divers arts: the treatise of Theophilus. Chicago: University of Chicago. Tornari, V., Bonarou, A, Esposito, E., Osten, W., Kalms, M., Smyrnakis, S. & Stassinopulos, S. 2001. Laser based systems for the structural diagnostic of artwork: an application to XVII Byzantine icons, Proc. SPIE 4402: 172–183. Tornari, V., Bonarou, A, Zafiropulos, V., Fotakis, C., Smyrnakis, S. & Stassinopulos, S. 2003. Structural evaluation of restoration processes with holographic diagnostic inspection. Journal of Cultural Heritage 4: 347s–354s. Vest, C.M. 1979. Holographic interferometry. 68–69. USA: John Wiley & Sons.

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Integrated digital speckle based techniques for artworks monitoring D. Ambrosini & D. Paoletti Università dell’Aquila, L’Aquila, Italy

G. Galli Università di Roma “La Sapienza”, Roma, Italy

ABSTRACT: In this paper we discuss artwork diagnosis by digital speckle based techniques, such as ESPI (Electronic Speckle Pattern Interferometry), speckle decorrelation, shearography and speckle photography. The drawbacks of a single diagnostic technique can be partially overcome by integration, proposed by different research groups in recent years. In this paper, technique integration is discussed from two points of view: a) integration of different methods in the same equipment and b) use of different methods in diagnosis (i.e. an integrated approach). With respect to the existing literature, emphasis is given to extend monitoring to different artworks (wooden panel): an extensive comparison on models is given.

1

INTRODUCTION

The ageing process in a work of art can have different effects depending on several factors such as the original materials, the surrounding environmental conditions and, above all, the conservation accidents and ancient restorations. In particular, the surface of artworks, interacting with the environment, can be modified over time. It is now generally acknowledged that preservation requires an integrated multidisciplinary approach based on effective diagnostic tools for both checking the state of conservation of the artworks and identifying the interventions for their conservation. In principle, the methods for monitoring artworks should have the following features: – Non-contact nature; – Fast and inexpensive operation; – Good sensitivity. In this paper we discuss artwork diagnosis by digital speckle based techniques, such as ESPI (Electronic Speckle Pattern Interferometry), speckle decorrelation, shearography and speckle photography. ESPI diagnosis is well documented in literature (Paoletti & Schirripa Spagnolo 1996, Ambrosini & Paoletti 2004) while a comparatively smaller attention has been paid to the structural diagnosis of artworks by decorrelation and speckle photography. The drawbacks of a single diagnostic technique can be partially overcome by integration; therefore, an

integration of techniques, often in a portable equipment, was proposed by different research groups in recent years (e.g. Schirripa Spagnolo et al. 1997b, Tornari et al. 2000, Tornari et al. 2007, Hinsch et al. 2007). In this paper, technique integration is discussed from two points of view: a) integration of different methods in the same equipment (integration of architectures) and b) use of different methods in diagnosis (an integrated approach). With respect to existing literature, emphasis is given to extend the monitoring to different works of art (wooden panel): an extensive comparison on artworks is given. Experimental qualitative and quantitative data analysis is considered, using FFT and correlation based algorithms. Furthermore, in the framework of an integrated approach, complementariness with traditional (e.g. visual inspection, tapping, raking light photography) and recent (e.g. infrared thermography) techniques should be considered. It is worth noting that the imaging diagnosis have the distinctive feature of portraying results in an easily understandable visual format. It will enable conservators not trained in non-destructive techniques (NDT) to use the results of modern diagnosis as well as the general public to understand and share the efforts for artworks protection and conservation. Furthermore, imaging NDT can share the advantages of the recent diffusion of multimedia technology and effective electronic imaging tools in the cultural heritage field.

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2

DIAGNOSTIC METHODS

Holographic interferometry (Vest 1979) is a wellknown NDT, which makes possible to map the displacements of a relative rough surface with an accuracy of a fraction of a micron, used as a diagnostic tool in artworks since 1974. It can furnish information about location of defects at an incipient stage in wooden panels, frescoes, mosaics and so on, as well as a precise monitoring of restoration processes (Tornari et al. 2003). However, despite the sensitivity and the high quality of the images, holographic interferometry was never really able to exit the research laboratories and to reach the restorers daily practice. One of the reasons is the cost, the stringent stability requirements and the need of an optically skilled operator. An important advance in flexibility and ease of use is represented by speckle-based techniques.

be digitally processed to extract a 3D map of the defect. It is then clear that, from an operational point of view, ESPI is faster and simpler to use than conventional holography. ESPI has proved to be a very attractive tool, especially for in situ investigation on frescoes, wooden panels and museum objects (Gülker et al. 1990, Boone & Markov 1995, Paoletti & Schirripa Spagnolo 1996, Lucia et al. 1997, Schirripa Spagnolo et al. 2003, Ambrosini & Paoletti 2004, Dulie-Barton et al. 2005), as well as on different artworks such as mosaics (Schirripa Spagnolo et al. 1998), canvas (Young 1999) and terracotta warriors (Gülker et al. 2001). ESPI can also be used to monitor the artwork conditions over time as well as the real object deformations due to microclimate variations (Schirripa Spagnolo et al. 1997a) or the effect of sunshine on frescoes (Hinsch et al. 2007). 2.2 Shearography

2.1

Electronic speckle pattern interferometry

ESPI, also called Video holography, was developed in the early 1970s as a method of producing interferometric data using video systems instead of holographic materials (Rastogi 2000). The experimental setup to perform ESPI diagnosis resembles holography, with the TV target replacing the glass plate as the recording medium. The reconstruction process is performed electronically with the computer. In practice, the intensity distribution in the detector plane is stored with the object in its reference state. The object is then deformed and a second frame is stored. The two frames are then subtracted electronically to give a resultant intensity distribution. If the speckle patterns from the first and subsequent frames are subtracted and the difference is squared, correlation live fringes are displayed on the TV monitor. By using narrow band filters, centred at the laser wavelength, ESPI systems can also operate in daylight conditions. The resulting fringes are similar in appearance to conventional holographic fringes but with a lower image quality, due to a much more evident speckle noise. For this reason, ESPI fringes are usually digitally treated for noise removal and contrast enhancement. Furthermore, due to the subtractive nature of the reconstruction process, many visible details of the artwork are lost and the precise location of the defect on the artwork can be difficult. This drawback can be alleviated by recording a visual image of the object using the same TV camera of the ESPI system and by superposing on the fringes the edge map of the object (Schirripa Spagnolo et al. 1997a). An advantage of ESPI is the possibility to follow the displacement visually on the monitor and to save a suitable record at any time. Finally, ESPI images can

Speckle shearography (Hung 1982) is an interferometric method to measure displacement gradients of a surface. Its significant advantages with respect to holographic techniques are the simplicity of the optical setup, more tolerance to environmental disturbances and reduced resolution requirement of the recording medium. Shearography can be simply performed inserting a Fresnel biprism in front of the lens as shearing device. Scattered light from the object surface passes through halves of the biprism and is focused on the CCD. The two laterally sheared wavefronts interfere on the CCD to form a speckle pattern. This speckle pattern is slightly altered if an object deformation occurs. Therefore, shearograms are obtained, in practice, by subtracting two speckle patterns, sequentially recorded, with a deformation in-between.The resulting fringe patterns depict displacement derivatives with respect to the direction of image shearing. Shearography has proved to be a very attractive tool for investigation of composite structures, such as tyres and honeycomb structures. It is also particularly effective in detecting delaminations (Steinchen & Yang 2003). Shearography can be combined in a single equipment with digital speckle photography to obtain full characterization of surface strain (Groves et al. 2005). A sophisticated shearography sensor, projected as part of a multi-functional sensor and capable to work on panel paintings and canvas, has been recently presented (Groves et al. 2007). 2.3 Speckle decorrelation We can roughly define the correlation between two speckle patterns as the capability to give fringes. Local correlation of laser speckles consists in the evaluation

400

Figure 1. Integrated speckle based experimental setup.

of a local parameter that estimates the decorrelation of speckles after any modification of the test object. Close correspondence exists between the object surface structure and the speckles in the image plane. For this reason, speckle correlation can characterize any physical or chemical mechanism that involves a surface alteration of the order of laser wavelength. To obtain the correlation pattern corresponding to the deformation field, two images are acquired and stored before and after the deformation. Then, a digital subtraction between these two images is performed. If the two images are perfectly correlated, they will cancel completely when subtracted; if there is some decorrelation, the subtraction will not be complete. Therefore, where non-correlation occurs, bright areas are visible, indicating the presence of defects. The technique can provide information about defects on wooden paintings, frescoes and mosaics (Schirripa Spagnolo et al. 1997b, Schirripa Spagnolo et al. 1997c, Schirripa Spagnolo et al. 2003a). It has less sensitivity and image quality than ESPI but it is comparatively cheaper and simpler.

2.4

Digital speckle photography

The previous speckle technique is largely qualitative. Better results are obtained by Digital Speckle Photography (DSP). DSP is a well established technique based on the calculation of the geometrical displacement of a speckle pattern (Sjödahl 2001). In the cross-correlation approach, subimages are extracted from the reference image IR and the deformed image ID . Then, the correlation surface is obtained using suitable correlation filters. The peak location in the correlation surface gives the relative displacement between the two subimages.

As a rule of thumb, one can think that the sharper the correlation peak, the more reliable the estimation of its position. This is not completely true, because noise tolerance is very important (Sjödahl 2001). An early version of DSP, still based on autocorrelation, was used to monitor efflorescence in stones (Schirripa Spagnolo & Paoletti 1996). More recently, it was applied to monitor displacements of historical leather tapestry (Hinsch et al. 2005). 3

INTEGRATION OF ARCHITECTURES

The possibility of integrating different diagnostic methods in the same equipment was suggested by several authors in literature, both in artworks diagnosis (e.g. Schirripa Spagnolo et al. 1997b, Tornari et al. 2000, Hinsch et al. 2007) and in flow measurements (e.g. Dubois et al. 1999). In this paper, we propose a system which is based on a previous integrated setup (Schirripa Spagnolo et al. 2001). The experimental setup (see Fig. 1) has been designed to combine ESPI, shearography and speckle decorrelation, as in Schirripa Spagnolo et al. (2001), plus digital speckle photography. Most optical elements are common to every diagnostic technique. The switching from one technique to the other is achieved by inserting or removing the appropriate element. Table 1 resumes the optical diagnostic techniques possible with the system and gives the specific elements to introduce or remove. 4

INTEGRATED DIAGNOSTIC APPROACH

Generally speaking, the integration of different techniques is useful in solving the typical problems

401

Table 1. A list of possible diagnostic techniques with the integrated setup. Optical tools

Reference beam

Fresnel biprism

ESPI Shearography Speckle decorrelation Speckle photography

yes no no no

no yes no no

Figure 2. Holographic interferogram on a wooden panel model.

associated with each single method. Integrated diagnostic approach is widely accepted (see Preusser 1991) for example, in fresco studies. Furthermore, all results should be collected in a digital interactive database for general public and researchers.

5

EXPERIMENTAL RESULTS

The proposed system has been tested on a model simulating a wooden panel painting with defects.The model consisted of a 15 cm × 21 cm × 2 cm panel of poplar wood, coated with the usual priming layers of canvas, gesso and glue and with artificially detached regions inside the layered structure. In the following, holographic interferometry is considered as the benchmark for optical testing. Figure 2 shows the holographic interferogram obtained after

Figure 3. ESPI (a) and decorrelation (b) on a wooden panel model.

stressing the model by a 150 W IR lamp at approximately 1 m of distance. Note the two debonds under the eye of the model. For a comparison, Figure 3 shows results obtained by ESPI (a) and decorrelation (b). Although these techniques are able to detect defects, there is an image degradation and a slight loss in sensitivity. For a comparison with shearography results on the same model, see Schirripa Spagnolo et al. (2001). Now, emphasis is given to digital speckle photography (DSP). In literature, DSP is mainly used to detect displacements (Hinsch et al. 2005) and the inplane displacement gradient components (Groves et al. 2005). In principle, defects can also be detected by monitoring in-plane displacements. To validate this idea, the experimental setup of Figure 1 was used in the DSP configuration. The TV camera was a Silicon Video® 2112 CMOS with PIXCI® D2X imaging board by EPIX with a resolution of 1288 x 1032 pixels. The camera was equipped with a Micro-Nikkor 55 mm lens.

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Figure 4. Magnitude of displacements on a wooden panel model.

Figure 5. Displacement arrows on a wooden panel model.

Images were evaluated using correlation algorithms based on the MATLAB® package MatPIV 1.6.1 by J.K. Sveen. Cross-correlation was calculated with three iterations through the images and a 50% overlap of the interrogation windows. A simple signal-to-noise ratio filter was used. Figures 4 and 5 show the results obtained with DSP: the debonds are clearly located (check the “8” figure). A comparison with simple qualitative decorrelation on the same object test (Fig. 3) shows the increase in sensitivity and performance. A final comparison, in the frame of an integrated approach, is made with infrared (IR) thermography. IR thermography is a non-destructive diagnostic technique, which has already been successfully and widely applied to historical buildings and masonry (Grinzato 2001). Regarding painted surfaces, the main difficulties concerning the thermograms are correlated with the influence of the different emissivity of the pigments.

Figure 6. IR thermography on a wooden panel model.

Figure 6 shows a thermogram obtained on the wooden panel model using a FLIR – S65 HS IR camera. Defects are once again located, together with some false alarms. A comparison with the same situation using an older IR camera, AVIO TVS-2000MkIILW (Schirripa Spagnolo et al. 2001) shows the increased performance in diagnosis. A similar approach in wall paintings diagnosis can be found in (Ambrosini et al. 2006). Finally, some diagnostic results on a real artwork, a mosaic (approximately 20 cm × 20 cm) of Roman age with marble tesserae, coming from Hadrian’s Villa (Tivoli, Rome, 125–134 AD), are reported. Mosaics are usually made of tesserae of different colours or materials; it may happen that in some regions the speckle patterns do not interfere. In this case, it may be useful to introduce carrier fringes in ESPI (Schirripa Spagnolo et al. 1998) or use an integrated approach. Figure 7 shows the results obtained by holographic interferometry and speckle decorrelation. A large defect is detected in the upper right region of the holographic image. The speckle decorrelation image (Fig. 7, bottom) indicates more clearly a large detached region that is an adhesion loss between the tesserae and the setting bed. For a comparison with ESPI and thermography results on the same artwork, see Schirripa Spagnolo et al. (1998).

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6

CONCLUSIONS

In this paper, digital speckle based techniques were considered for artwork diagnosis. Techniques integration is discussed from two points of view: a) integration of different methods in the same equipment (integration of architectures) and b) use of different methods in diagnosis (an integrated approach). With respect to existing literature, emphasis is given to extend monitoring to different artworks: an extensive comparison

Figure 7. Holographic interferometry and speckle decorrelation performed on a mosaic of roman age.

on artworks is given. Digital speckle photography (DSP) was find suitable to reveal defects. Further developments include the possibility to integrate, in the experimental setup shown in Figure 1, a contouring system based on a diffractive optical element (Schirripa Spagnolo & Ambrosini 2001). This equipment will be used to gain a 3D plot of the object as well as to locate defects through surface anomalies (Schirripa Spagnolo et al. 2003b). REFERENCES Ambrosini, D. & Paoletti, D. 2004. Holographic and speckle methods for the analysis of panel paintings. Developments

since the early 1970s. Reviews in Conservation 5: 38–48. Ambrosini, D., Paoletti, D., Quaresima, R. & Galli, G. 2006. Frescoes diagnosis: an integrated approach and a case study. In R. Sablatnig, J. Hemsley, P. Kammerer, E. Zolda, J. Stockinger (eds.) Digital Cultural Heritage – essential for Tourism, Proc. 1st EVA 2006 Vienna Conference, Vienna, 27–30 August 2006. Vienna: Austrian Computer Society. Boone, P. M. & Markov, V. B. 1995. Examination of museum object by means of video holography. Studies in Conservation 40: 103–109. Dubois, F., Joannes, L., Dupont, O., Dewandel, J. L., Legros & J. C. 1999. An Integrated Optical Set-up for Fluid-physics Experiments under Microgravity Conditions. Measurement Science & Technology 10: 934–945. Dulie-Barton, J., Dokos, L., Eastop, D., Lennard, F., Chambers, A. R. & Sahin, M. 2005. Deformation and strain measurement techniques for the inspection of damage in works of art. Reviews in Conservation 6: 63–73. Grinzato, E. 2001. Infrared and thermal testing for conservation of Historic Buildings. In X. P. V.Maldague (tech. ed.) & P. O. Moores (ed.) Non-destructive Testing Handbook, third edition: Volume 3. Columbus: ASNT. Groves, R. M., Fu, S., James, S. W. & Tatam, R. P. 2005. Single-axis combined shearography and digital speckle photography instrument for full surface strain characterization. Optical Engineering 44: 025602. Groves, R. M., Osten, W., Doulgeridis, M., Kouloumpi, E., Green, T., Hackney, S. & Tornari, V. 2007. Shearography as part of a multi-functional sensor for the detection of signature features in movable cultural heritage. In C. Fotakis, L. Pezzati, R. Salimbeni (eds.) O3A: Optics for Art, Architecture and Archaeology, Munich, 20–22 June 2007. Proc. SPIE 6618: 661810. Gülker, G., Hinsch, K. D., Holscher, C., Kramer, A. & Neunaber, N. 1990. ESPI system for in situ deformation monitoring on buildings. Optical Engineering 29: 816–820. Gülker, G., Hinsch, K. D. & Kraft, A. 2001. Deformation monitoring on ancient terracotta warriors by microscopic TV-holography. Optics and Lasers in Engineering 36: 501–513. Hinsch, K. D., Gülker, G., Hinrichs, H. & Joost, H. 2005. Artwork monitoring by digital image correlation. In K. Dickmann, C. Fotakis, J.F. Asmus (eds.) Lasers in the Conservation of Artworks, Springer Proceedings in Physics 100: 459–467. Berlin: Springer-Verlag. Hinsch, K. D., Gülker, G. & Helmers, H. 2007. Check-up for aging artwork: optical tools to monitor mechanical behaviour. Optics and Lasers in Engineering 45: 578–588. Hung, Y. Y. 1982. Shearography: a new optical method for strain measurement and non-destructive testing. Optical Engineering 21: 391–395. Lucia, A. C., Zanetta, P. M. & Facchini, M. 1997. Electronic Speckle Pattern Interferometry applied to the Study and Conservation of Paintings. Optics and Lasers in Engineering 26: 221–233. Paoletti, D. & Schirripa Spagnolo, G. 1996. Interferometric methods for artwork diagnostic. In E. Wolf (ed.) Progress in Optics XXXV: 197–255. Amsterdam: Elsevier. Preusser, F. 1991. Scientific and technical examination of the Tomb of Queen Nefertari at Thebes. In Sharon Cather (ed.), The conservation of wall paintings, Proc. Int.

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Symp., London, 13–16 July 1987.Los Angeles: The Getty Conservation Institute. Rastogi, P. K. (ed.). 2000. Digital Speckle Pattern Interferometry & Related Techniques. New York: Wiley. Schirripa Spagnolo, G. & Paoletti, D. 1996. Laser speckle correlation for monitoring building stone efflorescences. Journal of Optics 27: 133–137. Schirripa Spagnolo, G., Ambrosini, D. & Guattari, G. 1997a. Electro-optic holography system and digital image processing for in situ analysis of microclimate variations on artworks. Journal of Optics 28: 99–106. Schirripa Spagnolo, G., Ambrosini, D. & Paoletti, D. 1997b. Image decorrelation for in situ diagnosis of wooden artifacts. Applied Optics 36: 8358–8362. Schirripa Spagnolo, G., Paoletti, D., Ambrosini, D. & Guattari, G. 1997c. Electro-optic correlation for in situ diagnosis in mural frescoes. Journal of Optics A Pure and Applied Optics 6: 557–563. Schirripa Spagnolo, G., Ambrosini, D. & Paoletti, D. 1998. Optical methods for mosaics diagnosis. Journal of Optics 29: 394–400. Schirripa Spagnolo, G. & Ambrosini, D. 2001. Diffractive optical element-based profilometer for surface inspection. Optical Engineering 40: 44–52. Schirripa Spagnolo, G., Ambrosini, D. & Paoletti, D. 2001. Comparative study on the efficiency of some optical methods for artwork diagnosis. In Renzo Salimbeni (ed.), Laser techniques and Systems in Art Conservation, Proc. Int. Congr., Munich, 18–19 June 2001. Bellingham: SPIE. Schirripa Spagnolo, G., Ambrosini, D. & Paoletti, D. 2003a. An NDT electro-optic system for mosaics investigations. Journal of Cultural Heritage 4: 369–376.

Schirripa Spagnolo, G., Majo, R., Ambrosini, D. & Paoletti, D. 2003b. Digital moiré by a diffractive optical element for deformation analysis of ancient paintings. Journal of Optics A: Pure and Applied Optics 5: S146–S151. Sjödahl, M. 2001. Digital speckle photography. In P.K. Rastogi (ed.) Digital Speckle Pattern Interferometry and Related Techniques: 289–336. Chichester: Wiley & Sons. Steinchen, W. & Yang, L. 2003. Digital Shearography. Bellingham: SPIE Press. Tornari, V., Zafiropulos, V., Bonarou, A., Vainos, N. A. & Fotakis, C. 2000. Modern technology in artwork conservation: a laser-based approach for process control and evaluation. Optics and Lasers in Engineering 34: 309–326. Tornari, V., Bonarou, A., Zafiropulos, V., Fotakis, C., Smyrnakis, N. & Stassinopulos, S. 2003. Structural evaluation of restoration processes with holographic diagnostic inspection. Journal of Cultural Heritage 4: 347s–354s. Tornari, V., Bernikola, E., Osten, W., Groves, R. M., Marc, G., Hustinx, G. M., Kouloumpi, E. & Hackney, S. 2007. Multifunctional encoding system for assessment of movable cultural heritage. In C. Fotakis, L. Pezzati, R. Salimbeni (eds.) O3A: Optics for Art, Architecture and Archaeology, Munich, 20–22 June 2007. Proc. SPIE 6618: 66180W. Vest, C. M. 1979. Holographic Interferometry. New York: Wiley. Young, C. 1999. Measurement of the biaxial properties of nineteenth century canvas primings using speckle pattern interferometry. Optics and Lasers in Engineering 31: 163–170.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Laser-based structural diagnosis: A museum’s point of view E. Kouloumpi, A.P. Moutsatsou, M. Trompeta, J. Olafsdottir, C. Tsaroucha & A.V. Terlixi National Gallery – Alexandros Soutzos Museum, Athens, Greece

R.M. Groves Institute für Technische Optik (ITO), Stuttgart, Germany

M. Georges Centre Spatial de Liege (CSL), Liège, Belgium

G.M. Hustinx Optrion, Liège, Belgium

V. Tornari Institute of Electronic Structure and Laser (IESL), FORTH, Crete, Greece

ABSTRACT: This paper presents the museum’s point of view on the application of holographic techniques on portable panel paintings. The “Multifunctional Encoding System for Assessment of Movable Cultural Heritage” (Multi-Encode) is an European research program whose aim is to solve problems related to portable cultural heritage through the institution of a laser-based method for detection and recording of the artifacts’ structural condition. The unique property of holography to diagnose defects before they become visible to the naked eye and thus encoding them, offers solutions to long-lasting problems, such as continuous control of the condition of the artifact before and after its exhibition or loan; diagnostic information to the conservators about its preservation or authenticity inspection. The National Gallery – Alexandros Soutzos Museum, as an end-user, evaluates the usefulness of this type of structural diagnosis techniques and underlines the importance of their presence in a museum’s action line. 1

INTRODUCTION

The collection of information concerning the identity, the structure, the preservation condition and the pathology of an artwork, is a demand of every modern museum. Additionally, the museums’ policy for cultural exchanges demands the creation of temporary exhibitions and thus a loan strategy. Under these circumstances, the work of art has to deal with environmental changes, transportation, storage and handling, leading inevitably to deterioration. Moreover, later or even improper conservation treatments may affect the condition of the artifact, or even sometimes its authenticity can be at risk. Among the several types of artifacts, paintings – either on canvas or on wooden panel- consisting of many different layers can prove quite problematic. The various layers, comprising different and sometimes inhomogeneous materials (different density of components, different reaction rates, etc.) interact with each other, creating weak areas. Hence, when a defect is

formed in one layer, it is almost certain that it will affect the integrity of the subsequent layers as well. This often results in weakening of the layers, formation of defects and accelerated deterioration of the artifact. Structural diagnosis techniques are extremely valuable tools for every museum, since they can offer important information on the existing hidden defects and their evolution, the deterioration degree and the overall condition of the artifact. More important, their non-destructive nature does not contradict the conservation ethics (such as minimum intervention) and does not affect the integrity of the object of art. Laser-based structural diagnosis and especially laser metrology is a relatively new tool for the museums community. Multi-Encode is a research program based on the application of holographic methods on artworks aiming to form a structural diagnosis technique capable to encode the artifact, examine and control the evolution of the decay and determine a signature-authenticity element.The National Gallery – Alexandros Soutzos Museum, as part of the specific

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consortium, under the role of end-user together with Tate Gallery, is responsible for the definition of the museum’s requirements in order to apply this method onto museum objects, to provide the necessary samples and finally, to assess the integrated system. As it can be seen, there is a lack of instrumental techniques available on structural diagnosis. Conventional techniques such as radiography can indeed provide important information concerning the structure of a painting, such as location and direction of nails, condition of knots, etc., but they cannot offer in any case information about the stresses, strains and evolving defects. At this point, holographic techniques come to provide solutions and fill in the gap. 2

MULTI-ENCODE RESEARCH PROGRAM

The Multi-Encode (STREP 10.10.2003) project aims to provide solution to long-standing problems in movable cultural heritage strategic issues by developing a novel impact assessment procedure and by advancing the state-of-the-art in conservation monitoring procedures of panel and easel paintings on loan and display. Analytically, the aim of this project is to exploit the holographic technology advances and innovative tools by using a series of holographic techniques with different geometries, designs and characteristics. The proposed method relies on the original coded extraction of distinct features from a painting under conservation, transportation and loan that characterizes the state of preservation of the artwork and its originality. The coding and decoding of such characteristic features can be performed holographically and then optically and numerically transformed for digital archiving. The object features of the archived coded data forming the signatures of the object can be used for later comparison in order to record initial stages of deterioration and also produce secure-labelling for the artifacts. 2.1

NGA’s participation to MULTI-ENCODE

The Conservation Department of the NGA deals on a daily basis with a large number of paintings both on panel and canvas. It was decided that NGA would focus on panel paintings and Tate on easel paintings. The main reason was to cover a greater range of materials and therefore, investigate two distinct art styles. A summarized outline of the end-user’s participation is: a) To define and suggest the meanings of “signature defects” of artworks. b) To construct artificial samples and determine the rules of cycling and the effects of alteration on their structure. c) To review and assess the results of the investigation.

d) To provide guidance on system integration, userfriendly requirements, hardware construction to suit conservation and travel exhibition demands. e) To assist the generation of end-user protocols and procedure standardisation. 2.2 NGA’s MULTI-ENCODE research protocol The development of structural diagnosis techniques requires artificial samples that satisfactory simulate original objects. Thus, NGA produced artificial samples to imitate original panel paintings. The protocol for the construction of model panel paintings included: – Study of the typical structure of the paintings to be simulated. – Determination of the deterioration factors that can be reproduced. – Definition of the defects that have to be reproduced. – Construction of the samples. – Documentation and characterization of the artificial samples. – Ageing of the model panel paintings. Taking into consideration the typical structure of a panel painting (for example, a Byzantine icon), the factors of deterioration and the types of defects that usually occur, a strictly definite series of 18 artificial samples were produced (Table 1, Figs. 1, 2). The samples were divided in 6 groups; each group contained 3 identical samples for each technical partner. The aim was to cover all possible combinations in the structure of a panel painting, such as the presence of textile, nails, etc. and to produce all the common types of defects, such as cracked wood, detached ground and paint layers, etc. For the accelerated ageing, it was decided to follow the oxidative type of ageing through thermal treatment of the samples. Due to the lack of standards on accelerated ageing of painting on wooden panels, original experimental protocols were produced. According to previous papers (Wu 1998, Seves et al. 2000) and publications (Feller 1994), the kinetic properties can be used to estimate the rate of degradation of materials containing cellulose, employing the Arrhenius equation:

where Ea is the activation energy, R is the constant of gases, T1 is the accelerated ageing temperature and T2 is the reference temperature (25◦ C). It should be noted that the other deterioration mechanisms of cellulose, lignin and other wood components, i.e. hydrolysis and photo-oxidation, were not stimulated by this procedure. However, this accelerated ageing procedure is satisfactory serving the

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Table 1. Table of artificially made samples and their characteristics. # Technique

Layer Structure

Defects

1 Byzantine 1. wooden substrate 1. iron nails (with textile) 2. incise the sustrate 2. torn textile 3. textile 3. cracked wooden 4. preparation layer substrate 5. bole 6. layer of gold 7. paint layer 8. varnish 2 Byzantine 1. wooden substrate (with textile) 2. textile 3. preparation layer 4. bole 5. layer of gold 6. paint layer 7. varnish 3 Byzantine 1. wooden substrate (with textile) 2. textile 3. preparation layer 4. tracing – anthivolon 5. paint layer 6. varnish 4 Byzantine 1. wooden substrate (no textile) 2. preparation layer 3. bole 4. layer of gold 5. paint layer 6. varnish 5 Byzantine 1. wooden substrate (no textile) 2. preparation layer 3. bole 4. layer of gold 5. paint layer 6. varnish 6 Byzantine 1. wooden substrate (no textile) 2. preparation layer 3. tracing – anthivolon 4. paint layer 5. varnish

1. knots 2. partial loss of gilding 3. loss of ground layer

1. general ageing during preparation 2. superficial cracking of gesso and paint layers 1. iron nails 2. cracked wooden substrate Figure 1. An artificial sample (Sample 1-Group 1). 1. knots 2. partial loss of gilding 3. loss of ground layer 1. general ageing 2. superficial cracking of gesso and paint layers

Figure 2. Schematic cross-section of an artificial sample (Sample 1-Group 1).

holographic experiments since its aim was the cause of remarkable defects on the samples more than the controlled cause of a chemical phenomenon. Vice versa, the holographic experiments of the project were an opportunity to study the protocols that have to be followed in the case of accelerated ageing of model panel paintings. All the samples underwent the accelerated ageing procedure in an oven (Memmert Company) with maximum air recycling. The indoor relative humidity varied from 45 to 50% during the whole procedure. The first phase of the heating program for each sample or

group of samples is presented in Table 2. The cycling continues for a number of group samples. 2.3 Evaluation of samples The samples were examined by the technical partners before and after the ageing process. Among the purposes was to detect the defects produced, to differentiate the stable and unstable defects, to monitor the alterations produced with ageing, to develop the impact assessment procedure, to develop an integrated

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Continuous heating at 102◦ C, for 480 h

has become larger. Also a change from the smooth surface texture can be seen. One possibility is that the paint layer is being detached from the wood at these points. This type of observations of the evolution of the defects could not have been made by naked eye, or by any other structural diagnosis technique.

Heating at intervals at 102◦ C, for totally 66.5 h

3

Table 2. The heating protocol followed for the MultiEncode model paintings. Sample Group no Heating protocol 1

2

3

4

5

6

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

Continuous heating at 102◦ C, for 17.5 h Continuous heating at 102◦ C, for 46 h Continuous heating at 102◦ C, for 141 h Continuous heating at 85◦ C for 247 h

CONCLUSIONS

As it has been previously mentioned, structural diagnosis techniques are extremely valuable for every museum. Even though there is a variety of techniques adopted so far, none of them presents the advantages of laser interferometric techniques. For example, radiography can provide important information concerning the structure of a painting, such as location and direction of nails, condition of knots, etc., but it cannot offer in any case information about the stresses, strains and evolving defects or detachment; on the contrary, axial tomography can detect the detachments but it cannot distinguish the stable from the active ones; thermography cannot detect detachments unless they are active, while it can offer a series of structural information but their interpretation can be problematic, since they can be related to different causes (Drdacky & Lesak 2006). The NGA’s participation to Multi-Encode led to the conclusion that the holographic system as it has been developed along this project can be a valuable tool for the museum’s procedures. Among the number of advantages, the integrated system is a quick, non-destructive, non-invasive and non-contact, remote method, which can provide whole field analysis with high resolution, real time and temporal resolved analysis with low vibrational sensitivity. Apart from the fact that the holographic system demands certain acquaintance in order to interpret the results and apply the method, it presents a set of invaluable characteristics for the conservation scientists. Specifically:

1. Continuous heating at 60◦ C, for totally 66.5 h 2. Continuous heating at 90◦ C, for totally 148 h Heating at intervals at 102◦ C, for totally 66.5 h

Figure 3. Artificial sample 3-Group 3 after accelerated ageing at 102◦ C continuous heating for 141 h, examined by holographic interferometry at two different time periods (IR lamp loading by one front and one back lamp).

system and to create analytical protocols suitable for panel paintings. A typical example of the results obtained from holographic analysis on the model paintings is presented in Figure 3. As it can be seen, the change in the sample due to the ageing process is very clear. The vertical linear feature in the phase map is, by visual inspection of the top and bottom edges of the sample, a crack. The relative strain levels at the crack are higher in the aged sample (bottom phase map) indicating that the crack

– This system can be easily installed and work in a museum’s environment. – It can be used as a mapping tool for most type of defects. – It provides ability to assess in detail the condition of the object. – It leads the conservators to focus on certain problematic areas. – It can provide a “signature” when selecting the suitable (stable) defects like cracks or knots. In addition, it must be mentioned that this structural diagnosis system can be used not only for conservation purposes, but also for seasonal control, before and after a loan process and for authenticity checking.

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ACKNOWLEDGEMENTS

REFERENCES

The authors would like to thank the Multi-Encode project [006427 (SSPI)]. Thanks are also due to Eleni Kavalieratou and Lucila Torres from the National Gallery – Alexandros Soutzos Museum. Finally, thanks are extended to the following project partners for their invaluable contribution: Cedric Thizy from CSL-Belgium, Stephen Hackney and Tim Green from the Tate-UK, Roger Groves from ITO-Germany and Yannis Orphanos, Kostas Hatzigiannakis, Irini Bernikola from FORTH/ IESL-Greece.

Drdacky M. & Lesak J. 2006. Non-invasive survey of detachment of historic rendering. In R. Fort, M.A. de Buergo, M. Gomez-Heras, C. Vasquez-Calvo (eds.), Heritage, Weathering and Conservation: 591–597. London: Taylor and Francis. Feller, R.L. 1994. Accelerated Aging, Los Angeles: The Getty Conservation Institute. Seves, A.M. et al. 2000. Effect of thermal accelerated ageing on the properties of model canvas paintings, Journal of Cultural Heritage 1: 315–322. Wu,Y. 1998. Kinetic studies of thermal degradation of natural cellulosic materials, Thermochimica Acta 324: 49–57.

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High-resolution 3D laser digitisation of the Maiano terracotta roundels for documentation and condition monitoring K. Hallett, Z. Roberts & S. Julien-Lees Historic Royal Palaces, Surrey, United Kingdom

A. Geary University of the Arts, London, United Kingdom

ABSTRACT: The exterior of Hampton Court Palace is adorned with a significant set of terracotta Renaissance sculptures of Roman Emperors. Exposure to the weather for nearly 500 years has lead to deterioration and as a result, a programme of recording, monitoring, analysis and treatment has been initiated. A crucial component is the use of high-resolution 3D laser scanning technology and metric analysis for documentation and condition tracking. This paper considers the digitisation of the sculptures using close range high-resolution optical laser scanning. The use of specialist reverse engineering software for the production of 3D models and their metric analyses is presented. The documentation and visualisation objectives of the project require that the resulting 3D models have accurately mapped colour data. To achieve this, computer graphics techniques including UV coordinate generation and projection mapping are utilised.

1

INTRODUCTION

In 1521, Cardinal Thomas Wolsey commissioned the Florentine artist Giovanni da Maiano to produce eight terracotta roundels of Roman emperors to adorn his new palace at Hampton Court, Surrey. Widely accepted to be the UK’s earliest surviving architecturally sited Renaissance figural sculptures, the roundels are highly significant artifacts. Despite a complex history of movement, repair and addition, the majority of the set survives after almost five hundred years in their original context (Thurley 2003). However, constant exposure to the elements and other factors has resulted in deterioration in all of the roundels, with three now in a particularly fragile state. In October 2005, Historic Royal Palaces (the charity that cares for Hampton Court Palace) embarked on a four-year programme to record, monitor and investigate the roundels. The objectives of the programme are to find out more about their history, study the materials and technology of their manufacture, record and track their condition, identify the mechanisms causing the terracotta to deteriorate and define a longterm conservation treatment to arrest active decay. Laser scanning enables us to document these complex three-dimensional sculptures in an extremely detailed manner, accurately recording the physical nature of the objects as they survive today.

Figure 1. Terracotta roundel of Emperor Galba.

2

3D DOCUMENTATION ISSUES

3D digitisation is a milestone in the recording and monitoring of the roundels. The digitisation data can be used in a raw state for documentation and analysis

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applications and provides the basis for any 3D modelling and visualisation work. It also enables interrogation of the sculptures via virtual access in between periods of physical access; located more than twenty feet above ground level, regular inspection of the roundels is an impractical prospect. Alongside documentation, the scanning is a crucial tool in the condition monitoring of the roundels. In conjunction with conventional written and photographic condition recording, comparisons of the scan data will permit us to track and quantify the rate of deterioration over time, in a way that has not previously been possible. This process will help us to make important decisions about the options for the longterm preservation of the roundels and enable us to monitor the success and sustainability of conservation treatment, once implemented. Several non-contact 3D digitisation technologies, including photogrammetry, laser scanning and image based methods, have been available for several decades (Blaise 2004), but more recently, cost accessible systems that meet the high-resolution and portability needs typical in cultural heritage artifact visualisation and documentation, have emerged. Methods that permit high resolution capture, suitable for the fine surface documentation requirements of the project, include laser scanning and active image based systems. The latter employs patterned light projection to extract range information from stereo CCD images. For this project, laser scanning offered greater immunity to the ambient light variability on the exposed site combined with the accuracy, and rugged portability required. For further technical accounts of available 3D sensing technologies, the reader is referred to Blaise (2004) for a comprehensive review of the development of range finding since the early 1980s. Fowler (1985) provides a detailed account of the principals of laser range finding. 2.1

Data capture

The digitisation of the roundels was undertaken by The Scan Team Ltd., in situ, using optical laser scanning and facilitated by scaffold access (Fig. 2). The Konica Minolta VI-910 (Konica-Minolta 2004) laser scanner was the chosen system for digitisation of the roundels as it combines the portability required to accommodate the inaccessibility of the site, with a level of accuracy sufficient for the purposes of the project. The necessity of using a scaffold tower to access the roundels for scanning presented additional potential for error due to vibration and motion in the platform. This was minimized, as far as possible, by restricting access during scanning, and reinforcing the scaffolding by boarding out the platforms for increased stability. During scanning, each scan was immediately reviewed for motion error and, if detected, simply

Figure 2. The Scan team scanning the roundels.

Figure 3. Detail and rendered view (inset) of complete Hadrian dataset.

repeated to capture a corrected version. Some alterations were also necessary for the control of the wide range of ambient light and weather conditions experienced on the external scaffold site, although modern scanning devices have become sufficiently powerful and offer precise adjustment control, in order to accommodate this factor. Two datasets were captured from each of the roundels – a complete high-resolution dataset of the entire roundel (Fig. 3) and a set of very high-resolution local patches of selected unstable surface regions – for the purposes of deterioration monitoring (Fig. 4). In the former, the scanner was used with a standard 14 mm lens configuration to record each entire roundel to a resolution of 0.5 mm. In the latter case, a 25 mm telephoto lens was used to capture coordinates to a resolution of 0.1 mm. The VI-910 scanner has a limited field of view within the optimal scanning range of between 600–1200 mm. This, together with the requirement

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Figure 4. High-resolution monitoring patch data for Nero.

to reposition the scanner to observe different viewpoints to optimise capture of less accessible regions, means that many areas, or scan “shells”, are captured when recording a subject. For each of the roundels, an average of 145 shells were recorded. To reconstruct these data into the usable geometry of a complete roundel, multiple overlapping shells were first registered relative to their adjacent neighbours to produce a single, contiguous surface. Following registration, the raw data coordinates were refined to reduce any erroneous “noise” and remove any extraneous data. Subsequently, by a process of automatic triangulation, the raw coordinates are connected to produce a complete polygonal mesh surface model, usable for the various applications of the research. Care was taken to capture the surface data to as complete a degree as possible by repositioning and orienting the scanner to observe undercut and high curvature features. Inevitably, data in some hidden areas – where a line of sight for the scanner was not achievable – were omitted. If a contiguous “watertight” model is needed for applications such as facsimile reproduction, this can be facilitated by filling any holes in the model by means of curvature filling algorithms, which interpolate a plausible infill from the adjacent data.

3

DATA PROCESSING AND MANIPULATION

The scale of the datasets and the processing, required to produce usable 3D models for the various purposes of the project, presented a technical challenge. Specialist reverse engineering software, such as RapidForm™ (Inus 2007) and Geomagic Studio™ (Geomagic 2007), normally used in computer-aided design and automotive prototyping, was applied in the production of the 3D models and in their metric analysis. Such commercial applications were developed primarily for the highly invested markets of vehicle design and other manufacturing industries, where

Figure 5. Detail of a human readable image map associated with a human readable generated UV map.

reverse engineering or computer aided manufacture have important roles and the software design is targeted specifically at the needs of this users group. For the museum sector, application features and feature combinations that fall outside typical industrial design needs are required. For example, metric tools for volume and surface analysis may be available, but not in combination with suitable viewing and annotation tools or the image rendering and export capability needed for conservation assessments. During the roundels project, an adaptive approach is being taken, using selected aspects of available software to meet specific project needs such as 3D annotation and metric analysis tasks. 3.1 UV Mapping The documentation and visualisation objectives of the project additionally required that the resulting 3D models have accurately mapped colour data. To achieve this, high-resolution digital photographs were applied to the mesh geometry using computer graphics techniques including UV coordinate projection mapping. UV mapping is the technique of unwrapping a 3D mesh to form a 2D net, defined in a 2D coordinate system with U and V axes (to differentiate from the 3D coordinate space where the convention is to use X and Y). Once created, this net or “UV map” is used to assign the pixels of a two-dimensional bitmap image with precision to a 3D model (Fig. 5). The original 2D image is distorted to some extent to accommodate the geometry, in the same way as a sheet of rubber might be if it is stretched to fit over a curved surface. The UV coordinate information is stored in the 3D model, where the file format supports this, so when the resulting image is mapped to the geometry, it will be correctly positioned. In regions of high curvature in the model, disturbing distortions can arise in the image-mapped appearance. This can be overcome to

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Figure 6. A non-human readable image map associated with UVs generated in the RapidForm application, according to a metric parameter.

a large extent by creating “seams” in the UV map that divide the model up into coplanar regions and projecting a map perpendicular to the average plane for each region. Producing viable UV maps for very large polygon count 3D models is extremely computationally expensive and represents a challenge to the “unwrapping” algorithms available to perform this function. This currently necessitates that lower resolution models are used in the visualisation aspects of the project. However, research is ongoing (Zayer et al. 2007, Geary & Swann 2007) to resolve the angle based flattening problem associated with this process. If successful, such methods will allow high polygon count models, such as the roundels with several million polygons, to be processed in a realistic time scale to produce viable UV coordinates. In some existing applications, such as RapidForm, their approach to UV creation attempts to overcome inefficiency problems by sorting the UVs according to a simple parameter, such as the edge length of the mesh faces. This is efficient; however, it results in a UV map that retains no morphological visual relationship with the original model. The map and its associated image map will render correctly, but are quite unintelligible in respect of human visual readability (Fig. 6). Only human readable UV maps and associated images offer the possibility of manual editing and manipulation for a range of visualisation applications. Some commercial 3D laser scanning technologies allow accurate colour capture in conjunction with 3D digitisation. Obviously, this offers the ideal approach to colour mapping, whereby the image map can be reverse engineered from an accurate colour value associated with each 3D point captured. However, this still

requires the challenge of high-resolution UV generation to be overcome so that the full resolution accuracy of the data can be exploited. The UV associated colour maps are heavily interpolated as the polygon count is reduced, and can quickly degrade to below an acceptable quality of definition. Visualisation applications, such as virtual reconstruction or visual annotation, currently require a 3D modelling and animation environment subsequent to basic 3D data processing operations. In this area, the software situation is less problematic. A range of industry standard applications such as Maya® (AutoDesk 2007), Cinema4D® (Maxon 2007) and 3D Studio Max® (AutoDesk 2007), widely used in the film and computer games industries, are well suited to cultural heritage visualisation. In this approach, the outcome is the production of rendered movies or still images. This means of representation can produce high-quality visualisation; however, specialist consultation is usually required given the complexity of the software and production processes. Moreover, the 2D representation of 3D data can never be as satisfactory or complete as interrogating the actual dataset at first hand. However, research is currently pursuing the development of software that will be able to offer high fidelity, real-time rendering of high-resolution 3D models needed for cultural heritage visualisation applications (Geary 2007).

4

DATA EXPLOITATION

The 3D scan data captured using the digitisation process has proven to be a versatile and essential aid in several different parts of the conservation project. For maximum efficiency in reconciling file size, computer processing power and condition monitoring requirements, only areas of particular vulnerability were selected for scanning at higher resolution. Three patches of approximately 10 × 10 cm2 were identified on each roundel, typically chosen because they contain an unstable crack or because the terracotta surface has lost the fireskin and subsequently eroded. Metric analysis will be applied to the high-resolution patches, which will be re-scanned periodically to allow measurement of surface morphological changes such as erosion on selected cross-sections, changes in crack dimensions and material losses. Quantification of these factors will inform and guide the future conservation plan and customised treatment strategies for these highly significant sculptures. The 3D digitisation programme is set into the wider project context of scientific analytical techniques, as not all change is visually observable. While capturing current condition and enabling quantification of future loss was the primary aim of the digitisation programme, the data may also be used

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Figure 7. Emperor Trajan, with damage to top of head.

for other purposes. The digitisation programme is one strand of a larger conservation project, which will include the treatment of unstable areas of damage on each roundel with a repair mortar or similar. Two of the roundels have suffered significant losses to the tops of the heads (Fig. 7).The 3D scan data may be used to produce accurate moulds for the production of gap-fills for their conservation treatment. In the longer term, if the outcome of the project’s deterioration monitoring indicates that some, or all, of the roundels cannot continue to withstand an uncontrolled exterior environment, the datasets may be used in the production of replacement facsimiles. Additionally, the 3D data is being used for interpretative and visualisation projects, which will enhance public access to the roundels. Replicas can be produced for visitor handling on the ground that will allow visitors to view the roundels close-up, perhaps with indications of original decorative paint schemes. Future potential exploitation of the datasets includes animation of the 3D models. Interrogation of the data has also helped to develop our curatorial understanding. Metric analysis of the data sets produced for each roundel segment allows elucidation of manufacturing technique, and comparison between the roundels to determine which parts may have been produced from the same mould. The ability to manipulate and realign the 3D models also allows examination of perspectives and sections, which are physically impossible to view in the real sculptures.

5

CONCLUSIONS

This project marks the start of a monitoring process which may continue for decades through constantly

evolving technology, so complete raw data must be able to be viewed, manipulated and quantified using a wide variety of common software programs. The data files are archived concurrently in a number of different formats, which are a mixture of open-source and industry standard proprietary formats (including ASCII, PLY, VRML and OBJ) to help ensure that the data will be accessible in the future. In order to use the data to measure rates of change in the sculptures, the file formats need to be flexible. The human readable UV generation, to enable editable colour mapped geometry for the roundel models, has been undertaken at a level of resolution near the current limits of what is computationally achievable using consumer workstation technology and available algorithms. It is hoped that continuing work in this area will permit much higher resolution UV colour mapping, closer to that of the raw capture data, to be applied in the visualisation aspects of the project. The production of a complete set of 3D digitised images of the roundels is a benchmark in the documentation of these significant sculptures. It has captured a highly accurate “snap-shot” in time that, in conjunction with periodic repeat scans and metric analysis of high-resolution patches, enables for the first time a quantifiable judgment of rate of loss.

ACKNOWLEDGEMENTS The authors wish to express their thanks to colleagues who have contributed to this project: The Scan Team Ltd; Mike Halliwell (Textile Conservation Centre, University of Southampton); Kent Rawlinson and William Page (Historic Royal Palaces); Lucia Burgio and Charlotte Hubbard (Victoria & Albert Museum); Chris Doherty (Oxford University Research Laboratory for Archaeology and History of Art); Oxford Authentication Ltd; Patricia Jackson; Prof. John Ashurst and Catherine Woolfitt (Ingram Consultancy); Daffyd Griffiths and Beth Werrett (University College London); and Richard Roberts. This research is generously supported by the World Monuments Fund. REFERENCES Autodesk Inc. Autodesk Maya. Retrieved August 30, 2007 from http://images.autodesk.com/adsk/files/maya08_ broch_overview_v17_1_.pdf Autodesk Inc. Autodesk 3DS Max. Retrieved August 30, 2007 from http://images.autodesk.com/adsk/files/3ds_max_ 2008_overview_brochure.pdf Beraldin et al. 2002. Virtualizing a Byzantine Crypt by Combining High-resolution Textures with Laser Scanner 3D Data. In 8th International Conference on Virtual Systems

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and Multimedia (VSMM2002), Gyeongju, Korea, 25–27 September 2002: 3–14. Blaise, F. 2004. Review of 20 Years of Range Sensor Development. Journal of Electronic Imaging, 13: 231–240. Fowler, V. J. 1985. Laser Scanning Techniques, Laser Scanning and Recording. SPIE Proceedings 378: 38. Geary, A. 2007. Is Real-Time Photorealistic 3d Viewing on the Horizon? 3DVisA Bulletin 2, March 2007. http://www.viznet.ac.uk/3dvisa/ Geary, A. & Swann A. 2007. SCIMod Prototype Development. SCIRIA Internal Report, SCIRIA, University of the Arts London. Geomagic Inc., Geomagic Studio. Retrieved August 30, 2007 from http://www.geomagic.com/en/products/studio/

Inus Technology Inc. RapidForm2006. Retrieved August 30, 2007 from http://www.3dsolutions.fr/documentation/ 3dsolutions-inus-technology-rapidform2006.pdf Maxon Computer GmbH. Cinema 4D. Retrieved August 30, 2007 from http://www.maxon.net/pages/products/ cinema4d/cinema4d_e.html Thurley, S. 2003 Hampton Court: A Social and Architectural History. New Haven: Yale University Press. Zayer, R., Lévy, B. & Seidel, H., 2007. Linear angle based parameterization. In Proceedings of the Fifth Eurographics Symposium on Geometry Processing (Barcelona, Spain, July 04–06, 2007). ACM International Conference Proceeding Series, 257: 135–141. Aire-la-Ville, Switzerland: Eurographics Association.

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An SLDV/GPR/IR-T integrated approach for structural and frescoes investigation in the medieval monasteries of Moldavia E. Esposito, A. Agnani & M. Feligiotti Dip. Meccanica, Università Politecnica delle Marche, Ancona, Italy

A. del Conte Artemis srl, c/o DACS, Ancona, Italy

S. Goncalves Tavares Dep. de Engenharia Mechanica, Univ. Federal de Minas Gerais, Belo Horizonte, MG, Brazil

ABSTRACT: In the second half of July 2006, personnel from the Mechanical Department of the Polytechnic University of Marche and from Artemis srl, a spin off company from the same university, participated in the workshop organized by the CULTURE 2000 project “Saving Sacred Relics Of European Medieval Cultural Heritage”, financed by the European Commission under contract CLT 2005/A1/CHLAB/RO-488. They were in charge of carrying out damage detection in frescoes and structural investigations in the monasteries of Balinesti, Popauti and Sucevita, northeast of Romania, applying three innovative technologies: Scanning Laser Doppler Vibrometry (SLDV), Ground Penetrating Radar (GPR) and InfraRed Thermography (IR-T). Although the limited time period did not allow to fully explore the artworks under examination, a lot of interesting characteristics of the churches and their decorative parts have been put in evidence and in this work, several significant examples of the measurements results will be presented.

1

INTRODUCTION

During the last years, the growing importance of the correct determination of the state of conservation of artworks has been stated by all personalities in care of cultural heritage. There are many analytical methodologies and techniques to characterize the physical and chemical properties of artworks, but at present their structural diagnosis generally rely on the expertise of the restorer/technician and the typical diagnostic process is accomplished mainly through manual and visual inspection of the structure. For this reason, many innovative optical techniques have been tried and applied to this issue, also comparing their features to define the aptest for different diagnosis situations. In this respect, Digital Shearography, Holographic Interferometry and Scanning Laser Doppler Vibrometry (SLDV) (Tornari et al. 2001, 2005) show a good degree of complementarity, and are capable of investigations ranging from micro-scale objects to macro–scale ones, using different physical quantities such as surface deformations, deformation gradients or surface vibrations. However, these techniques are not always of practical use, for instance, in the case of investigations of very large areas (Crippa et al. 2005),

very distant areas or when defects and discontinuities are located very deep inside the structure of the monument (Agnani et al. 2006). In these cases, other instruments must be employed and in this paper we will show our work in the monasteries of Balinesti, Popauti and Sucevita, all placed in the northeast of Romania, around the town of Suceava, using not only SLDV but also Ground Penetrating Radar (GPR) and InfraRed Thermography (IR-T). In the three monasteries, the adhesion level of frescoes has been verified by the employment of SLDV: the resulting vibration maps of frescoes have clearly distinguished between the detached and the consolidated areas. Using the same SLDV setup, a structural investigation has been performed in the Balinesti monastery, measuring the pressure waves propagation velocity in the church walls. To validate the SLDV results, GPR has been employed on the same church walls. Moreover, GPR has been used with good results both for structural investigation in Balinesti church and Popauti monastery and to investigate if visible tomb plates in Sucevita monastery were coinciding with actual graves. Measurements sessions using IR-T have been conducted to verify the frescoes status and to evaluate materials and structures. This

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technique is able to perform the scanning of big areas in a shorter time than SLDV, but the disadvantage of IR-T is that the investigated surface has to be heated. The three techniques, where possible, have been utilised on the same areas, so that their complementary features have led to a better knowledge of the characteristics of the artifacts. In the following, we will give some details on GPR and SLDV, although we will not supply such information on IR-T, because this represents a very well known and consolidated investigation technique, with many publications to be found in the literature (Grinzato et al. 2002, Rosina et al. 2004, Tavares et al. 2006). 2 TECHNOLOGIES 2.1

possible without external intervention on the structure and may be performed quickly and with a high degree of accuracy. Another different, minimally invasive measurement procedure, that employs the vibrometer as a sensor, has been implemented for the measurement of the propagation velocities of mechanical waves induced by an impact hammer. This procedure allows approximately verifying the integrity of a structure and is also capable of evaluating the mechanical characteristics of materials. SLDV is quite sensible to out-of-plane components of surface vibrations so that the surface waves, namely Rayleigh waves, are the only ones to be effectively detected when excited by an impact and their characteristics may be usefully employed for diagnostic purposes (Agnani et al. 2006).

Scanning Laser Doppler Vibrometry – SLDV

For many years, the Department of Mechanics has been developing a non-invasive and non-contact diagnostic technique based on laser Doppler vibrometers (Castellini et al. 1998) to assess the conservation state of many types of artworks, with special attention to frescoes (Castellini et al. 1996, 2000, 2003). The basic idea is to substitute human senses and contact sensors with measurement systems capable of remote acquisition and, if necessary, of remote structural excitation: SLDV scans the fresco surface and identifies detached areas by their higher vibration velocity when they are excited appropriately, usually by acoustic sources. Laser vibrometers also identify structural resonance frequencies of such areas, leading to a complete characterization of these defects. The proposed technique is thus completely non-contacting, and even if a little bit disturbing for the operator, the acoustic excitation is completely harmless to the examined artwork, as it has been clearly demonstrated during past projects (Tomasini et al. 2002): the pressure on the examined surface is in the range of 1/10000 of the usually imposed by the knock of a restorer (Castellini et al. 1999, see also Castellini et al. 2000 for a description of the measurement procedure). The application to historical buildings is more recent and still limited but seems promising. Of course, there are still a lot of difficulties, mainly related to the non-optically collaborative surfaces of tested structures and the necessity of working at great distances to get representative data of the examined object. We must not forget the need to isolate the instrument from ground vibrations and the realisation of special excitation techniques but it has been already demonstrated the capability of LDV to unintrusively acquire vibrational data on non-treated surfaces up to 10–15 meters, a real asset when dealing with large structures (Agnani et al. 2006). Regular monitoring of important parameters related to the state of conservation of these huge objects, like resonant frequencies, is thus

2.2 Ground Penetrating Radar – GPR Ground Penetrating Radar is a well and widely accepted diagnostic technique for the study of historical monuments (Daniels 1997, Conyers & Goodman 1997, Binda et al. 2000). GPR is a non-destructive geophysical method that produces a continuous cross-sectional profile or record of subsurface features, without drilling, probing or digging. GPR profiles are used for evaluating the location and depth of buried objects and to investigate the presence and continuity of natural subsurface conditions and features. It operates by transmitting pulses of high frequency radio waves (microwave electromagnetic energy) down into the ground by an antenna. When the transmitted signal enters the ground, it reaches objects or subsurface layers with different electrical conductivities and dielectric constants. Part of the waves reflects off the object or interface, while the rest of the waves pass through to the next interface. The antenna is then set to receive the reflected signal waves and send them for storage, analysis and display to a digital control unit. The control unit registers the reflections against two-way travel time in nanoseconds and then amplifies the signals. The output signal voltage peaks are plotted on the ground penetrating radar profile as different colour or black and white bands by the digital control unit. GPR waves can reach depths up to 30 meters through low conductivity materials such as dry sand or granite. Clays, shale and other high conductivity materials, may attenuate or absorb GPR signals, decreasing the depth of penetration to 1 metre or less. The depth of penetration is also determined by the GPR antenna used: antennas with low frequencies (25–200 MHz) obtain deeper subsurface reflections (from 10 to 30 m or more), but have low resolution. These low frequency antennas are used for investigating the geology of a site, for locating sinkholes or fractures and to

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Figure 1. (a) Balinesti Church, (b) Popauti Monastery, (c) Sucevita Monastery. Table 1.

Figure 2. Photograph and vibration map of fresco at the church entrance (north wall).

Measured velocity values.

Velocity (m/s)

1

2

3

1129

1026

1327

locate large, deep buried objects. Antennas with higher frequencies (300–2000 MHz) obtain reflections from shallow locations (∼ 0 to about 10 m), and have higher resolution. These high frequency antennas are used to investigate surface soils and to locate small or large, shallow buried objects and rebar in concrete. 3

Figure 3. Photograph and vibration map of fresco at the church entrance (west wall).

MEASUREMENT DATA

In the following paragraphs some relevant examples of measured data obtained in the three sites of Balinesti, Popauti and Sucevita are presented (Fig. 1). In all sites we used a customised OmetronVPI4000 SLDV, in conjunction with a FBT MaxX400 loudspeaker system and a PCB impact hammer, a GSSI SIR3000 GPR and an AVIO TVS600 thermal camera. 3.1

Balinesti Church

In the Balinesti Church, different tests have been conducted: study of the atrium frescoes by SLDV, masonry characterisation by vibrometry and GPR, and evaluation of outdoor repairs and structures by IR-T. Figures 2, 3 show the photographs and the vibration maps obtained by acoustic excitation of the frescoes at the church entrance. In Figure 2, a small detached area, that presents the highest vibration (detachment) level in this room (0.3 mm/s), is evident in the upper area, while in the bottom left corner a problem with the flexible wood floor, leading to poor signal acquisition, can be noticed (Strean et al. 1996). In Figure 3, wide detached areas are clearly distinguishable in the upper part of the fresco, while a well-consolidated area is observable in the middle of the scan area. Again, problems with low signal are visible in the bottom-left corner. As previously mentioned, a SLDV test setup has been implemented for structural testing, using an instrumented impact hammer acting as a source of

Figure 4. Measurement of sonic velocity.

Rayleigh waves. Figure 4 shows an example of such measurements on the south walls, in which the SLDV and the impact hammer load cell recordings are reported. Table 1 displays the measured velocity values at different distances from the impact point. According to our experience, values above about 1000 m/s, as in this case, are indicating a wall in good conservation state (Agnani et al. 2006). Figure 5 illustrates two IR-T measurements done on the external wall of the apse with a time separation of 30 s, and past interventions done with materials different from the original ones are visible due to their lower temperature (areas in dotted circles). Figure 6 refers to the behaviour of the masonry of the tower at the entrance of the church during a heating/cooling cycle: partially exposed to the sun (Fig. 6a) and in the shadow (Fig. 6b). It can be easily noticed the presence of moisture, observable in the top-right

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Figure 5. IR-T measurements at 0 s (a) and at 30 s (b). Figure 8. Photograph and vibration map of fresco at the church entrance.

Figure 6. IR-T measurements: masonry exposed to the sun (a) and in the shadow (b). Figure 9. Vibration maps of frescoes on the south (a) and north (b) wall of the church entrance.

Figure 7. GPR inspection of the south apse. Figure 10. IR-T measurements: door filled with masonry.

corner of Figure 6a, and detachments, the delimited area in the same Figure 6a. The GPR examination, shown in Figure 7, done along the same south walls tested by SLDV, but conducted from the external side of the wall, confirms a structure with no deficiencies. Moreover, it has also identified important characteristics of the walls, such as the thickness of the structure, the presence of different masonries and the position of their joints. 3.2

Popauti Monastery

In the Popauti Monastery the same tests have been conducted: study of the atrium frescoes by SLDV, masonry characterisation by GPR and evaluation of outdoor repairs and structures by IR-T. In Figure 8 the photograph and the vibration map measured by SLDV of a small cleaned area of a fresco placed at the church entrance are shown. The fresco seems to be in general good state: in fact, the observable small detached areas present a vibration level lower than 0.1 mm/s.

Problems with low Doppler signal are visible in the delimited area to the right. In Figure 9, the recorded vibration maps of frescoes on the south (Fig. 9a) and north (Fig. 9b) walls show that these frescoes are almost completely detached from the wall. Figure 10 illustrates an IR-T measurement done on a closed door filled with masonry, where interventions done with different materials are visible: moving towards the centre, from low to high temperatures, the massive wall, the stone portal, different filling material and the masonry filling are detected. In Popauti, the restorers had a specific requirement for GPR, since before going on with their work, they had to know the exact profiles of the stones used at the base of the church. As required, it is possible to distinguish the thickness of each slab (the dashed line) and also to find out inclusions or structural discontinuities (black circles) in the GPR radargram shown in Figure 11.

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Figure 11. GPR inspection of stone plates.

Figure 14. IR-T measurements of external wall of Sucevita church: hidden window.

Figure 12. Photograph and vibration map of fresco on external wall of Sucevita church.

Figure 15. IR-T measurements of the external wall of Sucevita church: masonry texture under the fresco layer.

Figure 13. Vibration map of fresco of the Burial Chamber.

3.3

Sucevita Monastery

In the Sucevita Monastery, SLDV has been used to study the external frescoes, recently consolidated, and also those located in the Burial Chamber, while IR-T was used to evaluate materials and structures, and finally GPR to investigate the stone tombs inside the church. In Figure 12, the vibration map of an external fresco show diffused detachments even after the consolidation: this could be due to the restoration strategy based on spotted consolidation interventions, not on full fixing of the plaster/painting layers to the wall. In Figure 13 we present a vibration map of a fresco of the Burial Chamber, showing a good state of conservation, at least from an adhesion point of view. Problems with low Doppler signal are visible in the top-left corner.

Figure 14 and 15 illustrate the thermographic study of the external walls of the church showing both the likely presence of a closed window (question mark in Figure 14) and the masonry texture, clearly visible in the IR spectrum even under the fresco layers (Figure 15). Finally, a GPR investigation has been performed inside the Monastery church: a grid has been prepared (Fig. 16) to verify if visible tomb plates coincided with actual graves. Figure 17 shows the proof that the investigated tomb stone was the only one with a probable grave under it. The other tomb plates show no evidence of graves, but they were randomly positioned after some past destruction of the church floor. 4

CONCLUSIONS

The aim of this work was to present some results from the investigations done in the churches and monasteries of Balinesti, Popauti and Sucevita, investigations agreed with local restorers to determine the level of consolidation of the frescoes, to assess the state of the masonries and to solve other conservation-related problems, such as the tracing of walls profiles and to find out inclusions of materials different from the original ones. Three innovative technologies have been applied for these issues: Scanning Laser Doppler Vibrometry (SLDV), Ground Penetrating Radar (GPR) and

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– Masonries, as explored by GPR, SDLV and IR-T, seem in a good structural condition, so it should not be urgent to operate for their consolidation. Thus, the three techniques showed a good level of mutual integration, and being all based on images as the main output, they can easily be employed in monitoring plans based on digital data storage. All of them require very skilled operators and are still relatively costly, but their sensibility to early damage, robustness and transportability would make them ideal for brief measurement campaigns done at regular intervals.

Figure 16. GPR inspection: grid tracing.

ACKNOWLEDGEMENTS We thank Roxana Radvan for giving us the possibility of participating in the workshop “Saving Sacred Relics Of European Medieval Cultural Heritage” organized by the CULTURE 2000 project, contract CLT 2005/A1/CHLAB/RO-488, and financed by the European Commission. Figure 17. GPR inspection of tomb plate inside the Sucevita church.

InfraRed Thermography (IR-T). With our work we integrated these techniques as much as possible, to make use of their complementary features, to overcome the deficiencies of each of them when used individually. For example, SLDV has been used to detect delaminations of the frescoes both inside and outside the monasteries, a task that is impossible with GPR and that posed many difficulties to IR-T, due to the lack of adequate systems for artificial heating; it was also used to verify the structural integrity of the masonry of Balinesti church. This is a relatively recent application for this instrument, and, in fact, to validate this diagnostic investigation, a measurement by GPR, a widely accepted investigation technique for the study of historical monuments, has been performed. GPR has been generally used for masonry characterisation in all the churches and to verify the presence of stone tombs in Sucevita monastery. Unfortunately, we did not find any new grave under the investigated tombs. However, even if GPR is a really good instrument for such works, it was IR-T that detected interesting structural features, see Figures 14 and 15, in locations that were really hard to reach by the radar. Finally, even if it is not possible to present here the full report, as delivered to the responsible of the project, some general conclusions may be stated: – As regards the frescoes, the situation ranges from the generally good conservation state of those in Sucevita, to the very bad one of those in Popauti, while in Balinesti we find out detached areas but not so large as in the previous case.

REFERENCES Agnani, A., Del Conte, A., Esposito, E. & Naticchia, B. 2006. Non-destructive measurement systems for the characterization of ancient masonry: an SLDV/GPR integrated approach. Proceedings of 7th International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, Ancona, Italy, June 20–22nd, 2006. SPIE 6345: 6345–31 published on CD. Binda, L., Forde, M., Saisi, A., Valle, S. & Zanzi, L. 2000. Application of radar tests in the survey of the load bearing walls of the Torrazzo of Cremona. Proceedings of 5th International Congress on Restoration ofArchitectural Heritage, September 17–24, Florence. Castellini, P., Paone, N. & Tomasini, E. P. 1996. The Laser Doppler Vibrometer as an instrument for non-intrusive diagnostic of works of art: application to fresco painting. Optics & Lasers in Engineering 25: 227–246. Castellini, P., Revel, G. M. & Tomasini, E. P. 1998. Laser Doppler Vibrometry: a Review of Advances and Applications. The Shock And Vibration Digest 30: 443–456. Castellini, P., Esposito, E., Paone, N. & Tomasini, E. P. 1999. Non-invasive measurements of damage of frescoes paintings and icon by Laser ScanningVibrometer: experimental results on artificial samples using different types of structural exciters. 6th World Conference on NDT and Microanalysis in Diagnostics and Conservation of Cultural and Environmental Heritage, Rome, May 17–20. Download at: http://www.ndt.net/article/v04n12/tomasini/tomasini. htm Castellini, P., Esposito, E., Paone, N. & Tomasini, E. P. 2000. Non-invasive measurements of damage of frescoes paintings and icon by Laser Scanning Vibrometer: experimental results on artificial samples and real works of art. Measurement 28: 33–45. Castellini, P., Esposito, E., Marchetti, B. & Tomasini, E. P. 2003. New applications of Scanning Laser Doppler Vibrometry (SLDV) to non-destructive diagnostics of

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artworks: mosaics, ceramics, inlaid wood and easel painting. Journal of Cultural Heritage 4/S1: 321–329. Conyers, L. B. & Goodman, D. 1997. Ground-Penetrating Radar. An Introduction for Archaeologists. Alta Mira Press. Crippa, M. A., del Conte, A., Esposito, E. & Perrotta, P. 2005. Applicazione di sistemi ottici per la Diagnostica dello stato di adesione di Rivestimenti superficiali: il caso dell’edificio “Trifoglio” del Politecnico di Milano, IDN 58, Proceedings on CD of Conferenza Nazionale sulle Prove non Distruttive Monitoraggio Diagnostica – 11◦ Congresso Nazionale dell’AIPnD, Milan (IT), October 13–15. Daniels, D. J. 1997. Surface Penetrating Radar. London: IEE. Grinzato, E., Bressan, C., Marinetti, S., Bison, P. G. & Bonacina, C. 2002. Monitoring of the Scrovegni Chapel by IR Thermography: Giotto at Infrared. Journal of Infrared Physic and Technology 43: 165–169. Rosina, E., Avdelidis, N. P., Moropoulou, A., Della Torre, S., Pracchi, V. & Suardi, G. 2004. IRT monitoring in planned preservation of built Cultural Heritage. Proceedings of 16th WCNDT – World Conference on NDT, Montreal. Published on CD, download at www.ndt.net. Strean, R. F., Mitchell, L. D. & Barker, A. J. 1996. Global noise characteristics of a laser Doppler vibrometer part I: theory. Proc. of Second International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, Ancona, Italy, September 23.

Tavares, S. G., Agnani, A., Esposito, E., Feligiotti, M. & Andrade, R. M. 2006. Comparative study between infrared thermography and laser Doppler vibrometry applied to frescoes diagnostic. Proceedings on CD of the 8th International Conference on Quantitative InfraRed Thermography – QIRT 2006, Padova (IT), June 28–30. Tomasini, E. P., Castellini, P. & Esposito, E. 2002. Laserart: a european project for the protection of the cultural heritage. Proc. of the Int.al Conference “Archaeometry in Europe in the Third Millenium” Accademia Nazionale dei Lincei, Roma, 29–30 March. Contributi del Centro Linceo Interdisciplinare “Beniamino Segre” 105: 233–239. Roma: Accademia Nazionale dei Lincei. Tornari, V., Bonarou, A., Castellini, P., Esposito, E., Osten, W., Kalms, M., Smyrnakis, N. & Stasinopulos, S. 2001. Laser based systems for the structural diagnostic of artworks: an application to XVII century Byzantine icons. Proc. of Laser Techniques and Systems in Art Conservation, Munich 18–19 June. SPIE 4402: 172–183. Tornari, V., Tsiranidou, E., Orphanos, Y., Falldorf, C., Klattenhof, R., Esposito, E., Agnani, A., Dabu, R., Stratan, A., Anastassopoulos, A., Schipper, D., Hasperhoven, J., Stefanaggi, M., Bonnici, H. & Ursu, D. 2005. Multi-Tasking Non-Destructive Laser Technology in Conservation Diagnostic Procedures. Proc. of Lasers in the Conservation of Artworks: LACONA VI, Vienna, September 21–25, Springer Proceedings in Physics: 601–610. Heidelberg: Springer Verlag.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Development of an impact assessment procedure for artwork using shearography as a measurement tool R.M. Groves & W. Osten ITO Institut für Technische Optik, Universität Stuttgart, Germany

S. Hackney Conservation Department, Tate, London, UK

E. Kouloumpi Conservation Department, National Gallery – Alexandros Soutzos Museum, Athens, Greece

V. Tornari IESL-FORTH, Heraklion, Crete, Greece

ABSTRACT: Shearography is a full-field optical technique used for non-destructive testing and for the measurement of surface strains. This work presents the development of a novel Impact Assessment Procedure (IAP) using shearography to collect the data about the condition of the artwork. This is a multi-step process with the first step being the recording of reference phase maps and the determination of signature features (anomalies in the phase map profile). After the artwork has undergone an event which could possibly change its condition (e.g. transportation, malicious damage, conservation, ageing) it is re-measured and the data is compared with the reference. The information obtained can be used by conservators to guide future conservation strategies on the artwork. This manuscript describes the development of a custom shearography prototype sensor for this purpose and the development of an IAP using canvas and wooden panel painting samples.

1

INTRODUCTION

Optical techniques are important tools for the study of artworks. They are usually non-contact and the technology is ever evolving, driven by developments for engineering and manufacturing applications. Art objects, in addition to their aesthetic qualities, are often characterized by their non-homogeneous construction, their varying scattering and reflective properties and their location outside of an optics laboratory. These physical properties present a particular challenge to optics researchers, as they demand a more robust performance from the sensor and the manipulation of more complex data sets. The applications of laser techniques to cultural heritage were reviewed in a recent book (Fotakis et al. 2006). The applications cover many fields, e.g. laser cleaning, laser processing, spectroscopy and interferometry. Optical holography and speckle interferometry techniques can be used to make fullfield characterisation of artworks (Tornari et al. 2001, Tornari et al. 2005). In this paper, the focus is on the technique of shearography (Steinchen & Yang 2001),

as it provides a direct measurement of the displacement gradient, a parameter closely related to strain. Shearography is a full-field optical interferometry technique, often used for non-destructive testing and strain measurement (Kalms & Osten 2003, Groves et al. 2006). It incorporates a shearing interferometer to give self referencing from a laterally, or vertically, shifted region of the object surface. Because of this, the sensor is sensitive to displacement gradient, a parameter closely related to strain. Additionally, this self referencing makes the sensor operation more robust outdoors as it is insensitive to small rigid body motions. In the Multi-Encode Project, an interdisciplinary team of researchers in Applied Optics, Laser Applications in Art Conservation and Art Conservation has been assembled. The aim of the Multi-Encode Project is to develop a novel Impact Assessment Procedure (IAP) for artworks, using optical holography sensors to record information about the state of the artwork (Tornari et al. 2007). The first step was to optimise the sensor design for this application and this involved the development of a highly integrated

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prototype shearography sensor and associated standard measurement procedures. To perform the IAP, the artwork undergoes measurements at various times during its lifetime, with a special emphasis on performing measurements after an event that could cause an impact. In this work, sample representative artworks of wooden panel paintings and canvas paintings were constructed by the National Gallery, Athens and Tate, respectively. Accelerated ageing and simulated accidental and malicious damage were used to generate changes that could be detected by the sensor. The final step is the comparison of data with the reference data, recorded before the event occurred and to identify the criticality of the changes. This would guide both immediate remedial action and future conservation strategies for the artwork. An additional feature of the IAP is that forged artwork can be detected by comparison with reference data containing detailed information about the physical properties of the original. This manuscript contains a description of the design and construction of the shearography sensor and the artwork samples. Results from the measurement programme are presented and the current progress towards the full implementation of the IAP is discussed.

1996). Icons are a strong Hellenistic tradition and are found widely in South-Eastern Europe and Russia. A typical construction is a number of layers of material on a wooden base. The wood is usually softwood, with pine or cedar being commonly used. The first layers form an interface between the wood and the paint and stabilize the painting. These include stretching and gluing a canvas onto the wood and using priming layers such as gesso (a mixture of calcium carbonate and rabbit skin glue). The picture is formed by egg tempera paint, a mixture of egg yolk and pigments, and gold leaf. Egg also has a symbolic meaning, as it represents life and especially the creation of life. Finally, a varnish layer is applied to protect the painting. 2.2 Canvas paintings

The term ‘movable cultural heritage’ covers a wide range of objects with cultural value that may be loaded between museums and galleries. This includes different types of paintings, e.g. wooden panel paintings and canvas paintings, sculptures, installation art, books and manuscripts. These items can be of high value, both monetary and culturally, and could be subject to loss or damage during the loan procedure. For this reason, high levels of security and control are required during the loan and the project addresses this point. Wooden panel paintings and canvas paintings are given as examples in the measurement procedure.

The refinement of the technique of painting in oils on stretched canvas is credited to the Venetians – the technique itself is much older. Canvas painting allowed the size of the artwork to increase, while still remaining portable. Strong linen, similar to sailcloth, is stretched over a wooden frame and is tensioned using turnbuckles or wedges. The first layer is a sizing layer of animal glue, which acts as a support for the oil paint and as a barrier between the paint and the canvas. Priming layers, which can be chalk or gypsum mixed with either animal glue or linseed oil, are applied next and provide an optically neutral surface, typically white, on which to work. Oil paint, which is ground pigment suspended in linseed oil, is painted in layers on the canvas. The artist may modify this paint using turpentine or petroleum spirit. The drying process is slow. The first stage is by evaporation and the second more important stage is the chemical cross-linking of unsaturated oils in a polymerisation reaction. This creates a tough permanent film. Optionally, the paint is covered by a varnish layer. Over time the film becomes more brittle, as further chemical changes take place, and a craquelure pattern is formed.

2.1 Wooden panel paintings

3

2

MOVABLE CULTURAL HERITAGE

Funerary portraits from the Fayum region in Egypt in the early centuries AD depicted the dead person in encaustic on a wooden base. These subsequently influenced the wooden panel paintings, known as icons from the Greek εικoνα – image, which developed during the Byzantine Empire (Talbot Rice 1963). Icons depict religious themes and as such formed a link between a largely illiterate population and the underworld. Their mystical character was enhanced by their standardization and by their simple features, such as big eyes, long noses, frontal positions and lack of perspective, and by their materials, egg tempera paint with a semitransparent matt finish and gold leaf (Dizikirikis

SHEAROGRAPHY – SENSOR DESIGN

3.1 Theory Shearography, also known as speckle shearing interferometry, is a full-field speckle interferometry technique sensitive to displacement gradient. To perform a measurement, the object is illuminated by the expanded beam from a laser. The light scattered by the optically rough surface of the object forms a speckle pattern which is imaged through a shearing interferometer. This forms an interferogram, which can be recorded by a camera. Shearography is a double-exposure technique. Interferograms are recorded either before and after loading, or at two points in the loading cycle, and

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correlated by a computer. In this simplest form, this is by subtraction, but more commonly phase-shifting is used to recover the phase without ambiguity in the phase direction (Kreath 1993). The component of displacement gradient measured is determined by the shear direction and by the object illumination and viewing directions. The most common configuration is sensitive to one of the out-of-plane displacement gradients, as given by Equation 1:

where φ is the phase change in the interferometer, λ is the optical wavelength, δw/δx is the out-of-plane displacement differentiated in the horizontal direction, δw/δy is the out-of-plane displacement differentiated in the vertical direction, and dx and dy are the horizontal and vertical shear directions, respectively. 3.2

Development of the prototype

ITO has developed a portable custom shearography prototype for this application, based on the standard design of a shearography sensor. The sensor contains four modules: laser, sensor head, electronics controller and PC (desktop or laptop). The complete sensor system is shown in Figure 1a and a close-up of the sensor head is shown in Figure 1b. The beam from a frequency doubled Nd:YAG laser (532 nm, Coherent Verdi-V10, 10 W or Adlas 300, 100 mW depending on the optical power required) is expanded using a lens and used to illuminate the object. As the reference in shearography is from the object surface, the only connection required for the laser is for electrical power. The sensor head is mounted on a tripod and contains the interferometer (beamsplitter, shearing mirror and phase shifting mirror), camera lens and camera. The phase-shifting mirror is controlled by a Linos PSP-1 phase-shifter, powered by an in-house built amplifier in the electronics controller. The camera lenses are interchangeable and focal lengths between 16 and 50 mm have been used.The camera is also interchangeable and the configuration shown in the figure is with a Basler A102f (1392 × 1040 pixels, Firewire interface). The electronics controller is the interface between the sensor and the PC. It controls and powers the phaseshifter, infrared lamps for loading the object and some other peripherals, and has a USB connection to the PC. Also it acts as a power supply for the Firewire camera when it is controlled from the 4-pin (data only) Firewire laptop connection. A modular instrumentation control and image processing software was developed using Labview. This software allows flexibility in setting instrument parameters (camera exposure time, field of view, etc.),

Figure 1. (a) is a photograph of the ITO shearography instrument during laboratory testing at ITO. It shows the four modules: laser, sensor head, electronics controller and PC (desktop or laptop); (b) is a close-up of the tripod mounted sensor head, showing the mounted beamsplitter, shearing mirror, phase-shifting mirror, camera lens and camera.

switches the infrared lamps on/off to give a controlled heating time, performs the real time image processing and display of phase-maps and controls the automatic archiving of data. One key aspect in achieving realtime operation has been coding optimization for image processing tasks, such as phase unwrapping. 4

IMPACT ASSESSMENT PROCEDURE

The first stage of the Impact Assessment Procedure was to develop a standardised measurement procedure so that data recorded at two times during an object’s lifetime could be compared. This was performed in cooperation with the project partner CSL, Liège. Standard parameters for the infrared lamp positions, lamp powers, heating times, shear settings, sensor distance, etc. were established. Once determined these parameters were used for a measurement programme to assess the performance capability of the sensor for typical size defects. Reference wooden panel painting and canvas painting samples with characteristic defects were prepared by the project partners at the National Gallery, Athens (NGA) and Tate, respectively.

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The next stage is to identify events that could cause an impact on an art object and to consider how they may be simulated in the laboratory. For the wooden panel painting an accelerated aging approach was taken. The wooden panel painting samples constructed were approximately 200 × 150 × 10 mm in size to allow aging in a small environmental chamber. Full details are given by Kouloumpi et al. (2007). The approach for canvas paintings was to consider physical damage to the painting. In practice transit damage during a loan and mishandling present the most danger to the artwork. International cooperation between the major art galleries and museums is increasing and valuable artworks are increasingly being lent. Additionally malicious damage can occur and some simulated examples of this type of damage were studied during the measurement programme. A further danger is theft and in this case a returned sample may be damaged or be a forgery. In this case if a forgery is returned, it should be compared with reference data as soon as possible. Currently automatic procedures for the comparison of artworks at different stages in its lifetime are being developed to replace the manual procedures currently in place. Defects and features identified in the artwork are classified as stable or unstable. The identification of an unstable defect highlights a region in the artwork that requires attention from a conservation point of view and it can also be used to calibrate and determine the comparative size of new defects compared with those already present. Stable defects are continuously present in the artwork and are used for identification. 5

paint thickness and types. Delaminations and cracks (various sizes >1 mm) were usually (>75%) detected.

MEASUREMENT PROGRAMME

This section gives information about the detection capability, examples of the identification of stable and unstable defects in sample paintings and some preliminary measurements from a landscape painting. 5.1

Figure 2. (a) A photograph of the panel painting sample after ageing; (b) a diagram showing the location of the introduced defects; (c) shearography phase map before aging and (d) phase map after aging. The defect location is marked with a circle.

Sizes and types of defects detected

For wooden panel paintings typical defects introduced included nail holes, cracks, delamination of the paint layer from the substrate, etc. Measuring with the sensor these defects appear overlaid onto the underlying characteristics of the piece of wood, such as knots and wood grain. More details of the wooden panel painting sample preparation are given in Kouloumpi et al. (2007). All delaminations and cracks (> 0.5 mm length) in the samples prepared were detected by the sensor and the majority of wood knots (various sizes 2–20 mm) were located. For nail holes the detection was not reliable. Similarly canvas painting samples were prepared to contain typical defects. These included cracks of various sizes, minor impact or handling damage, in addition to characteristic features such as different

5.2 Stable defects The example given is from a wooden panel painting sample with a delamination of known size that was aged at 60◦ C for 66 hours. The object is 200 × 147 × 10 mm3 and was located approximately 1 m from the sensor. The object was loaded after recording of the reference interferogram, using two 150 W infrared lamps, which simultaneously heated the sample from the front and back. A photograph and diagram of the sample are shown in Figure 2 in addition to phase maps recorded 104 s after loading. The overall fringe density is approximately the same in both phase maps, and the defect location is clearly identified. 5.3 Unstable defects The second example is of an unstable defect introduced by hitting the canvas with a hammer. This simulates both malicious damage and impact damage over a small area that could occur during poor handling. The canvas is 450 × 450 mm and was constructed in the conservation laboratories at Tate. First, a reference

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Figure 4. (a) A photograph of a british landscape painting and (b) the experimental setup showing a wrapped phase map on the monitor display. Figure 3. Unwrapped phase maps obtained (a) before and (b) after impact damage with a hammer on the top right hand side, marked with an arrow.

phase map was recorded, with the sensor 2 m from the painting. The painting was then heated with two 150 W infrared lamps from the front and back, for 1 s. The phase map obtained is shown in Figure 3a. Then the painting was struck with a hammer approximately 100 mm from both the top and the right hand side of the canvas. Repeating the measurement procedure, the phase map in Figure 3b was recorded. The phase maps obtained show both the stable fringes that are present due to existing cracks in the canvas and the new strain loading due to the impact. Note that the relative strain levels present in the different defects can be extracted by comparing the magnitude of the phase changes. 5.4

Recording of reference data from a real painting

This example is the recording of reference data from a landscape painting. This painting is in the British

landscape style of the 19th century and depicts an idyllic rural scene with fields in the foreground and farm buildings in the middle foreground. Its size is 290 × 250 mm2 with a narrow wooden frame. It has a somewhat unknown history, but was believed to have a second canvas glued to the back of the original at some time in the past, although this is now quite old. A photograph of the painting and the experimental setup showing the fringe pattern obtained are shown in Figure 4. The canvas was thermally loaded using one front and one back infrared lamps. Experimental tests show that the temperature rise is approximately 1–2◦ C. An additional factor to be considered in assessing real artworks is the laser exposure time. There is concern that chemical reactions, leading to deterioration, are catalysed by light and that the free radical generation in the artwork is cumulative, depending on the total amount of illumination that the artwork receives during its lifetime. An exposure time of 650 klx hr over

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the lifetime of the artwork is given in the literature (Thompson 1994). From the sensor point of view, approximately 300 to 500 mW of laser light (532 nm) are required to measure an artwork of this size and surface reflectivity. The actual exposure can be estimated from the optical power, the artwork size (expansion of the laser beam) and the measurement time. A major contribution to avoid unnecessary light exposure can be made by switching on the laser illumination only during the measurement period. The amount of light exposure that the artwork has received during the measurement is saved with the measurement data.

6

DISCUSSION

The sensor development programme has been successful and a shearography sensor adapted to performing measurements on artworks has been designed and constructed. The shearography sensor has proved to have a good sensitivity to defects in the artwork, with the sensor mounted on a tripod. The development of the instrumentation software and image processing algorithms required a balance between speed and robustness of the algorithms to noise. Under normal operating conditions the sensor displays the output phase maps at a rate of one every 2–3 seconds and is robust to phase-shifting errors and external influences, such as room light. The speed of sensor operation is also a factor in minimising the exposure of the artwork to unnecessary light from the laser. The Impact Assessment Procedure development was a major part of the work. The first step was to identify suitable standard conditions for detecting defects of various types and sizes in panel paintings. Then the IAP was developed to identify both stable and unstable defects in two types of artworks. It was found that defects could be classified as stable and unstable defects and could be assigned into these categories by the measurement procedure. Additionally, it showed that certain defect types, such as small scratches on the surface are not detectable by the shearography sensor. This is because the small scratches do not have a significant effect on the surrounding strain field. To summarise, there is a number of key advantages for incorporating shearography in a multi-functional sensor for cultural heritage applications. Shearography is able to perform measurements on gently vibrating canvas paintings without stabilisation of the painting. This allows a truly non-contact measurement if loading by means of infrared lamps is used.The sensor operates on a lightweight tripod. The sensor has an adjustable sensitivity to the measurand, displacement gradient, and the sensor gives direct information on the strain changes in the sample.

7

CONCLUSIONS

The programme to develop the shearography sensor for cultural heritage applications has been successful in the first part of the project. Current work aims to further optimise and automate the image processing. ACKNOWLEDGEMENTS This research is supported by the European Union funded Multi-Encode Project (006427 SSPI). The authors would like to thank Marc Georges and Cèdric Thizy at the Centre Spatial Liège and Kostas Hatzigiannakis, Eirini Bernikola and Yannis Orphanos at IESL for their contributions to the development of the measurement procedures. REFERENCES Creath, K. 1993. Temporal phase measurement methods. In D.W. Robinson & G.T. Reid (eds.) Interferogram analysis, digital fringe measurement methods. Bristol: Institute of Physics. Dizikirikis, G. 1996. Byzantium and the West, cultural and aesthetic matters of painting. Athens: Aigokeros. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2006. Lasers in the preservation of cultural heritage: principles and applications. Boca Raton: CRC Press. Groves, R.M., Furfari, D., Barnes, S.E., James, S.W., Fu, S., Irving, P.E. & Tatam, R.P. 2006. Full-field Laser Shearography Instrumentation for the Detection and Characterisation of Fatigue Cracks in Titanium 10-2-3, J. ASTM Int. 3. Kalms, M. & Osten, W. 2003. Mobile shearography system for the inspection of aircraft and automotive component, Opt. Eng. 42: 1188–1196. Kouloumpi, E., Moutsatsou, A., Trompeta, M., Olafsdottir, J., Tsaroucha, C., Doulgeridis, M., Groves, R.M., Georges, M., Hunstix, G. M. & Tornari, V. 2007. Laserbased structural diagnosis: a museum’s point of view, in this volume. Steinchen, W. & Yang, L. 2001. Digital Shearography. Bellingham: SPIE Press. Talbot Rice, D. 1963. Art of the Byzantine Era. London: Thames & Hudson. Thompson, G. 1994. Museum Environment. ButterworthHeinemann. Tornari, V., Bonarou, A, Esposito, E., Osten, W., Kalms, M., Smyrnakis, S. & Stassinopulos, S. 2001. Laser based systems for the structural diagnostic of artwork: an application to XVII Byzantine icons, Proc. SPIE 4402. Tornari, V., Tsiranidou, E., Orphanos, Y., Farsari, M., Kalpouzos, C., Fotakis, C. & Doulgeridis, M. 2005. On Interference Generated Defect Indicative patterns for the Validation of Application on Artworks Structural Diagnosis, Journal of Cultural Heritage. Tornari, V., Osten, W., Groves, R. M., Georges, M., Hustinx, G. M., Kouloumpi, E. & Hackney, S. 2007. Multifunctional encoding system for the assessment of movable cultural heritage, ICEM13, Alexandroupolis, Greece, 2007.

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Imaging and Documentation

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Ultra high-resolution 3D laser color imaging of paintings: The Mona Lisa by Leonardo da Vinci F. Blais, J. Taylor, L. Cournoyer, M. Picard, L. Borgeat, G. Godin, J.A. Beraldin & M. Rioux National Research Council Canada, Ottawa, Ontario, Canada

C. Lahanier Centre de Recherche et de Restauration des Musées de France, Paris, France

ABSTRACT: During the autumn of 2004, a team of 3D imaging scientists from the National Research Council of Canada (NRC) was invited to Paris to undertake the 3D scanning of Leonardo’s most famous painting. The objective of this project was to 3D scan the Mona Lisa – obverse and reverse – in order to provide high-resolution 3D image data of the complete painting to help in the study of the structure and technique used by Leonardo. This paper describes some challenges associated with scanning the Mona Lisa and presents results of the modeling and analysis of the 3D data including preliminary measurements of the thickness of the varnish layer.

1

INTRODUCTION

At the request of the Paintings Department of the Louvre Museum, the Centre de Recherche et de Restauration des Musées de France (C2RMF) undertook the study on the Mona Lisa. This study coincided with the move of the painting to the new Salle des États and is considered the most extensive scientific examination on a painting ever undertaken. A team of 39 specialists with backgrounds in art history, conservation, photography, physics, engineering, chemistry, optics and digital imaging from seven institutions took part in this study (Mohen 2006). As part of this project, the NRC developed a portable “high resolution” 3D camera system optimized for the scanning of paintings. In May 2004, testing of the portable color prototype system was done at the C2RMF by scanning a series of Renoir paintings (Blais 2005). The 3D scanning of the Mona Lisa was subsequently undertaken over two nights, October 18–19, 2004. The main objectives of this work were: (1) to document and precisely measure the distorted shape of the poplar panel on which the Mona Lisa is painted, (2) to examine surface features of the composition, the craquelure in the paint layer, the split in the panel, surface lacunae and, (3) to help the study of both the painting’s state of conservation and Leonardo’s technique, in particular his sfumato. 3D imaging is a completely new unconventional approach for the detailed examination of paintings and other objects, and as a conservation tool (Godin 2002, Taylor 2003). It creates a very accurate archival

quality virtualized 3D model of the object which allows detailed exploration and study without any risk of damage. One of the key objectives of this research also included the demonstration and comparison of this new emerging technology with existing well proven imaging methods. It compares advantageously to other more conventional methods such as color and infrared photography, infrared reflectography, and emissiography. In the case of the Mona Lisa, 3D imaging corroborated key historical findings. Using historical documents and information obtained from new scientific images, art historian Bruno Mottin demonstrated that the cloth she was wearing was an unambiguous physical proof that Mona Lisa had given birth before the commissioning of the painting (Mohen 2006) as suggested by several authors (Zöllner 2005, Kemp 2006). These observations were also corroborated using 3D laser imaging (Blais 2007). Another potential property unique to 3D imaging is the potential of measuring the thickness of the transparent varnish. This physical property was unexpected and currently is the subject of further investigations. 2

3D IMAGING

The primary advantage of using a high-resolution optical 3D color laser scanner for the recording of works of art such as the Mona Lisa is that it yields a very accurate archival quality “3D Digital Model”

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Figure 1. The high resolution color 3D scanner.

of the exact shape as well as the color reflectance of the object. 3D imaging seems a strange solution for the study of paintings which are assumed to be intrinsically bidimensional. Subtle heights variations due to brush strokes or paint thickness, cracks, wood grains, and warping make precise shape information invaluable. This record can be used to make very accurate measurements and to monitor changes over time; it can also be studied for art history and conservation applications. Another advantage of the 3D laser scanner technology is that, as an optical technique, it does not contact the surface of the object. A custom built 3D high-resolution portable color laser scanner capable of acquiring 3D images at a depth resolution of 10 µm (about 1/10 the diameter of a human hair) was brought to Paris to scan the complete painting – obverse and reverse. In operation, the system scans a small (less than 100 µm diameter) “white” laser spot from a RGB (red, green, blue) laser source over the complete surface of the painting in order to produce a high-resolution archival quality 3D digital model of both shape and color of the painting’s surface (Fig. 1). The laser is low power and safe for scanning works of art (equivalent exposure of 20 minutes at 50 lux). The triangulation based detection system simultaneously records the shape (X,Y,Z) measurements and the color (R,G,B) reflectance from the spot on the painting in perfect registration. Details of the triangulation principle and scanning method are available in (Taylor 2003, Blais 2005). In the maximum resolution configuration used for this project, the system provided a lateral spatial

Figure 2. The virtual 3D Mona Lisa – obverse.

(X and Y) resolution of 0.060 mm and a depth uncertainty of 10 µm (0.010 mm). This resolution, as well as the lower one used for the reverse, were imposed by time constraints: priority was given to acquiring the obverse at the higher resolution. The laser scanner head is mounted on a linear translation stage on a rail supported by two tripods. The translation stage moves the scanner across the painting to digitize a band of approximately 20 cm in length and 4 cm in width. A total of 72 sequential bands for the obverse and 68 for the reverse and sides were recorded over the entire painting, stitched and merged by software to form a complete 3D and color model. The obverse (front) side was scanned with the frame in place during the first night. The back and sides were scanned during the second night. A first band in the back was scanned then one traverse removed that resulted in an important change in the shape of the painting of 3 mm due to the pressure applied by the frame (Mohen 2006). Finally the frame was completely removed, and the reverse (back) and the four sides were measured: the scans were registered, stitched and blended to complete the 3D model. The distortion induced by the pressure exercised by the frame was numerically compensated. Each scanning session was subject to specific security conditions. With very few exceptions, the number of individuals allowed in the immediate proximity of

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Figure 4. The 3D Mona Lisa is virtually cut to highlight the warping of the poplar panel.

Figure 3. The virtual Mona Lisa – Reverse.

the painting at one time was limited to four in order to maintain the temperature and humidity conditions surrounding the painting. All manipulations of the Mona Lisa were carried out by restorers. The physical dimensions of the poplar panel are 79.4 cm × 53.4 cm, at a 3D sampling resolution of 60 µm (0.06 mm): this corresponds to an image of 12800 × 8800 pixels or the equivalent of a 113 million pixels camera. Figures 2 and 3 show views of the 3D model of the Mona Lisa which consists of 330 million 3D polygons, the basic geometrical primitive used by 3D graphic processor boards for rendering. 3 THE VIRTUAL 3D MONA LISA The shape data recorded by the scanner can be used to generate contour plots and color coded elevation maps as demonstrated in (Mohen 2006, Blais 2007) which provide convenient and familiar representations of the overall shape of the panel. Figure 4 clearly shows the pronounced curvature of the poplar panel. Here, the 3D model was virtually cut in half to reveal the detailed profile of the panel. A method frequently used to enhance the detailed shape of the surface of the panel is synthetic shading of the model. One or several light sources can be directed from any direction to examine the surface relief on the painting. Using a low angle of incidence on the surface simulates raking light, a standard method for the examination of paintings. Ambiguities caused by changes in surface color are completely removed using only shape information. Figure 5 illustrates such an artificially shaded monochrome image of the obverse

Figure 5. Some details of the pictorial layer.

side. The surface relief due to the wood grain structure is clearly visible. The 12 cm split from the top edge to the head, which has been stabilized during an earlier conservation treatment, is apparent. A faint outline of the head, the crack patterns and some other elements of the landscape are also visible. The presence of a barb, a crest of paint located between the poplar panel and the frame is highlighted in the 3D image (Fig. 6) demonstrating that Leonardo painted the panel after it had already been set into a frame. The back of the panel also shows clear evidences of saw marks documenting previous attempts at trimming the panel (Fig. 7). Similar observations can be made for the insect cavities in the back and sides of the panel (Fig. 6). A closer examination of the painting shows that apart from the surface relief due to the wood grain structure, previous restorations and craquelure features, particularly in the landscape areas, very few surface relief details relating to the painting composition itself are apparent. As such, in contrast to other paintings scanned previously and which typically records the surface relief details from brushstroke, there is very little pictorial composition 3D relief on this painting. The second aspect, which is also closely related, concerns the application of multiple thin semitransparent layers or glazes using the sfumato technique. The absence of brushstrokes and the very subtle heights variations detected is an example of Leonardo’s famous technique of applying successive

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4 THE COMPOSITION

Figure 6. Presence of a barb, shaved panel, and details of the craquelure pattern.

Figure 7. Saw marks on the back of the poplar panel.

extremely thin semi-transparent layers of glaze. The delicate shadows in the face around the eyes, nose, and mouth are the results of extremely flat layer composition called “sfumato” or “smoke like” appearance. More information is available in (Mohen 2006).

Three conventional 2D imaging techniques that were used by the C2RMF to examine and analyze details of the composition of the Mona Lisa are infrared photography, infrared reflectography and electron emission radiography or emissiography. Collectively these techniques are used to examine features on works of art which are difficult or impossible to see with the naked eye due to the fact that the varnish layer has yellowed and darkened or that the original composition has been changed and over painted by the artist. To date, Mona Lisa’s clothing has been little studied. In 1625, Cassiano dal Pozzo complained about the dark varnish that made it difficult to interpret. As art historian Bruno Mottin has observed using infrared reflectography: “The model’s whole body is covered in transparent veils that spill onto the left shoulder, fall onto the back of the chair, and run alongside the line of the right arm. According to Jacqueline Herald, this transparent overlayer was called a guarnello and was an indoor garment worn only by young children, pregnant woman, or woman that recently gave birth. Lisa Gherardini (Mona Lisa) did give birth to her second son, Andrea, on December 1, 1502, before the painting was commissioned in 1503.” (Mohen 2006). These observations are corroborated by the red color laser data recorded by the 3D scanner. The red laser wavelength (658 nm) penetrates the pictorial layer deeper than the blue (442 nm) or green (532 nm) wavelength but less than IR. Also, because the laser beam is very well focused it will penetrate deeper than conventional illumination by reducing the scattering between the different particles of pigments. Figure 8 shows the result of extracting the red wavelength from the virtual 3D model of the Mona Lisa; simple contrast enhancement techniques were used to remove much of the uniform background light and to amplify the details of the dress as well as the bonnet and the balustrade. It shows clearly the semi-transparent over layer guarnello dress that covers the shoulders. In the back of Mona Lisa’s head is also the evidence of a bonnet that holds the hair (Mohen 2006, Blais 2007). A few locks of the undulating hair on both sides of the face, rather than a loosely flowing mass of hair, is another indication of the presence of a bonnet to hold the hair up in a bun that was evident in both infrared photography and in the laser image. It is even more interesting to note that the red laser image provides information that is not obvious in conventional imagery and, in several cases, barely visible even in infrared. For instance the balustrade or railing of the loggia crossing transversally the whole painting behind Mona Lisa is clearly visible in the laser image of Figure 8. One horizontal line can be seen in infrared reflectrography; the laser image shows

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Figure 9. Superposition of color, range and telemicroscope images showing details of the craquelure pattern close to left eye of Mona Lisa.

Figure 8. The red laser component of the virtual 3D model is used to highlight details and the transparency of the dress.

parallel lines that appear to correspond to the low wall and balustrade. Other observations can be made for what appears to be two rocks visible in the river bed hidden by the hills (highlighted in Fig. 8) and for the two columns, although a more thorough analysis is needed to confirm these observations. Both laser and infrared reflectrography images also highlight other details of the planning of the composition that shows clearly that Leonardo changed his mind at a few occasions (Mohen 2006). 5 THE VARNISH LAYER The presence of small raised localized features in the 3D images, especially in the areas of the hair on either side of the face, had been left unexplained in (Mohen 2006). But subsequent experimental measurements in October 2005 on the Mona Lisa provided clues of an unexpected optical property of 3D laser imaging. Figure 9 combines information from three different sources in perfect registration for the detailed study of the painting: shape information, color, and images acquired using a telemicroscope. Registration of images is easy to do in 3D because perspective and scaling are perfectly known. The range image shows

Figure 10. Simplified model of the paint layer including a translucent layer of finely cracked old varnish and/or transparent glaze.

geometrical “bubbles” that are not visible in the telemicroscope image (center) and are white and floating over the paint layer in the color image (left). Figure 10 shows a simplified model of the pictorial layer showing a finely cracked translucent layer of old and modern shiny varnish/glaze. Let assume a directional light source (A-B) illuminating the painting. Because the surface of the varnish is shiny, an important amount of light will be deflected (C) while some will be diffused by the pigment surface (D). A very small deformation on the surface can potentially generate white specular reflections, either back to the measuring instrument (E-F-G) or an observer (H). The thickness of the transparent layer can be obtained by measuring the height of the reflections (E-F-G) with respect to the background (D) and compensating for the index of refraction of the medium. This property was verified by measuring a surface through a finely scratched microscope slide cover. Comparing the height of these white structures with the background colored image yields variations in the

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Figure 11. Reflections from ambient lights on the Mona Lisa during the 3D scan.

was part of the first extensive experimental study performed since 1952. One of the key objectives was to obtain a very accurate and detailed 3D model to provide an archival quality record of the real object. But even more important is that experts can now manipulate and analyze the virtual object at their own leisure, which is practically impossible on real works of art such as the Mona Lisa. The study of the shape of the panel and the details of the pictorial layer presented in this paper are just a few examples of uses of the 3D data for the detailed analysis of the overall shape of the painting, the technique of the artist and the effects of time on the pictorial layer such as for the cracks and the split. The red laser wavelength showed interesting optical properties of penetration through the pictorial layer that corroborates other imaging techniques and assists in better understanding the painting such as the dress and the bonnet, and some details of the background that are becoming visible behind the personage. The thickness of the varnish is another important potential measurement offered by 3D scanning. These interesting physical properties of 3D laser imaging are still under investigation, but they have already been shown to help further explain Leonardo’s painting technique. REFERENCES

Figure 12. The virtual 3D Mona Lisa fine craquelure pattern from the painting (darker color) and from the varnish layer (white reflections).

thickness of the translucent layer of the Mona Lisa between 0.11 mm and 0.17 mm on average. It is important to note that these values are still approximate until the exact index of refraction and the effects of penetration in the opaque paint layers are perfectly mastered. 6

CONCLUSIONS

The advantages of using 3D imaging have been clearly demonstrated on many occasions in the past for the study of some of the world’s most celebrated works of art. The Mona Lisa project has brought a completely new perspective to this emerging field. This project

Blais, F., Taylor, J., Cournoyer, L., Picard, M., Borgeat, Godin, G., Beraldin, Rioux, M., J.-A., Lahanier, Mottin, B. 2007. More than a poplar plank: the shape and subtle colors of the masterpiece Mona Lisa by Leonardo”, Videometrics IX, Electronic Imaging, San Jose, CA, 649106 1–10. Blais, F., Taylor, J., Cournoyer, L., Picard, M., Borgeat, L., Dicaire, L.-G., Rioux, M., Beraldin, J.-A., Godin, G., Lahanier, C., Aitken, G., 2005. “Ultra-High Resolution Imaging at 50 µm using a Portable XYZ-RGB Color Laser Scanner,” Intl. Workshop on Recording, Modeling and Visualization of Cultural Heritage. Centro Stefano Franscini, MonteVerita.Ascona, Switzerland, May 22–27. Godin, G., Beraldin, J.-A., Taylor, J., Cournoyer, L., Rioux, M., El-Hakim, S., Baribeau, R., Blais, F., Boulanger, P., Picard, M., Domey, J. 2002. Active Optical 3-D Imaging for Heritage Applications, IEEE Computer Graphics & Applications: Special Issue on Computer Graphics in Art History & Archaeology, 22: 24–36. Kemp, M., 2006. Leonardo daVinci, Oxford University Press. Mohen, J.-P., Menu, M., Mottin, B. Ed., 2006a. Mona Lisa: Inside the painting, Abrams, New-York. Mohen, J.-P., Menu, M., Mottin, B., 2006b. Au coeur de La Joconde, Léonard de Vinci décodé, Gallimard, Musée du Louvre, Paris, France. Taylor, J., Beraldin, J.-A., Godin, G., Cournoyer, L., Baribeau, R., Blais, F., Rioux, M., Domey, J., 2003. NRC 3D Technology for Museum and Heritage Applications, Journal of Visualization and Computer Animation, 14: 3. Zöllner, F. 2005. Leonardo da Vinci, the Complete Paintings, Taschen, Köln.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Characterization and virtual reconstruction of polychromed alabaster sculptures A. Sarmiento, K. Castro, M. Angulo, I. Martinez-Arkarazo, L.A. Fernández & J.M. Madariaga Department of Analytical Chemistry, University of Basque Country, Bilbao, Spain

J.M. Gonzalez-Cembellín & M. Urrutikoetxea Barrutia Diocesan Museum of Sacred Art, Bilbao, Spain

ABSTRACT: Several polychromed sculptures have been analysed by means of portable Raman microprobe spectroscopy in order to verify the nature of the support (alabaster) and to characterize the remaining polychromy. The Raman microprobe spectroscopic study was performed directly over the surface of the alabaster relieves in a non destructive way. The microscopic grains of pigments as well as the painting layers were successfully characterised by comparing the experimental Raman spectra with our e-VISART database of vibrational spectra of Artist’s Materials. The results obtained were very useful to check the origin and date of the artworks and gave us the opportunity to simulate the original palette used in the manufacturing of the specimens through a virtual reconstruction of them.

1

INTRODUCTION

Alabaster, a fine-grained form of gypsum, is a marblelike stone that became popular in the Middle Ages for the carving of religious sculptures, being England an important European centre of alabaster based artworks production during the 14th and 15th centuries. Initially, the artistic activity was centred on the funerary sculpture. Later, its employment was generalized and the pieces gathered together in diptychs, integrated polyptychs, formed part of altarpieces or even appeared as free pieces. As a consequence of the popularity of these sculptures the industry spread from Nottingham and surroundings up to the workshops of York and London (Cheetham 2003). These pieces were of small size and therefore easily transportable. This fact, in addition to its relative low cost (they were manufactured in series and the alabaster was inexpensive), turned them into very popular elements and, therefore, artworks made of this material were usual in many churches and chapels all over Europe (Alcolea 1971). Nowadays, it is possible to find more alabasters distributed through the European continent than in England, where most of them were destroyed in the 17th century during the reformist period (Cilla-Lopez et al. in press). In fact, only few artworks have survived until our days. It must be taken into account that alabaster is a very fragile and delicate material.

Besides, these artworks were also used for the private cult of rich families and all of them have suffered all kind of vicissitudes (Yarza 1993). These artworks were covered with a polychromed layer that was, in many cases, very rich, with some parts painted even with gold leaf. Although it is still possible to recognize this polychromy in some of the surviving artworks, almost all sculptures have partially or totally lost it and no colour can be seen to the naked eye. However, microscopic remains are still present on the surfaces of the alabasters and can be the basis for microscopic analysis in order to reconstruct the original palettes. In this work we present the preliminary results obtained from the analysis of several specimens of such English alabaster sculptures by using portable, non-destructive, non-invasive, “in situ” Raman microprobe spectroscopy. 2

INSTRUMENTATION

The Raman analysis was carried out using a portable Renishaw RA100 microprobe system that implements a 785 nm excitation laser and a CCD detector. Thermal decomposition of the samples was avoided using neutral density filters to give laser powers between 5 and 50 mW at the source. The microprobe was coupled to ×20 and ×50 objectives that allows to focus

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Figure 1. Left to right: Saint John the Baptist’s head, the last judgement, Saint Michael and Saint George.

Figure 2. Raman spectrum of the back surface of an alabaster sculpture.

3.3 Saint Roch (1420–1460) even on little grains of pigments with the help of a micro-TV camera implemented on the probe. Thus, it was possible to detect not only the microscopic grains of remaining pigments in the analysed artworks but also the surviving painting layers. 3

3.4 Saint John the Baptist’s head (1420–1460)

SPECIMENS

Several polychromed alabaster sculptures (see some of them in Fig. 1), located in the Diocesan Museum of Sacred Art of Bilbao (North of Spain), were selected for analysis by Raman spectroscopy. The analysis was performed in situ, that is, the Raman instrument was brought to the Museum. Some of the specimens were even analyzed while they were hanging on the exhibition wall. Although this fact could be a handicap, the good results obtained demonstrated the usefulness of this Raman setup.

3.1

This sculpture is original from the city of Orduña (North of Spain) and was provided with a canopy. It shows a good state of conservation and abundant remains of polychromy can be observed at simple sight.

Saint George, Saint Michael (1420–1460) and The Last Judgement (1460–1480)

The design shown by this sculpture, coming from the chapel of Nuestra Señora de las Nieves de Aguirre (Gorliz, North of Spain), is one of the most frequent among the English sculptures in alabaster. Almost a hundred copies of this composite scheme (with small variations) can be found all over the world. All these specimens together with some others that have not been included in this work compose the most numerous collection of English alabasters from the 14th and 15th centuries that can be appreciated in a Spanish Museum. There are several reviews about English alabaster sculptures in Spain (HernándezPerera 1958, Hernández-Perera 1970, Franco 1999). 4

These specimens were originally located in the Church of Santa Eufemia (Bermeo, North of Spain) over a grave from the late middle ages. However, these samples could have belonged to a polyptych, as it was the case in other examples of English alabaster works. The polychromy had almost disappeared.

RAMAN ANALYSIS

The Raman analysis of the specimens took into account not only the polychromy but also the support in order to check if some of the sculptures could be forgeries. The collected unknown spectra were compared with our e-VISART Raman spectra database (Castro et al. 2005).

3.2 The Altarpiece of the Passion (1440–1460)

4.1 Analysis of the support

It is composed by five pieces that represent the Passion of Jesus Christ (The Betrayal, The Flagellation, The Crucifixion, The Saint Burial and The Resurrection) and it was located on the Church of Santa María Magdalena (Plentzia, North of Spain). It is the most completed Altarpiece of the Passion that can be seen in Spain and, in fact, some of the pieces are unique in the country. It is in a well state of conservation and the polychromy still remains (De Echegaray 1912, Lahoz 1991).

Several measurements were carried out on the back surface of the pieces in order to characterize the support and to confirm that they were really sculpted in alabaster. The rear zone of the pieces was chosen to avoid possible cross-contamination by the presence of pigmentation and/or external pollutants (plasters, cement, concrete, etc) which could distort the results. The obtained Raman spectra showed a clear band around 1008 cm−1 characteristic of alabaster stone (CaSO4 · 2H2 O). Besides, less intense absorption

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Figure 3. Detail of the flowers in the alabasters and Raman spectrum of the white colour.

Figure 4. Raman spectra of red zones: a) red polychromy and b) remains of red dark painting.

bands located at 1134, 671, 620, 493 and 414 cm−1 (see Fig. 2) could be seen in all the spectra. 4.2 Analysis of the polychromy The Raman results obtained from the analysis of the polychromy are presented below sorted by colour. When it was required, X-ray Fluorescence (XRF) measurements were also performed. 4.2.1 White colour The analysis of the white polychromy by means of Raman spectroscopy revealed the presence of a band centred around 1048 cm−1 , which confirmed the presence of white lead (2PbCO3 · Pb(OH)2 ). This colour was present principally in the flowers that can be appreciated in most of the alabasters (see Fig. 3) as decorative motifs. 4.2.2 Red colour The Raman analysis of the red polychromy revealed the presence of vermilion (HgS), in agreement with the bands at 342, 283 and 251 cm−1 . This red colour was one of the most used colours in the manufacture of English alabasters (Prigent 1998) and it can be found in diverse areas of the pieces (robes, faces, etc). In the back part of several sculptures, abundant remains of another red dark pigment were found. The spectra of these areas showed bands located at 409, 293 and 226 cm−1 , showing the presence of red iron oxide (see Fig. 4). 4.2.3 Golden areas The golden colour present in these pieces has been also analyzed by means of XRF in order to find possible gold in its composition. These zones were also analysed by Raman spectroscopy to detect the possible presence of pigments used as substitutes of the gold leaf, which were known as “false gold”, such as SnS2 also called “mosaic gold”. No signals were obtained in the Raman spectra from the golden colour areas. In contrast, X-ray Fluorescence (XRF) analyses demonstrated the presence of gold (Au), iron (Fe) and

Figure 5. Raman spectrum of orange polychromy.

lead (Pb). Thus, with these results, it can be said that the surface of these artworks was covered by authentic gold leaf. The existence of iron and lead in these areas may be related to the presence of a preparation layer composed by some bole what could not be detected by means of Raman spectroscopy because the laser could not penetrate through the gold leaf. 4.2.4 Orange colour In one of the sculptures some orange colour remains of polychromy were found in a lateral zone. In agreement with the Raman spectra of such areas (see Fig. 5), the colour was formed by a mixture of minium (Pb3 O4 ), with Raman bands at 550, 457, 390, 312 and 229 cm−1 and a small quantity of vermilion whose principal Raman band is located at 251 cm−1 . This mixture is said to be very common in Western Europe during the middle ages, being the minium added to vermilion, as the filler, because this last pigment was very expensive and difficult to obtain. 4.2.5 Black colour Some of the figures that formed a part of the different designs had certain features (eyes, nose, mouth, hair, folds, etc) outlined with a black colour. To our surprise, a mixture of two pigments was always detected. Systematically the mixture was composed by carbon black (with two characteristic broad Raman bands at 1300 and 1600 cm−1 ) and iron black (confirmed by XRF).

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Figure 6. Raman spectrum of green colour showing both the copper oxalate and the calcium oxalate dihydrate.

4.2.6 Green colour One of the most interesting facts in this work was the presence of copper oxalate in the green colour of the alabasters (most intense Raman bands at 559 and 1517 cm−1 ). This compound is a very strange natural mineral (moolooite), first reported in 1985 with a very scant occurrence, and formed after microbiological attack on carbonate based copper minerals. As moolooite was not known at the time of manufacturing the alabasters under study, the presence of this compound can only be explained as being a decay produced from the original pigment. The absence of other bands from blue or yellow pigments and the XRF results suggest that the original pigment was a green copper pigment such as copper resinate or malachite. Nevertheless, that green original pigment is unknown for the moment because no Raman signal could be recorded. Moreover, the presence of calcium oxalate dihydrate (weddellite) in some of the spectra of these green areas (Raman band at 1476 cm−1 ) suggests a possible biodeterioration process. Calcium oxalate can be detected in natural outcrops of different climate as a consequence of the metabolic activity (oxalic acid excretions) from different microorganisms, epylithic lichens and blue algae, and subsequent reactions with calcium carbonate (Del Monte et al. 1987, Holder et al. 2000). Colonisation of such organisms on historical building façades and wall-paintings produce also calcium oxalate, being considered one of the most important effects of biodeterioration (Edwards 1991, Edwards et al. 1995, Perez-Alonso et al. 2006). The quality of the spectra from these areas is rather low, because of the difficulty to obtain good Raman spectra of green materials with an excitation laser of 785 nm like the one used in this work, but the Raman signals are clear enough to address the presence of both oxalate compounds (see Fig. 6).

5 VIRTUAL RECONSTRUCTION The Raman characterization performed over the sculptures together with some microphotographs taken with

Figure 7. Current aspect of Saint John the Baptist’s head (up) and virtual reconstruction of polychromy (down).

an optical microscope and with a video camera allowed us to simulate the original polychromy for some of the pieces. Although the polychromy in these alabaster sculptures is very scarce, with the obtained results and the help of imaging treatment software, the images presented in Figure 7 (Saint John the Baptist) and Figure 8 (The Betrayal) were elaborated. This virtual reconstruction may be the first step in the process of getting 3D reproductions of these pieces to be showed at the Museum and/or in their original locations.

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ACKNOWLEDGEMENTS This work has been financially supported by the PIETRA project (Ref. UE05-A09), funded through the Universidad-Empresa program of the Basque Country Government. A. Sarmiento acknowledges his predoctoral fellowship from the Spanish MEC. Dr. K. Castro is grateful to the Ministry of Education and Science for his contract at the UPV/EHU (PTA 2003-02-00050). Authors gratefully acknowledge the support of the Diocesan Museum of Sacred Art of Bilbao. REFERENCES

Figure 8. Virtual reconstruction of the polychromy of The Betrayal.

6

CONCLUSIONS

Raman spectroscopy has demonstrated its usefulness in the non-invasive, non-destructive, in situ analysis of alabaster sculptures. As the Raman analysis does not need sampling, a complete mapping of Cultural Heritage specimens can be carried out, acquiring as many spectra as required for a complete characterization process. Thus, in order to confirm the spectra of a given colour shade, several analyses in different areas of the same shade are currently made. In the case of the alabaster analysed in this work, the detected pigments were the common ones in paintings from the Middle Ages: vermilion, lead white, carbon black, iron oxide, minium and a green copper pigment. It is very important to remark the presence of two oxalate compounds in the green areas and it would be very interesting to confirm their biological induced formation. Finally, the virtual reconstruction of ancient objects is nowadays a new way of recovering the brilliance lost by the passage of time and gives to Museums the possibility to enhance their collections.

Alcolea, S. 1971.Relieves ingleses de alabastro en España: Ensayo de catalogación. In Archivo Español de Arte, n◦ XLIV: 137–153. Castro K., Perez-Alonso M., Rodríguez-Laso M.D. & Madariaga J.M. 2005. “On line FT-Raman and dispersive Raman spectra database of artists’ materials (e-VISART database). Analytical and Bioanalytical Chemistry, 382: 248–258. Cheetham, F. 2003. Alabaster images of medieval England. The Boydell Press. Woodbridge. Cilla-Lopez, R. & Gonzalez-Cembellín, J.M. Guía de la colección. Museo Diocesano de Arte Sacro, Bilbao. In press. De Echegaray, C. 1912. Alabastros de Plencia. In Boletín de la Comisión de Monumentos de Vizcaya, n◦ XIII: 37–39. Del Monte, M., Sabbioni, C. & Zappia, G., 1987, Science of the Total Environment, 67: 17–39. Edwards, H.G.M. 1991. Spectrochimica Acta, Part A, 47A: 1531–1539. Edwards, H.G.M., Russel, N.C., Seaward, M.R.A. & Solarke, D. 1995. Spectrochimica Acta, Part A, 51A: 2091–2100. Estrade, M. 1994. Dos alabastros góticos ingleses. In Memoria 1994. Museo Arqueológico, Etnográfico e Histórico Vasco. Bilbao: 85–94. Franco, A. 1999. El retablo gótico de Cartagena y los alabastros ingleses en España. Caja Murcia. Madrid. Hernández-Perera, J. 1958. Alabastros ingleses en España. In Goya, 22: 216–222. Hernández-Perera, J. 1970. Mas relieves góticos de alabastro en España. In Homenaje a Elías Serra Rafols. Universidad de La Laguna. La Laguna: 251–264. Holder, J.M., Wynn-Williams, D.D., Rull-Perez, F. & Edwards, H.G.M. 2000. New Phytol., 145: 271–280. Lahoz, M.L. 1991. Un retablo de alabastro inglés en Plencia (Bizkaia). In Kobie (serie Bellas Artes), n◦ VIII: 73–81. Perez-Alonso, M., Castro, K. & Madariaga J.M. 2006. Investigation of degradation mechanisms by portable Raman spectroscopy and thermodynamic speciation: the wall painting of Santa Maria de Lemoniz (Basque Country, North of Spain). Analytica Chimica Acta, 571: 121–128. Prigent, C. 1998. Les sculptures anglaises d’albâtre au musée national du Moyen Âge-Thermes de Cluny. Musée de Cluny. Paris. Yarza, J. 1993. Alabastros esculpidos y comercio InglaterraCorona de Castilla en la baja edad media. In Homenaje al profesor Hernández Perera. Comunidad Autónoma de Canarias – Dirección General de Patrimonio Histórico. Madrid: 605–615.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

ITR: A laser rangefinder for cultural heritage conservation applications with multi-sensor data integration capabilities R. Ricci, M. Ferri De Collibus, G. Fornetti, M. Francucci, M. Guarneri & E. Paglia ENEA, Frascati, Rome, Italy

ABSTRACT: The acronym ITR (Imaging Topological Radar) identifies a family of AM (amplitude-modulated) laser rangefinder prototypes designed and realised at ENEA. The AM rangefinding technique enables to acquire in a single scan an accurate range image and a shade-free, high resolution – grey-scale or native-RGB – reflectivity image, which can be used in 3D rendering as colour information for enhanced realism. ITR systems provide submillimetric range measures up to several tens of meters and can be operated in hostile environments. Particularly important for cultural heritage conservation applications is the capability of ITR systems to integrate 3D models with data acquired independently by other sensors (LIF scanners, vibrometers etc.). As a demonstration, we report the results obtained in a recent measurement campaign carried out in the Sucevita monastery (Romania), concerning the integration of 3D models with vibration maps produced with the Scanning Laser Doppler Vibrometer developed by the Polytechnic University of Marche.

1

INTRODUCTION

Laser scanning (Strand 1985, Amann et al. 2001) is a mature technology widely used for 3D digitization in an increasing number of industrial, scientific and cultural applications. This technology has demonstrated to be particularly well suited to the needs specific of the cultural heritage domain (Fantoni et al. 2000, Ricci et al. 2003 a,b). Laser scanners provide here a powerful yet versatile means to realise faithful computer models of rare, sometimes unique, delicate artworks, as well as of entire scenes of cultural or archaeological interest (building façades and interiors, archaeological sites etc.). Once in digital format, these models can be used for various purposes, e.g. for cataloguing and/or computer-aided restoration. In order to better exploit the new possibilities made available by laser scanning in the cultural heritage domain, it is particularly important to define methods enabling the accurate integration of a 3D model’s geometric information with data obtained by using various diagnostic techniques, such as e.g. laser induced fluorescence, thermography, vibrometry etc. (Ricci et al. 2005). The availability of integrated tools for combined 3D inspection and diagnostics is expected to bring enormous benefits to computer-aided cultural heritage conservation and restoration processes and pave the way for unprecedented applications. In recent years, a class of laser rangefinders with multisensory data integration capabilities, collectively

identified by the acronym ITR (Imaging Topological Radar), have been developed by the Artificial Vision group of the ENEA National Research Laboratories in Frascati (Italy). ITR laser scanners are based on the AM (amplitude-modulated) rangefinding technique, which enables to acquire in a single scan an accurate range image and a shade-free, high resolution reflectivity image. This makes ITR systems particularly suited to applications that require enhanced viewing capabilities combined with submillimetric range measurements, at distances from some tens of centimetres to several tens of meters. Thanks to the one-to-one correspondence with range measures, reflectivity data can be used in 3D rendering as vertex colour information, resulting in highly realistic models. Particularly important for cultural heritage applications is the capability of ITR systems to integrate, under specified yet sufficiently general conditions, 3D models with multi-diagnostic data. This is accomplished by means of dedicated companion software applications, also developed at ENEA, which enable to perform, among others, a precise matching of geometric information with data acquired independently by other sensors (LIF scanners, metric cameras, thermographic and vibrometric systems etc.). As a demonstration of ITR capabilities, we report the results of a measurement campaign carried out in Romania in 2006 in the framework of the European program CULTURE 2000, namely the 3D digitisation of the frescoed walls of the Burial Chamber

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in the Sucevita monastery. We also report results, obtained in collaboration with the Polytechnic University of Marche (UNIVPM) andArtemis srl, concerning the integration of Sucevita 3D models with vibration maps that were acquired during the same campaign by using the Scanning Laser Doppler Vibrometer (SLDV) technology developed in UNIVPM. 2 2.1

ITR LASER RANGEFINDERS Functioning principle

ITR systems belong to the category of AM laser rangefinders. Range information is obtained by determining the round trip time delay of an AM sounding beam via the measurement of the phase delay ϕ of the signal photocurrent with respect to a reference signal. For each sampled point, the distance d between the scanning mirror and the target is given by the formula:

where νm is the modulation frequency, c the velocity of light in vacuum and n the refraction index of the transmitting medium. For laser optical powers such that the signal shot-noise dominates over all other noise sources in the detection process, the accuracy of measurements can be showed to increase with the modulation frequency νm , according to the formulae (Nitzan et al. 1977):

where σR is the “intrinsic” error (i.e. the minimum attainable error in optimal experimental conditions), m is the modulation depth and (SNR)i is the current signal-to-noise ratio, depending on the laser frequency ν and collected power PS , the integration time τ, the detector’s quantum efficiency η and the overall optics merit factor . Due to the cyclic nature of the modulation process, the AM range finding method is generally affected by the so-called “folding” ambiguity. Univocal distance measurements are only possible within a welldetermined range window, equal to half the value of the corresponding modulation wavelength λm . In order to overcome this inconvenience, two modulation frequencies are simultaneously used. Provided the low-frequency range window is chosen large enough to encompass the whole scene of interest, unambiguous, yet less accurate, low-frequency measurements can be used to remove the ambiguity that affects the corresponding high-frequency, more accurate, measurements. This technique enables to measure distances at the level of accuracy permitted by the high-frequency mode but well beyond its intrinsic range and can profitably be used to obtain accurate digitisations of large scenes, such as building façades or interiors, vaults etc.

2.2 General features ITR systems are designed so as to have a passive optical head, mainly constituted by mirrors and lenses, separated by the active opto-electronic system, which includes the laser source and the detector with dedicated electronics. The core of the electronic subsystem is a Stanford SR-844 lock-in amplifier unit, which modulates the laser source and digitalises range and reflectivity target information. Unlike laser scanners based on optical triangulation, in ITR systems the launching optics is coaxial with the receiving optics in a strictly monostatic configuration. This feature enables to avoid self-occlusions during the scanning process and eliminate off-axis optical aberrations, resulting in an improvement in lateral resolution. The active and passive subsystems are optically coupled trough optimized optical fibre connections. This modular setup enables to place the passive module in the position that is more convenient for the measure, without compromising the overall functioning of the system even in extreme or hostile conditions, such as at high or low temperature or in presence of intense ionizing radiation background. A typical ITR optical head is shown in Figure 1. Several ITR prototypes have been designed and realised during the last years. Beside metrological and viewing applications in manufacture and nuclear research (Bartolini et al. 2000), ITR systems have proved to be very well suited to accurate 3D digitization of valuable and/or hardly accessible objects (Fantoni et al. 2003).

3

MULTI-SENSOR DATA INTEGRATION

In the context of cultural heritage applications, an emergent research field is the precise mapping of data collected using various diagnostic tools and techniques onto the geometric models provided by laser scanners. This is to be distinguished from simple texturing techniques, where the correspondence of the superimposed image with the underlying model is approximate and

Figure 1. Scheme of a typical ITR optical head.

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usually only aimed at presentation purposes. Multisensor data integration is particularly important for cultural heritage preservation, since it enables restorers to reference diagnostic information directly onto the 3D model of the object under restoration, thus making it possible to fine-tune potentially invasive interventions and limit possible damages. ITR systems offer different data integration functionalities, depending on the type of data and the characteristics of systems and procedures used to collect them. Data mapping can be realised: – by adopting a “hybrid” protocol, which consists in the application of special algorithms to data collected by following a specified acquisition procedure, aimed at directly referencing the second sensing system (SSS) with respect to the ITR; – exclusively via software, for imaging data acquired in whatever conditions, provided at least part of the visual aspect of the target is conserved. The first method, beside providing in general better control over the accuracy of final results, can be applied also to diagnostic information that show no immediate relationship with the superficial appearance of the target, at the cost of a constrained acquisition setup and a possibly significant computational load. The second method requires no particular tricks during data collection, but is conversely less accurate and not generally applicable. The two techniques are described in more detail in the following sections. 3.1

ITR-SSS referencing protocol

The aim of this method is to directly relate the SSS’s and ITR’s reference frames, by using the latter to accurately measure the position of the former. This technique permitted to achieve good results (Ricci et al. 2005b, Colao et al. 2005) in integrating geometric information with laser induced fluorescence (LIF) data. The general idea underlying the integration method is to try to determine the linear transformation (rototranslation) that relates the ITR and the SSS local reference frames. Once this transformation is known, it can be used to refer to the ITR frame the view line under which each data pixel is measured by the SSS. This in turn enables to determine, for each view line of the SSS, the closest vertex in the geometrical model and to associate the corresponding information to that vertex. In all cases where the SSS nodal point is directly accessible to the ITR, the afore mentioned LIF system fell in this category, the overall procedure can be schematically summarised as follows: 1. identify at least three reference points on the target scene; 2. use the ITR to determine the vector lying between the two systems’ “nodal points”, typically, the centres of the scanning mirrors;

3. for each reference point, measure corresponding angles in the second reference frame; 4. for each reference point, measure corresponding vectors (angles and distances) in the ITR frame; 5. use the data collected in steps 2 to 4 to calculate the distances of the reference points from the origin of the second frame; 6. use the data obtained in steps 2 to 5 to calculate the rototranslation transformation that links the two reference frames (Horn 1987). Alternatively, distances of reference points in the second frame, needed to calculate the rototranslation transformation, can be evaluated without measuring the vector between nodal points, but using instead the relative distances between the reference points themselves, which can be easily calculated in the ITR frame. The problem of determining the distances of n points from a fixed “centre”, given the relative distances of the n points between each other and the angles formed by the line segments connecting the points with the centre, is conceptually equivalent to the so called “PnP” problem, well-known in photogrammetry and completely solved (Linnainmaa et al. 1988), at least in the case of three points (P3P). This variant of the method has some obvious advantages, namely: – the nodal point of the second system needs not be accessible to the ITR; – it is not necessary for the two systems to coexist at the same time in the same place: measurements can be performed at different times with no particular positioning constraints for the second system. On the other hand, the determination of distances in the second method is mostly based on calculations, with possible round-off effects also due to the complexity of the P3P problem that demands for a numerical approach.

3.2 Direct Linear Transformation A completely different technique, which exploits the so called “Direct Linear Transformation” widely used in photogrammetric applications (see e.g. Hartley & Zisserman 2004), can be used for the superposition of data acquired with sensors such as high-resolution digital metric cameras, thermocameras, vibrometers etc. This technique does not require that the position of the nodal point of the second system in the ITR reference frame is known in advance and so can be effectively applied whenever this information is not available, under certain conditions that are made explicit in the following. In the DLT method a certain number of “control points” are used in order to determine the set of parameters L1 to L11 that relate the coordinates of image

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pixels (u, v) to those of object points in 3D space (x, y, z) in the system of equations:

Since the coordinates of control points must be known in both reference frames, the method can only be applied if it turns out to be possible to locate a sufficient number of corresponding points on the 2D image, that represents the external data to be mapped, and the 3D model. This is generally not possible for most laser scanning systems, which are only capable of acquiring the surface geometry of the target. In the case of ITR systems, on the contrary, corresponding points can easily be determined by visual inspection, provided the 2D data image conserves a resemblance with the target, due to the availability of reflectivity information. We report in the next section an example of the results obtained by using this technique on vibrometric data.

4

RESULTS

In July 2006 personnel from the ENEA “Artificial Vision” laboratory participated at the workshop “Saving Sacred Relics of European Medieval cultural heritage”, organized in Bucovina (Romania) by the CULTURE 2000 project and financed by the European Commission under contract CLT 2005/A1/CHLAB/RO-488. They were in charge of performing a complete 3D digitization of the interior of the Burial Chamber in the Resurrection Church of Sucevita Monastery, as well as of the external wall surrounding the apse on the left hand side. In the same occasion, a considerable amount of diagnostic data was collected by other researchers in the monasteries of Balinesti, Popauti and Sucevita – near Suceava, also in Romania – by using various imaging techniques, such as LIF imaging and acoustic interferometry. One of the objectives was to demonstrate the advantages of an integrated multisensorial approach for the purpose of computer-aided restoration, where one can benefit of the 3D models produced by the ITR as "geometric supports" for the precise referencing of all collected diagnostic information. In particular, researchers from the Polytechnic University of Marche (UNIVPM) carried out structural investigation and fresco analysis activities by Scanning Laser Doppler Vibrometry (SLDV). This technique is briefly outlined in next section.

4.1

Laser Doppler vibrometry

A Laser Doppler Vibrometer (LDV) is an optical interferometer for the remote measurement of velocity and displacement of vibrating structures (Castellini et al. 1998). In monostatic configuration, i.e. when launching and receiving optics are collinear, velocity V is related to the measured Doppler frequency shift νD according to the formula

where λ the laser wavelength, and  the angle between the laser beam and the instantaneous direction of movement of the target point illuminated by the laser. A scanning version of the instrument (SLDV) is obtained by supplementing the system with two moving mirrors driven by galvanometric actuators, which make it possible to direct the laser beam to the desired measurement points. In typical applications, slight vibrations of the structure under examination are excited by mechanical and acoustical actuators, while the SLDV quickly perform a series of velocity measurements on a grid of points, producing a 2D or 3D image. SLDV technology has been applied to different types of decorative artworks, like frescoes, icons and mosaics (Castellini et al. 2003). In the case of frescoes, e.g., delaminated areas can be easily spotted by a SLDV, since velocity is higher where these defects occur than in neighbouring areas. The SLDV can non-intrusively acquire vibrational data on nontreated surfaces up to 10–15 metres. Regular monitoring of important parameters related to the state of conservation of historical buildings, like frequencies of resonance, is thus made possible at a high degree of accuracy with no external intervention on the structure.

4.2 3D model reconstruction and vibrometric data integration During the Sucevita campaign, the entire Burial Chamber was digitised and reconstructed by using a monochrome ITR system (see Table 1). Data acquisition lasted approximately 30 hours, with the ITR positioned at a height of about 1.80 m in order to avoid problems related to the presence of visitors during day operation. The resulting 3D model (∼1 million vertices, see a detail in Fig. 2) offers the possibility of analysing the chamber at various levels of detail: it can be used e.g. to determine the precise location of painted features, architectonic elements or surface damages (bulges, scratches, discolorations). Structural defects ascribable to the original construction or to progressive degradation can be highlighted once the model is

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Table 1. Configuration and settings of the ITR system used for the 3D digitalization of the Burial Chamber in the Resurrection Church at Sucevita monastery (Romania). Optical wavelength Transmitted laser power Modulation frequency (low | high) Spot size Distance from target Pixel sampling rate Average range resolution Acquisition time Average lateral resolution

830 nm 7 mW 5 | 200 MHz 700 µm–3 mm 1–15 m 10 ms 200 µm ∼30 hours 17 µm–2.6 mm

Figure 2. Part of the (decimated) 3D model of the Burial Chamber in the Resurrection Church of Sucevita Monastery (Romania).

examined in term of curves of level, or selected sections are compared to the ideal geometrical structure. Discrepancies between the 3D model and suitable fit functions can be interpreted in terms of fresco’s irregularities (suspect detachments) which may suggest the need for a restoration action. In the same campaign extensive vibrometric data were collected on both internal and external frescoed walls, some of which were also 3D-digitised by means of the ITR. Since SLDV and ITR data acquisition could not be carried out simultaneously due to logistic difficulties, data integration has been realised by using the purely algorithmic method described in section 3.2. A detail of the results is shown in Figure 3, where data collected by the SLDV system, represented as a falsecolour image, with an indication of vibration isolevels, are shown overlapped onto the 3D model.The achieved overlapping accuracy is quite satisfactory, despite the much lower lower resolution of the images acquired by the SLDV scanner as compared with the ITR.

Figure 3. An example of vibrometric data integration: (a) vibrometric image with an indication of vibration isolevels; (b) 3D model of the same surface element acquired by means of the ITR; (c) result of the superposition of vibrometric data onto the 3D model.

5

CONCLUSIONS AND FUTURE WORK

The results of the 2006 campaign in Romania have largely confirmed that ITR systems are optimally suited to applications in the cultural heritage domain, by evidencing ITR’s good performances in terms of resolution (yet at the price of long scanning times) and

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versatility, thanks to the modular design and monostatic configuration. Particularly relevant for artwork conservation applications is the ITR’s imaging capability, which largely facilitates the merging of other images independently collected by using heterogeneous diagnostic techniques, even in the absence of any additional information (optical markers, structured scenes etc.). The Artificial Vision Laboratory is currently involved in the realization of a native-RGB ITR system, suitable to be used for accurate colorimetric analyses. A first very promising three-stimulus prototype has already been realised (Bartolini et al. 2007), by using three independent pigtailed laser sources kept together by means of a patented mechanical device (ferrule). Research is being carried out to develop a system exploiting a single “white” laser beam instead of three separated R, G and B beams.

ACKNOWLEDGEMENTS We thank Prof. Enrico Esposito and his collaborators from Department of Mechanics, Polytechnic University of Marche, Ancona, Italy, for providing vibrometric data and support for the realisation of this work. REFERENCES Amann, M.C., Bosch,T., Lescure, M., Myllylä, R. & Roux, M. 2001. Laser ranging: a critical review of usual techniques for distance measurement. Opt. Eng. 40: 1–10. Bartolini, L., Bordone, A., Coletti, A., Ferri De Collibus, M., Fornetti, G., Lupini, S., Neri, C., Riva, M., Semeraro, L. & Talarico, C. 2000. Laser in vessel viewing system for nuclear fusion reactors. International Symposium on Optical Science and Technology Proc. SPIE. 4124: 201–211. Bartolini, L., Ferri De Collibus, M., Fornetti, G., Francucci, M., Guarneri, M., Nuvoli, M., Paglia, E. & Ricci R. 2007. Color (RGB) Imaging Laser Radar. To be presented inISPDI 2007 International Symposium on Photo-electronic Detection and Imaging, September 9–12 (Beijing, China). Castellini, P., Revel, G.M. & Tomasini, G.M. 1998. Laser Doppler Vibrometry: a Review of Advances and Applications. The Shock and Vibration Digest. 30: 443–456. Castellini, P., Esposito, E., Marchetti, B. & Tomasini, E.P. 2003. New applications of Scanning Laser Doppler Vibrometry (SLDV) to non-destructive diagnostics of artworks: mosaics, ceramics, inlaid wood and easel painting. Journal of Cultural Heritage. 4: S321–S329.

Colao, F., Fantoni, R., Fiorani, L., Gomoiu, I. & Palucci, A. 2005. Compact scanning lidar fluorosensor for investigations of biodegradation on ancient painted surfaces. Proc. of the 8th Romanian Biophysics Conference, 7: 3197–3208. Fantoni, R., Palucci, A., Ribezzo, S., Borgia, I., Bacchi, E., Caponero, M., Bordone, A., Businaro, L., Ferri De Collibus, M., Fornetti, G. & Poggi, C. 2000. Laser diagnostics developed for conservation and restoration of Cultural Inheritance. ALT’99 Proc. SPIE 4070: 2–7. Fantoni, R., Bordone, A., Ferri De Collibus, M., Fornetti, G., Guarneri, M., Poggi, C. & Ricci, R. 2003. High resolution laser radar: a powerful tool for 3D imaging with potential applications in artwork restoration and medical prosthesis. ALT’02 International Conference on Advanced Laser Technologies Proc. SPIE 5147: 116–127. Hartley, R. & Zisserman, A. 2004. Multiple View Geometry in computer vision. Cambridge University Press. Horn, B.K.P. 1987. Closed-form solution of absolute orientation using unit quaternions. Journal of the Optical Society of America A, 4: 629–642. Linnainmaa, S., Harwood, D. & Davis, L.S. 1988. Pose determination of a three-dimensional object using triangle pairs IEEETrans. on PatternAnalysis and Machine Intelligence, 10: 634–647. Nitzan, D., Brain, A.E. & Duda, R.O. 1977. The Measurement and Use of Registered Reflectance and Range Data in Scene Analysis. Proc. IEEE 6: 206. Ricci, R., Bordone, A., Fantoni, R., Ferri De Collibus, M., Fornetti, G., Guarneri, M. & Poggi, C. 2003a. Development of a high-resolution laser radar for 3D imaging in artwork cataloguing. Third GR-I International Conference on New Laser Technologies and Applications Proc. SPIE 5131: 244–248. Ricci, R., Bordone,A., Fantoni, R., Ferri De Collibus, M., Fornetti, G., Guarneri, M. & Poggi, C. 2003b. High-resolution laser radar for 3D imaging in artwork cataloging, reproduction, and restoration. Optical Metrology for Arts and Multimedia Proc. SPIE 5146: 62–73. Ricci, R., Bartolini, L., Ferri De Collibus, M., Fornetti, G., Guarneri, M., Paglia, E. & Poggi, C. 2005a. Amplitudemodulated laser range-finder for 3D imaging with multisensor data integration capabilities. ALT’04 International Conference on Advanced Laser Technologies Proc. SPIE. 5850: 152–159. Ricci, R., Ferri De Collibus, M., Fornetti, G., Guarneri, M., Paglia, E. & Poggi, C. 2005b. ITR: an AM laser range finding system for 3D imaging and multi-sensor data integration, Proc. of ICST 2005 International Conference on Sensing Technology, Palmerston North, New Zealand, 21–23 November 2005 Strand, T.C. 1985. Optical three-dimensional sensing for machine vision. Optical Engineering 24: 33–40.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Multispectral and multi-modal imaging data processing for the identification of painting materials A. Pelagotti Art-Test s.n.c., c/o Incubatore d’Impresa, Polo Scientifico e Tecnologico, Navacchio (PI), Italy

A. Del Mastio & V. Cappellini MICC – Media Integration and Communication Center, Center of Excellence of the University of Florence, Florence, Italy

ABSTRACT: Nowadays there are several multispectral acquisition systems which have been proposed for imaging artworks.With multispectral imaging it is in principle possible to achieve, for instance, higher colour quality. However, what is perhaps most interesting is that it paves the way to image spectroscopy, where the final goal is to achieve the spectral signature of each imaged element of the artwork. This is a crucial step to identify pigments and binding media in a totally unobtrusive way. Whatever the system used to acquire multispectral images is, the information must be processed, and an appropriate algorithm must be used to differentiate the areas within the image which have similar spectral signature and therefore similar chemical composition. The more efficient the algorithm used, the more accurate the results. However, when the system chosen has a limited number of filters, still some problem of metamerism may appear, and the algorithm used may not be able to give a correct answer in all cases. In order to have a more efficient use of a system with a limited number of filters, and therefore a quite coarse approximation of the reflectance spectral profile of the materials in the artwork, we chose to include different modalities. In order to obtain a larger amount of data, we analyzed also the radiation reflected by the artwork in the IR range, and the fluorescence emitted in the visible range when the radiating source was UV light. In this way, we could profitably compare the available data and search the same profile registered at a given location all over the painting surface. Having more modalities allowed a more reliable segmentation of the image.

1

INTRODUCTION

The most emphasized characteristic of paintings is that they are images. Image processing seems therefore to be a natural candidate to deal with them. Actually, image processing can benefit the art field in several ways, for example in the implementation of virtual restoration, and in the pursuing of unobtrusive diagnostics. Non-invasive diagnostic techniques are, in fact, highly desirable in a field where owners are not entrusting too invasive attentions, and most of the current analytic techniques imply microsampling of the painting. The key idea is to infer images data by exploiting the fact that materials reflect, absorb and emit electromagnetic radiation in ways that depend on their molecular composition and shape. With optical techniques and image processing, the entire surface of a painting can be analyzed, and the materials within can be localized with good accuracy, using an approach which is similar to what is commonly used in remote sensing,

and exploits multispectral imaging in the visible and infrared range, and not only standard illumination source, but also UV light.

2

IMAGING TECHNIQUES

Depending on the application and its requirements, the devices employed for image acquisition use passive or active detection schemes, and several kinds of sensors and radiation sources (lasers, X-ray tubes, halogen and UV lamps, etc.). However, most of the current developments are concentrated on processing and analyzing the visible, near IR and UV range images.

2.1

Multispectral digital acquisition in the visible range

For scientific imaging in the visible range, commercial RGB digital cameras are sometimes employed, where

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each sensor element generally has either a red, green or blue filter in front of it, and a simultaneous RGB image is acquired. More frequently, custom devices for spectral imaging are developed. They are mainly based on two approaches, i.e. using either appropriate filters or a dispersive element in front of the detector. For those systems using filters, interferential or tunable, a (small) number of monochromatic images are gathered, one for each transmission band chosen. The number of filters in the solutions proposed in literature varies from 7 to 32. The choice of these filters is a compromise between narrow-band filters, providing specific information within a spectral region, and broad-band filters, transmitting sufficient light intensity and preventing an impractical number of imaging modes. Typically narrow wavebands are about 10 nm, and wide bands about 40 nm. For systems of this type, the sensor may be a single element, a row of sensors (Ludwig 2004) or a two dimensional array (Imai et al. 2001), with a suitable optical system and a positioning device. For systems using imaging spectrographs, a line polychromator is coupled to a sensor array, and mounted on a moving device. In this way, the image is scanned and a simultaneous acquisition of the wavelengths of a painting line is obtained with high spectral resolution. With multi-spectral imaging it is possible to achieve higher colour quality (Sharma 1997), however the killer application is image spectroscopy, where the final goal is to achieve the spectral signature of each imaged element of the artwork. In the framework of one of the very first projects to tackle this subject (Martinez et al. 2002) filters were used. More recently, systems with 13 (Ludwig 2004), 18 (Bruni et al. 2002) and 32 (Carcagni et al. 2004) were designed. This sort of devices and methodology may also be used to record UV induced fluorescence images of paintings, one of the most significant branch of research, since this fluorescence emission lies in the visible range. The challenges implied in this sort of application are both on the image acquisition part, since the signal to be collected is rather low, and on the post-processing side which is necessary to correct the acquired data and eliminate the signal due to visible stray light. For all the systems described, the final spatial resolution depends on the number of active elements in the detector, on the optical path and its Modular Transfer Function (MTF), or on the sampling grid, if the system is realized with a single element sensor which is moved by translation slides and scans the acquisition area. For the systems using line polychromators, the horizontal and vertical resolutions are generally different, since they depend on the sensor and the optics and in the other case, on the dimension of the polychromator slit. The systems reviewed above show different performances, as far as the acquisition and the

post-processing required are concerned. Our aim was to develop a system and an associated image processing method, which could be easy to use (and not particularly expensive), and could still be used for reliable material recognition and localization. 2.2 Multispectral IR reflectography Some of these considerations hold also for imaging in the near IR range, or IR reflectography. This technique allows revealing the drawings hidden beneath the paint layers, and helps to determine the materials used in their creation, clarifying artists’ working methods. There are several devices currently used to acquire an image of the IR radiation reflected by a painting. They can be distinguished as the other imaging methods, by the attainable spatial resolution, the intrinsic geometrical distortion of each acquired image, and its dependency on illumination, but moreover they differ in the waveband characteristic of the sensor used, and therefore the “penetration” inside the paint layers which can be attained. Generally speaking, there are 2 digital devices which are quite spread, and 3 which are less used, for different reasons. The 2 popular methods are lead sulphide infrared Vidicon tube cameras and the Sony DSC-F828 (or similar) digital cameras with the night shot option. The 3 less common devices are: planar scanners with InGaAs sensor (single point or array), professional cooled CCD cameras and platinum silicide cameras. All these devices can be used to acquire a single large band, or several images corresponding to filtering the signal with interferential filters. Every method has advantages and disadvantages. Multispectral imaging systems in the near IR range are quite new, although they proved to provide fairly interesting results. 3 THE DEVELOPED METHOD Our goal was to analyze the spectral signature of the materials on the painting’s surface and to distinguish the areas within the image which have similar spectral signature and therefore similar chemical composition. To this purpose we developed a fast and reliable processing algorithm, as described in the next section. When the system chosen has a limited number of filters, still some problem of metamerism may appear, and the algorithm used may not be able to give a correct answer in all cases. In order to have a more efficient use of a system with a limited number of filters, we chose to process at the same time images acquired by different modalities. The application software we developed is able to analyze the spectral signature in the visnear IR range and the UV induced visible fluorescence image together with the visible reflectance.

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The software accepts 8 images for each of the 3 categories: Visible Reflectance: images record the reflectance of the artwork when illuminated by means of a white light, within spectral bands of 50 nm, spaced about 50 nm, from 400 to 750 nm; Near Infra Red Reflectance. representing the reflectance of the artwork when illuminated by means of infrared light, within spectral bands of 50 nm, spaced about 50 nm, from 800 to 1100 nm; UV Visible Band Fluorescence: representing the fluorescence of the artwork when illuminated by means of a high pressure mercury-vapour lamp, emitting in the UV part of the spectrum; such UV source excites visible fluorescence emission of most of the artwork materials. It is not necessary to have all of them as the software also works on a subset of images. 3.1

Estimates for the joint and marginal distributions can be obtained by simple normalization of the joint and marginal histograms of the overlapping parts of both images (Maes et al. 1997). The joint histogram hα (x, y) is obtained by binning the pixel intensity value pairs (X(p),Y(Tα (p))) for all the pixels in the overlapping region of X and Y . Since very often the registered pixel position Tα (p)will not coincide with a grid position, an interpolation of the reference image will be required to obtain the pixel valueY (Tα (p)). Next, the following distributions can be estimated:

Registration step

All the methods using an optical system in front of the actual sensor, produce very often data which are misaligned, due to the different length of the optical path. The goal is to compare all of these data, ensuring that each pixel on the various images correspond to the same point in the real artwork; thus, data images need to be correctly registered among each other. In our work we employed an automatic registration technique to determine the correct displacement (that is a geometrical transformation including subpixel translation, rotation and scaling) to align points from one image with the corresponding points of another image of the same object or scene (Cappellini et al. 2005, Zitovà & Flusser 2003). The registration technique is based on the computation of Mutual Information (MI), which is a similarity measure coming from the information theory. Mutual information is a measure of the amount of information contained in one image as compared with another one. The Maximization of the MI (MMI) approach states that the MI computed between two images, is maximum when the images are correctly registered. Given two images X and Y related by the geometric transformation Tα , with α such that the pixel p of X with intensity value x correspond to the pixel Tα (p) of Y with intensity value y, their MI is:

where pXY (x, y) is the joint distribution, pX (x) and pY (y) the marginal ones. The mutual information registration criterion states that the images are geometrically aligned by the transformation Tα∗ where:

By using these values in Equation 1 it is possible to derive the Mutual Information I (X , Y ), whose maximization will give us the optimal registration parameter α∗ . 3.2 The SAM algorithm In the developed method, the segmentation algorithm is based on the well known similarity measure called SAM – Spectral Angle Mapper, which has been extensively applied to hyperspectral information processing. For each pixel of the images under analysis, the value is considered as the component of a n-dimensional vector. Two vectors are judged to be close when the angle between them is small enough, i.e. it is smaller than a predefined threshold (Schowengerdt 1997, Keshava 2004). The angle between two vectors is computed according to the subsequent equation:

where x and y are the two vectors to be compared. This method is insensitive to illumination, since the SAM algorithm uses only the vector direction and not the vector length. However, since the multispectral images have been acquired in the same session or in sessions showing similar lighting conditions, all the images show similar illuminations; this means that the information concerning the vector length can be used as a further comparison term. Thus, the proposed technique allows the user to compare points both by a suitable chosen threshold on the angle and length. The vector related to the reference point is compared with all other vectors associated with each pixel

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within the image. All the points which are similar to the one selected by the user as reference point, are then labelled as belonging to the same class. In our implementation of the algorithm also the amplitude of the vectors, using a different and suitable threshold, is used in order to compute the similarity measure over a broader criterion. 3.3

Interface

Great importance has been given to the development of a suitable interface. Since the software is devoted to the analysis of artworks, which is often performed by restorers or, generally speaking, by people which are not IT specialists, the interface has to be simple and user-friendly. This implies that the user will need to perform only few actions in order to get the final result. On the other side, however, the user will still have the possibility of setting some parameters, related to the above mentioned thresholds, in order to increase or to refine the analysis. Here in brief how the system works. First of all, the user has to identify a reference image, which usually is a colour image of the artwork; this helps to identify the pigment (i.e. the reference point) the user wants to query about. After that, the user has to load the various images; the interface put at disposal a grid with a guideline, for instance, “Visible Band at 450 nm”. (see Fig. 1). Once all the images to analyze have been loaded, the user has the chance to select a point on the reference image, in order to point out the material of interest, or can set the coordinates of the point to analyze. The procedure starts automatically once the reference point has been identified. As a result, the segmented image is shown, where all the points similar to the selected one have been set as white, and also the reference point is highlighted with a cross shaped marker. As mentioned above, the user is also able to change the performance of the software, by setting a different threshold for the angle and enabling and setting a threshold for the amplitude (Fig. 2). 4

EXPERIMENTAL RESULTS

We tested our system on a set of samples of painting materials, as selected and realized by the Opificio delle Pietre Dure (OPD) in 1994. The samples have been spread on a wood panel (see Fig. 3) and their chemical composition is known. They are grouped attending to colour. We acquired the reflectance images, in the visible and near-IR range, and the UV fluorescence images. Figure 4a refers to the result of the test using only images in the visible range. Figure 4b when also the

Figure 1. Interface for the loading of the images to be analyzed.

Figure 2. Detail of the selectable tolerance for the angle and for the amplitude of the SAM analysis.

IR band is used and Figure 5 when the three modalities were jointly analyzed. As it is shown in the pictures, the localization of the painting material was much more reliable in the latter case. In particular there were much less false positive results. In case the IR range is also taken into account, the presence of the underdrawing is clearly influencing the results, since the paint layer is at least partially transparent to IR radiation. This system was used to acquire and process images of the painting ”Giannettino Doria” by Agnolo Bronzino (attributed), now in the collection of Palazzo del Principe in Genua (Fig. 6). The results of the analysis are shown in Figure 7, localizing a restoration intervention performed near the mouth. In Figure 8, it was possible to localize a restoration intervention on the background area whereas an intervention on the right arm was localized as well (Fig. 9). 5

CONCLUSIONS

We presented a multispectral image analysis method, and a software application, based on a slightly

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Figure 3. Panel with samples of painting materials (courtesy of OPD – Opificio delle Pietre Dure in Florence).

Figure 5. Result of the segmentation on the Cerulean Blue plus white pigment, using all the images which are at disposal (the visible reflectance, the infrared reflectance and the UV visible fluorescence images).

Figure 4. Result of the segmentation on the Cerulean Blue plus white pigment, using only the visible reflectance images (a) and using the visible reflectance images and the infrared reflectance images (b).

Figure 6. Bronzino Portrait of Giannettino Doria (oil on panel), Palazzo del Principe, Genua.

modified version of the SAM similarity measure method. Our target was to localize, within an artwork surface and in a totally unobtrusive way, materials presenting a similar chemical composition.

The input for our system consists of multispectral images acquired with a fairly limited number of quite broad wavebands filters (about 50 nm), both in the visible (8) and near infrared (7) range. In order to have a more efficient use of this acquisition system and of

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on the well known OPD painting materials samples, and on a painting by Agnolo Bronzino. ACKNOWLEDGEMENTS We are grateful to Alfredo Aldrovandi from Opificio delle Pietre Dure (Florence), for providing the pictorial materials samples on which the tests were carried out, and for useful discussions on this subject. We are also grateful to Laura Stagno, curator of the collection of Palazzo del Principe (Genua), for allowing us to present here the results of the investigation. Figure 7. Localization of a restoration intervention near the mouth (the visible reflectance, the infrared reflectance and the UV visible fluorescence images have been used).

REFERENCES

Figure 8. Localization of a restoration intervention on the background area (the visible reflectance, the infrared reflectance and the UV visible fluorescence images have been used).

Figure 9. Localization of a restoration intervention on the right arm (the visible reflectance, the infrared reflectance and the UV visible fluorescence images have been used).

Bruni, S., Cariati, F., Consolandi, L., Galli, A., Guglielmi, V., Ludwig, N. & Milazzo, M. 2002. In situ and laboratory spectroscopic methods for the identification of pigments in a northern-italy XIth century fresco cycle. Applied Spectroscopy 7: 827. Cappellini, V., DelMastio, A., DeRosa, A., Pelagotti, A. & Piva, A. 2005. An automated registration technique for cultural heritage images. Proc. of 8th International Conference on Non Destructive Investigations and Micronalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage, Lecce (Italy). Carcagnì, P., Patria, A. D., Fontana, R., Greco, M., Mastroianni, M., Materazzi, M., Pampaloni, E. & Pezzati, L. 2004. Realization of a new spectrophotometer for multispectral imaging and colorimetric characterization of pantings. Proc. of OPDIMON04 International Conference on Optical Diagnostics and Monitoring, Bacoli (NA), Italy. Imai, F., Rosen, M. & Berns, R. 2001. Multispectral imag ing of Van Gogh’s self-portrait at the National Gallery of Art Washington, D.C. Proc. of IS&T PICS Conference: 185–189. Montreal, Quebec, Canada. Keshava, N. 2004. Distance metrics and band selection in hyperspectral processing with applications to material identification and spectral libraries. IEEE Transactions on Geoscience and Remote Sensing 42: 1552–1565. Ludwig, N. 2004. Tecniche di spettroscopia d’immagine. Ph. D. thesis, Florence: University of Florence. Maes, F., Collignon, A., Vandermeulen, D., Marchal, G. & Suetens, P. 1997. Multimodality image registration by maximization of mutual information. IEEE Trans. Medical Imaging 16: 187–198. Martinez, K., Cupitt, J., Saunders, D. & Pillay, R. 2002. Ten years of art imaging research. Proceedings of the IEEE 90: 28–41. Schowengerdt, R. A. 1997. Remote Sensing: Models and Methods for Image Processing Academic Press. Sharma, G. 1997. Digital colour imaging. IEEE Transaction on Image Processing 6: 901–932. Zitovà, B. & Flusser, 2003. Image registration methods: A survey. Image and Vision Computing 21: 977–1000.

the images obtained, we include in the analysis the UV fluorescence multispectral images. In this way the performance of the system is noticeably improved giving reliable results, as it was shown in the tests performed

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Recovering colour and volume from relics in restoration tasks J. Finat, J.I. San José, J.J. Fernández, J.D. Pérez-Moneo, J. Martínez, F. Gutiérrez-Baños, L. Giuntini & F.M. Morillo Valladolid University, Valladolid, Spain

ABSTRACT: An accurate knowledge of the current status of art pieces, by means of Shape and Content Analysis, is a crucial aspect before planning and executing conservation or restoration tasks. Image-based and range-based are the most common non-contact approaches for the shape recording, including colour and volume information. In this work, we show some results coming from different recording devices (cameras and time of flight 3D laser scanners), different methodologies (image-based and range-based approaches) and their fusion in a common framework given by the software platform UvaCad. Its main contribution is the development of software tools for the joint management of geometric and radiometric dense information. The information fusion and its advanced visualization to different levels of detail are performed on a 3D support which can be modified interactively, allowing the user to supply lacking elements in the current state. We illustrate our approach on surveying and proposing interventions for old mural paintings in a wall of the church of Wamba, a small village in Valladolid, Spain. The developed applications are applied in restoration tasks in the diagnostic, monitoring and simulation of interventions, as a component of more advanced cultural heritage Information Systems.

1

INTRODUCTION

A general problem for surveying cultural heritage is related with the gap in concerns and needs between information users (conservation specialists and managers) and the information providers (photogrammetrists and cultural heritage recorders) (Letellier et al. 2002). Digital formats provide a durable support for inserting different kinds of multimedia contents, and for allowing remote consultation, analysis and diagnosis of damaged pieces. The incorporation of laser devices provides very accurate navigable global three-dimensional models where analysis tools can be applied. All of them configure an Advanced Visualization Framework (AVF) for integrating methods and procedures developed by different sources and information providers. The AVF can be applied for identifying, tracking and monitoring degradations. The goal of this AVF is to assist the analysis, planning and implementation of the most appropriate strategies for conservation and restoration tasks, following a user centred methodology. The multidisciplinary character of conservation and restoration tasks requires the development of a flexible and modular Information System able to a) adapt itself to the whole piece, despite irregularities or structural defects with respect to a model, and b) fulfill the requirements of different experts (such as interactive annotations) which facilitate collaboration, communication and information sharing

as a first step. Databases of simple geometric entities (such those appearing in typical CAD software tools) are not enough for the high complexity of objects which must be managed by an information system of cultural goods. The design and implementation of a common software platform can be developed by following a methodology which is similar to threedimensional geographic information systems, able of combining top-down (arising from models) and bottom-up (arising from features) approaches, but in a collaborative environment, able of integrating geometric/architectural data and interpretations linked to context (as metadata). In a geometrically oriented digital information system, any information relative to 2D or 3D objects is referred to a coordinate system as an additional information layer. In particular the complex spatial object supports different layers of information including geometric, iconic and semantic aspects, following increasing complexity criteria. Their geometry provides a precise and robust support for annotating and simulating possible interventions in terms of iconic aspects linked to well-documented models. Thus, geometrical aspects are the easiest of recognizing and semi-automatic annotated labelling is meaningful for iconic representation. However, the lack of a commonly accepted semantics for cultural heritage interventions is an important bottleneck for the interoperability between different advanced visualization frameworks which can be useful for conservation

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and restoration purposes. Their integration is not an easy task, because it involves dealing with different sources in digital repositories, different image-based or range-based hardware devices for information capture, different software tools for information processing working on different 2D and 3D supports and different methodologies answering demands arising from different experts. Some related important issues require higher developments of: 1) an object-oriented approach capable of managing the very large morphological diversity of digital information with cultural heritage contents; 2) image-based or range-based models for cultural heritage surveying including a photogrammetric network; 3) interoperability solutions between software tools for processing, integrating and extracting information; 4) a Semantic Service Oriented Approach (SSOA) focused towards cultural heritage interventions. The two first points are well-known, with a generalised agreement for 3D modelling as the best option based in discrete (dense clouds of points) or continuous models (superimposed structures to clouds of points given by meshes or smooth surfaces). The UvaCad platform developed by the DAVAP cluster of the University of Valladolid (Spain) provides a multi-scale adaptive modular solution with increasing functionalities for cultural heritage issues for the third point mentioned above. A general framework for SSOA is provided by Semantically Oriented Cultural Information Systems (SOCIS). A SOCIS requires 1) a knowledge of a specific lexicon for knowledge domain; 2) an agreement on thesauri between experts of different countries to be used in different tasks (documentation, diagnostic, monitoring); and 3) a development of taxonomies for identifying and generating relations based on pieces and their context (design and implementation of expert systems for assisting documentation, conservation and restoration tasks). Once again, the main problem is the lack of interoperability between software tools. This problem involves formats, software processing tools and embedded contents and its solution is far from being reached. This work is mainly focused towards interoperability between image- and rangebased cultural heritage Information Systems (CHIS) and software tools (CAD) for assessing conservation planning. The UvaCad methodology (UvaCad 2007) provides software tools for 3D modelling, navigating and information extraction in architectural surveying tasks. By way of illustration, it is applied in this work to an architectural Mozarabic context and a simulated restoration of original mural drawings from surviving relics of Pre-Romanesque style in the visigothic church of Wamba, a small village in Valladolid, Spain. The resulting laser-based 3D model can be navigated and modified according to the analysis of

different experts (high level information users) which are the primary stakeholders for the application development. This work is organised as follows. Paragraph 2 is devoted to the architectural framework, providing the architectural context for paintings. Paragraph 3 provides some key aspects for the understanding of deteriorated mural paintings. The methodology performed for a multilayered 3D surveying is developed in paragraph 4. Finally, paragraph 5 shows the proposed reconstruction. We conclude with some final remarks and comments about the work in progress. 2 TOWARDS AN INTEGRATED ARCHITECTURAL FRAMEWORK An integrated architectural framework includes geometric, structural and historical aspects. The architectural surveying is the key point for integrating different aspects and providing the support for a high level semantic approach. Geometric aspects of architectural surveying provide a low-level interpretation relative to morphological and visible stylistic aspects, from iconic description based in models. Structural aspects provide information for identifying the role of different components in the original construction, and for analyzing, planning and executing conservation or restoration interventions for a secure and robust recovery of the fabric. Historical aspects are crucial for understanding the cultural influences, functions and the use (semantic aspects), not only of components, but also the whole building in its environment. However, sometimes only some vestiges are preserved, and thus a reinterpretation is required in terms of better conserved vestiges corresponding to other buildings with similar style. In this case, it is necessary a comparative study for advanced visualization including a magnified reality. Mozarabic art is developed by Christians coming from the areas of the Iberian Peninsula occupied by Muslims, or inspired on Hispano-Moresque stylistic features. This leads us to comment briefly the political situation of Spain in the 10th century, i.e., at the time when the church of Santa María de Wamba was erected. Christian kingdoms live a complex and even anarchical situation from the political, social and artistic points of view. It is the time of the maximum splendour of the muslim domination, but it is also a time in which christian kingdoms start gaining territory. In these newly conquered territories christians lived formerly with the muslims, or christians transferred to these territories from areas still dominated by the muslims. These christians, the same as their liturgy, writing or art, are called ‘mozárabes’ (mozarabic). The Church of Santa María de Wamba (Fig. 1) was founded by Abbot Fruminio, former Bishop of Leon, retired in this place, in 928. The chevet and transept

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Figure 2. Wall painting from the church of Santa María de Wamba. Figure 1. General scheme of the Church of Santa María de Wamba.

belong to the Mozarabic style, but the rest of the church, including façade and nave, was rebuilt in the romanesque era. First of all, the oldest part of the church consists of a triple apse arrangement of square plan and also of a transept of three corresponding sections. The original plan would have included an additional section preceding the transept and with three narthex or entrance lobbies. It shows all the distinguishing signs of the Mozarabic architecture: a) Horseshoe arch, of Hispano-Moresque (and, at last, Visigothic) tradition. b) Barrel vaults in the transept. In the presbytery the barrel vault is generated by a horseshoe arch. c) Proportions: the arches are relatively low, not surpassing the double of their wide; however, the vaults raise up to three time the width of the ships. d) Scarce decoration: there is almost no sculpture, although the walls were decorated with paintings. The oldest part of the church was reconstructed in 1996 due to structural problems.

3 AN ICONIC APPROACH FOR GUIDING RECOVERY AND RESTORATION TASKS The paintings on the east wall of the presbytery of the church of Wamba were conceived as a fictitious hanging in order to dignify outmost of the area behind the altar (i.e., the most sacred area of the church) at a time when no altarpieces or sculpted images were in use. These paintings imitate a medallion pattern textile, a sort of luxurious silk textile that originated in the Sassanian milieu and that was perpetuated in the Byzantine and Islamic world. Their quality and design, together with their exotic origin, justified the high esteem of these products in Western Christendom during the High Middle Ages. Imitations of medallionpattern textiles are known in the Iberian Peninsula from

the Visigothic era (relieves) to the 13th century (wall paintings). The murals in Wamba have been ascribed (although not unanimously) to the 10th century, to the period of the so-called Mozarabic art, and are especially outstanding as one of the few samples of wall paintings prior to the Romanesque period. The bad conservation state of the wall paintings has required a careful selection of illumination and the alignment of several views for composing the highresolution photo-mosaic displayed in (Fig. 2). The wall paintings of Wamba consist of two registers framed by a lozenge-pattern. Each register is divided in five rooms. Apart from the axial rooms (to be discussed after), all other rooms contain medallions differently designed, but always employing the same motifs: dots, circles, chevrons, lozenges, etc. The reconstruction of all eight medallions causes no problem, but the restoration of their content is much more difficult. Their content is preserved only in the fourth room of the upper register (a geometric flower of eight petals) and in the second and fourth room of the lower record (a beast). A comparison with other medallion pattern textiles and with the Spanish art of the Early Medieval period provides arguments for a reasonable restoration proposal. In the upper register, the geometric flower of eight petals has been copied in the first, second and fifth rooms, so creating a serial pattern comparable to the one to be found in one of the sculpted friezes of the Visigothic church of Quintanilla de las Viñas (although some remains in the second room make this proposal conjectural). In the lower register, two beasts have been copied in the first and fifth rooms, but symmetrically with respect to the two beasts preserved, so creating a pattern of facing animals, very common since the time of the Sassanian and their predecessors (and perfectly known in the Iberian Peninsula, as many Visigothic relieves prove). Some pigments remaining in the fifth room suggest that these figures of animals were coloured in red. The axial rooms of wall paintings are much more difficult to restore. The lower room is shown in a former reconstruction by Martín González (Martín González 1966) as displaying a cross, but the remains

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requires an advanced graphics user interface for the joint management of geometric, iconic and structural aspects in a global semantic approach.

4

Figure 3. Chrismon from a niche-plaques of the Visigothic era, used in the reconstruction of the central element in the wall painting.

do not actually fit and this hypothetical cross is quite different from all other crosses known from Early Medieval Spain. Having just followed the guidelines provided by the remains of the wall paintings, we have to confess that our proposal is not completely satisfactory but, by now it is not possible to make better-founded assumptions. Much more difficult was the case of the upper room. It was completely destroyed some centuries ago on the occasion of the enlargement of the axial window of the presbytery. Nowadays, the wall has been restored to the original condition of the 10th century, but it was impossible to offer something for the gap in the case of the wall paintings. So we were forced to risk a proposal, even assuming its purely conjectural character. Considering the prominent position of this room, our decision was to include a especially significant motif and, having in mind the so-called niche-plaques of the Visigothic era, usually carved with a chrismon (Fig. 3) and sometimes considered as having been used to monumentalize altars, we decided to place in this room a chrismon (also in correspondence with the supposed and problematic an iconic tradition of Early Medieval Spain). The one finally drawn is copied from a plaque from Toledo (Palol 1968). Next problem is how to insert and display this iconic information in the global 3D model following advanced visualization principles referred to vector information (coordinate systems). This integration

METHODOLOGICAL ASPECTS FOR RANGE MODELLING

Data capture is performed with different kinds of cameras and 3D laser devices for image- and range-based approaches. A large number of high resolution views have been taken and merged in a mosaic after correcting distortions corresponding to perspective effects, so it is possible to obtain a mosaic for surveying zones with different pathologies, such as partial occlusions or impossible scanning. Relative to range-based approach, three kinds of laser devices have been used: a time-of-flight Ilris 3D (Optech) for global external structure, a difference-of-phase Faro for internal mid-level data and a triangulation-based Minolta 910 for the most accurate data relative to small details in mural paintings. Different clouds of points are patched together by means of manual identification of a low number of control points. The accuracy range of the global model varies from 2 cm to 0.01 mm depending on the applied laser device and the chosen resolution along the capture. The UvaCad software platform allows managing and merging the local and global models with different resolutions. The discrete global 3D model is supported on a dense cloud of points with a non-uniform density. Different geometric structures can be superimposed on the discrete models giving triangular meshes or smooth surfaces. Two important contributions are the software modules for reducing information (proportional or adaptive sampling) or increasing information (re-projection of high resolution views) depending on the desired resolution. An advanced visualization module has been developed in UvaCad for different kinds of applications such as interactive navigation, projective operations (sections and projections) for generating high resolution CAD planimetry (including interactive drawing for improving lacking details), and extraction of metric or geometric information relative to different objects of the model. Metric information is referred to a coordinate system which is centred on the object, not the observer. Geometric information allows comparisons between the real objects and ideal simple objects (currently planes and some simple squares) for identifying structural defects based on the difference with the expected shape. This module has been successfully used for analysis, planning and execution of interventions performed in several buildings of the region (San José et al. 2007). The fusion of global image- and range-based information is performed as follows. First, the cloud of

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shape and structure, and the management of advanced software tools for interactive drawing and simulation of substantial modifications before deciding their applications. Next step involves the development of an integrated information system for surveying and management of cultural goods following a semantic approach for incorporating iconic and historical aspects as a whole. 5

Figure 4. Different working layers cloud of points, rectified photography and drawings.

points must be reoriented according to the highresolution rectified mosaic. Secondly, a low number of homologue control points are manually selected in the 2D mosaic and the oriented view of the 3D model in order to finally re-project the 2D mosaic on the global 3D model. The lifted view of the 3D model inherits its metric information. In this way, we generate an augmented three-dimensional (A3D) model which incorporates image information avoiding the advanced technicalities of 3D reconstructions based in computer vision. The A3D model can be visualised with different levels of detail using the basic functionalities described above (navigation, consult and intervention planning). The A3D model provides the support for superimposed continuous structures (triangulations, meshes and smooth surfaces), allowing the insertion of labels, comments or diagnosis in linked cultural heritage information systems. The UvaCad software allows an interactive visualization of regularly distributed slices for surveying structural aspects of the building. Such slices show corresponding perpendicular planes to a direction selected interactively by the user. In this way, it is possible to perform monitoring and interventions planning with a relatively low cost and with a variable resolution. Furthermore, different layers on particular elements can be annotated and superimposed to the A3D. In this way, real and restored data corresponding to the wall paintings can be visualized as superimposed layers projected on a 3D model with different resolutions (Fig. 4). The availability of a high resolution A3D model presents multiple functionalities which are interesting for experts and citizens in general. Indeed, this model provides a recreation based in a non-intrusive low-cost visualization about an ideal restoration for everybody, and it allows identifying mutual influences between geographically distant cultural traditions. For experts, it allows the identification of problems relative to the

FIRST STEPS TOWARDS AN INTEGRATED INFORMATION SYSTEM

Due to multiple factors, large-scale integration of information systems with cultural heritage contents poses many kinds of problems, including legal (copyright), organizational (hardware and software architectures, repositories) and technical (digital conservation and interoperability between tools and repositories) issues. If we limit ourselves to low-scale integration relative to a building, the main problems are technical ones. Even in this case, integration can involve quite different aspects such as the changing nature of related cultural heritage information, the design and implementation of software tools with different users interacting on local and remote repositories, or the interoperability between different software tools applied to the same object. GIS are increasingly used for cultural heritage. However, most GIS-based approaches take as unit a territorial environment (see Petrescu 2007 and references therein) and often there are not enough collaborations, making difficult the contents creation. Our approach is focused towards the application of extended GIS principles to a building. The main contribution of GIS arises from software tools for different layers which are superimposed to a 3D object. Most of these layers involve the visible spectrum; but we have incorporated a magnified module supporting a visualization of possible interpretations linked to the conserved archaeological remains or reliques. The visualization of these interpretations has been added after a comparative iconic and historic study and it improves the understanding of possible reconstructions of the original state. The visualization is intentionally remarked in a quite different colour, but due to the digital nature of the support, it is possible to recover the original texture and reconstruct it on the wall. Furthermore, the geometric nature of the support allows the addition of layers corresponding to the non-visible spectrum with information arising from non-intrusive techniques (Fig. 5). 6

CONCLUSIONS AND FUTURE WORK

The standardisation in conservation and restoration tasks requires a semantic service-oriented approach,

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mixed reality technique can be labelled as interactive reconstruction and is being integrated in the UvaCad platform for conservation and restoration interventions on buildings. The problem is not of elemental nature, because usual design tools (CAD) do not support clouds with hundred of thousands of 3D points such those appearing in cultural heritage surveying. The CHIS which is currently being developed in the DAVAP cluster, incorporates tools for the distributed and remote management of database with 2D/3D geometric support. In the next future, integrated information systems will incorporate software tools for the in situ and management of semantic information relative to surveyed objects. ACKNOWLEDGEMENTS This project has been financed with funds from the Spanish Ministry of Science and Education R&D project MAPA COD VIA2004-08392-C02-01.

Figure 5. Projection of the reconstructed wall painting in the original cloud of points and perspective view.

including the design and implementation of modular architectures and distributed devices (technologies, interfaces) for supporting them. The UvaCad software platform provides a scalable and multi-resolution environment for navigation, consultation, superposition of high-resolution views on dense clouds of 3D points. Furthermore, this software platform provides a support for larger integrated cultural heritage information systems including the management of data and metadata and the interoperability with CAD software for assisting the design of restoration tasks. The proposed strategy has been applied to the mural painting of the church of Santa María de Wamba. The geometric nature of motives appearing in walls has contributed to visualize an ideal reconstruction and to set out some guidelines for restoration having into account such ideal reconstructions from the available colour and volumetric information. Next step involves the development of tools to improve the magnified models by means of development of software tools for planning, simulating, visualizing and evaluating in an easy way more complex interventions on cultural heritage objects. This

REFERENCES Letellier, R. & Gray, C. 2002. Bridging the Gap between Information Users and Information Providers, Recording, Documentation and Information Management (RecorDIM) Initiative, GCI-ICOMOS-CIPA, Roundtable, Los Angeles, Californa, March 2002 (available at www. getty.edu/conservation/publications/pdf_publications/recordim.pdf). Martín González, J. J. 1966. Pintura mural de la iglesia de Santa María de Wamba (Valladolid), Bol. Seminario de Estudios de Arte y Arqueología 32: 435–436. Palol, P. 1968. Arte hispánico de la época visigoda, ed. Polígrafa (Biblioteca de Arte Hispánico), Barcelona. Petrescu, F. 2007. The use of GIS technology in Cultural Heritage, XXI Intl CIPA Symposium, CIPA, Athens, Greece. SanJosé, J. I., J.J.Fernández-Martín, J. J., Pérez-Moneo, J. D., Finat, J. & Martínez-Rubio, J. 2007. Evaluation of Structural Damages from 3D Laser Scans, XXI Intl CIPA Symposium, CIPA, Athens, Greece. UvaCad Software Platform, 2007. DAVAP cluster, MoBiVA Group, Laboratory of Architectural Photogrammetry, University of Valladolid, http://uvacad.no-ip.org.

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Multi IR reflectography R. Fontana, M. Greco, M. Mastroianni, M. Materazzi, E. Pampaloni & L. Pezzati Istituto Nazionale di Ottica Applicata (CNR – INOA), Firenze, Italy

P. Carcagnì Istituto Nazionale di Ottica Applicata (CNR – INOA) – Sez. di Lecce, Arnesano (LE), Italy

ABSTRACT: Optical techniques are widely disseminated in the field of painting diagnostics because of their effectiveness and safety. Among them, infrared reflectography is traditionally employed in painting diagnostics to reveal features underlying the pictorial layer thanks to the paint layer transparency to NIR radiation. Highresolution reflectography was introduced in the 1990s at the Istituto Nazionale di Ottica Applicata, where a prototype of an innovative scanner was developed, working in the 900–1700 nm spectral range. This technique was recently improved with the simultaneous acquisition of the reflectogram and the color image, perfectly superimposed. Here we present a scanning device for multi-spectral IR reflectography, based on contact-less and single-point measurement of the reflectance of painted surfaces. The back-scattered radiation is focused on square-shaped fiber bundle that carries the light to an array of 14 photodiodes equipped with pass-band filters so to cover the NIR spectral range from 800 to 2500 nm.

1

INTRODUCTION

Infrared reflectography is one of the most suitable optical techniques traditionally employed in nondestructive diagnostics of ancient paintings (Van Asperen de Boer 1966, 1968). Generally applied to panel, canvas and wall paintings, this analysis can show aspects of a painting underneath the visible surface thanks to transparency characteristics to NIR radiation (0.8–2 µm) of the materials composing the paints. The technique consists in irradiating the painting with halogen lamps, and in detecting the backscattered radiation with a suitable device; the acquired image is called the reflectogram. When performing multi-spectral imaging, radiation is detected within spectral intervals by means of filters or spectrographs. The basic concept of multi-spectral imaging is that differences in material composition of any particular object often become much more apparent when the object is viewed in a wavelength region other than normal visible light. Where two materials might have the same characteristics for the reflection or absorption of light in the visible wavelength region, it is highly unlikely that this is also the case in the infrared region. Infrared analysis is thus able to show any variation during the artwork preparation (the so called pentimenti), the underdrawing, the presence of restoration interventions and repainting carried out by means of modern pigments, fills where the original paint is lost,

and, generally speaking, the conservation state of the artwork. This technique gives, thus, precious indications both on the realization phases and on the actual state of the artwork; furthermore it helps dating the painting and in some cases it can confirm or deny the attribution to an artist (Strehelke et al. 2002). When a painting is illuminated with a broadband continuous source (white light), the VIS part of the spectrum is reflected by the paint layer, whereas the IR radiation can pass through it, being reflected by the preparation surface and absorbed by the underdrawing, depending on the materials used. Underdrawing visibility is a function of the transparency of the paint layers to NIR radiation and of the underdrawing contrast: IR absorption is high when carbon is present in the drawing (charcoal, graphite, carbonaceous pencils and inks), otherwise in presence of iron-gallium inks the underdrawing can be invisible, even if the paint layer is transparent. Reflectivity is high when the preparation is chalk-and-gypsum based. Transparency depends on the composition and depth of the paint layers, and on the radiation wavelength used. Paints transparency generally increases with wavelength: for λ ∼ 1.6–1.7 µm and paint thickness ∼ 0.1 mm, nearly all ancient pigment compounds are at least partially transparent. It is in 1990 that the first prototype of scanner based on single-point detection is developed at the National Institute for Applied Optics (Bertani

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et al. 1990). This innovative device, equipped with an InGaAs photodiode, is capable of acquiring reflectograms with a spatial resolution of 16 points/mm2 and a tonal dynamic that are unparalleled by other techniques traditionally used for infrared reflectography. Besides that, point-by-point surface sampling assures uniform lighting and entails off-axis aberration free images (Falletti & Nelson 2002). Its performances were recently improved with the addition of a three-channel head for the simultaneous acquisition of the RGB color image, perfectly superimposing the reflectogram (Fontana et al. 2003). In this work we present a multi-spectral scanner, to be considered as an “upgrade” of the INOA’s old scanner, for multi-band IR reflectography. Its working principle is based on a point-by-point acquisition of the surface under study, differently from acquisition with imaging detectors where all single elements (pixels) are acquired simultaneously. The use of a single sensitive element solves a series of problems with respect to traditional detection systems, such as uniform lighting of the sampled area, chromatic aberration, geometric deformations, non-uniform detector response, luminosity fall-off from the centre to the border of the image due to the lens. Its drawbacks are instrument huge dimensions and weight, and low acquisition rate. When choosing the proper instrument for IR reflectography the following parameters should be considered: instrument price and portability, spectral sensitivity (the wider the spectral range, the more contrasted and rich of details is the reflectogram), spatial resolution, acquisition and processing time for the reflectogram of the whole artwork under investigation. Depending on the aim of the analysis, one is to be enhanced with respect to the other.

2

INSTRUMENTS

The block diagram of the scanner is shown in Figure 1; the scanning system moves both the lighting system and the collecting optics. This latter is connected to an optical demultiplexer that carries light to a detector array (split into two modules, a VIS spectrophotometer and a NIR spectrometer), and the whole system is computer controlled. Lighting system and collecting optics are placed in a 45◦ /0◦ illumination/observation geometry as suggested by CIE for reflectance measurements. The lighting system is composed of two low-voltage current-stabilized halogen lamps (20 W) irradiating an area of about 5 cm2 . One of them has a 4100 K colour temperature in order to enhance the blue emission that is critical when measuring with the visible module, and a dichroic coating transmitting backwards large part of the generated IR radiation, thus minimizing the heating of the painting surface. The other lamp is a standard

Figure 1. The diagram of the multi-spectral IR scanner.

halogen lamp with an aluminium back-reflector. The beam divergence is, in both cases, 10◦ according to CIE specifications for the 45◦ /0◦ illumination/observation geometry. The collecting optics is a catoptric objective lens made of two faced spherical mirrors, thus, the chromatic aberration is equal to zero. The working entrance f# is 3.7, corresponding to an acceptance angle of less than 16◦ (according to CIE specifications); the effective f# is 4.7 because of the secondary mirror darkening. The working distance is about 12 cm with a depth of field of ± 750 µm and a unitary magnification. The collecting optics gathers the radiation scattered from the measured point on the painting and focuses it on a 16-fibres squared-shaped bundle that carries the light to the detector array. The fibre used is a multimode optical fibre with core and clad diameter of 200 µm and 230 µm, respectively, with transmission in the UV-VIS or VIS-NIR spectral range depending on the module to which it is connected: 14 fibres carry the NIR signals to an array of photodiodes, one of the remaining two is used to carry VIS signals, and the other one will be used either to extend the device capabilities in the UV spectral region or as reserve. Fibre light guiding minimizes the load on the scanning system, thus limited to the lighting system and the collecting optics whose simultaneous motion limits surface heating and, together with point-by-point surface sampling, avoids any problem concerning uniform lighting. The termination of the optical fibre bundle is lodged on the image plane by means of a purposely made connector. The detection system is a detector array composed of 3 Si and 11 InGaAs photodiodes, each equipped with an interferential filter kept in a filter holder, designed to have the best matching between the detector sensitive area and the fibre. The filter FWHM bandwidth ranges from about 50 to 100 nm, except for the first channel that has a 10 nm FWHM filter in order to match the spectrum obtained with the VIS module. The last channel is wider than the others because of the poor photodiode efficiency in that region. The overall spectral response covers the range from 800 to 2300 nm

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Figure 2. Panel painting by an unknown Florentine author: (a) picture, (b) detail of the measured area (white rectangle), (c) NIR multi-spectral scanner during measurements and (d) the 14 acquired images.

(the fibre cutoff sets the upper sensitivity limit). In the region beyond 1700 nm, due to the low signal and to the high dark current typical in this spectral region, thermoelectrically-cooled InGaAs sensors are used. These sensors require a heat sink to dissipate the heat generated by the cooler, an amplification circuit and a temperature controller to hold the detector at a constant temperature. As the instrument is supposed to be used for in situ measurement campaigns, compactness and robustness are important requirements, in order to be easily transported. A purpose-build sub-rack unit was then designed to keep all the detectors together. It also comprehends a power supply unit, a pre-amplification and offset circuit, a sample-and-hold board and an ADC (Analog-to-Digit Converter). The XY scanning system, composed by two high-precision motorized translation stages mounted orthogonally, allows to measure continuously areas up to 1.5 m2 with a spatial resolution of 16 dots/mm2 . The acquisition time for 1 m2 area is of about 8 hours at typical acquisition rate of 500 Hz. In the acquisition phase our scanner is probably time-consuming compared with most of existing devices, but this time loss is regained because data do not need neither

geometrical nor intensity corrections, and thanks to hardware registration the 14 monochromatic images are automatically superimposed. The digitized data are recorded and handled via a dedicated software running on a standard PC and can be displayed as 14 monochromatic images. Storing the images in a 8-bit standard format can be chosen either to convert all the grey scale, thus compressing the intensity resolution, or to convert a limited part of the grey scale, to have the highest intensity resolution in selected parts of the image. An auto-focus system, besides keeping the painting surface focused during the scanning even in case of irregular surfaces, will allow the simultaneous acquisition of the painting shape. Channel equalization was made on a reference standard surface with a certified reflectance of 40%, and the very first laboratory tests were carried out on a painting that is presently under restoration at the Opificio delle Pietre Dure in Florence, where the Art Diagnostic Group has the operative laboratory. 14 multi-spectral images were acquired on a detail of a panel painting of the XV century (unknown Florentine author) and they are shown in Figure 2, together with a picture of the painting.

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Figure 3. Virgin with Child (Cimabue), detail of the Child’s head. a) Reflectogram obtained with the IR scanner; b) CH2 image at 850 nm; c) CH14 image at 2265 nm; d) CH14-CH2 and e) CH14/CH2.

3 APPLICATIONS The first multi-spectral acquisition in the NIR spectral region was carried out on a panel painting by Cimabue (second half of XIII century) representing the Virgin with Child. The painting, that is under repair at the Opificio delle Pietre dure in Florence, was subjected to a series of analyses that are currently part of the diagnostic investigation process. The 14 monochromatic images were analyzed separately, and pixel-by-pixel differences and ratios between different wavelength images were carried out in order to highlight details that are not visible in the reflectogram obtained with the INOA old scanner. A few examples are shown in Figure 3, where a detail of the Child’s head is shown. In the reflectogram are clearly visible two light spots on the Child’s forehead that are present also in the image at 850 nm, whereas they are absent in the image at 2265 nm, where a mark of the golden leaf appears as a white spot above the eye. Considering, for instance, channels number 14 and 2, and making the pixel-by-pixel difference and ratio, all the above mentioned details are clearly visible both in the difference and in the ratio image. Moreover, the decoration in the Child’s halo, that was scarcely visible

in the separate channels, becomes here evident. In the ratio CH14/CH2 image a mark of a non-homogeneity along the Child’s arm makes its appearance. In Figure 4 the detail of the Child’s arm is presented: similarly to what happened in the previous example, in the CH2 and CH14 images different spots and retouched regions are evident on the arm and on the hand, and in this latter case the spot is invisible in the reflectogram. Once again, by making the difference and the ratio images all the described details become visible and they are enhanced. Concerning the information contained in the ratio image, we evaluated the contribution of the different channels in order to emphasize different details. In Figure 5 the detail of the Virgin’s bust is presented, taking into account the ratio image obtained with channels number 14 and 2, and channels number 10 and 3. In the CH14/CH2 image, for instance, a disuniformity in the Virgin’s mantle on the right shoulder is enhanced, whereas dishomogeneities in the mantle on the Virgin’s head are highlighted in the CH10/CH3 image, as well as the spots present on the Virgin’s face. The complementary information in the two ratio images helps also to study and characterize the big retouch that crosses the painting along its width.

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Figure 4. Virgin with Child (Cimabue), detail of the Child’s arm. a) Reflectogram obtained with the IR; b) CH2 image at 850 nm; c) CH14 image at 2265 nm; d) CH14-CH2 and e) CH14/CH2.

Figure 5. Virgin with Child (Cimabue), detail of the Virgin bust. a) Reflectogram obtained with the IR; b) CH14/CH2; c) CH10/CH3.

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4

CONCLUSIONS

In this work we have presented a scanning device for multi-spectral IR reflectography, based on contact-less and single-point measurement of the reflectance of painted surfaces in the spectral band ranging from 800 to 2300 nm. The instrument acquires the radiation back-scattered from a painting surface in 14 spectral bands, with a FWHM bandwidth of 50–100 nm. The simultaneous motion of the lighting system and the collecting optics limits surface heating and, together with point-by-point surface sampling, assures uniform lighting. Moreover, single point detection entails off-axis aberration free images. Thus, even if in the acquisition phase our scanner is probably timeconsuming compared with most of existing devices, the time loss is regained because data do not need neither geometrical nor intensity corrections, and thanks to hardware registration the 14 monochromatic images are automatically superimposed. Extending the spectral sensitivity beyond 1.7 µm, that is the cut-off wavelength of the old INOA scanner, seems to add interesting information about the painting status. A few examples of applications were then presented, as well as a very first data handling. Pixelby-pixel difference and ratio images were calculated to enhance the presence of retouches and re-paintings. As the main drawback of our system is heaviness, a new mechanics will be designed. The fulfilment of

a new bundle is also planned so to make the procedure repeatable. We are studying a method for producing bundles with fibres aligned within a 3 micron error. An auto-focus system is at present under development, that will keep the surface of the painting focused during the scan, thus allowing the acquisition even in case of irregular surfaces, and will simultaneously provide a measurement of the surface shape. REFERENCES Bertani, D., Buzzegoli, E., Cecchi, S., Cetica, M., Kunzelman, D., Poggi, P. & Puccioni, P. 1990. A Scanning Device for Infrared Reflectography. Studies in Conservation 35: 113–115. Falletti, F. & Nelson, J.K. 2002. Venus and LoveMichelangelo and the new ideal of beauty. Firenze Italy: Giunti Gruppo Editoriale. Fontana, R., Gambino, M.C., Greco, M., Marras, L., Materazzi, M., Pampaloni, E., Pezzati, L. & Poggi, P. 2003. New high resolution IR-colour reflectography scanner for painting diagnosis. Proc. SPIE, 5146: 108–115. Strehelke, C.B. & Frosinini, C. 2002. The Panel Paintings of Masolino and Masaccio. The role of techniques. Milano Italy: 5 Continents publisher. Van Asperen de Boer J.R.J. 1966. Infrared reflectograms of panel paintings. Studies in Conservation 11: 45–46. Van Asperen de Boer J.R.J. 1968. Reflectography: a Method for the Examination of Paintings. Applied Optics 7: 1711–1714.

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Miscellaneous

Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Imaging and mass spectrometry of microparticles generated during surface decontamination of an ancient parchment sample by laser radiation R. Wurster Universität Hohenheim, Institut für Physik und Meteorologie, Stuttgart

S. Pentzien Bundesanstalt für Materialforschung und – prüfung, Berlin

W. Kautek Universität Wien, Institut für Physikalische Chemie, Wien

ABSTRACT: Both identification of artworks and monitoring the laser cleaning process of artworks can be based on the investigation of microscopic and submicroscopic particles released from the contaminated and degraded surface of an artwork. The microparticles were passively collected on ultrathin supporting foils covering electron microscopical grids. Single particle characterization was performed by means of high resolution scanning electron microscopy and laser microprobe mass spectrometry. At least three different particle types could be clearly discriminated using morphological and mass spectrometric features.

1

INTRODUCTION

Laser matter interaction causes the release of a great variety of signals, e.g. electromagnetic radiation, electrons, ions, atoms, molecules, clusters, liquid and solid microparticles, that may be analytically or technically exploited. Optical emission spectrometry of the electromagnetic radiation or mass spectrometric detection of the ions on the one hand provide a deep insight into the physical processes of the laser target interaction (Bäuerle 2000) and into the actual state of a running laser treatment of a sample surface. On the other hand, thin films having a tailored composition can be produced by pulsed laser deposition of the ejected gas and plasma plumes. Disruption and recondensation of the material also can yield micro- and nanoparticles that may have quite different or similar and stoichiometric identical compositions as the targeted bulk material strongly depending on the pulse duration (ns and fs regime) (Lee et al. 2004, Ostrowski et al. 2007, Barcikowski et al. 2007). As microparticles and nanoparticles can be incorporated in the human respiratory tract, health risk assessment during laser cleaning also has become an essential issue. In situ monitoring the laser decontamination process of surfaces of artworks still is an important and difficult object. Laser induced breakdown spectrometry (LIBS) can be a straightforward and smart solution (Winefordner et al. 2000). A less direct method aiming

at the characterization of laser generated microparticles that uses both microscopical and microanalytical methods may also be a meaningful and sparkling source of information. A case study on a delicate ancient parchment was performed by help of a computer controlled laser cleaning system (Kautek et al. 2005). Particle sampling and microanalytical characterization followed a well established procedure successfully used in aerosol research (Kaufmann et al. 1980).

2

EXPERIMENTAL

Figure 1 schematically shows the experimental setup of the laser cleaning station as being combined with a particle collection device. The cleaning laser (Nd-YAG) was operated in its frequency doubled mode at λ = 532 nm. A total integrated radiation flux of 0.5 J was scanned across an area of 5 mm × 5 mm. Further laser parameters were a pulse duration of 8 ns, a focus diameter of 170 µm, a pulse energy of 0.24 J, and a laser fluence of 0.54 J/cm2 . Particles were collected passively on 8 electron microscopical grids that are covered by ultrathin (20 nm) substrate films (Pioloform® ). The grids were mounted on a special mechanical device in order to catch particles ejected from the interaction site into different directions as related to the incident laser beam

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Figure 1. Experimental set-up: the laser beam is controlled by a PC-based scanning system, annular double stage device for passively collecting particles emitted from the processed area.

Figure 3. Hymn-book parchment (15th century, Southern Germany; private W. Kautek) with yellowish laser-cleaned square (5 × 5 mm2 ) intentionally positioned apart of letters and notes. Total cleaning time: about 100 s.

positive or negative ions are recorded by a secondary electron multiplier detector mounted at the end of the TOF tube. The amplified electronic signal is stored by a computer controlled digital transient oscilloscope.

3

Figure 2. Photograph of the particle sampling device: Two of eight optional Cu-grids are visible. Laser generated particles passively will be deposited on the thin Pioloform® film covering each grid.

(Fig. 2). Scanning electron microscopical imaging of the deposition plates provides a reliable characterization of geometrical particle features like size and shape. The high resolution scanning electron microscope (ABT DS150F, Topcon, Tokyo) was operated at a beam voltage of 20 kV using medium magnifications. Imaging was possible without applying any anticharging metallic sputter layer. Eventually laser induced mass spectrometry by means of the laser microprobe mass analyser (LAMMA 500, Leybold, Cologne) is applied to individual microparticles. Laser microprobe mass spectrometry (LMMS) is based on the interaction of pulsed laser radiation (10−8 s, 265 nm, 1010 W/cm2 ) that is focused onto the microscopic features of a specimen or onto a single microparticle. Heating, melting, vaporization and ionization of the specimen will occur. The ions are accelerated and drift along the TOF tube. Time-of-flight (TOF) mass spectra of laser induced

RESULTS AND DISCUSSION

Laser cleaning parameters were selected based on an earlier investigation of the same parchment (Puchinger et al. 2005) in order to avoid any destruction of the integrity of the original sample surface. The grey value picture (Fig. 3) of a section of the parchment shows the result that becomes visible to the naked eye after a 100 s treatment of an area of 5 mm by 5 mm. The cleaned square looks brighter. Obviously soot traces in the right part of the square are also decontaminated. Light microscopical screening of the eight electron microscopical grids did not show any preference as to the direction where the particle immission takes place. The number density of deposited particles does not vary significantly. A possible anisotropic emission pattern of the laser generated particles seems to be smoothed by microturbulences that are caused at the site of the laser spot. The region of turbulent mixing may have a spatial extension that completely encloses the eight deposition plates. Typical examples of the particle deposition are shown in Figure 4 at two different magnifications. The scanning electron microscopical picture (top) reveals a sufficiently large number density of deposited particles obtained during the 100 s cleaning period. The enlarged section (bottom) shows a few individual microparticles. Obviously this SEM picture reveals differences as to geometrical particle features like shape (spherical, irregular) and size. Fortunately these

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Figure 5. Typical TOF mass spectra obtained from particles with spherical shape. (Top: spectra of positive ions; bottom: spectra of negative ions.)

Figure 4. The scanning electron microscopical (SEM) picture taken at low magnification (top) shows a suitable number density of collected laser-induced particles. Higher magnification (bottom) reveals at least two different particle morphologies (spherical and irregular shape).

different types of particles partly can also be discriminated by light microscopical observation as is used in the LAMMA instrument prior to mass spectrometric analysis. TOF mass spectra of positive and negative ions obtained from single microparticles by means of the laser microprobe mass analyser reveal the existence of at least three particle types as observed by SEM imaging. Different chemical signatures clearly appear in the mass line pattern both of positive and negative ions. TOF spectra of positive (upper row) and negative ions (lower row) obtained from spherical

and irregularly shaped microparticles are shown in Figure 5 and Figure 6, respectively. Elemental peaks (Na+ , K+ , Ca+ , Fe+ , etc.) predominantly appear in the spectra of positive ions. Obviously many more mass lines densely populate the spectra of negative ions causing problems as to their qualitative interpretation. Molecular fragment ions of the collagen fibers mainly forming the base membrane of parchment may be assigned to the mass lines at 103, 119, and 179. Sometimes however as in this example a clear discrimination between different particle chemistries based on the bare visual comparison of mass line patterns is possible and sufficient. The regular sequence of mass peaks at 24, 36, 48, 60, 72, 84, 96, . . . corresponding to negatively charged C2 , C3 , C4 , C5 , C6 , C7 , . . . ions (Fig. 7) most probably points to a soot particle.

4

CONCLUSIONS

Laser cleaning by its very nature is a destructive process in respect to the contaminant that will always be accompanied by the generation of microparticles. Particles collected during a 100 s laser treatment of an ancient parchment were characterized by SEM and LMMS. At least three different particle types

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Figure 7. TOF mass spectrum of negative ions obtained from a small irregularly shaped microparticle showing a mass line pattern with a mass number period of 12.

REFERENCES

Figure 6. Typical TOF mass spectra obtained from particles with irregular shape. (Top: spectra of positive ions; bottom: spectra of negative ions.)

could be discriminated according to morphological and compositional differences. The preliminary results have shown the benefit of using aerosol measurement techniques to laser generated single particles obtained from a contaminated surface of an ancient parchment. As in-situ active particle sampling can be performed at a higher time resolution (of a few seconds) by help of miniaturized cascade impactors analytical information e.g. on a surface contamination becomes available at an increased lateral and depth resolution. Future experiments will combine in-situ time resolved particle size spectrometry and active particle sampling for off-line microanalytical single particle characterization to get a more detailed ”image” of the cleaning state.

Bäuerle, D. 2000. Laser Processing and Chemistry. Springer Verlag-Berlin Heidelberg, New York. Barcikowski, St., Bärsch, N. & Ostendorf, A. 2007. Generation of nanoparticles during laser ablation: risk assessment of non-beam hazards during laser cleaning, Lasers in the Conservation of Artworks VI. Springer-Verlag Heidelberg 116: 631–640. Kautek, W, & Pentzien, S. 2005. Laser cleaning system for automated paper and parchment cleaning, Lasers in the conservation of artworks V. Springer-Verlag Heidelberg 100: 403–410. Kaufmann, R., Wieser, P. & Wurster, R. 1980. Application of the laser microprobe mass analyser LAMMA, Aerosol Research.Scanning Electron Microscopy II: 607–622. Lee, D. W. & Cheng, M. D. 2004. Particle generation by laser ablation during surface decontamination, in Aerosol Science 35: 1527–1540. Ostrowski, R., Marczak, J., Strzelec, M., Barcikowski, St., Walter, J. & Ostendorf, A. 2007. Healthrisks caused by particulate emission during laser cleaning, Lasers in the Conservation of Artworks VI. Springer-Verlag Heidelberg 116: 623–630. Puchinger, L., Pentzien, S., Koler, R. & Kautek, W. 2005. Lasers in the conservation of artworks V. Springer-Verlag Heidelberg 100: 51–58. Winefordner, J. D., Gornushkin, I. B., Pappas, D., Matveev, O. I. & Smith B. W. 2000. Novel uses of lasers in atomic spectroscopy, J. Anal. At. Spectrom. 15: 1161–1189.

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Laser for removing remains of carbonated matrices from Pleistocene fossils L. López-Polín & A.Ollé Institut Català de Paleoecologia Humana i Evolució Social – Àrea de Prehistòria, Universitat Rovira i Virgili Tarragona, Spain

J. Chamón & J. Barrio Departamento de Prehistoria y Arqueología, Universidad Autónoma, Madrid, Spain

ABSTRACT: This paper studies the feasibility of using lasers to remove carbonated sediment layers from Pleistocene bones. The laser facility is a portable Nd:YAG laser system with optical fibre. Laser cleaning is compared with mechanical cleaning with a scalpel, a tool that is normally used to prepare fossil remains. This comparison takes into account precision and the time needed to achieve similar results with both laser and scalpel. To assess precision, the changes on the surface of the bones are monitored using Optical and Electronic Microscopy, Scanning Electron Microscopy (SEM) and Environmental Scanning Electron Microscopy (ESEM). The results show that laser cleaning is feasible and that, in some cases, it is more precise than mechanical cleaning.

1

INTRODUCTION

The study of fossil remains often depends on specimens which have been treated by conservators. For fossils to be classified taxonomically and fully or partially observed, they need to be cleaned. Compacted and hardened sediments are common in many sites, particularly in those in which the infiltration of calcium carbonate is usual. Calcium carbonate leads to the presence of hard deposits on the surface of the fossils which are difficult to remove. Traditionally the enclosing matrix has been removed by mechanical tools and chemicals, which are also often used for final surface cleaning. Although lasers have been used in other fields of conservation for many decades, they have not been used (at least to such an extent) in the field of ancient bone remains. In fact, very little work has been done on the application of lasers to palaeontological specimens (Landucci et al. 1999, 2000, 2003,Asmus 2000, Cornish & Jones 2003, Cornish et al. 2004) and there are very few papers that deal with lasers being used to remove particles of sediment (Landucci et al. 1999, 2000, 2003). This paper presents the results of some tests on Pleistocene bones with two specific aims: to determine whether it is feasible to use lasers to remove the remains of a carbonated matrix; and to compare the effectiveness of lasers and traditional manual tools.

In this study, effectiveness is considered to be a combination of speed and precision. In order to achieve these aims, separate test pits were made in some specimens with both laser and scalpel. Then, the effectiveness of each of the techniques was compared by monitoring how long they took and the final appearance of the fossils (particular attention was paid to the degree of cleaning and the respect for the original surface of the bones).

2

MATERIALS AND METHODS

2.1 The fossil sample The fossil sample consisted of macromammal bones from two Pleistocene sites: Gran Dolina (Sierra deAtapuerca, Burgos, Spain) and La Cansaladeta (La Riba, Tarragona, Spain). In this study, twelve samples were treated.Ten bones were from two different levels of the Gran Dolina site: TD6 (c. 800,000 years ago) andTD10 (c. 350,000 years ago) (Carbonell et al. 1999). One bone was from level K of La Cansaladeta (c. 400000 years ago) (Angelucci et al. 2004). All these bones had carbonated sediment remains on their surface. Finally, a fragment of bone from TD10 of little scientific value was used for destructive assays.

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The concretions that had to be removed were made up of carbonated sediment, mainly clay, but also silt and sand. The colour of the concretions (red) was different enough from the colour of the bone (white) to allow laser treatment. 2.2 The laser treatment The laser equipment was a portable Nd:YAG laser system with optical fiber. It emits near infrared radiation (1064 nm) in the Short Free Running mode (SFR). In some cases, prior to laser cleaning, the concretions were reduced with mechanical tools to obtain a layer that was approximately 1 mm thick. Some initial tests were made on numerous samples (on more than the twelve selected for this experiment) to determine the range of suitable parameters for safe and effective cleaning. As a result of these tests, the following parameters were chosen: energy between 0.7–1 J; a spot of 6 mm; fluences between 2.5 and 3.5 J/cm2 ; frequency between 4 and 10 Hz. The laser cleaning was performed in water-assisted conditions: that is to say, the surface was either kept wet during irradiation or the sample was totally immersed. 2.3

Surface examination techniques

The surface of the bones was examined after the treatment by means of optical and electronic microscopy to evaluate the precision of both the laser and scalpel cleaning techniques. The following equipment was used: an Olympus SZ11 stereoscope (with a digital photo system Infinity X), a JEOL 6400 Scanning Electron Microscope, and a FEI Quanta 600 Environmental SEM.

Figure 1. Fossil showing damage induced by laser. The dark bands correspond to laser sweeps carried out using different parameters. The box indicates the area shown in Figures 2a and 2b.

0.2 J; spot of 10 mm; fluence of 0.3 J/cm2 . The damage threshold of the wet substrate, however, was considerably higher because a higher fluence of 4 J/cm2 (energy of 0.5 J and 4 mm of spot) only left a slight mark. This test showed the considerable difference between the effects of the dry and the wet cleaning. As other authors have pointed out (Landucci 2000, 2003), wetting is necessary to minimize the heat effect of laser irradiation. This, then, limits the usefulness of laser treatment, as sometimes wetting specimens is not possible. 3.2 Laser cleaning

3

RESULTS AND DISCUSSION

3.1 Induced damage To characterize the potential alterations caused by laser irradiation, the surface of one of the samples was deliberately damaged. The laser was used to irradiate the clean surface of a bone. A systematic sweep (making lines) was made using different parameters (Fig. 1). Half of the bone was dry, while the other half was wet. In the dry half, the sweep produced visible grey or black lines. Furthermore, excessive exposure of bone to the laser produced small holes, craquelures, and a molten appearance. These surface modifications seem to have been caused by the heating of the surface under the laser spot. On the other hand, the wet half of the bone was mostly unaltered using the same parameters. Thus, the damage threshold depends significantly on the dampness of the surface. In this specific sample, the damage threshold for the dry substrate was established using the minimum parameters tested: energy of

In some bones, laser cleaning gave excellent results (Fig. 3) whereas in other apparently similar bones from the same sites, results were not so good (at least no better than results obtained with a scalpel). Results seem to be related more to how the sediment layer is joined to the bone surface than to its composition: concretions of similar compositions were treated but the results depended on the degree of adhesion. In these samples, the minimum fluence for removing encrustation was 2.5 J/cm2 . Taking into account the higher damage threshold in wet conditions, safe cleaning is possible. This value cannot be considered universal as both the original substrates and the removable layers vary considerably. In fact, this parameter may need to be changed even when working on a single sample. Thus, an individual test is needed before any treatment is applied. As mentioned above, surface wetting is needed to achieve safe cleaning. Furthermore, not only wetting but complete immersion of the bones in water gave good results. Immersion provides .continuous cooling of the surface and prevents heating. Immersion

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Figure 3. Mandible of bovidae from level TD6 of Gran Dolina site. Detail of the same area before cleaning (a), and after cleaning with laser (b).

also causes the formation of bubbles, which seems to improve the mechanical action. Laser is especially useful in difficult access areas (nooks) or in rough zones where other tools (e.g. scalpels) cannot access or are not selective enough. 3.3 Laser versus scalpel

Figure 2. Detail of the bone surface after laser treatment at 800 mJ, 8 Hz, spot 4 mm, a) the spot under the stereoscopic microscope, b) the same area under the ESEM, low vacuum mode, c) enlarged view of the effects of laser on the bone surface (area marked by a white box in 2b), small holes, craquelures, and a molten aspect are the features of the thermal alteration produced by an excessive exposure of bone to the laser.

This study has compared treatment by laser and by scalpel. Both these tools are used to treat thin layers and provide precise cleaning (scalpels in particular are commonly used by conservators). In some cases the appearance of the treated surfaces and the working times after using a laser or scalpel were similar in the same sample but in other samples the differences were considerable. In some cases, when the carbonated sediment was strongly adhered to the bone surface, the laser was quicker than the scalpel (and apparently more precise) (Fig. 4). Furthermore, on softer bones, the scalpel commonly left some marks while the laser did not. In some cases, then, the laser is a clear improvement. To summarize, each case has its own characteristics. Acceptable results can be provided by lasers depending

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Figure 4. Fossil from La Cansaladeta site (La Riba, Tarragona), completely covered by sediment strongly adhered to its surface (a), first, the sediment was removed with an ultrasonic piezoelectric device, until a thin layer of carbonated sediment had been revealed (1 mm approximately). Then one half was cleaned with a laser (right) and the other half with a scalpel (left), in order to compare effectiveness as well as damage to the bone surface.

on the combination of such factors as the sedimentbone adhesion, the hardness or weakness of the bone, the kind of surface (rough), etc. Finally, although the techniques have been used separately for purposes of comparison, a complete treatment would certainly involve some sort of combination.

4

CONCLUSIONS

Lasers can be effective at removing carbonated sediment layers up to 1 mm thick on this kind of fossil bone. The working irradiation parameters have to be adjusted to the characteristics of the sample which is to be cleaned, which must be wetted for good results. Laser is especially useful in difficult access areas or in rough zones. Furthermore, as laser is a non-contact tool, it is a good solution when the bone is fragile and thin. With conventional tools pressure is inevitably exerted on the specimen and this can sometimes be a risk. In some samples, lasers are as precise, effective and efficient as scalpels, that is to say, results with both tools can be similar. In others, lasers are quicker and more precise than scalpels, and they leave fewer marks on the bone surface, which is a clear improvement. At times, however, they are as effective as scalpels only when high energy parameters, which may damage the bone surface, are used. Further research about the chemical composition of the removable layers and their adhesion to the substrate is needed to better understand when the laser technique is really helpful.

Figure 5. Details of the bone surface of the fossil in Figure 4 under stereoscope microscope once cleaned. The white line separates the area treated by scalpel (left) from the one cleaned by laser (right). Enlarged view under ESEM (low vacuum mode) of both areas (b and c respectively). The laser did not leave visible marks (although some microscopic detaching can be observed on the bone surface after the treatment).

Finally, whether results are good or not depends on what is known about the material treated (both the fossils and the characteristics of the enclosing rock matrices), on previous experience in conservation treatments, and, especially, on the user’s skill in handling the laser cleaning equipment.

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ACKNOWLEDGEMENTS We are grateful to M. Moncusí and M. Stankova of the Microscopy Unit of the Scientific and Technical Service of the URV. We also thank E. Catalán and A. I. Pardo for their assistance at the Conservation Laboratory of the Universidad Autónoma de Madrid. L.L-P. is the recipient of a pre-doctoral research grant from the Rovira i Virgili University. J.Ch. received a predoctoral research grant from the Spanish Ministry of Education and Science. Thanks are also given to EL. EN. (Italy) and Laser Tech Ibérica.

REFERENCES Angelucci, D. E., Cáceres, I., Lozano, M., Ollé, A., Rodríguez, X.P. & Vergès, J.M. 2004. El jaciment de la Cansaladeta (la Riba, Alt Camp) en el marc del Plistocè mitjà català. Cypsela 14: 151–170. Asmus, J. F. 2000. Laser divestment for natural history museum collections. Journal of Cultural Heritage 1: 259–262. Cornish, L. & Jones, C.G. 2003. Laser cleaning of natural history specimens and subsequent SEM examination. In J. H. Townsend, K. Eremin & A. Adriaens (eds),

Conservation science 2002 : papers from the conference held in Edinburgh, Scotland, 22–24 May 2002. London: Archetype. Cornish, L., Miller, G. & Jones, C. 2004. Pulsed laser cleaned natural history specimens with reference to the removal of conductive coatings. In K. Dickmann, C. Fotakis & J. F. Asmus (eds), Lasers in the Conservation of Artworks. Lacona V Proceedings, Osnabrück, Germany, September 15–18, 2003. Springer Verlag. Carbonell, E., Esteban, M., Martín, A., Mosquera, M., Rodríguez, X. P., Ollé, A., Sala, R., Vergès, J. M., Bermúdez de Castro, J. M. & Ortega, A. I. 1999. The Pleistocene site of Gran Dolina, Sierra de Atapuerca, Spain: a history of the archaeological investigations. Journal of Human Evolution 37: 313–324. Landucci, F., Pecchioni, E., Pini, R., Siano, S. & Salimbeni, R. 1999. A new laser approach in the conservation of paleontological findings. In A. Guarino (ed.) 2nd International Congress on Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin, 5–9 July 1999, Paris, France: Proceedings. Paris: Elsevier. Landucci, F. Pini, R., Siano, S., Salimbeni, R. & Pecchioni, E. 2000. Laser cleaning of fossil vertebrates: a preliminary report. Journal of Cultural Heritage 1: 263–267. Landucci, F., Pecchioni, E., Torre, D., Mazza, P., Pini, R., Siano, S. & Salimbeni, R., 2003. Toward an optimised laser cleaning procedure to treat important palaeontological specimens. Journal of Cultural Heritage 4: 106–110.

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Lasers in the Conservation of Artworks – Castillejo et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Environmental optical sol-gel sensors for preventive conservation of cultural heritage N. Carmona, E. Herrero & M.A. Villegas Institute of History, CCHS-CSIC & National Centre for Metallurgical Research, CENIM-CSIC, Madrid, Spain

J. Llopis Faculty of Physics, UCM, Madrid, Spain

ABSTRACT: The conservation of cultural heritage depends on the control and evaluation of environmental parameters which affect degradation mechanisms of materials and objects. The most accurate and easy to handle devices for monitoring the main environmental parameters (acid pollutants, humidity, light and temperature) are chemical sensors, particularly sensors with some kind of optical response. The present work deals with the design and preparation of reversible environmental sol-gel sensors based on the encapsulation of sensitive dopants able to respond optically. The final foreseen application deals with the environmental monitoring of works of art and other materials and objects of high historical and cultural value, both outdoor and indoor (monuments, show-cases, museums, buildings, exhibition halls, storage rooms, etc.) for their correct preventive conservation.

1

INTRODUCTION

Conservation and restoration of cultural heritage objects is a very important task, because most of materials (stones, metals, alloys, glasses, ceramics, textiles, leathers) are susceptible to ageing and weathering (Carmona et al. 2005b, 2006a, 2006b, García-Heras et al. 2003, 2005a). Preservation of this valuable legacy against the pass of time must be done ensuring the better conditions for their proper conservation and, thus, enlarging their life time and avoiding any risk of damage (Caen 1998). The presence of some environmental agents and/or non-adequate exhibition or storage conditions endangers historical materials, no matter their use or location (indoor and outdoor) (Davison 2003, Fernández Navarro 1996). Humidity and polluted environments, as well as thermal shock and intense light are parameters which make difficult conservation strategies (Carmona et al. 2005a). When such environmental parameters occur jointly, the risk of damage is even higher, since synergic effects can take place increasing both the intensity and variety of degradation mechanisms. Degradation of historical buildings and statues located outdoor can be diminished controlling the traffic near the monuments, when possible. Conservation difficulties inside museums include a high number of places to be monitored, e.g. exhibition halls, interior of showcases and storage boxes, etc. Moreover, restricted access to sophisticated measurement equipments (Camuffo et al. 2002, Thomson 1986)

and correct aesthetic appearance are common subjects that increase the problems of monitoring the museums environment. In general, negative effects of environmental conditions could be partially avoided by controlling the corresponding parameter (relative humidity (RH), pollutants, temperature and light). In this way, a preventive integral conservation strategy can be achieved by using sensors. Sensors are detection systems to alert on possible risks for the historical and cultural objects correct conservation, previous to any degradation. Moreover, they are no invasive with respect to the original object, ensuring the added value of the cultural heritage to the community (Römich & Fuchs 1992, García-Heras et al. 2004). Currently, conventional sensors involve some sort of devices, apparatus, wires and batteries (Baron & Elie 2003, Martucci et al. 2003). Furthermore, sometimes sensors need to be placed in special conditions or limited space, view, position, thereby adding a relative high number of sites to be monitored. Finally, in most cases, low-cost monitoring devices are required. Sol-gel technology is a process for obtaining glassy layers with wide range of applications, such as coatings (Pilz & Römich 1997, Carmona et al. 2006c), fibres, aerogels and monoliths (Fernández Navarro 2003). Sol-gel materials are suitable as host matrices for encapsulation of many types of dyes and molecules preserving their chemical properties (García-Heras et al. 2005b, Matsui 2005, Mac Craith et al. 1997, Villegas & Pascual 1999, Villegas et al. 2002). After

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hydrolysis and polycondensation of alkyl-alkoxide precursors, a three-dimensional polysiloxane glassy network is obtained. The encapsulation of dopants, dyes or other chemical species is accomplished when a thermal densification is carried out. Thus, sol-gel technology seems to be a promising alternative route for obtaining low cost and easy to handle environmental sensors (Carmona et al. 2004, Carmona et al. 2007, Herrero et al. 2007). The present work deals with the design and preparation of environmental sol-gel sensors able to response optically by means of a change of colour. The main purpose is the simultaneous monitoring of environmental acidity, relative humidity, light intensity and temperature in the most critical ranges for the final application: the preventive integral conservation of historical, artistic and cultural heritage assets. 2 2.1

EXPERIMENTAL

Figure 1. Overview of some sensors.

Sensors preparation

Sensors design has been carried out by different ways, depending on the environmental parameter to be monitored. Sensitive phase of sensors are amorphous sol-gel oxide systems (inorganic or hybrid organicinorganic polysiloxane networks) which act as hosts for the immobilization of optical sensitive dopants. For each particular sensor, organic or inorganic compounds sensitive to the environmental parameter to be monitored have been selected. Four different silica sols were prepared starting from alkoxides and/or alkylalkoxides as precursors into which four different dopants were independently added. After hydrolysis and polycondensation of the hydro-alcoholic solution, a transparent colourless inert matrix for the corresponding dopant encapsulation was obtained. Dopants have been previously selected depending on their sensitivity threshold against one or several environmental parameters mentioned above and the possibility to be immobilized in the sol-gel matrix preserving their properties. Thin coatings were obtained for the acidity sensor and the relative humidity sensor. The coatings were applied by dip-coating upon common glass slides and then partially densified at 60◦ C during 3 days. Non-densified monoliths, i.e. gels, hermetically sealed in polystyrenecuvettes were prepared for the light sensor and the temperature sensor. Figure 1 shows the final appearance of some acidity, relative humidity and light sensors. 2.2

concentration in a Kesternich-chamber model VCK300. Other environmental measurements were carried out with a data collector Multilog Pro DB-Lb 4.1, equipped with the following conventional sensing devices: a thermometer in the −25–110◦ C range; a hygrometer for recording relative humidity in the 0–100% ± 2% range; a pH-meter measuring the entire pH range and two luxometers in the 0–300 lux and 0–130,000 lux ranges. Additional measurements of light intensities were performed with a portable luxometer Promax model IL-185. The optical response of the sensors was recorded with a UV-VIS spectrophotometer Shimadzu model 3100, attached with an integrated sphere (recording range 300–800 nm). 2.3

General measuring procedure

Sensors were located inside the corresponding environment (e.g. Kesternich climatic chamber) and initial conditions were recorded. As the corresponding environmental parameter to be monitored varies (light, temperature, acidity or relative humidity), the sensor reacts and acquires the corresponding colour. Intermediate spectra were recorded for a changing parameter while all the other parameters remained unchanged. Finally, a calibration curve for each parameter can be built. Such calibration curves can be used for monitoring real conditions later on (e.g. museums indoors, outdoor, near historical stained glass windows, etc.).

Characterization techniques

General characterization of the sensors produced has been undertaken by conventional procedures and techniques. The sensors were submitted to different artificial conditions in the laboratory. Acid environments were simulated by controlling different SO2

3

RESULTS AND DISCUSSION

3.1 Acidity sensor The acidity sensor consists of a thin inorganic silica sol-gel coating doped with an organic dye applied on

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Table 1.

Summary of sensors response and properties. Optical response

Sensor Acidity Relative humidity Light

Temperature

Properties

Colour A (Mean Absorption peak position)

Colour B (Mean Absorption peak position)

Violet (λ = 570 nm) Green (λ = 425/625 nm) Dark pink (λ = 535 nm) Light pink (λ = 516 nm)

Sensitivity range

Response time

Orange (λ = 430 nm) Blue (λ = 590 nm) Light yellow (λ = −)

7.0–6.4 of pH scale 40–95% RH

1 min

Blue (λ = 665 nm)

Accuracy

Remarks

10 min

0.1 of pH scale ±1% RH

100–2000 lx

10 min

±20 lx

10–60◦ C

15 min

±2◦ C

Thin film upon glass slide. Thin film upon glass slide. Monolithic gel in hermetically sealed cuvette. Monolithic gel in hermetically sealed cuvette.

A corresponds to the minimum value of the parameter to be measured. B corresponds to the maximum value of the parameter to be measured.

a common glass substrate. The sol-gel coating was partially densified. Initially it shows an absorption band peaked at λ = 570 nm (violet colour) (Table 1). After 1 min exposure to different controlled SO2 concentrations in the Kesternich chamber (T = 23◦ C; 35.5% relative humidity), the environmental acidity is detected and the absorption band at 570 nm diminishes, while other absorption band at λ = 430 nm increases, enhancing the appearance of orange colour. The relative decreasing of the absorption intensity at 570 nm corresponds to a certain concentration of acid species in the air (in this case SO2 ). Calculation of the environmental pH value for the corresponding SO2 concentration (García-Heras et al. 2005c) yielded a pH value in the pH range between 6.4 and 7.0 for the 0–50 ppm SO2 concentration range. Detection threshold under gaseous media (atmospheric air) is about 1 ppm SO2 , which corresponds to 0.1 on average in the pH scale. In this way, by recording the absorption spectrum initially and after exposure to different environmental acid pollutants, an absorbance increase is observed. Thus, following the steady line in the calibration curve (Fig. 2), an estimated value of the environmental pH can be obtained. 3.2

Relative humidity sensor

In a similar way that the acidity sensor, the relative humidity sensor is composed of a thin inorganic silica sol-gel coating, doped with an organic dye applied on a glassy substrate and partially densified. Initially this sensor shows two absorption bands at λ = 425 and 625 nm, which provide green colour (T = 25.5◦ C, under absence of SO2 ). In the presence of a certain relative humidity, the intensity of the 425 nm absorption

Figure 2. Calibration curve of the environmental acidity sensor: relative absorbance decreasing at λ = 570 nm as a function of the environmental pH.

band decreases (and hence the yellow component of the final colour), whereas the intensity of the absorption band at 625 nm increases and shifts to 590 nm. As a result, the sensor becomes deep blue. After 10 min of exposure, the increasing of the absorption band intensity at 590 nm allows the estimation of the environmental relative humidity. Figure 3 shows the calibration curve for this sensor in the 20–90% relative humidity range. The steady line allows the estimation of a given relative humidity value. 3.3 Light sensor The light sensor is composed of a polysiloxane hybrid sol-gel matrix doped with a photochromic substance.

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Figure 3. Calibration curve of the relative humidity sensor: relative absorbance increase at λ = 590 nm as a function of the relative humidity percentage.

Figure 5. Calibration curve of the temperature sensor: absolute absorbance increasing at λ = 665 nm as a function of the temperature.

which co-relates the sensor optical response (in terms of the absorption decreasing measured at 535 nm, i.e. the sensor bleaching from pink to yellow), as a function of the lighting intensity in the 100–3000 lx range. Once the sensor is exposed and the absorbance decreasing recorded, it is possible to estimate the luminance of the environment by following the steady line plotted in Figure 4. 3.4 Temperature sensor

Figure 4. Calibration curve of the light sensor: relative absorbance decreasing at λ = 535 nm as a function of the illuminance.

The sensitive material was cast into hermetically sealed cuvettes, in order to avoid interferences with pollutants and humidity. This arrangement also prevents the sensor ageing. The calibration experiments were carried out at room conditions: T = 26◦ C; 35.5% relative humidity; absence of SO2 (springtime in Madrid, Spain). The initial spectrum of the sensor in darkness shows one main absorption band with a maximum at λ = 535 nm (the sensor shows an intense pink colouring). When the sensor is irradiated, the absorption band decreases and becomes flat. Then the sensor shows pale pink colour, salmon and finally clear yellow, depending on the intensity and time of lighting. Figure 4 shows the corresponding calibration curve,

The temperature sensor consists of a sol-gel inorganic matrix doped with an inorganic salt. Once prepared, the sol was cast into cuvettes and hermetically sealed. This material behaves as a sensitive thermochromic system. The optical response is based on a change of colour from light pink to deep blue in the 10–50◦ C range. The experiments to attain a calibration curve were carried out at room conditions: 35.5% relative humidity and absence of SO2 (springtime in Madrid, Spain). When the temperature is low (T = 10◦ C, pink colour), the initial spectrum showed a sole absorption band peaked at λ = 516 nm. As the temperature increases, a second absorption band grows at 665 nm (blue colour). Figure 5 shows the calibration curve for the temperature sensor in the most critical range to be monitored for preventive conservation of historical objects (10–50◦ C). By recording the corresponding absorption spectra and comparing the colour of the sensor initially and after 15 min exposure, it is possible to estimate the environmental temperature following the steady line of Figure 5. 3.5

Other properties of sensors

The environmental sensors prepared have the following additional properties: 1) they are reversible and

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reusable; 2) they have fast response times (between 1 and 15 min, Table 1); 3) their life time, in terms of estimated using period, is longer than 4–6 months and 4) they can be used both in indoor and outdoor atmospheres. Reversibility of sensors was tested submitting them to successive cycles carried out with extreme values of the corresponding parameter to be measured, e.g. between darkness and 3000 lux for the light sensor, or by means of consecutive immersion in solutions of pH = 2 and pH = 10, for the acidity sensor. Sensors gave accurate absorption values without optical fatigue. The main advantages of these sensors are connected with their stability against chemicals and cleaning products. Moreover, they are discrete with a small size: 5.0 × 5.0 × 0.2 cm3 for the environmental acidity sensor and the relative humidity sensor, and 1.0 × 4.5 × 1.0 cm3 for the light sensor and the temperature sensor. On the other hand, they have been produced by a low cost method and they are easy to handle, yielding optical responses able to be interpreted by non-specialists.

4

CONCLUSIONS

Four different sol-gel systems, doped each one with a particular sensitive compound, have been designed and prepared. The silica based materials obtained are sensitive to environmental acidity, relative humidity, light and temperature, respectively. Consequently, they behave as environmental sensors and are adequate for the easy and one-fast-step monitoring of those parameters that damage historical, artistic and cultural heritage assets. Therefore, these sensors can help to improve preventive conservation strategies. The sensors developed have small size, easiness for handling, fast response time, reversibility and re-using ability. Thus, they can be applied as useful devices for environmental monitoring in museums, storage rooms, showcases, exhibition halls, monuments and others. Finally, the sensors can work both indoor and outdoor, since their optical response ranges can be modulated according to the particular user needs.

ACKNOWLEDGEMENTS Authors acknowledge the support from EU project MC-ERG-CT-2004-516436 and Spanish project CICYT-MAT-2006-04486. NC thanks CSIC-ESF I3P program for a postdoctoral contract. The professional support of the CSIC Thematic Network for Cultural and Historical Heritage is also acknowledged.

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Author Index

Abraham, M. 227 ÁDics o, . 271 Acquaviva, S. 317 Agnani, A. 419 Álvarez, C. 115, 199 Ambrosini, D. 399 Andreotti, A. 253 Angulo, M. 177, 441 Appolonia, L. 191 Arroyo, M. 297 Artigas, D. 15 Asmus, J.F. 1 Aura, E. 237 Aze, S. 11 Bajraszewski, T. 61 Baldwin, A.M. 285, 291 Baldwin, K.G.H. 49 Baonza, V.G. 89 Barcikowski, S. 263 Baronnet, A. 11 Barrera, M. 323 Barrio, J. 297, 477 Barup, K. 169 Bellucci, R. 101 Beraldin, J.A. 435 Bernikola, E. 381, 393 Beyer, E. 35 Bilmes, G.M. 361 Birkholzer, K. 221 Blais, F. 435 Boldura, O. 357 Borgeat, L. 435 Boukos, N. 309 Brunetto, A. 191 Camaiti, M. 253 Caneve, L. 149 Cap, N. 361 Cappellini, V. 453 Carcagnì, P. 101, 465 Carmona, N. 73, 483 Casaccia, A. 101 Castellucci, E. 253 Castillejo, M. 41, 133, 357, 371 Castro, K. 177, 441 Catalán, E. 297 Cavaletti, R. 231 Cañamares, M.V. 29 Cecchi, G. 157, 163, 169 Cedric, T. 381

Chamón, J. 297, 477 Chmielewski, K. 259 Climent-Font, A. 323 Colao, F. 149 Colombini, M.P. 253 Colombo, C. 109, 243 Conradi, A. 345 Conti, C. 109, 169 Cooper, M. 277 Cormack, I.G. 15 Cournoyer, L. 435 Curran, C. 329 Dajnowski, A. 209, 303 De Giorgi, M.L. 317 de la Paz, F. 79 De la Roja, J.M. 89 De Marchi, M. 243 Dean, C. 227 deCruz, A. 253 del Conte, A. 419 Del Mastio, A. 453 Delaporte, P. 11, 49 Della Patria, A. 317 Dement’ev, A.S. 263 Detalle, V. 11 Dickmann, K. 127, 249, 353, 367 Dicsö, Á. 271 Dobai, C. 271 Domenech, A. 237 Domenech, M.T. 237 Domingo, C. 41 Dornheim, S.D. 89 Doulgeridis, M. 381 Drakaki, E. 309 Dre´scik, D. 215 D’Agata, R. 67 D’Anna, E. 317 ÉGalambos, . 271 Engel, P. 263 Escudero, C. 323, 337, 375 Esposito, E. 419 Fantoni, R. 149 Fassina, V. 231 Feligiotti, M. 419 Fernández, J.J. 459 Fernández, L.A. 177, 441 Ferri De Collibus, M. 447 Ferro, D. 133

489

Finat, J. 459 Fiorani, L. 149 Fontana, R. 101, 465 Fornetti, G. 447 Francucci, M. 447 Freeman, D. 49 Freisztav, C.M. 361 Froidevaux, M. 277 Galambos, É. 271 Galli, G. 399 Gambino, M.C. 101 García, A. 41, 337 García-Heras, M. 73 Gaspard, S. 41 Gaudini, G. 231 Geary, A. 413 Geller, B. 353 George, M. 381 Georges, M. 407 Giuntini, L. 459 Godin, G. 435 Gomoiu, I. 157 Goncalves Tavares, S. 419 Gonzalez-Cembellín, J.M. 441 Grasso, G. 67 Grauby, O. 11 Greco, M. 465 Green, T. 381 Grosse, Th. 35 Groves, R.M. 381, 407, 427 Grönlund, R. 169 Guarneri, M. 447 Guedes, J. 263 Gugolya, Z. 263 Gutiérrez-Baños, F. 459 Góra, M. 23, 61 Hackney, S. 381, 427 Hallett, K. 413 Herrero, E. 483 Hildenhagen, J. 127, 249, 353, 357, 371 Hipólito, A. 263 Hustinx, G.M. 381, 407 Hällström, J. 169 Iglesias, M. 183 Ihlemann, J. 55 Iwanicka, M. 61 Jasiunas, K. 263 Jiménez, P. 73

Johansson, A. 169 Julien-Lees, S. 413 Kaminska, A. 95 Karaszkiewicz, P. 141 Kautek, W. 357, 371, 473 Kisapáti, I. 263 Klotzbach, U. 35 Kolar, J. 263, 357 Korenberg, C. 221, 285, 291 Koss, A. 203, 215, 259, 263 Kouloumpi, E. 381, 393, 407, 427 Kowalczyk, A. 23, 61 Križnar, A. 79 Krok, F. 141 Krüger, J. 345 Kántor, Z. 263, 271 Köhler, W. 35 Lahanier, C. 435 Leitner, H. 35 Lentjes, M. 127, 353, 367 Leona, M. 29 Licciardello, M.P. 67 Liese, S. 35 Ljubic, V. 357, 371 Llopis, J. 483 Lognoli, D. 157, 163, 169 Loza-Alvarez, P. 15 López, A.J. 115, 121, 199 López-Polín, L. 477 Madariaga, J.M. 177 441 Madsen, N.R. 49 Maguregui, M. 177 Mainusch, N. 55 Maracineanu, W. 133, 249, 357, 371 Marczak, J. 23, 203, 215, 259, 263 Martinez-Arkarazo, I. 177, 441 Martínez, A. 375 Martínez, J. 459 Martínez, M.A. 337 Masotti, L. 163 Mastroianni, M. 101, 465 Mateo, M.P. 115, 121, 199 Materazzi, M. 465 Matteini, M. 253 Mazur, M. 259 Melniciuc, N. 357, 371 Moreno, P. 41, 337 Morillo, F.M. 459 Moutsatsou, A. 381 Moutsatsou, A.P. 407 Murauskas, E. 263 Muñoz, M.V. 79

Márton, Z. 263, 271 Méndez, C. 41, 337 Navarro, J. 73 Nevin, A. 393 Olafsdottir, J. 407 Ollé, A. 477 Orsetti, A. 361 Osten, W. 381, 427 Ostrowski, R. 215, 263 Oujja, M. 41, 133, 357, 371 Paglia, E. 447 Pajagic Bregar, G. 357, 371 Palazzo, M. 243 Palmer, R. 253 Palombi, L. 157, 163, 169 Palucci, A. 149 Pampaloni, E. 101, 465 Panzner, M. 35 Paoletti, D. 399 Pardo, A.I. 297 Pelagotti, A. 453 Pentzien, S. 345, 473 Perrie, W. 329 Pezzati, L. 101, 317, 465 Pflugfelder, C. 55 Picard, M. 435 Piccirillo, A. 191 Piccolo, R. 101 Pingi, P. 101 Pires, M. 263, 329 Platt, P. 277 Pouli, P. 285, 291 Prauzner-Bechcicki, J.S. 141 Prieto, C. 337 Psilodimitrakopoulos, S. 15 Pérez-Moneo, J.D. 459 Rabal, H. 361 Radvan, R. 133, 249, 357, 371 Raimondi, V. 157, 163, 169 Ramil, A. 115, 121, 199 Rampazzi, L. 109 Realini, M. 109, 243 Remondino, F. 387 Respaldiza, M.A. 79 Ricci, R. 447 Rioux, M. 435 Rizzo, B. 109 Roberts, Z. 413 Rode, A.V. 49 Rouba, B. 61 Rycyk, A. 23, 215, 263 Salimbeni, R. 263 San Andrés, M. 89 San José, J.I. 459

490

Sansonetti, A. 243, 253 Santos, S. 263 Sanz, A. 337 Sarmiento, A. 441 Sarzy´nski, A. 263 Sawczak, M. 95 Serafetinides, A.A. 309 Serrado, L. 15 Siano, S. 191, 231, 263 Simileanu, M. 133, 357, 371 Simone, S. 67 Sinigalia, T. 357 Skrzeczanowski, W. 263 Slavinskis, N. 263 Smirniou, M. 221 Spoto, G. 67 Striová, J. 253 Strlic, M. 263, 357, 371 Strzelec, M. 203, 215, 259, 263 Svanberg, S. 169 Švedas, V. 263 Szambelan, R. 263 Szelagowska, K. 141 Szkulmowski, M. 61 Szymonski, M. 141 Sáiz, B. 183, 237 Sánta, I. 263, 271 Targowski, P. 23, 61 Taylor, J. 435 Terlixi, A.V. 407 Theuer, M. 35 Tornari, V. 381, 393, 407, 427 Trompeta, M. 407 Trtica, M. 263 Tsaroucha, C. 407 Tymi´nska-Widmer, L. 61 Urrutikoetxea Barrutia, M. 441 Uteza, O. 49 Vallet, J.M. 11 van Dalen, P. 367 Vega, M. 79 Villegas, M.A. 73, 483 Viöl, W. 55 Vlachou-Mogire, C. 309 Wain, A. 49 Walczak, M. 141 Watkins, K. 277, 329 Wojtkowski, M. 61 Wurster, R. 345, 473 Ynsa, M.D. 323 Yáñez, A. 115, 121, 199 Zafiropulos, V. 133, 357, 371 Zergioti, I. 309