Lasers in the Conservation of Artworks VIII

LASERS IN THE CONSERVATION OF ARTWORKS VIII

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PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON LASERS IN THE CONSERVATION OF ARTWORKS VIII (LACONA VIII), 21–25 SEPTEMBER 2009, SIBIU, ROMANIA

Lasers in the Conservation of Artworks VIII

Editors

Roxana Radvan National Institute of Research and Development for Optoelectronics—INOE 2000, Romania

John F. Asmus Physics Department, University of California, San Diego, La Jolla, CA, USA

Marta Castillejo Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

Paraskevi Pouli Institute of Electronic Structure and Lasers, Foundation for Research and Technology—Hellas, Heraklion, Crete, Greece

Austin Nevin Courtauld Institute of Art, London, UK

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CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2011 Taylor & Francis Group, London, UK Typeset by Vikatan Publishing Solutions (P) Ltd., Chennai, India Printed and bound in Great Britain by Antony Rowe (a CPI Group Company), Chippenham, 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 publisher. 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: CRC Press/Balkema P.O. Box 447, 2300 AK Leiden, The Netherlands e-mail: [email protected] www.crcpress.com – www.taylorandfrancis.co.uk – www.balkema.nl ISBN: 978-0-415-58073-1 (Hbk) ISBN: 978-0-203-81866-4 (eBook)

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

Table of contents

Preface

ix

Permanent scientific committee

xi

Gauguin, Mucha, and Art Nouveau J.F. Asmus

1

Innovative approaches in laser cleaning researches and instrumentation development The effect of ultrafast lasers on laser cleaning: Mechanism and practice K.G. Watkins, P.W. Fitzsimons, M. Sokhan & D. McPhail Spectral analysis of the effects of laser wavelength and pulse duration on tempera paints M. Oujja, M. Castillejo, P. Pouli, C. Fotakis & C. Domingo The role of the substrate in the laser cleaning process: A study on the laser assisted removal of polymeric consolidation materials from various substrates S. Kogou, A. Selimis, P. Pouli, S. Georgiou & C. Fotakis

9 15

23

Compact short pulsed fiber laser offers new possibilities for laser cleaning J. Hildenhagen & K. Dickmann

29

Decontaminating pesticide-exposed museum collections J.F. Asmus

33

Laser cleaning of burial encrustation and aged protective coating on Egyptian leather: Optimization of working conditions A.A. Elnaggar, P. Pouli, A. Nevin, M.A. Fouad & G.A. Mahgoub

39

The practical use of lasers in removing deteriorated Incralac coatings from large bronze monuments A. Dajnowski & A. Lins

47

PROCON TT 49: Laser cleaning of ancient Egyptian wall paintings and painted stone surfaces B. Graue, S. Brinkmann & C. Verbeek

53

The influence of paper type and state of degradation on laser cleaning of artificially soiled paper S. Pentzien, A. Conradi & J. Krüger

59

Laser cleaning studies for the removal of tarnishing from silver and gilt silver threads in silk textiles B. Taarnskov, P. Pouli & J. Bredal-Jørgensen

67

Thickness of ablation control by structured light method R. Sitnik, J. Rutkiewicz & J. Marczak

75

213 nm and 532 nm solid state laser treatment of biogenetical fibrous materials M. Forster, S. Arif, C. Huber, W. Kautek, S. Bushuk, A. Kouzmouk, H. Tatur & S. Batishche

79

Free-running Er:YAG laser cleaning of mural painting specimens treated with linseed oil, “beverone” and Paraloid B72 J. Striova, E. Castellucci, A. Sansonetti, M. Camaiti, M. Matteini, A. deCruz, A. Andreotti & M.P. Colombini

85

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Studies on the UV femtosecond ablation of polymers: Implications for the femtosecond laser cleaning of painted artworks I.A. Paun, A. Selimis, G. Bounos & S. Georgiou

93

Monitoring the laser cleaning process of ornamental granites by means of digital image analysis J. Lamas, A.J. López, A. Ramil, B. Prieto & T. Rivas

99

Optimization of laser cleaning parameters for the removal of biological black crusts in granites A.J. López, J. Lamas, A. Ramil, A. Yáñez, T. Rivas & J. Taboada Bronze putti from Wilanów Palace garden façade—conservation studies and tests of laser cleaning H. Garbacz, E. Fortuna, Ł. Ciupiński, K.J. Kurzydłowski, A. Koss, J. Mróz, A. Zatorska, K. Chmielewski, J. Marczak, M. Strzelec, A. Rycyk & W. Skrzeczanowski

105

111

Comparative studies: Cleaning results of short pulsed Nd:YAG vs. fibre J. Hildenhagen & K. Dickmann

119

Laser cleaning of iron: Surface appearance and re-corrosion of model systems C. Korenberg & A.M. Baldwin

123

Reversion of darkened red lead-containing wall paintings by means of cw-laser irradiation: In situ tests and first application S. Aze, J.-M. Vallet, V. Detalle & O. Grauby Comparative study on the irradiation methods against fungal colonization case study S.A. Abd Abd El Rahim

129 135

Investigation and diagnostics methods Absolute LIBS stratigraphy with Optical Coherence Tomography P. Targowski, E.A. Kwiatkowska, M. Sylwestrzak, J. Marczak, W. Skrzeczanowski, R. Ostrowski, E. Szmit-Naud & M. Iwanicka

143

Database of complex paint spectra decomposed by principal component analysis, for identification of artwork colours Zs. Márton, T. Tóth, É. Galambos & R. Mingesz

149

Study of matrix effect in the analysis of pigments mixtures using laser induced plasma spectroscopy M.P. Mateo, T. Ctvrtnickova, A. Yañez & G. Nicolas

155

Pomerania Laboratory—A solution for the cultural heritage research and conservation A. Iwulska, I. Traczyńska, R. Jendrzejewski, M. Sawczak, G. Śliwiński & A. Kriegseisen

161

THz-Time-Domain Spectroscopy—A new tool for the analysis of artwork M.J. Panzner, U. Klotzbach, E. Beyer, G. Torosyan, A. Schmid & W. Köhler

167

19th century paints of Richard Ainè used by Jan Matejko (1838–1983). Analysis of preserved paints from tubes, palettes and of paintings’ surfaces and paint-layer M. Wachowiak

173

Study of the effect of relative humidity on the identification conditions of paper soiling by means of the NIR technique M. Sawczak, G. Rabczuk, A. Kamińska & G. Śliwiński

177

Monitoring, imaging and documentation of artwork Experimentation of a three-focal photogrammetric survey system as non invasive technique for analysis and monitoring of painting surfaces decay condition P. Salonia, A. Marcolongo & S. Scolastico RGB-ITR: An amplitude-modulated 3D colour laser scanner for cultural heritage applications R. Ricci, L. De Dominicis, M.F. De Collibus, G. Fornetti, M. Guarneri, M. Nuvoli & M. Francucci

185 191

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3D laser reconstructions of Buddhist temple from Ladakh D. Ene & R. Rădvan

199

Robotized structured light system for automated 3D documenting of cultural heritage R. Sitnik, M. Karaszewski, W. Załuski & P. Bolewicki

203

Through-glass structural examination of Hinterglasmalerei by Optical Coherence Tomography M. Iwanicka, L. Tymińska-Widmer, B.J. Rouba, E.A. Kwiatkowska, M. Sylwestrzak & P. Targowski

209

Editing protocol for the digital mapping of related imagistic investigations L.M. Angheluta

215

U-ITR: A 3D laser scanner prototype aimed at underwater archaeology applications R. Ricci, L. De Dominicis, M.F. De Collibus, G. Fornetti, M. Guarneri, M. Nuvoli & M. Francucci

221

Author index

227

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

Preface

The meeting of cultures should offer our children and grand children ‘the privilege of belonging to several worlds in a single life’, to quote a French anthropologist (Serge GRUZINSKI). The International Conference on Laser in Conservation of Artworks—LACONA VIII follows the prestigious series of conferences initiated by Prof. Costas Fotakis organizing LACONA I—1995 in Heraklion, Greece. This was followed by LACONA II—1997 in Liverpool UK, LACONA III—1999 in Florence, Italy, LACONA IV—2001 in Paris, France, LACONA V—2003 in Osnabruck, Germany, LACONA VI—2005 in Vienna, Austria, LACONA VII—2007 in Madrid, Spain. The success of these unique conferences motivated the members of the Permanent Scientific Committee to stress the international addressability of the biennial event and honoured Romanian partners to organize the eigth edition for the first time in one of the East-European countries. The LACONA VIII event—an invitation to dialogue between specialists from different geo-cultural area, and a dialogue between different professional fields for the benefit of a common heritage- was organized in Sibiu, between 21 and 25 September 2009, by INOE – The National Institute of Research and Development for Optoelectronics with precious collaboration of local authorities and cultural institutions. Today, a new affirmation regarding the role of science in Cultural Heritage conservation is almost a truism, but I am sure that the research field dymanics and tendencies are of great interest. I am glad to see that new knowledge boundaries are defined and the papers of scientific community proove a strong and interrelated international activity. The public discussions and the open dialogue during Round Table Session reaffirmed the inexhaustible efforts for correlation of analytical investigation, for trainings and know-how transfer by recognized professional authorities, for international certification of validated methods. Due to their accuracy, high sensibility, and versatility, the laser methods are recommended for remote controlled investigation, for transportable, portable, and tele-operated instrumentation. This Volume of Proceedings comprises selected contributions related laser induced phenomenas, laser investigations and documentation of most recent laboratory studies and on-site applications. For this volume I was honoured to receive a priceless and generous support from all co-editors: John F. Asmus, Marta Castillejo, Paraskevi Pouli, Austin Nevin. I Iike to put in light a very discrete and special person who supports with enthusiasm our activities from the first moment of the first project. The team of CERTO Department is thankful to Dr. Roxana Savastru for her full involvment, for never lost energy, and not less for the wonderful lesson of professional and management commitments. Roxana Radvan LACONA VIII Chair Bucharest, September 2010

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

Permanent scientific committee

Margaret Abraham

Los Angeles County Museum of Art, 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, UK

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, Vienna, 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, Romania

Renzo Salimbeni

Istituto di Fisica Applicata Nello Carrara CNR, Florence, Italy

David Saundersn

The British Museum, Department of Conservation, Documentation and Science, London, UK

Manfred Schreiner

Academy of Fine Arts Vienna, Austria

Matija Strilic

University College London—Centre for Sustainable Heritage The Bartlett School of Graduate Studies, London, UK

Veronique Verges-Belmin

Laboratoire de Recherche des Monuments Historiques, Champs-sur-Marne, France

Kenneth Watkins

Department of Engineering, University of Liverpool, UK

Vassilis Zafiropulos

Technological Educational Institute of Crete & Center for Technological Research—Crete, Sitia, Crete, Greece

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

Gauguin, Mucha, and Art Nouveau J.F. Asmus University of California, San Diego, La Jolla, CA, USA

ABSTRACT: The Art Nouveau movement traces its earliest stirrings to the middle of the 19th Century and several of that era’s well-known impressionist painters. In the year 1895 Alfons Mucha’s Sarah Bernhardt poster burst upon the Paris theater scene and this Art Nouveau sensation captured the public imagination. A few months earlier in Brittany, Paul Gauguin had been composing female images for Tiffany until he became bedridden in the aftermath of a fistfight with three sailors. During the twomonth convalescence Gauguin’s model, “Annah la Javanaise”, stole his belongings in order to raise money through sales in Paris. We have performed digital computer image analyses of surviving items by Mucha and Gauguin revealing that one of Gauguin’s artworks fell into Mucha’s hands and was photographically copied. Thus, Mucha’s famous Gismonda poster for the Bernhardt performance in the Sardou play had been plagiarized and Gauguin, instead, was the actual source of the Art Nouveau phenomenon. 1

INTRODUCTION

of 1894 (Reade, 1963). The manager of the Theatre de la Renaissance called Mucha’s printing shop asking if anyone there could design a poster for Gismonda as his artist was ill. It was stipulated that the poster had to be composed, executed, and published within five days as Madame Bernhardt insisted that it appear on December 31. In spite of this crushing deadline and tremendous pressure the posters were drying by December 31 and distributed throughout Paris on January 1. It was an immediate success and Parisians applauded the launching of the school of the “new art”. Soon “Style Mucha” became an international movement known as “Art Nouveau” and the unknown Czech artist, Alfons Mucha, became his native country’s national hero. The accidental discovery of the postage-stamp collage raises the specter (from historical, stylistic, and technical perspectives) of an act of plagiarism. These aspects of the issue are explored and analyzed in the forthcoming sections of this paper.

A small collage of paper fragments pasted onto the glazed face of a ceramic tile appeared in 1954 at an auction of objects from a San Marino, California estate. The collage design portrays the actress, Sarah Bernhardt, in the title role of Gismonda in Sardo’s 1894 Paris theater production. A careful inspection under magnification reveals that the figure was created by gluing snippets from at least sixty French postage stamps onto the of a common household ceramic tile. It appears that the stamps have all been cancelled, however the cancellation marks are undecipherable due to the manner in which the very tiny fragments are dispersed throughout the entire composition. No paint or ink is used to outline the lady. The borders consist of dark lines cut from postage stamps. In 1967 the tile with the attached collage was shown to the French art dealer Oscar Meyer (Tyler, 1988). He immediately exclaimed: “My God, that’s by Paul Gauguin!” He identified the Gauguin selfportrait as a jester appearing behind the banner on the plinth and the “GP” initials at the lower right-hand corner. (He had seen receipts signed by Gauguin in exactly that manner). In addition another “GP” is formed by Bernhardt’s right hand and sleeve, and various “PG” initials appear in the drapery of the gown. These discoveries planted the first seeds of what was to grow into a theory that Gauguin, not Alfons Mucha, was the authentic father of the Art Nouveau movement. According to Mucha’s son, the events leading to the overnight public infatuation with the school of Art Nouveau began during the Christmas holidays

2

LATE 19th CENTURY PARISIAN ART

French art and culture rebounded in a wave of vigor and optimism following the trauma and agony of the Franco-Prussian War, the capitulation of the Imperial Army in 1870, and the ensuing political and civil upheavals. French Impressionism became the worldwide focus of a new renaissance in arts and architecture. Artists, dealers, collectors, and bohemians flocked to Paris in order to celebrate the sense of a new and explosive vitality. The Eiffel Tower symbolized a revolutionary

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stones at Concarneau, when I was walking with Annah. I knocked down with two punches a pilot who attacked me. I took them all on, and kept the upper hand, until my foot caught in a hole and in falling I broke my leg.” He was confined to bed for two months and his inability to paint led to long hours, days, and weeks of boredom. To another friend he replied: “Your letter surprised me in utter idleness: in front of me a heap of unanswered letters growing higher each day.” We speculate that this “heap” constitutes the material of the stamp collage. Although Gauguin is best known for his easel paintings, he worked in various media including wood and metal. At the time of his stay in Brittany he was also collaborating in designs for stained glass that contributed to the art of Tiffany. Thus, creating a collage would be in keeping with his broad interests in diverse media (Hoog, 1987). During Gauguin’s convalescence it is known that he was largely limited to reading his favorite monthly publication, “Le Figaro Illustre”, which featured a color picture of Sarah Bernhardt as she appeared between acts of the play Izeyl. She was greatly admired by Gauguin, as, in addition to her acting, she was an accomplished painter and sculptress. It is doubtful that he saw her except in photographs as in the le Figaro illustration. (However, there are historical hints that Gauguin may have traveled to London a few months earlier, specifically, to witness a Bernhardt stage performance there). It is plausible that Gauguin was inspired by the Bernhardt photograph, his anticipation of her appearance in the Sardou (his idol) Gismonda production, and the widely admired pose of Liberty Enlightening (Statue of Liberty, also depicted in Le Figaro) by Bartholdi. The Gismonda collage presents itself as a synthesis of these three elements. Gauguin’s confinement, inability to paint, mountain of postage stamps, infatuation with the subject, and access to a ceramic tile (the renovation of his rooming house: Figure 3) make him a likely candidate composer of the collage. Nevertheless, this raises the speculative question as to how this artifact (if created by Gauguin) might have fallen into the hands of Alfons Mucha in Paris. Gauguin’s letters to friends suggest the following scenario. With each passing day of Gauguin’s confinement and recuperation his model and mistress, Annah la Javanese, became progressively more restless and less agreeable. She was probably bored with Gauguin and his country life. She longed to return to the excitement of Paris and did so in September with any effects of value that she was able to loot from Gauguin’s possessions. (Perhaps, the original artwork composed upon the tile was among these items). Upon Annah’s return to Paris she began modeling for Mucha, became his mistress, and took up residence in his studio (Figure 4).

Figure 1. Typical Parisian posters from the era preceding the appearance of Mucha’s Gismonda (1894) that heralded the Art Nouveau movement. On the left is a liquor advertisement (1893) by Henri Guydo. Henri de Toulouse-Lautrec produced the poster on the right to promote a performance (1892) by the cabaret singer, Aristide Bruant.

Figure 2. Sarah Bernhardt (left), whose performance in Sardo’s Gismonda inspired the emergence of Art Nouveau. One of Mucha’s most famous designs (La Plume) illustrating the pinnacle of his “Style Mucha” (right).

fusion of art and industry. Names such as Degas, Manet, Monet, Vuillard, Van Gogh, Gauguin, and Renoir became the most recognizable icons of Western Art. Henri de Toulouse-Lautrec became famous as a prolific practitioner of the “Poster Art” branch of Impression. Figure 1 reproduces images of the styles typical of the poster art before the appearance of the “New Art” (Style Mucha/Art Nouveau) of Alfons Mucha and his portrayals of Sarah Bernhardt and her theater appearances (Figure 2). 3

HISTORICAL MILIEU

In May 1894 Paul Gauguin moved from Paris to Pont-Aven in Brittany. While on a stroll with friends and fellow artists, his bizarre attire provoked a shouting match with three passing sailors. In a letter to a friend Gauguin related the climax of the encounter: “They started throwing

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4

GEOMETRICAL COMPARISON OF THE TILE AND POSTER

The famous 1894 Gismonda poster by Mucha appears to scale juxtaposed with the postage-stamp collage in Figure 5 (black and white reproductions in both instances). In determining the probable origin of the Gismonda poster design it is relevant to compare the precise geometries and details of the collage with those of the poster. In principle the flicker technique, photogrammetry, and 3-D video imaging are both convenient and informative in such comparative studies. However, for hardcopy transmission to scholars (viz., art historians) and convenient interpretation we found that a colorcoded overlay provides the most revealing display for our purposes. This may be accomplished by means of a photographic double exposure in which one image is in one color and the other is in another. Thus, when the two images match, the colors combine (e.g., red and green may combine to produce yellow). In regions where image contours are not superimposed the degree of color displacement reveals geometrical image differences. With the advent of digital computer image processing this type of operation is much more readily and precisely performed with a computer than by means of photographic reproduction. Our first step in implementing such a bicolor Gismonda superposition was the digitization of highquality photographic reproductions of the poster and collage images. A high-pass image-processing (edge detection) algorithm was applied to both images in order to enhance outlines and details for comparison. The (5 × 5) kernel in the transformation was:

Figure 3. A Pont-Aven street (left) showing the irregular pavement where Paul Gauguin may have fallen and broken his leg during a brawl with three seamen. The rooming house (right) in Pont-Aven where Paul Gauguin lived with Annah la Javanese and recuperated from his broken leg in 1894.

Figure 4. Photographic self-portrait of Alfons Mucha in his Paris apartment/studio in early 1894 (left). Mucha photograph of some friends in his studio in 1894 (right). His future model and lover, Annah la Javanese, is in the rear at the center. Gauguin is in front.

−1 −1 −1 −1 −1

The Bernhardt poster emergency fell upon Mucha’s print shop several weeks after Annah joined him in Paris. It is plausible that in the urgency of those final hours of December Mucha came upon a desperate solution to his dilemma. He would have placed the Gismonda tile in photographic enlarger, traced the poster-sized image on paper, and use the copy to prepare the lithographic press. There would have been no time or sensible rationale for seeking Gauguin’s permission for use of his pilfered property. This hypothetical scenario establishes a circumstantial case for the provenance of the Gismonda tile and its role in the genesis of Art Nouveau. However, the superficial plausibility of this explanation does not constitute a defensible proof. Toward this end, digital computer Image Processing (IP) has been applied to the study of the Gismonda image of the tile collage and the Mucha poster. We also employ IP to analyze Gauguin’s signature and compare it to scratches on a cane from the same estate lot of the tile.

−1 −1 −1 −1 −1

−1 −1 25 −1 −1

−1 −1 −1 −1 −1

−1 −1 −1 −1 −1

Figure 5. A black and white reproduction of the sensational 1894 Gismonda poster by Alfons Mucha (center). The postage stamp collage is shown (black and white) to scale on the right.

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which signifies that the intensity value for the pixel in question is multiplied by 25 and the 24 surrounding values are multiplied by −1. The central pixel value is replaced by the normalized sum of all 25 numbers. As the original two photographs from which Figure 5 was produced were of different magnifications, it was necessary to scale the digital image files to identical sizes. This was accomplished by adjusting the respective scales so that face-neck lengths were equal numbers of pixels. The horizontal scale was checked by means of the separation of the hands. The initial geometrical comparison was accomplished next by digitally subtracting the enhanced Mucha design from the enhanced collage design. The result is shown in Figure 6 (left) with the original red/green pseudo color-coding replaced by a gray scale for this publication. In Figure 6 the Mucha poster design edges are displayed in gray. The collage design edges appear as white lines. A careful inspection of the Figure 6 (left) overlay reveals that there is an almost perfect match between the outlines of the two compositions for the top halves. Specifically, by following the profiles of each palm leaf or gown detail it is seen that they are represented by light-dark line pairs that follow identical trajectories in most instances. On the other hand, when the entire superposition is considered (rather than just the top half), an entirely different picture emerges. The entire lower half exhibits a major vertical displacement. This suggests that a central horizontal strip of the Mucha poster is missing (with respect to the collage composition). In order to illustrate this hypothesis that the Mucha design is incomplete, this image was sliced at the center and the upper and lower portions were separated (in the computer’s digital

Figure 7. Typical design detail from the front of the Gismonda gown (left). The same gown design detail at the location of the apparent break at the middle of the Gismonda poster (right) identifying the elements that have been removed.

image file). Figure 6 (right) displays the consequences of such an operation. The top and bottom halves of the Mucha poster image conform perfectly to respective portions of the collage image. This shows that the poster is missing a band of detail at the center constituting about 5% of its length. This conclusively establishes that the collage cannot be a copy of the poster. On the other hand this opens the door to the possibility that the poster could be an enlarged copy of the collage. Thus, it is technically possible that the collage may be the template from which the poster design was derived. In producing the Figure 6 (right) overlay the poster image was arbitrarily sliced at the center as a cursory inspection indicated this level as the origin of the discontinuity. However, we were able to determine the precise position of the dislocation by considering design details of the gown. The front panel of the Gismonda gown has a repeating sea creature/foliage design with four cycles. Figure 7 (left) reproduces one floral element in one cycle of the complete branch with leaves. This appears in a complete form in all four cycles on the collage and three of the four cycles on the Mucha poster. However, on the poster the fourth of the cycles is presented in an abridged and unsymmetrical form (Figure 7, right). This feature appears at the second cycle from the top of the gown panel. This is at the level of the proper right hand of the female figure. This element is complete on the collage. 5

WALKING STICK PROVENANCE

We developed another piece of evidence to consider in addition to the expert attribution of the collage tile to the hand of Paul Gauguin (Section 1). There was an antique hand-carved walking stick in the estate sale with the tile collage. Clearly, Gauguin would have required such assistance once his leg had healed enough for him to leave his bed. The handle end of this hand-carved cane exhibits faint traces of deliberate scratches. A black and

Figure 6. Overlays of the edge-enhanced outlines of the Gismonda figures from the Mucha poster (gray) and the postage-stamp collage (white). The overlay on the left reveals that the poster figure is shorter in proportion than the collage. On the right the poster image has been split and the lower half has been moved downward in order to match the collage at both the top and the bottom.

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are virtually identical except for a horizontal band that is omitted at the center of the poster. As the two Gismonda artworks are so different in size, the only 19th Century technology capable of producing a scaled copy of such high fidelity would be an optical photographic enlarger such as those available to Mucha in his employer’s print shop. If one supposes that the tile collage was a photographically guided copy of the Mucha poster, there is no sensible explanation for adding a strip in the middle. Conversely, the lithographic process is such as to quite easily lose a strip in going from an enlarged image of the tile to the poster. (It is customary to print large posters with two lithographic plates and detail at the joint between the two plates is lost). A third possibility is that both pieces are copies of some common ancestor. However, on the one hand there is no evidence for such a work. On the other this would involve two independent optical copy procedures with the attendant doubling of the degradation in spatial fidelity. This is belied by the observed precision of the spatial match. Further, the mechanics of collage making are such that it would be vastly more difficult to produce from an optically projected image than would be a drawing. Lastly, evidence that the collage is by Gauguin is compelling as the autographs on both the collage and the associated walking stick conform to those known to be of the artist. The inescapable conclusion to be drawn from this study must be that Gauguin is the originator of the design of the Gismonda poster that triggered the rise of the Art Nouveau movement. What remains to be resolved is whether Gauguin (in distant Tahiti) knew of Mucha’s use of the design, whether he gave his friend permission to use it, or whether it was simply an act of plagiarism.

Figure 8. A photograph of the handle of a wooden walking stick (top) from the same estate sale as the postage-stamp collage. Computer-enhanced (FFT, edge enhancement, gain-bias adjustment) scratches on the handle of the walking stick (center). Typical Paul Gauguin signature from a painting (bottom).

white copy of a color photograph of this cane handle is reproduced at the top of Figure 8. We employed computer image enhancement to each of the scratches in an effort to decipher them. We employed FFT and Hi-Pass spatial filters as well as pseudo-color and gain-bias transformations. The results of these numerical operations are shown at the center of Figure 8. Gauguin’s writing style was notably idiosyncratic so it seemed implausible to suppose that our performing cross correlations with diverse alphabet styles would prove fruitful. Consequently, we relied on a subjective visual comparison of the enhanced scratches with a characteristic Gauguin signature (Figure 8, bottom). We concluded that the walking stick scratches are probable representations of Gauguin’s signature. It is known that Gauguin worked with several different media in his art. It follows that it is plausible to surmise that he may have carved the walking stick himself. In late 1894 when Annah left Gauguin and stole his property she probably realized that his art was beginning to attain some marketable value (as did one of his landladies who refused to relinquish some of his paintings). It seems that Annah pilfered both the cane and the Gismonda collage from Gauguin’s possessions and took them to Mucha’s studio when she moved in with him. Then Mucha had a perfect opportunity to copy Gauguin’s collage composition under the emergency situation of having only days to create a commercial poster for Bernhardt. The association of the collage with the walking stick supports this scenario. 6

ACKNOWLEDGEMENT Mr. Charles Tyler of Brentwood, California proposed and developed the Art Nouveau genesis theory summarized above. He also paid the mainframe computer (CRAY) time charges associated with the digital image manipulations of this investigation.

REFERENCES Hoog, M. 1987. Paul Gauguin Life and Work. New York: Rizzoli. Reade, B. 1963. Art Nouveau and Alphonse Mucha. London: Her Majesty’s Stationery Office. Tyler, C. 1988. An Art Nouveau Phenomenon. Los Angeles: Charles B. Tyler Publisher.

CONCLUSIONS

In conclusion, it emerges from this computer-aided comparison of the two designs that the geometries

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Innovative approaches in laser cleaning researches and instrumentation development

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

The effect of ultrafast lasers on laser cleaning: Mechanism and practice K.G. Watkins & P.W. Fitzsimons Laser Group, Department of Engineering, University of Liverpool, Liverpool, England, UK

M. Sokhan Department of Conservation, City & Guilds Art School, London, UK

D. McPhail Department of Materials, Imperial College, London, UK

ABSTRACT: The increased availability of picosecond (ps) and femtosecond (fs) laser sources opens the prospects of new tools for art restoration. However, the use of such ultrafast lasers has most often been investigated in application areas such as material removal by ablation which is more proper to engineering requirements rather than conservation. The purpose of this investigation is to consider the effectiveness of these ultra fast laser types under the specific requirements of the conservation activity and to determine whether new mechanisms leading to strictly limited material removal on the nano and micro size scales are operational. A dielectric layer of TiO2 is considered and the results are analysed by means of surface analysis techniques in order to deduce the operative mechanism in each case and to assess the applicability of ultra short pulse lasers in art conservation. 1

could play an increasing role in art conservation (S. Barcikowski et al., 2006; A.V. Rode et al., 2006; S. Georgiou et al., 2008; S. Gaspard et al., 2008). As conservators use these laser types, what mechanisms will be involved? Will the mechanisms be the same, given that the pulse length is now much shorter (200 fs–20 ps compared with

INTRODUCTION

Most conservation work with lasers is approached empirically. However, there are good reasons to enquire about what mechanisms are operating when material is removed from the artefact surface. Surprisingly, there is still a shortage of knowledge about what is actually taking place when an art object is cleaned by laser. For the case of the laser types with pulse length varying from continuous wave down to nanosecond pulses that are most often currently employed, it is clear that more than one mechanism is involved (K.G. Watkins et al., 2005). As shown schematically in Figure 1, a range of mechanisms can be employed depending on the laser absorbed intensity and the pulse length. Lasers with much shorter pulse length— picosecond and femtosecond lasers—have been available in research laboratories and in industry for some time but these are bulky, expensive and mainly unavailable in art conservation. Very recently, new, low cost, small size picosecond and femtosecond lasers have become available that could extend the tool set available for conservation. The short pulse passively mode locked fibre lasers that are becoming available are of small form and of low cost compared with earlier short pulse laser types and

Figure 1. Absorbed intensity versus interaction time diagram showing schematic regimes of candidate laser cleaning mechanisms (K.G. Watkins et al., 2005).

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to stop at the layer—substrate interface, it could be self limiting. A key advantage is that the extent of material removal could be controlled down to the nanometer scale. This would amount to a new process of ultra-fine laser cleaning. Because the pulse repetition rate is so large in these new ultrashort pulse laser types, cleaning rate in terms of area coverage (as the laser is rastered over the work piece) is expected to be high. Stoian et al. (2008), investigating laser ablation of Al2O3 by a coulombic repulsion mechanism with 100 fs laser pulses, found an initial gentle ablation regime in which a ‘few nanometers’ is removed per laser shot, followed by a strong ablation regime characterised by an order of magnitude increase in ablation rate. The crossover point was after 20 laser pulses, the transition attributed to a buildup of mechanical deformation (F. Aumayr et al., 1999). Both regimes may be valuable in art conservation. The objective of this preliminary study is to establish evidence for the controlled removal on a nanometer scale of material from titania by the use of femtosecond laser pulses and to offer this as a contribution to the recognition of the new laser types as valuable new tools in art conservation. The text should fit exactly into the type area of 187 × 272 mm (7.36" × 10.71"). For correct settings of margins in the Page Setup dialog box (File menu) see Table 1.

10 ns for Nd:YAG) and the repetition rate much higher (up to 100 MHz compared with 10 Hz for Nd:YAG)? At these very short pulse lengths, laser intensity at the treated surface will be very large −1010–1014 W/cm2. This compares with laser intensity at the treated surface of 107–109 W/cm2 for typical Nd:YAG cleaning. At the high intensity produced by the ultrashort pulse laser types, new mechanisms become possible. This may lead to new forms of controlled removal of material from a surface. The mechanisms involved are likely to be as complex in scope and scale as those summarised for pulse lengths down to nanoseconds, as shown in Figure 1, and it is unwise at this stage to be restrictive. It is more proper to consider candidate mechanisms. One such candidate is the coulombic repulsion mechanism (R. Stoian et al., 2002). The key concept is that under highly intense laser radiation, the highly energetic photons can remove electrons from the bound atoms or molecules in the surface of the materials and the atoms / molecules there will obtain a net positive charge. If the resulting repulsive force is large enough to overcome the bonding forces, material will be removed by coulombic repulsion. Note that there would be no melting; the process of removal would be more akin to sublimation (phase change without melting). In most cases in art conservation, avoidance of melting is desirable. More than one photon may be required to promote an electron from the valence band to the conduction band (multiphoton ionisation). To achieve this laser intensity approaching 1012 W/cm2 is required. Coulombic repulsion is unlikely to be active in metals (J. Cheng et al., 2009). Metals have abundant free electrons that would simply replace the electrons removed by ionization. However, in art conservation, interest is normally in removing non metals—even on metals it is the oxides and sulphides that require removal. If the process was

Figure 2.

2

EXPERIMENTAL

The laser used in this study was a Clarke MXR CPA 2010 1 W average power femtosecond laser with a 1 kHz repetition rate operating at a wavelength of 387 nm. The output pulse energy was controlled via an external attenuator and ranged between 0.638–3.46 μJ with a temporal pulse

Schematic diagram of the experimental arrangement.

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Table 1.

Depth of ablation (nm) of TiO2 with fs pulses as a result of varying the fluence and pulse number. Number of pulses

Fluence (J/cm2)

0

5

10

15

20

30

40

50

0.32 0.48 0.72 1.02 1.29 1.73 2.15

0 0 0 0 0 0 0

– – 21.20 26.24 38.62 45.48 38.30

– – 24.40 32.66 41.80 45.80 42.24

– 29.74 25.18 36.98 41.70 46.74 –

15.90 31.4 27.80 40.02 45.72 – –

31.60 38.08 28.76 44.98 43.24 – –

37.80 34.36 30.82 46.68 X – –

34.50 39.96 41.44 X X – –

Legend: no results.

This shows that it is possible using fs pulses to ablate material from the TiO2 layer on a nanometre scale. The minimum depth recorded was 15.90 nm (recorded at 0.32 J/cm2 and 20 pulses) and the maximum was 46.74 nm (recorded at 1.73 J/cm2 and 15 pulses). Significantly all the material removal was less than 100 nm in depth, showing the possibility of ultrafine laser cleaning. At fluences of 0.32 and 0.48 J/cm2 the onset of surface ablation did not occur until 15 and 10 pulses, respectively; this indicates that the number of pulses is a contributing factor in the ablation process. As the surface was exposed to successive pulses the ablation threshold of the TiO2 was altered; so that after an incubation period ablation was initiated at the surface. Figure 3 is a white light interferometric measurement of the TiO2 surface post treatment; all depth measurements were made on the Y profile (Figure 3c). The average depth was recorded with respect to the original layer height. This measurement was taken at the lowest fluence recorded and shows that 50 pulses achieved an average ablation depth of 34 nm; this corresponds to an average ablation depth per of 0.68 nm per pulse if the incubation period was not considered. However, it was observed that the surface roughness at the centre of the ablated region is very high; this type of surface quality post processing would not be ideal for conservation needs. Figure 4 shows the ablated region after the surface had been exposed to 15 pulses at 1.73 J/cm2; the quality of the ablated surface was significantly improved over the surface quality observed at 0.32 J/cm2. The average depth of ablation recorded was also 34 nm; however this was achieved at significantly lower pulse numbers than Figure 3. The average depth of ablation recorded per pulse was 2.2 nm. It is immediately apparent that the surface quality in the ablated area is much improved over those observed at the lower fluence (0.32 J/cm2); surfaces of this quality would be much more applicable to conservation.

length of 180 fs (FWHM). The number of pulses was controlled using a shutter with an accuracy of ±3 ms. The focussed spot size was determined to be 29 μm. A 3-axis motion control system (Aerotech) was used in conjunction with NView MMI software to manipulate samples. TiO2 samples were prepared by sol-gel deposition onto a glass substrate via spin coating. A sol-gel of TiO2 was synthesised and added dropwise onto a silicon wafer. The sample was spun at 2000 rpm for one minute. An amorphous film was formed by heating the sample at 80°C for ten minutes and then at 320°C for a further thirty minutes. The process of adding the solution drop wise and heating was repeated until the desired number of layers had been added; at this point a final heat treatment of 500°C for sixty minutes was carried out to produce a consolidated film. The depth of the applied layer was measured to be approximately 500 nm. A single sample of TiO2 was exposed to an increasing number of pulses (5, 10, 15, 20, 30, 40 and 50) at increasing fluences (0.32, 0.48, 0.72, 1.02, 1.29 and 1.73 J/cm2). Through variation of these parameters and recording the average depth of ablation observed, it was possible to tabulate and quantify the effect of both the fluence and number of pulses upon the TiO2 layer. Surface analysis and depth profiling was performed using an optical microscope (Nikon) attached to a CCD camera and white light interferometry (WYKO NT1100). A Phenom (FEI) Scanning Electron Microscope (SEM) system was utilised to provide a highly detailed image of the ablated region. 3

RESULTS AND DISCUSSION

Table 1 shows the depths of ablation recorded on the single TiO2 sample with increasing fluence and pulse number; all observed ablation depths are given in nanometres.

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Figure 3. Ablated region of TiO2 surface. A fluence of 0.32 J/cm2 and 50 pulses were used. From top left clockwise: a) Topographic image; b) cross sectional profile in the X-plane; c) cross sectional profile in the Y-plane including depth measurement; d) 3D image of ablated region.

Figure 4. Ablated region from TiO2 surface. A fluence of 1.73 J/cm2 and 15 pulses were used. From top left clockwise: a) Topographic image; b) cross sectional profile in the X-plane; c) cross sectional profile in the Y-plane including depth measurement and d) 3D image of ablated region.

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calculated to be between 5.36 × 1011 and 1.81 × 1012 W/cm2. At intensities of around a terawatt, the optical properties of materials can be altered, leading to nonlinear absorption (R.L. Sutherland et al., 2003). This effect is characterised by a well defined ablated region with little or no damage to the immediate surrounding area. From the cross sectional profiles provided by white light interferometry and the SEM images, it was observed that the ablated region matches this expected criteria and that nonlinear absorption is likely. The mechanism for material removal utilising ultra short pulse lengths would not proceed under the same conditions observed at longer pulse durations (i.e. nanosecond). In work on alumina reported by Ashkenasi et al. (2000), which utilises intensities and pulse lengths similar to those presented here, the ablation process observed was coulombic repulsion, as determined by the detection of fast ions using mass spectrometry and the transition from a weak to a strong ablation regime (R. Stoian et al., 2000; R. Stoian et al., 2001; D. Ashkenasi et al., 2000) where the depth of ablation increases significantly. The mechanism requires the high optical intensities (A.V. Rode et al., 2006) provided by ultrashort pulses. To confirm the presence of this mechanism in the present case requires further study, specifically of the ablated matter produced in the first few moments after ablation.

Figure 5 shows the variation of ablation depth with number of pulses. It can be seen that generally as the number of pulses and fluence increases so did the depth of ablation observed. However, the depth of ablation observed for increasing number of pulses at 0.72 J/cm2 did not follow the expected trend; the observed values at this fluence were lower than the majority of the results recorded for 0.48 and 0.32 Jcm2. This could be due to the large variation in surface roughness observed at 0.32 J/cm2. In the white light interferometry results, measurement of the depth of ablation was not taken from the lowest point recorded by the interferometer. Two artefacts at the edge of the ablated region which rise above and below the surface are visible (see Figure 6); these features are not present on the surface. This was shown to be the case by viewing the ablated region using an SEM in backscattered electron imaging mode (Figure 7). The debris on the surface of the sample is due to the fracture process required to examine the surface under SEM. It is not possible to describe an exact mechanism of ablation based on the recorded results. However, it is possible to suggest a mechanism based on the pulse length, intensity and images of the surface. From the recorded pulses energies and the measured spot size, the intensity on the surface is 60

50

40

30

20

10 0

10

20

30

40

50

60

Figure 5. Variation of ablation depth with increasing pulse numbers. Legend shows the Fluence (J/cm2).

Figure 7. SEM images of the ablated region. The top row shows the ablated holes under full illumination at a low (left) and high (right) magnification. The bottom row consists of a high magnification hole under topographic illumination.

Figure 6. Expanded X-axis profile from Figure 4 highlighting the artefacts.

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4

CONCLUSIONS

F. Aumayr, J. Burgdorfer, G. Hayderer, P. Varga and HP. Winter, “Evidence against the “Coulomb Explosion” Model for Desorption of Insulator Surfaces by Slow Highly Charged Ions”, Physica Scripta, Vol. T80, 240–242, 1999. J. Cheng, W. Perrie, B. Wu, S.P. Edwardson, M. Sharp, G. Dearden, and K.G. Watkins, “Ablation Study on Metallic Materials with a Picosecond Laser: Experimental and Simulation Analysis”, 2009. K.G. Watkins, and W.M. Steen, “Laser Materials Processing, Chapter 9: Laser Cleaning”, Third Edition, Springer-Verlag, 2005. R. Stoian, A. Rosenfeld, D. Ashkenasi and I.V. Hertel, “Surface Charging and Impulsive Ion Ejection during Ultrashort Pulsed Laser Ablation”, Phys Rev Letters, Vol. 88, 9, 2002. R. Stoian, D. Ashkenasi, A. Rosenfeld and E.E.B. Campbell, “Coulomb Explosion in Ultrashort Pulsed Laser Ablation of Al2O3”, Volume 62, Number 19, Physical Review B, 2001. R.L. Sutherland, “Handbook of Nonlinear Optics” 2nd Edition, Marcel Dekker, 2003. S. Barcikowski, N. Barsch, T. Burmester, J. Bunte, J. Ulrich, A. Gervais and M. Meier, “Femtosecond Laser Cleaning of Metallic Antique Artworks– Advantages, Limits and Economic Aspects”, Laser Cleaning II, D.M. Kane, World Scientific Publishing Ltd. 2006. S. Georgiou, D. Anglos and C. Fotakis, “Photons in the service of our Past: Lasers in the preservation of Cultural Heritage”, Contemporary Physics, Vol. 49, No. 1, 1–27, 2008. S. Gaspard, M. Oujja, P. Moreno, C. Mendez, A. Garcia, C. Domingo, and M. Castillejo, “Interaction of Femtosecond Laser Pulses with Tempera Paints”, Appl. Surf. Sci. 255, 2675–2681, 2008. X.C. Wang, G.C. Lim, H.Y. Zheng, F.L. Ng, W. Liu, and S.J. Chua, “Femtosecond pulse laser ablation of Sapphire in ambient air”, Appl. Surf. Sci. 228, 221–226, 2004.

Using fs pulses it is possible to remove nanometre layers from the surface of a dielectric through ablation. Removal of material on such a scale holds many potential benefits for conservation; i.e. the removal of over paints, tarnish layers and oxides from metals with no damage to the underlying substrate. Ablation with fs pulses produced well defined removal with no lateral damage to the surrounding surface. It is suggested that the candidate mechanism of ablation operative with the parameters used in this study is coulombic repulsion; however more experimentation is needed to confirm this. The number of pulses impinged on the surface plays an important role in the ablation mechanism. From Table 1, it can be seen that ablation can be initiated at higher pulse numbers when previously there was no ablation; this indicates that the ablation threshold of the target is altered by successive pulses. This effect is also present at higher fluences where the number of pulses influences the total depth of ablation. REFERENCES A.V. Rode, N.R. Madsen, E.G. Gamaly, B. Luther-Davies, K.G.H. Baldwin, D. Hallam, A. Wain and J. Hughes, “Ultrafast Laser Cleaning of Museum Artefacts”, Laser Cleaning II, D.M. Kane, World Scientific Publishing Ltd. 2006. D. Ashkenasi, R. Stoian, and A. Rosenfeld, “Single and multiple ultrashort laser pulse ablation threshold of Al2O3 (corundum) at different etch phases”, Appl. Surf. Sci. 154–155, 40–46, 2000.

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

Spectral analysis of the effects of laser wavelength and pulse duration on tempera paints M. Oujja & M. Castillejo Instituto de Química Física Rocasolano, CSIC, Madrid, Spain

P. Pouli & C. Fotakis Institute of Electronic Structure and Lasers (IESL), Foundation for Research and Technology-Hellas (FORTH), Crete, Greece

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

ABSTRACT: The application of laser cleaning methodologies on light-sensitive cultural heritage substrates (paintings, documents on paper and parchment, textiles, etc) requires the study of the physicochemical effects that may be induced upon laser irradiation to the constitutive materials. We present here work carried out to analyze the influence of laser wavelength and pulse duration on the modifications induced on egg yolk based tempera paints by using pulses of 150 picoseconds (at 1064 and 213 nm) and 10 nanoseconds (at 213 nm) on unvarnished aged model samples of unpigmented and artist’s pigment temperas (vermillion and lead chromate). A multianalytical approach was chosen based on the use of colorimetry for the quantification of colour changes, and spectrofluorimetry and Fourier Transform Raman spectroscopy (at 1064 nm) to assess possible laser induced chemical changes. An important observation from these experiments is that discoloration at 213 nm/150 ps is limited, fact that gets particularly important in the case of vermillion which is a well known laser-sensitive pigment. Raman bands attributed to the pigment in the tempera sample remain unchanged upon laser irradiation, except in the case of vermillion where the pigment bands tend to disappear from the spectra. Absence of amorphous carbon bands rule out carbonization or charring of the paint layer upon irradiation. Comparison of the obtained results using the pulse durations and wavelengths of this study with those previously available obtained with UV and IR nanosecond pulses illustrate the participation of mechanisms of diverse origin according with the pigment chemical nature and highlights the importance of the optimization of the laser parameters, mainly fluence and wavelength, in conservation treatments. 1

INTRODUCTION

(Athanassiou et al. 2000, Castillejo et al. 2001, 2002, 2003a & 2003b, Cooper et al. 2002, Chappé et al. 2003, Gordon Sobott et al. 2003, Pouli et al. 2000, 2001 & 2003, Teule et al. 2003, Zafiropoulos 2003). Furthermore, it was noticed that both the raw material (Pouli et al. 2001) and the paint system are affected (Weeks 1998), while the composition of the pigment and/or the irradiation parameters may affect the extent of the induced discoloration. It is interesting to note that, although verdigris was found to be laser insensitive (Castillejo et al. 2003, Pouli et al. 2001), vermillion (which in any case is one of the most known light-sensitive pigments) was found to discolour, in all studied wavelengths, at relatively low fluence values. Initially, given the sensitivity of paint systems to heat, photo-thermal phenomena were considered to explain the discoloration. This hypothesis was soon reconsidered as

This study aims to re-assess, on the basis of new advances in laser technology, the laser induced discoloration of pigments, which is considered an important drawback for the use of lasers in the conservation of paintings and polychromes. Since the early times of the laser cleaning applications (Weeks 1998) the scientific community was concerned with the sensitivity of pigments to laser irradiation and in this respect, a series of studies have been focused on the understanding of the phenomena related to the laser induced pigment discoloration. Experimentation using UV, VIS and IR laser wavelengths on a wide range of pigments both in raw form as well as in paint-systems (in various binding media) indicated that darkening of the pigment particles occurs in all the investigated wavelengths

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with a mixture of chalk and gypsum. They were prepared according to the procedure described elsewhere (Castillejo et al. 2002). Two different pigments, widely used in artistic painting practice with various chemical characteristics, were selected. These are vermillion (HgS) and lead chromate (PbCrO4), a traditional red pigment and yellow modern pigment respectively. Samples of unpigmented tempera (egg yolk) were also prepared to perform a comparison with results obtained in the binding medium itself.

reduced states of pigment compounds have been detected by means of surface specific analysis (i.e. X-Ray Photoelectron Spectroscopy, XPS) (Chappé et al. 2003, Pouli et al. 2001, Teule et al. 2003). Reduction mechanisms were further supported by the fact that darkening of several pigments (i.e. lead pigments (Pouli et al. 2001, Cooper et al. 2002)) was reversed in oxygen-rich environments and oxidised states of the laser induced products were detected. The introduction of ultra-short laser pulses of picosecond (ps) and femtosecond (fs) pulse duration in the field significantly enhanced the laser restoration possibilities, as they were found to overcome many disadvantages of the nanosecond (ns) laser pulses. Their superiority as regards the minimisation of photo-thermal, photo-mechanical and photo-chemical phenomena, independently of the optical properties of the treated material, and the optimization of morphological aspects, was shown on a variety of cultural heritage materials (Andreotti et al. 2006, Bartoli et al. 2006, Burmester et al. 2005, Gaspard et al. 2008a & 2008b, Pouli et al. 2007 & 2008) including paint systems (Castillejo et al. 2002). Initial studies on tempera paints using pulses of 120 fs at 795 nm (Gaspard et al. 2008a & 2008b) have shown the high degree of control that may be achieved in comparison to ns UV (248 nm) pulses. In continuation of this work, further experimentation on the same type of technical samples was undertaken by employing IR (1064 nm) pulses of 150 ps and UV (213 nm) pulses of 15 ns and 150 ps pulse duration. We present here results obtained in a selection of systems (Oujja et al. 2010), unpigmented, vermillion and lead chromate using colorimetry, spectrofluorimetry and Fourier Transform (FT) Raman spectroscopy, chosen to assess the physical and chemical modifications induced upon laser irradiation. Given the previous experience from the laser irradiation of the same type of samples (excimer laser at 248 nm, 25 ns pulse duration (Castillejo et al. 2002 & 2003a) and Ti:Sapphire laser at 795 nm, 120 fs pulse duration (Gaspard et al. 2008a & 2008b)), this study is expected to discuss the role of the operative wavelength and pulse duration on the laser irradiation of egg-yolk based paints, and thus to significantly approach the discoloration issue indicating the laser parameters that may overcome such effect. 2 2.1

2.2

Laser irradiation

Laser irradiation of the unvarnished tempera samples was carried out using two Nd: YAG laser systems. Pulses of 150 ps at 1064 and 213 nm were delivered by an EKSPLA, SL-312 system, while pulses of 15 ns at 213 nm were produced by a Lotis II, LS-2147 system. Irradiation tests were performed on a singlepulse basis for various fluence values (F), in order to determine ablation (Fth) and discoloration thresholds, as well as on a scanning basis in order to achieve a homogeneously irradiated area on which further analysis could be performed. Once the ablation and discoloration thresholds were determined for each tempera paint at the two irradiation wavelengths and pulse durations studied, a set of fluence values ranging below and well above the ablation threshold fluences was chosen for the scanned areas. Figure 1 shows images of the areas created on the tempera paints by irradiation at 1064 and 213 nm (with pulses of 150 ps). The irradiated surfaces (approximately 10 × 10 mm2) were created upon scanning of the beam (focused by a cylindrical planoconvex quartz lens of f = 150 mm to a spot about 10 × 1 mm2) along its largest spot dimension, with a 5% overlap between successive pulses. 2.3

Analytical techniques

Physicochemical changes induced by laser irradiation were assessed by using a combination of spectroanalytical techniques. Spectrocolorimetry served to measure the change of colour, while spectrofluorimetry and FT-Raman spectroscopy provided information on chemical modifications induced on the irradiated surfaces. A Minolta CM 2002 spectrocolorimeter served to measure the chromatic properties of the samples and specifically the changes induced by laser irradiation. Five spectra were obtained in each zone and averaged to obtain one data point. The CIELab colour space was used to measure colour shifts expressed in three variables, namely, ΔL*, Δa* and Δb*. The magnitude of the overall colour change is given by ΔE* = [(ΔL)2 + (Δa)2 + (Δb)2]1/2.

EXPERIMENTAL Samples description

The samples used for this investigation consist on unvarnished egg-yolk based tempera paint (thereafter tempera paints) applied on white panel constituted by a commercial card substrate primed

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intervals of 1 nm and excitation/emission spectral slits of 2.5 nm. All the samples were analysed at 30º from the sample axis. The emission spectra presented were taken with the excitation wavelength of 350 nm. FT-Raman spectra were recorded with a RFS 100/S–G Bruker spectrometer equipped with a cooled Ge detector. The excitation source consists in a Nd:YAG laser emitting at 1064 nm. Low laser power outputs, in the range of 10–20 mW, were used to prevent damage to the samples. Each data point was the result of the accumulation of 200 scans. The wavenumber resolution was 8 cm–1. 3 3.1

RESULTS Discoloration and ablation thresholds

The discoloration thresholds were determined by measuring the energy at which darkening of the irradiated area is observed under the optical microscope and are reported in Table 1. On the other hand, ablation thresholds were calculated by applying the spot regression method (Liu 1982). For laser pulses with a Gaussian spatial beam profile, the maximum laser fluence, F, on the sample surface and the diameter, D of the ablated area are related by D2 = 2ω0ln(F/Fth), where ω0 is the 1/e2 radius of the Gaussian beam distribution and Fth the ablation threshold. The diameter of the ablated area was determined as observed by optical microscopy. From a plot of D2 versus ln E, Fth and ω0 can be determined. Ablation thresholds of the different paint systems are reported in Table 1. It is observed that the thresholds of the pigmented systems are lower than those of the unpigmented tempera due to the fact that the effective absorption of the paint layer increases in the presence of a pigment. At a fixed wavelength (213 nm), the fluence thresholds are reduced by a factor of 1.3–1.8 when using shorter pulses of 150 ps. It is also observed that the ablation thresholds of the different systems are higher at 1064 nm than at 213 nm, due to the increase of the effective absorption of both the binding Figure 1. Tempera paint samples with zones irradiated at different fluences with 150 ps pulses: a unpigmented; b vermillion and c lead chromate (numbers near irradiated zones correspond to F/Fth). For each pigment, the upper and lower rows display areas irradiated at 1064 and 213 nm respectively.

Table 1. Ablation and discoloration (in parentheses) thresholds in mJ/cm2 for the treated tempera paints. The estimated errors are around 10%. (*) No discoloration observed. Wavelength

Spectrofluorimetric measurements were performed with a scanning Jobin-Yvon (FluoroMax-4) system with excitation provided by a Xenon arc lamp. The scanning was carried out with an integration time of 0.2 s per point in

213 nm 1064 nm

Pulse UnpigLead duration mented Vermillion chromate 15 ns 150 ps 150 ps

450 (*) 250 (*) 800 (*)

260 (*) 160 (*) 400 (70)

250 (60) 150 (50) 400 (80)

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medium and the used pigments upon irradiation in the UV region (Castillejo et al. 2002 & 2003a, Nevin 2008). 3.2

of ΔE* even at fluences below ablation thresholds. For vermillion, overall colour changes of ΔE* = 35 and 40 were measured in areas irradiated at fluences of 0.65 Fth and 1.25 Fth respectively. Under irradiation below ablation threshold, we observed darkening of the red colour and above threshold, the pigment acquires a grey metallic aspect. For the lead chromate based paint, irradiation at 1064 nm induces an appreciable degree of discoloration both at fluences below and above ablation threshold (ΔE* = 13 and 19 for fluences of 0.36 Fth and 1.1 Fth respectively). The irradiation of the unpigmented tempera paints with pulses of 150 ps at 1064 nm results in different discoloration degrees. However, examination under the optical microscope indicates that in this system the observed discoloration is due to colour changes induced to the underlying panel.

Colorimetry

Colorimetric measurements were performed on the different non-irradiated and irradiated tempera paint areas. Table 2 displays the values of ΔE* as a function of the irradiation fluence. The overall colour change of the paints irradiated at 213 nm with 150 ps and 15 ns pulses is limited in the explored fluence range, with values of ΔE* ≤ 10. Lead chromate displays the highest degree of discoloration under these irradiation conditions. In particular, the discoloration induced in lead chromate by irradiation with 213 nm, 150 ps pulses at a fluence of 1.06 Fth is ΔE* = 10. Picosecond irradiation at 1064 nm of the vermillion and lead chromate paints yields high values

3.3

Table 2. Irradiation conditions of tempera paints. F and Fth indicate the fluence used to create the irradiated areas and the ablation threshold measured for each system respectively. ΔE* indicates the overall colour change. Pigment

Wavelength and pulse duration

213 nm, 15 ns

Unpig mented

213 nm, 150 ps

1064 nm, 150 ps

213 nm, 15 ns

Vermillion

213 nm, 150 ps

1064 nm, 150 ps

213 nm, 15 ns

Lead chromate

213 nm, 150 ps

1064 nm, 150 ps

F/Fth

ΔE*

0.52 1.05 2.00 0.13 0.32 1.62 0.35 0.71 0.88 1.88

2.0 2.7 3.2 1.8 2.3 1.7 8.3 14.6 17.6 –

0.83 1.66 2.00 0.12 0.31 1.56 0.15 0.30 0.65 1.25

1.4 3.1 3.5 1.2 2.5 3.0 29.8 35.0 35.4 40.6

0.50 1.17 2.00 0.02 0.20 1.06 0.16 0.36 1.11

1.2 2.8 8.0 1.2 3.3 10.0 5.3 13.2 19.8

Spectrofluorimetry

The fluorescence spectrum of the unpigmented system consists of a broad feature extending from 375 to 650 nm (Figure 2a). This wide emission is the resulting contribution of different fluorofores that participate in the composition of egg yolk (Gaspard et al. 2008, Lakowicz 2006, Mills et al. 1994). Upon excitation at 350 nm, the fluorescence from the three aminoacids of proteins, tyrosine, tryptophan and phenylalanine, is not excited, as this wavelength is above their absorption maxima. Therefore the contribution of these compounds to the observed fluorescence band can be ruled out. In fact, in the region below 500 nm, the emission is due to the products of photo-oxidation, combination and modification of aminoacids, such as dityrosine, 3,4 dihydroxyphenylalanine (DOPA) or N-formylkynurenine (NFK) and kynurenine (Gaspard et al. 2008, Nevin et al. 2006a & 2006b, Palumbo et al. 2004, Wisniewski et al. 2007). Dityrosine, a dimeric species of tyrosine, has an emission maximum at 410 nm (Gaspard et al. 2008), while DOPA, another photo-oxidation product of tyrosine displays a characteristic emission at 480 nm (Palumbo et al. 2004, Wisniewski et al. 2007). NFK and kynurenine are products of the oxidation of tryptophan and emit at around 435 nm (Nevin et al. 2006a & 2006b). At longer wavelengths, above 520 nm, phospholipids are the main species that contribute to the fluorescence emission observed (Palumbo et al. 2004). Fluorescence spectra of the unpigmented paint recorded on areas irradiated at 213 (with pulses of 150 ps and 15 ns) and 1064 nm (with pulses of 150 ps), as shown in Figure 2a, reveal the changes induced by laser irradiation. A relative increase of intensity in the region of 400–480 nm is observed,

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pigment, which prevents the photo-oxidation of the compounds constituting the binding medium. The fluorescence spectra recorded in nonirradiated areas of lead chromate tempera (not displayed in the figure) contain two bands. The broad one, ranging from 375 to 525 nm attributed to the binding medium and the intense narrow one centred at 557 nm is assigned to the lead chromate pigment (emission due to the chromate ion chromophore, CrO42−) (Castillejo et al. 2002 & 2003a). No changes were observed for this paint system upon irradiation at 213 nm (with pulses of 150 ps and 15 ns) or 1064 nm (with pulses of 150 ps), even with fluences above ablation threshold. 3.4

FT-Raman spectroscopy

Figure 3 shows the FT-Raman spectra of the paint systems, once the spectrum of the underlying panel was adequately subtracted. Figure 3a display the spectra of the unpigmented system showing the characteristic bands of: tryptophan at 767 and 873 cm−1, the aminoacid phenylalanine at 1003 cm−1, the methylene groups of lipids and amino acids at 1302 and 1445 cm−1, the amide III and amide I of the proteins backbone at 1240 and 1656 cm−1, the C = O stretching at 1744 cm−1 assigned to fatty acid esters and the aliphatic compounds in the 2750–3100 cm−1 region (assigned to the ν(C-H) mode) (Bell et al. 1997, Drake et al. 2004, Nevin et al. 2007, Osticiolo et al. 2008, Vandenabeele et al. 2000). Irradiation at 213 nm (either with pulses of 150 ps or 15 ns) of the unpigmented tempera induces a slight decrease in the intensity of bands in the 600–1750 cm−1 region (Figure 3a), in particular those assigned to tryptophan, phenylalanine, amide III and I and fatty acid esters. However, a mild increase in the intensity of aliphatic compounds (2750–3100 cm−1 region) is observed. Irradiation at 1064 nm (150 ps) induces similar effects to those detected after irradiation at 213 nm. The decrease in the intensity of the bands in the region 600–1750 cm−1 is indicative of the degradation of aminoacids. The modifications induced by laser irradiation of the vermillion paint at 213 nm (with pulse duration of 150 ps and 15 ns), as reported in Figure 3b, show an insignificant decrease in the intensity of the pigment bands observed at 252, 282 and 343 cm−1 (Burgio et al. 2001). However irradiation at 1064 nm (150 ps) induces the total disappearance of the pigment bands indicating the degradation of the pigment. FT-Raman spectra of the lead chromate tempera do not show significant modifications upon laser irradiation at 213 nm (150 ps and 15 ns) and 1064 nm (150 ps) (Castillejo et al. 2002) in

Figure 2. Normalized fluorescence spectra taken at non-irradiated and laser irradiated areas of tempera paints: a unpigmented and b vermillion. The spectra were taken with 2.5 nm resolution at the excitation wavelength of 350 nm.

indicating the corresponding increase of photodegradation products emitting in this region. Figure 2b shows the fluorescence spectra recorded in areas of the vermillion system which are mainly pictured by two broad bands. The first one, in the region from 375 to 525 nm, is attributed to the binding medium while the second one, centred at 607 nm, is assigned to luminescence of the HgS semiconductor (Castillejo et al. 2002 & 2003a). Irradiation in the UV at 213 nm with pulses of 150 ps or 15 ns does not modify the emission spectra. However irradiation at 1064 nm in the ps regime results in the decrease of the emission corresponding to the pigment at fluences below ablation threshold and eventually its disappearance above ablation threshold, indicating that the pigment is chemically degraded. Contrary to what is observed in the unpigmented system at this wavelength, the fluorescence band of the binding medium in the painting mixture remains unaltered, due to the high absorption of laser light by the vermillion

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(Gordon Sobbot et al. 2003, Teule et al. 2003). In particular, in previous works, samples similar to the ones treated here were irradiated with KrF excimer laser pulses (248 nm, 25 ns) (Castillejo et al. 2002 & 2003a) and with a Titanium:Sapphire laser (795 nm, 120 fs) (Gaspard et al. 2008a, b). The colorimetric, spectrofluorimetric and FTRaman measurements provide the basis for discussion of the chemical changes induced on the different tempera paints at the two irradiation wavelengths and pulse durations. As mentioned, enhanced photodegradation of compounds of the egg yolk based binder, mainly dityrosine, DOPA, NFK and kynurenine is observed. However, in the pigmented temperas, the fluorescence bands of the binding medium remain unaltered due to preferential absorption of laser light by the pigment related chromophores, which prevents photo-oxidation of the binding medium components. Irradiation at 248 nm (25 ns) and 795 nm (120 fs) of this type of samples (Castillejo et al. 2002 & 2003a, Gaspard et al. 2008a, b) yielded a similar effect of preservation of the binding medium in presence of the pigment. The reduced extent of changes in the spectroscopic properties of the binding medium in the pigmented samples gives further indication of the stability of the binder in combination with the different studied pigments. These results provide a strong evidence of the prominence of a photochemical mechanism upon laser irradiation at 213 nm (150 ps and 15 ns), with minor contribution of further thermal effects on the surface of the samples, due to the high absorption coefficient of the binding medium at this wavelength (2550 cm−1) (Nevin 2008). Further evidence of the reduced contribution of a thermal mechanism at 213 nm is the absence of carbonization or charring that in contrast does occur at 248 nm, as observed by the appearance of Raman bands characteristics of amorphous carbon (Castillejo et al. 2002). The effect of laser irradiation on the pigmented systems under the conditions of this work is extremely dependent on the composition of the pigment itself. This fact is well illustrated by the comparison of the characteristic spectral features of vermillion and lead chromate in the spectrofluorimetry and FT-Raman results. Strong discoloration in the former, accompanied with the disappearance of the pigment bands, is in contrast with the slight discoloration and unaltered spectral bands in the latter. The limited colour changes (ΔE* ≤ 3.5) observed upon irradiation at 213 nm (150 ps and 15 ns) on the vermillion system are in contrast with those measured upon irradiation at 1064 nm (150 ps), at 248 nm (25 ns) (Castillejo et al. 2002 & 2003a) and 795 nm (120 fs) (Gaspard et al. 2008b).

Figure 3. FT-Raman spectra taken at non-irradiated and laser irradiated areas of tempera paints: a unpigmented and b vermillion. The spectra were taken with 8 cm−1 resolution at the excitation wavelength of 1064 nm.

the position and intensity of the pigment bands, observed between 338 and 403 cm−1 and at 839 cm−1. No bands of the binding medium were observed in the FT-Raman spectra of the pigmented systems, except the corresponding to the aliphatic compounds in the 2750–3100 cm−1 region assigned to the ν(C-H) mode (not shown in the spectra). 4

DISCUSSION

The results presented above obtained under different laser irradiation conditions can be discussed in reference to previous results (Castillejo et al. 2002 & 2003a, Gaspard et al. 2008a, b), although care should be taken when comparing with those obtained on samples prepared with different binding media and under different ageing conditions

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These differences can be ascribed to differences in the absorption coefficient of the binding medium (2550, 1000 and 100 cm−1 at 213, 248 and 795 nm respectively) (Nevin 2008), resulting in a high absorption of laser photons by the latter at shorter wavelengths. Possible mechanisms of darkening of the vermillion pigment have been the subject of various studies. The detection of traces of black cubic system (α’-HgS) by X-ray diffraction (XRD) upon fs laser irradiation (800 nm, 100 fs), suggested the formation of black meta-cinnabar (Zafiropoulos et al. 2003). On the other hand, XPS measurements performed on samples irradiated with pulses of 1064 nm, 6 ns, have detected a significant increase of the Hg/S ratio which has been attributed to the reduction of HgS to form the darker compound Hg2S (Pouli et al. 2001 & 2003). Hence, the results obtained upon irradiation of vermillion at 1064 nm, 150 ps, could be attributed to the participation of the two mentioned mechanisms. These results also give strong evidence of the little influence of the pulse duration when this pigmented system is irradiated with infrared wavelengths. In contrast, upon irradiation in the UV region, the extent of induced effects is strongly dependent on the absorption coefficient of the binding medium. Upon UV irradiation, and in the presence of strongly absorbing binder, damage to pigments is prevented. On the other hand, the effect of discoloration observed for the lead chromate system at 213 nm (150 ps and 15 ns) and 1064 nm (150 ps) is in agreement with previous observations obtained by irradiation at 248 nm, 25 ns (Castillejo et al. 2002 & 2003a) and at 1064 nm, 6 ns (Gordon Sobbot et al. 2003). This indicates that, even in the presence of an UV absorbing binder, high absorption of laser light by the pigment related chromophore takes place, hence the induced discoloration observed in this pigment. Previous XPS measurements performed on lead chromate showed that 248 nm, 25 ns, laser irradiation induces a noticeable decrease in the CrO42−/Cr3+ ratio (Castillejo et al. 2002 & 2003a) which indicates the formation of chromium (III) oxide (Cr2O3). Oxide formation by reduction of the original lead chromate could explain the colour shift to grey-green as observed by colorimetry. The differences reported concerning the behaviour of the two studied pigmented systems are directly related to differences on the light absorption properties of the pigment chromophore (Castillejo et al. 2002, Johnson et al. 1970, Zhou et al. 2004), HgS semiconductor for vermillion and the CrO42− ion for lead chromate, to their chemical composition and to their sensitivity to oxidation or reduction.

5

CONCLUSION

Various degrees of discoloration and chemical changes were observed upon laser irradiation at 213 nm (pulses of 150 ps and 15 ns) and at 1064 nm (pulses of 150 ps) of different aged and unvarnished tempera paint systems (unpigmented, vermillion and lead chromate). The differences are attributed mostly to differences in the absorption coefficient of the binding medium (2550 cm−1 at 213 nm and negligible at 1064 nm) and less importantly to the pulse duration (150 ps versus 15 ns). The chemical changes induced in the binder are mainly due to photodegradation of the proteins and lipids of egg yolk. It was observed that the extent of chemical change in the binding medium is reduced in the presence of the pigments, due to the effective absorption of the laser pulse energy by the pigment component of the mixture. The vermillion system behaves differently at the two studied wavelengths; the pigment features remain unaffected upon irradiation at 213 nm and disappear by irradiation at 1064 nm. Lead chromate is discoloured at all studied irradiation conditions with more intense modifications induced by IR irradiation (1064 nm). In the systems studied, extra bands attributed to amorphous carbon, indicative of carbonization and charring are absent, in agreement with previous observations upon irradiation with 795 nm, 120 fs pulses, and in contrast with irradiation with 248 nm, 25 ns pulses. More work is in progress on a larger range of pigmented eggyolk based temperas using short laser pulses also in the femtosecond range. In particular, work aiming at studying the laser-pigment interaction, using the raw pigments, will provide a better understanding of the mechanisms involved.

ACKNOWLEDGMENTS Funding from MEC (Projects CTQ2007-60177C02-01/PPQ and CONSOLIDER CSD200700058) and CAM (Programa Geomateriales S2009/Mat-1629) are gratefully acknowledged. One of the authors (MO) acknowledges the CSIC RTPHC for support. For this study, the treated samples were prepared by our collaborator, the late R. Hesterman.

REFERENCES Andreotti, A., Colombini, M.P., Nevin, A., Melessanaki, K., Pouli, P. & Fotakis, C. 2006. Laser Chem. Article ID 39046. Doi:10.1155/2006/39046. Athanassiou, A., Hill, A.E., Fourrier, T., Burgio, L. & Clark, R.J.H. 2000. J. Cult. Herit. 1: S209.

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Bartoli, L., Pouli, P., Fotakis, C., Siano, S. & Salimbeni, R. 2006. Laser Chem. Article ID 81750. Doi:10.1155/2006/81750. Bell, I.M., Clark, R.J.H. & Gibbs, P.J. 1997. Spectrochim. Acta A 53: 2159. Burgio, L. & Clark, R.J.H. 2001. Spectrochim. Acta A 57: 1491. Burmester, T., Meier, M., Haferkamp, H., Barcikowski, S., Bunte, J. & Ostendorf A. 2005. Proc. 5th Int. Conf. on Lasers in the Conservation of Artworks (LACONA V), Dickmann, K., Fotakis, C., Asmus, J.F. (Eds.), Springer Proceedings in Physics 100: 61. Castillejo, M., Martín, M., Oujja, M., Silva, D., Torres, R., Domingo, C., García-Ramos, J.V. & Sánchez-Cortés, S. 2001. Appl. Spectr. 55, 8: 992. 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. Anal. Chem. 74: 4662. Castillejo, M., Martín, M., Oujja, M., Santamaría, J., Silva, D., Torres, R., Manousaki, R., Zafiropulos, V., Van den Brink, O.F., Heeren, R.M.A., Teule, R. & Silva, A. 2003a. J. Cult. Herit. 4: 257S. Castillejo, M., Martín, M., Oujja, M., Rebollar, E., Domingo, C., García-Ramos, J.V. & Sánchez-Cortés, S. 2003b. J. Cult. Herit. 4: 243. Chappé, M., Hildenhagen, J., Dickmann, K. & Bredol, K. 2003. Proc. 4th Int. Conf. on Lasers in the Conservation of Artworks (LACONA IV), J. Cult. Herit. 4: 264S. Cooper, M.I., Fowles, P.S. & Tang, C.C. 2002. Appl. Surf. Sci. 201: 75. Drake, A. & Moore, K. 2004. J. Vib. Spectrosc. 2: 2. Gaspard, S., Oujja, M., Castillejo, M., Moreno, P., Méndez, M., García, A. & Domingo, C. 2008a. Lasers in the Conservation of Artworks, in: Castillejo M, Moreno P, Oujja M, Radvan R, Ruiz J (Eds.), Proceedings of the LACONA VII, Taylor & Francis Group, CRC Press/Balkema, The Netherlands 41. Gaspard, S., Oujja, M., Moreno, P., Méndez, C., García, A., Domingo, C. & Castillejo, C. 2008b. Appl. Surf. Sci. 255: 2675. Gaspard, S., Oujja, M., Abrusci, C., Catalina, F., Lazare, S., Desvergne, J.P. & Castillejo, M. 2008c. J. Photochem. Photobiol. A 193: 187. Gordon Sobott, R.J., Heinze, T., Neumeister, K. & Hildenhagen, J. 2003. J. Cult. Herit. 4: 276S. Johnson, L.W. & McGlynn, S.P. 1970. Chem. Phys. Let. 7: 618. Lakowicz, J.R. 2006. Principles of Fluorescence Spectroscopy, 3nd edn, Springer, New York. Liu, J.M. 1982. Opt. Lett. 7: 196. Mills, J.S. & White, R. 1994. The Organic Chemistry of Museum Objects, 2nd edn, Butterworth Heinemann, Oxford London.

Nevin, A., Cather, S., Anglos, D. & Fotakis, C. 2006a. Anal. Chim. Acta 573–574: 341. Nevin, A. & Anglos, D. 2006b. Laser Chem. ID 82823. Nevin, A., Osticioli, I., Anglos, D., Burnstock, A., Cather, S. & Castellucci, E. 2007. Anal. Chem. 79: 6143. Nevin, A. 2008. PhD. Thesis, Courtauld Institute of Art, University of London, United Kingdom. Osticioli, I., Nevin, A., Anglos, D., Burnstock, A., Cather, S., Becucci, M., Fotakis, C. & Castellucci, E. 2008. J. Raman. Spectrosc. 39: 307. Oujja, M., Pouli, P., Fotakis, C., Domingo, C. & Castillejo, M. 2010. Appl. Spectrosc. In press. Palumbo, G. & Pratesi, R. 2004. Lasers and Current Optical Techniques in Biology, Comprehensive Series in Photochemistry and Photobiology, Royal Society of Chemistry, Cambridge UK. Pouli, P. & Emmony, D.C. 2000. J. Cult. Herit. 1: S181. Pouli, P., Emmony, D.C., Madden, C.E. & Sutherland, I. 2001. Appl. Surf. Sci. 173: 252. Pouli, P., Emmony, D.C., Madden, C.E. & Sutherland, I. 2003., J. Cult. Herit. 4: 271S. Pouli, P., Bounos, G., Georgiou, S. & Fotakis, C. 2007. Proc. 6th Int. Conf. on Lasers in the Conservation of Artworks (LACONA VI), Nimmrichter J, Kautek W, Schreiner M (Eds.), Springer Proceedings in Physics 116. Pouli, P., Paun, I.A., Bounos, G., Georgiou, S. & Fotakis, C. 2008. Appl. Surf. Sci. 254: 6875. Teule, R., Sholten, H., Van den Brink, O.F., Heeren, R.M.A., Zafiropulos, V., Hesterman, R., Castillejo, M., Martín, M., Ullenius, U., Larsson, I., Guerra-Librero, F., Silva, A., Gouveia, H. & Albquerque, M.B. 2003. J. Cult. Herit. 4: 209S. Tonon, C., Duvignacq, C., Teyssedre, G. & Dinguirard, M. 2001. J. Phys. D: Appl. Phys. 34: 124. Vandenabeele, P., Wehling, B., Monees, L., Edwards H, De Reu M. & Van Hooydonk, G. 2000. Anal. Chim. Acta 407: 261. Weeks, C. 1998. Studies in Conservation 43: 101. Wisniewski, M., Sionkowska, A., Kaczmarek, H., Lazare, S., Tokarev, V. & Belin, C. 2007. J. Photochem. Photobiol. A: Chem. 188: 192. Zafiropoulos, V., Balas, C., Manousaki, A., Marakis, Y., Maravelaki-Kalaitzaki, P., Melesanaki, K., Pouli, P., Stratoudaki, Th., Klein, S., Hildenhagen, J., Dickmann, K., Luk’Yanchuk, B.S., Mujat, C. & Dogariu, A. 2003. J. Cult. Herit. 4: 249S. Zhou, G., Lü, M., Gu, F., Wang, S., Xiu, Z. & Cheng, X. 2004. J. Cryst. Grow. 270: 283.

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

The role of the substrate in the laser cleaning process: A study on the laser assisted removal of polymeric consolidation materials from various substrates S. Kogou & A. Selimis IESL-FORTH, Vassilika Vouton, Heraklion, Crete, Greece Department of Physics, University of Crete, Heraklion, Crete, Greece

P. Pouli & S. Georgiou IESL-FORTH, Vassilika Vouton, Heraklion, Crete, Greece

C. Fotakis IESL-FORTH, Vassilika Vouton, Heraklion, Crete, Greece Department of Physics, University of Crete, Heraklion, Crete, Greece

ABSTRACT: This study aims to examine and visualize the role of the substrate in the laser cleaning process and consequently in the choice of the optimum irradiation parameters for a successful cleaning application. Towards these objectives, a series of studies were undertaken on model samples specially prepared to simulate a particularly delicate and challenging issue in conservation practice; the removal of aged and degraded polymeric coatings used upon past and/or unsuccessful consolidation treatments on a variety of surfaces. The model samples comprised of substrates of diverse physicochemical properties (wood, aluminum, quartz slides etc.) covered with thin polymeric consolidation films (Paraloid B72). These samples were irradiated with a variety of laser parameters in order to investigate the presence and extent of any side-effects that may be induced to the substrate at the different cleaning conditions. Herein the results obtained upon UV (from 193 nm up to 355 nm) irradiation of naturally dried polymeric films are presented. Particular attention is given in both the absorptivity of Paraloid B72 film at the operative wavelength as well as the role of pulse duration in the ablation process. Under this scheme laser cleaning tests were also comparatively performed with pulse durations ranging from several nanoseconds (ns) down to several hundreds of femtoseconds (fs). 1

INTRODUCTION

Although the response of some of these materials (i.e. dammar and mastic varnishes) to commonly employed laser irradiation protocols (i.e. 248 nm) has been studied in the past (Srinivasan et al. 1989, Georgiou et al. 1998, Fotakis et al. 2006), little attention has been paid to the composition and properties of the underlying surfaces and their role to the cleaning process. Their contribution gets particularly important in the case of cleaning interventions in which the whole film of the polymeric coating must be removed (i.e. altered consolidation treatments). For strongly absorbed wavelengths, efficient coupling of the applied laser energy to the irradiated material enables layer-by-layer material removal with minimal thermal load or damage to the substrate. On the other hand, under moderate or weak absorption conditions a significant amount/ percentage of the laser energy is inevitably diffused into the bulk. This gets particularly important in

The use of polymeric coatings in conservation is mostly associated with protective and consolidation practices aiming to protect delicate surfaces and to strengthen and support fragile structures. Initially coatings were made of natural resinous solutions (i.e. from tree saps), while in the last few decades synthetic polymers were gradually introduced. These synthetic materials have been extensively used on a variety of substrates with different physicochemical properties such as paintings, stonework (Selwitz 1992), metallic objects (Munger 1984), wood (De Witte et al. 1984) etc. Ageing of these materials and their exposure to environmental conditions influence drastically their degradation, while surface phenomena originating from unsuccessful applications may hinder legibility and integrity of the original surface, making thus their removal indispensable.

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Figure 2. The technical samples employed for the laser tests with Paraloid B72 films on a) quartz slide, b) aluminum plate and c) wooden coupon. The diameter of all samples is 5 cm. Figure 1. Schematic representation of the two absorbing scenarios; a) the laser irradiation is relatively weakly absorbed by the over-layer and b) conditions of strong absorption and/or ultra-short laser pulses.

absorption of the solution by the substrate material, films of variable thickness were acquired (180–220 μm on quartz, 160–200 μm on metal samples and 110–140 μm on the wooden coupons). All samples were subsequently dried in room conditions for at least 48 hours. Initially, irradiation tests were performed on naturally dried samples. Still, in order to address the situation of aged and deteriorated consolidation films these experiments are currently repeated on artificially light-aged samples.

the case of ultra-thin over-layers (i.e. thin varnish or consolidation layers on paintings) with detrimental results into the laser sensitive paint layers. Nevertheless, the use of ultra-short laser pulses (Küper et al. 1987, Bäuerle 2000, Pouli et al. 2008) has been shown to be able to process even nominally transparent materials, while the effective optical penetration depth and consequently any side-effects on the substrate are significantly reduced. In Figure 1 a schematic representation of these scenarios is attempted, in which IO refers to the incident laser intensity while IR is reflected and IA the absorbed intensity. 2

2.2

Laser irradiation parameters

In this study the results of laser irradiation of Paraloid B72 films in the Ultraviolet (UV) region is presented. Additional to the ns irradiation, irradiation using ultra-short pulses (of pico- and femto-second duration) was also performed. Table 1 shows the technical characteristics of the laser systems employed and the parameters of the irradiation series. Irradiation tests were performed in air, on a spot basis using 1, 2 and 10 pulses, while in several cases (i.e. at 193 nm) a sequence of 40 up to 100 pulses was also considered. The employed wavelengths were chosen in order to investigate different absorption scenarios. As it can be seen from Figure 3, which shows the absorbance of the Paraloid B72 used in this study in the UV region (recorded using a Cury 50 UV-Vis spectrometer on an ultra-thin film of about 2.8 ± 0.2 μm), this material absorbs strongly at 193 nm, while its absorption is very weak at 248 nm (more than one order of magnitude) and becomes practically transparent at 355 nm. Table 2 shows the values estimated for the absorption coefficient and optical penetration depth of Paraloid B72, at the studied wavelengths, based on the absorption spectra shown in Figure 3 according to the Beer-Lambert’s law (Pouli et al. 2005). Thus, for the thicknesses of the films studied herein it is expected that a significant amount of the 248 nm and 355 nm irradiation penetrates the Paraloid film and affects the substrate. This may lead to significant and unpleasant side-effects.

EXPERIMENTAL PART

2.1 Test materials To examine the role of the substrate in the laser cleaning process a series of studies on model samples was undertaken. Technical samples consist of polymeric over-layers applied on different substrates (Figure 2). Paraloid B72 (a copolymer of ethyl methacrylate and methyl acrylate) was employed as over-layer material due to its broad use in conservation interventions on Cultural Heritage objects (Hories 2000, Gettens et al. 1966). Moreover its physicochemical properties are well known and studied (Chiantore et al. 2001, Miliani et al. 2001, Favaro et al. 2006). Thin films were prepared by casting solutions of Paraloid B72 in acetone (20% wt) on Quartz slides (Q), Wooden coupons (W) and Aluminum plates (A). Quartz slides do not absorb UV radiation and thus these samples are indicative of the response of the Paraloid B72 material to the various irradiation schemes, allowing thus direct comparison with the other substrates. In all cases the amount of the applied polymeric solution was the same, still due to the differential

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Table 1. The laser systems and irradiation parameters employed in this study. Wavelength (nm)

Range of operative Pulse fluences duration (J/cm2)

355

10 ns

0.30–2.80 (quartz) 0.15–2.50 (aluminum) 0.06–2.10 (wood)

QS Nd:YAG 355 (EKSPLA, SL312)

150 ps

0.45–1.30 (quartz) 0.10–1.30 (aluminum) 0.15–1.30 (wood)

KrF Excimer (Lambda physik compex 110)

248

25 ns

0.10–3.80 (quartz) 0.10–3.80 (aluminum) 0.10–12.30 (wood)

KrF Excimer dye system (Laser lab Göttingen)

248

500 fs

0.2–2.1 (quartz) 0.2–2.1 (aluminum) 0.2–2.1 (wood)

ArF Excimer (Lambda physik, compex 110)

193

Laser systems QS* Nd:YAG (Spectron, SL805)

25 ns

Table 2. The absorption coefficient and optical penetration depth of Paraloid B72 at the studied wavelengths. Wavelength (nm)

Absorption coefficient (cm−1)

Optical penetration depth (μm)

193 248 355

2600.0 ± 50 115.0 ± 15 4.1 ± 0.2

3.85 ± 0.05 87.00 ± 5 2440.00 ± 100

2.3

Evaluation of the irradiation tests

Initial studies involved a thorough investigation of the laser ablation rates as well as the morphology of the irradiated surfaces. Etching depth rates were measured with a mechanical stylus profilometer (Perthometer) while microscopic observations were performed using an optical microscope (OM) (Nikon, ME 600) employing both transmitted (TL) and reflected (RL) light. 3

RESULTS AND DISCUSSION

Serious damage to the metallic and wooden substrates was observed upon irradiation of the technical samples with the 355 nm (QS Nd:YAG laser, 10 ns pulse duration). Given that this wavelength is not absorbed by the polymeric over-layer (Fig. 3) almost all the laser radiation penetrates the film and interacts with the substrate. This results into severe alterations to the underlying surfaces such as melting, discoloration and surface modifications. The degree and extent of these damages, as well as their threshold values, are closely related to the individual properties of the underlying materials and the pulse duration. For the metallic substrate it was shown that for fluences below 0.6 J/cm2 the laser beam penetrates through the Paraloid B72 film and alters/damages the metal surface. For these fluences no removal of the Paraloid film is observed, still cracking of the polymer was recorded. Figure 4 shows the result of a single pulse of 0.5 J/cm2 in this wavelength, focused on both the metallic and the polymeric film surfaces. It is obvious that under this irradiation condition, no ablation of the polymeric material took place, while serious damage to the metal is visible as discoloration of the aluminum plate. More pulses of the same fluence resulted into polymer material ablation though in an irregular way (craters with uneven and rough edges) and further damage to the substrate. Above 0.6 J/cm2, ablation of the Paraloid film occurs upon single pulse irradiation still the crater morphology appears irregular and with intense

0.25–3.4 (quartz) 0.25–5.3 (aluminum) 0.3–3.9 (wood)

*QS = Q-switched.

Figure 3. The UV-Vis absorption spectra of Paraloid B72 and wood reference (Hon 1991).

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thermal and/or possibly mechanical alterations (Figure 5a, where the surface around the crater is visibly altered either due to melting and/or stress and strain deformations). It is interesting to note here that the singlepulse ablation threshold for the polymeric film (on quartz) is very high (Fabl = 2.6 J/cm2) while for accumulative pulses (i.e. 10 pulses) the lowest fluence that could result into visible phenomena (bubbling, swelling etc.) was 0.8 J/cm2. The fact that material is removed at lower fluence values in the case of metallic and wooden substrates denotes the presence of secondary effects originating at the interface of the two materials, which are totally uncontrollable. Similar observations were recorded at the ps regime; single-pulse ablation threshold of the neat polymeric film is very high (far beyond the maximum fluence tested, 1.3 J/cm2) while for the metallic and wooden samples damage to the substrate appears, still it is less intense than in the ns regime. Micro-bubble and foaming formation was observed upon irradiation of Paraloid B72 at 248 nm (KrF Excimer laser, 25 ns pulse duration) independently of the substrate (Pouli et al. 2009). In the case of the quartz samples (neat polymeric film) single pulse irradiation resulted into swelling of the polymer for fluences above 0.6 J/cm2, while further pulses were shown to remove material gradually. Figure 6 shows the morphology of areas irradiated in this regime with accumulative number of pulses for 1.95 J/cm2; the 1st pulse causes surface swelling in the range of 20–25 μm while one additional pulse initiates material removal (10 ± 5 μm),

Figure 5. Photo-micrographs of Paraloid B72 film upon 355 nm, 10 ns irradiation using a single pulse of F = 2.8 J/cm2 on (a) the quartz slide and (b) the metal plate (OM- RL, spot diameter 1.5 mm).

Figure 6. Photo-micrographs of Paraloid B72 film on quartz upon 248 nm, 25 ns irradiation at F = 1.95 J/cm2 (a) one, (b) two & (c) ten pulses (OM- TL, spot diameter 1.2 mm). The insets refer to profilometric measurements on the irradiated areas.

which eventually (after 10 pulses) results into a crater of 52 ± 7 μm. Similar phenomena were observed for the other substrates; swelling on the first pulse at all fluences tested (Figure 7) and initiation of etching from the second pulse and onwards. In addition in both cases, no damage occurred on the substrates, as long as they were not exposed to laser irradiation (i.e. due to the smaller optical penetration depth), indicating that the effects of irradiation are restricted in the polymer. Craters with clean edges but structured bases resulted upon 248 nm irradiation in the fs regime (500 fs pulse duration). In contrast to the ns regime the ablated areas show no foaming formation, still nano-holes were evidenced upon SEM observation (Paun 2009). In this regime etching resolution is very fine (in the order of 0.2 ± 0.05 μm) and thus a higher number of pulses is necessary in order to remove the whole thickness of the polymeric film. Still it can be seen that accumulative number of pulses does not cause melting in/or around the crater (Figure 8). Lift-off phenomena were interestingly observed on the sample with the metallic substrate upon multi-pulse (100) irradiation at 2.0 J/cm2 while no damage was detected on the metallic surface. Very fine etching resolution was also found upon irradiation at 193 nm (ArF Excimer laser, 25 ns pulse duration). As this wavelength is strongly absorbed by the polymeric material, it was

Figure 4. Photo-micrographs of Paraloid B72 film on metal upon 355 nm, 10 ns irradiation using (a–b) a single pulse and (c–d) two pulses of F = 0.5 J/cm2. Variable focusing (a–c) on the metal surface and (b–d) on the polymeric film surface reveals the phenomena taking place upon irradiation (OM- RL, spot diameter 1.5 mm).

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Figure 7. Photo-micrographs of Paraloid B72 film upon single pulse irradiation at 248 nm, 25 ns, F = 3.8 J/cm2 on (a) wood and (b) metal substrates (OM- RL, spot diameter 0.7 mm).

Figure 10. Photo-micrographs of Paraloid B72 film upon 193 nm, 25 ns irradiation on (a) quartz slide at F = 1.25 J/cm2 single pulse (OM- TL, spot area 0.6 × 1.3 mm) and (b) on metal coupon at F = 1.25 J/cm2, 100 pulses (OM- RL, spot area 0.5 × 1.8 mm). The inset refers to profilometric measurements on the irradiated area.

Figure 8. Photo-micrographs of Paraloid B72 film on quartz upon 248 nm, 500 fs irradiation at F = 2.1 J/cm2 (a) single pulse and (b) 100 pulses (OM- RL, spot side length 0.9 mm). Figure 11. Photo-micrographs of Paraloid B72 film on quartz upon 193 nm, 25 ns irradiation (a) 80 pulses at F = 2.5 J/cm2 (OM- TL, spot area 0.5 × 1.8 mm) and (b) 40 pulses at F = 4.5 J/cm2 (OM- TL, spot area 0.4 × 0.7 mm).

with ultra-fine cleaning resolution, in order to remove a layer of rather significant thickness, either a multi-pulse protocol must be followed, or instead, pulses of intense laser fluence should be applied. Both these methodologies may be associated with unpleasant side-effects and thus their choice must be carefully done in order to meet every specific case requirement. Figure 11 shows the phenomena that are associated with multipulse and intense fluence cleaning methodologies. In the first case splashes indicating undesired melting are clearly visible, while, on the other hand, intense fluences may result into cracking and other mechanical alterations. The etching depth per pulse for the 248 nm (both of ns and fs pulse duration) and 193 nm (25 ns) are shown in Figure 12. All depths were determined upon single pulse irradiation except the 248 nm/25 ns case in which single pulse irradiation was shown to result into swelling and bubbling of the surface. In this case the average value of 10 pulses was considered. From the graph it is obvious that upon the 248 nm/25 ns irradiation, etching depth is significantly higher than in the

Figure 9. Photo-micrographs of Paraloid B72 film on metal upon 248 nm, 500 fs irradiation at F = 2.0 J/cm2 (a) single pulse and (b) 100 pulses (OM- TL, spot side length 0.9 mm).

expected that laser radiation would be effectively coupled into the material and “clean” ablation with minimal influence to the surrounding surfaces would be possible. Figure 10 shows the craters obtained upon single pulse irradiation at 193 nm at F = 1.25 J/cm2. In this photo the crater borders and morphology are not easily discernible still their presence is established by profilometric measurements. Actually, the difficulty to differentiate the irradiated area on the polymer surface is the strongest evidence for a clean ablation with sharp crater edges and absence of any thermal or mechanical side-effects both in the crater base and the surrounding area, which is actually the ultimate goal of such an intervention. An important issue raised here is the following: as the strongly absorbed wavelengths are associated

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REFERENCES Bäuerle, D. Laser Processing and Chemistry (Springer-Verlag: Berlin, 2000). Chiantore, O. & Lazzari, M. 2001. Photo-oxidative stability of paraloid acrylic protective polymers. Polymer 42: 17–27. De Witte, E., Terfve, A. & Vynckier, J. 1984. The consolidation of the waterlogged wood from the GalloRoman boats of Pommeroeul. Studies in Conservation. 29: 77–83. Favaro, M., Mendichi, R., Ossola, F., Russo, U., Simon, S., Tomasin, P. & Vigato, P.A. 2006. Evaluation of polymers for conservation treatments of outdoor exposed stone monuments. Part I: Photo-oxidative weathering, Polymer Degradation and Stability 91: 3083–3096. Fotakis, C., Anglos, D., Zafiropulos, V., Georgiou, S. & Tornari, V. 2006. Lasers in the Preservation of cultural heritage. New York: Taylor and Francis. 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 738: 127–129. Gettens, R.J. & Stout, G.L. Painting Materials (Dover, New York, 1966). Hon, D.N.S. Weathering and Photochemistry of Wood (in Hon, D.N.S. & Shiraishi, N. (eds) Wood and cellulosic chemistry, Marcel Dekker, New York 1991). Hories, C.V. Materials for conservation (Butterworth Heinemann, Oxford 2000). Küper, S. & Stuke, M. 1987. Femtosecond UV laser ablation Applied Physics B 44: 199–204. Munger, C.G. Corrosion Prevention by Protective Coatings, (NACE International, Houston, Texas, U.S.A, 1984). Miliani, C., Ombelli, M., Morresi, A. & Romani, A. 2001. Spectroscopic study of acrylic resins in solid matrices, Surface and Coatings Technology 151–152: 276–280. Paun, I.-A., Selimis, A., Bounos, G. & Georgiou, S. 2009. Studies on the UV femtosecond ablation of polymers: Implications for the femtosecond laser cleaning of painted artworks, current volume proceedings. Pouli, P., Melessanaki, K., Giakoumaki, A., Argyropoulos, V. & Anglos, D. 2005. Measuring the thickness of protective coatings on historic metal objects using nanosecond and femtosecond LIBS depth profiling. Spectrochimica Acta Part B 60: 1163–1171. Pouli, P., Paun, I.-A., Bounos, G., Georgiou, S. & Fotakis, C. 2008. The potential of UV femtosecond laser ablation for varnish removal in the restoration of painted works of art. Applied Surface Science 254: 6875–6879. Pouli, P., Nevin, A., Andreotti, A., Colombini, P., Georgiou, S. & Fotakis, C. 2009. Laser assisted removal of synthetic painting conservation materials using UV radiation of ns and fs pulse duration: Morphological studies on model samples, Applied Surface Science 255: 4955–4960. Selwitz, Ch. Epoxy Resins in Stone Conservation (The Getty Conservation Institute, 1992). Srinivasan, R., Braren, B. 1989. Ultraviolet laser ablation of organic polymers, Chemical Reviews 89: 1303–1316.

Figure 12. Etching depth vs. fluence for Paraloid B72 upon irradiation at 193 nm (25 ns) and 248 nm (25 ns and 500 fs).

other two irradiation regimes. On the other hand, in these latter irradiation schemes, which correspond to strong absorption conditions (193 nm/25 ns) and to ultra-short laser pulse ablation (248 nm/500 fs), etching resolution is finer and a plateau of etching depth values is reached. 4

CONCLUSIONS

The role of the substrate in the determination of the cleaning methodology and the laser parameters to be followed upon the laser assisted removal of polymeric over-layers, is herein examined. It is clearly shown that the choice of a laser wavelength which is strongly absorbed by the material to be removed is crucial and in combination with the physicochemical properties of the substrate and the geometry of the over-layer may lead to an efficient cleaning intervention with minimal, if any, damage to the underlying authentic surface. Paraloid B72 is practically transparent to the 3rd harmonic of a Q-Switched Nd:YAG laser at 355 nm and thus a significant amount of the incident laser radiation inevitably interacts with the substrate resulting into detrimental effects. Consequently, cleaning with this wavelength should be avoided particularly in cases in which the underlying surface is laser-sensitive i.e. paintings. The best results were shown to occur upon ns irradiation at 193 nm and fs irradiation at 248 nm. In both cases clean ablation with very high etching resolution (in the order of 0.2 ± 0.05 μm) is achieved and thus both these irradiation schemes fulfil the criteria for a safe cleaning intervention, especially in the case of particularly sensitive and delicate problems such as ultrathin over-layers and highly sensitive substrates. Furthermore, detailed studies are undertaken in order to establish the advantages and limitations of both these two laser ablation regimes, while studies on accelerated aged polymer coatings are also currently performed.

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Lasers in the Conservation of Artworks VIII – Radvan et al. (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58073-1

Compact short pulsed fiber laser offers new possibilities for laser cleaning J. Hildenhagen & K. Dickmann Laser Center (LFM), Münster University of Applied Sciences, Steinfurt, Germany

ABSTRACT: Fiber lasers constitute a young but rapid growing category of laser technique. Many advantages such as compact size and no need for maintenance let this technique takes a huge distribution. There by compact short-pulse systems with low output power appear to be interesting for laser cleaning. However, the ablation process takes place by thousands of pulses per second with a laser spot in μm-scale. For applications in the field of cultural heritage especially the possible heat input has to be considered very well. This study was carried out by practical studies and FEM-simulation. It has turned out that the cleaning process easily gains critical temperature and only choosing specific parameter opens the way to clean a limited number of materials. 1

INTRODUCTION

The continuous enhancement of fiber laser sources strengthens their importance in the field of laser materials processing. These laser sources are benefiting from their compact and reliable construction and the cost-effectiveness. However, up to now hardly any efforts have been carried out to apply these laser sources for the cleaning of artworks. First experimental work has demonstrated the potential of cleaning thermally uncritical substrates, e.g. marble, with use of a continuous wave fiber laser source (PL = 300 W) /1/. In addition to continuous wave (cw) fiber laser sources novel pulsed fiber lasers with pulse durations of less than 200 ns and variable pulse shapes (e.g. SPI G3) are available. The low pulse energy (