Laser Cleaning (Master's Thesis)

COLUMBIA UNIVERSITY

LASERS AND CONSERVATION IN THE UNITED STATES

AN EXPLORATION OF THE LIMITED USE OF LASER TECHNOLOGY FOR CLEANING STONE

CATHERINE LAWSON SMITH

A THESIS SUBMITTED TO THE FACULTY OF THE HISTORIC PRESERVATION PROGRAM IN THE GRADUATE SCHOOL OF ARCHITECTURE, PLANNING & PRESERVATION IN CANDIDACY FOR THE DEGREE OF MASTER OF SCIENCE

NEW YORK, NEW YORK MAY 2010

COPYRIGHT 2010 Catherine L. Smith

For information about this work, please contact Catherine L. Smith at catlawsmith@gmail.com.

Permission is hereby granted to reproduce and distribute copies of this work for nonprofit educational purposes, provided that copies are distributed at or below cost, and that the author, source, and copyright notice are included on each copy. This permission is in addition to rights of reproduction granted under Sections 107, 108, and other provisions of the U.S. Copyright Act. Before making any distribution of this work, please contact the copyright owner to ascertain whether you have the current version.

June 9, 2010

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CONTENTS Title Page Copyright Table of Contents Abstract Acknowledgments Index of Figures Epigraph

i ii iii iv v vi viii

1. Introduction 2. Overview of Laser Cleaning

1 5

5 8 13

2.1. History 2.2. Technology 2.3. Current Practice

3. Research Methodology 4. Published Literature

4.1. Historical Overview

4.2. Publications 4.3. Author Affiliations

20 23 23 26 28

5. Laser Systems: Manufacture & Expense 6. Cleaning Stone: Lasers & Conventional Methods

32 42

42 43 49 50

6.1. Cleaning as a Conservation Tool 6.2. Conventional Cleaning Methods 6.3. Laser Cleaning 6.4. Lasers Used with Other Cleaning Techniques

7. Case Studies 8. Research, Training Programs, Conferences, & Funding Support

8.1. Research

8.2. Training Programs 8.3. Conferences 8.4. Funding Support

9. Practitioners

9.1 Professional Backgrounds

9.2 Nationalities 9.3 Training 9.4 Feedback

53 68 68 78 79 81

87 87 87 88 89

10. Conclusions

94

Bibliography Appendix

99 108

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ABSTRACT Lasers And Conservation In The United States An Exploration Of The Limited Use Of Laser Technology For Cleaning Stone Author: Catherine L. Smith Advisor: George Wheeler This research topic emerged as the focus of a master’s thesis through a discussion on contemporary cleaning techniques with Columbia conservation professor George Wheeler.

Despite 40 years of technological development and research, the number of conservation applications of lasers to clean stone in the United States is miniscule compared to what is taking place in Europe. This thesis articulates the reasons contributing to the disproportionate geography of projects utilizing lasers to clean stone, with particular focus on the preservation of historic architecture. Analysis is presented on areas of training, funding, literature, case studies, research, history of laser cleaning development, equipment, and other cleaning techniques.

This topic is particularly relevant because laser cleaning is a promising technology that is not evolving in a consistent way among international practitioners. The information presented here will hopefully contribute to an expansion of dialog between American and international conservation practitioners that will increase the potential for collaboration, training, education and, ultimately, more informed treatment choices for the conservation of stone.

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ACKNOWLEDGMENTS I am deeply grateful for the aid and advisement of numerous individuals whose shared expertise and professional experience were instrumental to the content development of this thesis. Above all, I thank my advisor George Wheeler for his guidance, wisdom, and patience. His suggestion of this focus for my research was guided by a recognition that there is a real question surrounding the uneven use of laser cleaning in architectural conservation; I hope my efforts have dignified his gift of this topic.

I have deep gratitude for Adam Jenkins and Norman Weiss, who shared the tasks of guidance and critique throughout this process. In particular, Mr. Jenkins provided invaluable feedback on aspects of technology, and greatly increased my understanding of the scientific processes of laser cleaning mechanics.

I am also indebted to several individuals who graciously gave me their time: Giancarlo Calcagno in Bassano del Grappa, Italy; Jonathan Hoyte, Paolo Pagnin, and Elvira Boglione in Venice, Italy; Kyle Normandin in New York; and Tony Sigel and Holly Salmon in Boston. Mr. Sigel invited me to experience the technology firsthand, and I thank him for providing this incredible opportunity.

Finally, I’d like to thank the many conservators who provided feedback through various digital channels, whether by survey, email inquiry, or online appeal. In particular, feedback and resources provided by the individuals listed below were vital to the successful completion of this report.

Margaret Abraham Timothy Allanbrook Anna Brunetto John Carr Jason Church Martin Cooper Isabel Costabal Andrzej Dajnowski J. Claire Dean Adele DeCruz Carole Dignard Frances Gale Mark Gilberg John Griswold Pamela Hatchfield

Nathan Jonjevic Daniel Jöst Deborah Lau Begoña Saiz Mauleón Timothy Niemeier Claudia Porcellana Paraskevi Pouli Mark Rabinowitz Matthew Reilly Malgorzata Sawicki Mary Striegel Jeanne Marie Teutonico John Twilley Constantinos Vasiliadis Véronique Vergès-Belmin

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INDEX OF FIGURES 1.1

2.1

2.2

2.3 2.4 2.5 2.6 2.7

3.1 3.2 3.3 4.1 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 7.1 7.2 7.3

Sculptural detail partly cleaned with a laser. Credit: Fotakis et. al., Lasers in the Preservation of Cultural Heritage: Principles and Applications.

Example of marble yellowing possibly induced by laser cleaning. Credit: Pouli et. al., “The laser-induced discoloration of stonework; a comparative study on its origins and remedies.”

Photo of emitted line from oscillating mirror. Credit: Dajnowski, “Laser cleaning of the Nickerson Mansion: The first building in the U.S. entirely cleaned using laser ablation.”

Nickerson Mansion before laser cleaning. Credit: Driehaus Museum Nickerson Mansion after laser cleaning. Credit: Driehaus Museum Close up of marble pediment sculpture at New York Public Library. Credit: Wiss Janney Elstner Associates Marble pediment sculpture at New York Public Library. Credit: Wiss Janney Elstner Associates Detail of Mausoleo di Teodorico before, during, and after cleaning. Credit: Pini et. al., “Application of a new laser cleaning procedure to the mausoleum of Theodoric.”

Porta della Carta at the Palazzo Ducale in Venice, Italy. Credit: Catherine Smith Church of the Maddalena in Venice, Italy. Credit: Catherine Smith Operating Harvard’s Lynton Phoenix laser. Credit: Catherine Smith Timeline marking growth in LACONA literature from 1995 to 2007.

CL 20 Backpack Laser from CleanLaser. Credit: CleanLaser CL 20/50 Laser from CleanLaser. Credit: CleanLaser CL 150/300/500 Laser from CleanLaser. Credit: CleanLaser Coherent Infinity laser at NCPTT. Credit: NCPTT EKSPLA NL200 Laser. Credit: EKSPLA El.En.’s Smart Clean II. Credit: El.En. Group El.En.’s EOS 1000 LQS. Credit: El.En. Group El.En.’s Thunder Art. Credit: El.En. Group El.En.’s Bramante. Credit: El.En. Group Lambda Scientifica’s ArtLaser. Credit: Lambda Scientifica Lambda Scientifica’s Art Light II. Credit: Lambda Scientifica Lambda Scientifica’s ArtINY. Credit: Lambda Scientifica Lambda Scientifica’s ArtDuo. Credit: Lambda Scientifica Litron Lasers’ Nano TRL model. Credit: Litron Lasers Lynton Lasers’ Phoenix Laser. Credit: Catherine Smith Lynton Lasers’ Compact Phoenix Laser. Credit: Catherine Smith New Wave Tempest laser. Credit: New Wave Quantel’s LaserBlast 1000 Laser. Credit: Quantel Example of black encrustation on stone. Credit: Altech Pressurized water cleaning. Credit: Central Parks Conservancy Chemical cleaning. Credit: Central Parks Conservancy Poultice paste application. Credit: Central Park Conservancy Mechanical scrubbing. Credit: Central Park Conservancy Sponge jet cleaning. Credit: Central Park Conservancy Cleaning tests: laser and poultice. Credit: Isabella Stewart Gardner Museum Cleaning tests: laser, gommage, and water. Credit: Columbia University Amiens Cathedral before restoration. Credit: chicagomontreal.wordpress.com Amiens Cathedral after restoration. Credit: webshots/pfjc Portal Finial of the Arch-Collegiate Church in Tum before and after laser cleaning. Credit: Koss et. al., Smith

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7.4 7.5 7.6 7.7 7.8

7.9

7.10 7.11 7.12 7.13

7.14

7.15 7.16 7.17 7.18 7.19 7.20 7.21 7.22 7.23

7.24

7.25

7.26

7.27

7.28

7.29

7.30 7.31 7.32 8.1

“Arch-Collegiate Church in Tum: Laser renovation of priceless architectural decorations.”

Church of the Maddalena before cleaning. Credit: COST G7/Nimmrichter Church of the Maddalena after cleaning. Credit: Altech Sculpture panel at Ince Blundell before laser cleaning. Credit: National Museums Liverpool Sculpture panel at Ince Blundell after laser cleaning. Credit: National Museums Liverpool Mausoleo di Teodorico during laser cleaning. Credit: Pini et. al., “Application of a new laser cleaning procedure to the mausoleum of Theodoric.”

Mausoleo di Teodorico during laser cleaning. Credit: Pini et. al., “Application of a new laser cleaning procedure to the mausoleum of Theodoric.”

Nickerson Mansion cornice prior to laser cleaning. Credit: Driehaus Museum Nickerson Mansion cornice during laser cleaning. Credit: Driehaus Museum Nickerson Mansion cornice after laser cleaning. Credit: Driehaus Museum Laser cleaning relief on a marble block from the Parthenon West Frieze during laser cleaning. Credit: COST G7/Nimmrichter Detail of marble block from the Parthenon West Frieze during laser cleaning. Credit: Fotakis et. al., Lasers in the Preservation of Cultural Heritage: Principles and Applications, p289 St. George’s Hall relief panels before cleaning. Credit: National Museums Liverpool St. George’s Hall relief panels after cleaning. Credit: National Museums Liverpool St. George’s Hall after restoration. Credit: Panoramio/Michael Ashton St. Stephen’s Church during laser cleaning treatment. Credit: Altech St. Stephen’s Church during laser cleaning treatment. Credit: Altech St. Stephen’s Church before laser cleaning. Credit: Credit: COST G7/Nimmrichter St. Stephen’s Church before laser cleaning. Credit: Credit: COST G7/Nimmrichter St. Stephen’s Church in Vienna, Austria. Credit: Credit: skyscrapercity.com Cathedral of Notre Dame in Paris sculptural detail during cleaning. Credit: Bromblet, et. al., “Diversity of the cleaning procedures including laser for the restoration of carved portals in France over the last 10 years.”

Sculpture cleaning at Poitiers Cathedral. Credit: Bromblet, et. al., “Diversity of the cleaning procedures including laser for the restoration of carved portals in France over the last 10 years.”

Portal sculptures at Cathedral Basilica in St. Denis, France. Credit: Bromblet, et. al., “Diversity of the cleaning procedures including laser for the restoration of carved portals in France over the last 10 years.”

Laser cleaning treatment of the capitals of Pavilion II at the University of Virginia. Credit: University of Virginia.

Cleaning tests on the Church of St. Michel in Bordeaux, France. Credit: Bromblet et. al., Laser cleaning of stones Sigismund Chapel during laser cleaning. Credit: Koss et. al., “Experimental Investigations and Removal of Encrustations from Interior Stone Decorations of King Sigismund’s Chapel at Wawel Castle in Cracow.”

Terra cotta sculpture ornament at the Cathedral of Seville. Credit: Oujja et. al., “Laser cleaning of terra cotta decorations of the portal of Palos of the Cathedral of Seville.”

New York Public Library pediment sculpture after cleaning. Credit: Wiss Janney Elstner Associates New York Public Library pediment sculpture before cleaning. Credit: Wiss Janney Elstner Associates New York Public Library pediment sculpture during cleaning. Credit: Wiss Janney Elstner Associates List of laser cleaning projects. Credit: Salimbeni et. al., “The European community research concerning laser techniques in conservation: results and perspectives.”

Index of Figures

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Cleaning is a critical part of the conservation process. It serves not only to improve the aesthetic appeal of an object or building but also to reveal its true condition so that appropriate action may be taken to ensure that it survives for many future generations to enjoy.

— Martin Cooper, Recent Developments in Laser Cleaning, 1997

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1. Introduction

Lasers are everywhere. From barcode scanners and computer data burners to skin resur-

facing and corrective eye surgery, lasers are familiar to just about everyone. Niche industries have adapted the technology for distinct purposes, and the field of objects conservation is no exception. Lasers are used to clean objects through a process called ablation. Ablation involves the use of pulsed light energy to remove soiling from the surface. For conservators who frequently handle historic objects in fragile condition, the development of laser technology as a cleaning method is a promising alternative to traditional cleaning techniques that can sometimes result in the loss of material. Historic architecture, in particular, stands to benefit from the technology in applications on soiled and deteriorating masonry.

As with many technologies, when first introduced, laser cleaning of stone struggled to

gain a foothold among architectural conservators who routinely use more accessible methods with longer track records. In operation as a conservation tool for fewer than 40 years—with minimal research development and use in the 1970s and 1980s—laser technology’s slow rate of acceptance is unsurprising. What is surprising, however, is the geographic distribution of its application. While lasers have been used on some stone conservation projects in the United States, the number of domestic sites is greatly outnumbered by projects completed abroad, particularly in Europe. This is an unexpected circumstance given historic trends that place the United States and Asia at the forefront of technological development and innovation. This thesis examines the use of laser cleaning of stone with particular focus on the U.S. and Europe, in an effort to understand the evolution of this preservation technology and its acceptance by conservators.

Laser-based cleaning techniques fill a void in the arsenal of stone-cleaning methods and

are ideal for use on severely delaminated surfaces, intricate sculptural detail, and thick pollution Smith 1

encrustations. Unlike chemical and mechanical cleaning processes, laser ablation targets surface soiling with precision and allows the conservator to stop the cleaning process once the desired effect has been achieved, thus greatly reducing the potential for “over cleaning.” This is not to say that lasers are appropriate for all masonry cleaning projects. In most architectural applications, laser ablation is used in tandem with other cleaning techniques where it is utilized to clean intricate carvings or friable surfaces, or as a secondary cleaning method.

As a cleaning method, laser techniques are not limited to use on stone. Metals, paper,

glass, textiles, wood, and painted surfaces are among the numerous materials that respond well to laser cleaning. Museum collections generally include objects of many different materials, so a cleaning tool with as broad an application scope as a laser is attractive. Of the limited number of laser cleaning projects taking place in the United States today, most have been undertaken in museum conservation labs.

Primary factors hindering the widespread use of laser cleaning include cost and mobility

of the equipment, training required for operators, and time required for a project’s completion.

Laser cleaning equipment is expensive. The least-expensive models are priced from $23,000 and higher-end equipment can exceed $300,000; most sales incorporate a long-term maintenance plan, which further adds to the high cost. The equipment is generally bulky and unwieldy, which can present tricky (if not insurmountable) logistical problems for projects with scaffolding. This issue has been addressed through modifications to the equipment, such as by installing long fiber cables,1 and the more recent introduction of smaller, portable machines. Proper training is essential for successful laser ablation applications. Inexperienced practitioners run the risk of damaging the stone by using an energy setting that exceeds the tolerance of the stone’s surface.

Furthermore, the equipment is expensive to repair and is particularly susceptible to malfunction if not used and maintained properly.

Introduction

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In addition to the costs of equipment and training, time can also be an expensive com-

modity on a project utilizing lasers to clean stone. While some studies argue that laser cleaning is a faster treatment method than traditional cleaning techniques,2 it is generally considered to be a much slower method. The small beam size of most laser delivery systems requires great precision and, usually, considerable operating time. For projects with tight schedules, laser ablation may not be viewed as the most favorable cleaning method.

Surmounting the prohibitive cost of laser cleaning equipment and cost of operator time

ultimately depends to the availability of funding. Privately funded projects—often financed through fundraising efforts—are less likely to utilize expensive treatment methods. Projects supported by government funding, on the other hand, have a better chance of obtaining support for extended research and employment of a more expensive technology, particularly when the issuing sponsor is a steward of cultural heritage. No region in the world has greater command of its historic resources than Europe, a status largely made possible through the abundance of government or government-funded cultural heritage organizations.

To accomplish the aim of this thesis, it is important to determine the elements influencing

the worldwide use of laser cleaning techniques to thereby isolate the factors precluding widespread use in the United States. A comprehensive study of training and education, geography of use, and funding for laser cleaning projects was undertaken. Synopses of published case studies are augmented by additional projects reported by individual conservators who use lasers for cleaning stone.

An online survey was performed to cull this experiential information, which also yielded valuable insight pertaining to training, preferred laser manufacturers, and the nationalities and professional backgrounds of practitioners. Interviews and site visits were conducted, and additional outreach extended to conservators who are not familiar with laser technology through email contact and an online forum distribution list. Museums and conservation organizations worldwide were polled to Introduction

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determine familiarity with and use of lasers, and training centers were queried to gauge the makeup of their audience. Published research provided invaluable information not only about individual experiments and case studies, but author and publication origin and funding sources as well.

The field of laser cleaning is richly collaborative, as is characteristic with many conservation

activities. The inter- or multidisciplinary nature of this field is the key to increasing awareness and perception of laser technology as a practical cleaning method: scientists, engineers, and conservators bring valuable insight from their fields of expertise, enhancing the potential for greater shared knowledge. International conferences such as LACONA (Lasers in the Conservation of Artworks) and publications dedicated to disseminating new research in laser use provide a strong foundation for continued growth and education.

Fig. 1.1 Architectural sculpture detail partly cleaned with a laser at 1064 nm.

1. Pini, Roberto, Salvatore Siano, Renzo Salimbeni, Valter Piazza, Marco Giamello, Giuseppe Sabatini, Fabio Bevilacqua. “Application of a new laser cleaning procedure to the mausoleum of Theodoric.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 93–97.

2. Jankowska, Marta and Gerard Sliwinski. “Is the laser cleaning of historical stone cost-effective?” Proceedings of SPIE Vol. 5120 XIV International Symposium on Gas Flow, Chemical Lasers, and High-Power Lasers (2003): 679–683.

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2. Overview of Laser Cleaning: History, Technology & Current Practice 2.1 History

The potential for lasers as a cleaning tool first surfaced in 1965, when American physicist

Arthur L. Schawlow vaporized black ink from white paper with his “laser eraser.”1 In 1972 John Asmus, an American physicist from the University of California, San Diego, experimented with lasers to create three-dimensional holographic images and, subsequently, to remove pollution crusts from marble sculptures in Venice, Italy. Asmus knew of Schawlow’s vaporizing laser experiment; both men were active in the emerging field of laser technology in the 1960s and early 1970s. Hired by the Italian Petroleum Institute2 to work in Venice, Asmus was using a ruby laser to create threedimensional holograms of important cultural resources. “In 1971, with the first inklings of sea level rise, there was concern about Venice being drowned. I was asked to go to Italy and use lasers to make holograms of the city, so people could see what it had once been like, when it was gone.”3

As a member of the Orion atomic energy program team in the 1960s, Asmus—who has

a PhD in quantum electronics and physics—worked to create alternative uses for atomic energy, ultimately proposing the use of lasers for nuclear propulsion of the Orion spacecraft. The 1962 Nuclear Test Ban Treaty led to the Orion project’s cancellation,4 however the research Asmus conducted through this project would eventually lead to his experimentations with lasers in Venice.

While undertaking the holographic mapping project, Asmus puzzled over the thick black crusts on many of the city’s marble artifacts, wondering if he might apply lasers to the stones to resurrect their surface. “With Orion, we had to direct millions of watts of power at a rocket in order to propel it into space without destroying it. I hypothesized that it would also be possible to shoot this hundred-million-watt laser beam at a fragile statue and preserve its structure.”5 Giulia MusuSmith 5

meci, a sculpture restorer in Venice employed by the superintendent of monuments, was particularly interested in Asmus’ hypothesis, as were the Victoria and Albert Museum’s Kenneth Hempel and Sir Ashley Clark, director of the Venice in Peril Fund.6 These individuals were professionally vested in the conservation of Venetian artifacts, and therefore were among Asmus’ greatest advocates. (This encounter with Hempel precedes later project collaborations between Asmus and the London’s Victoria and Albert Museum, which established a long-term laser cleaning research program that is still ongoing.)

Working with Lorenzo Lazzarini, L. Marchesini, and others, Asmus tested his theory by

aiming the ruby laser at the surfaces of several deteriorated marble sculptures. “We zapped the black stones with a laser and white spots appeared immediately. We’d rediscovered Nobel laureate Arthur Schawlow’s famous ‘laser eraser,’ which he developed to correct typing errors.”7 The cleaning tests showed great potential, however extensive research and experimentation would be necessary before laser cleaning could become a realistic consideration for use in conservation.

Immediately after Asmus’ cleaning demonstration in Venice, the Samuel H. Kress Foun-

dation and Smithsonian Institution provided funding to begin a research program at the University of California, San Diego, to determine the safety of laser use in conservation cleaning applications. The study concluded that marble, limestone, sandstone, stucco, concrete, terra cotta, some metals, leather, vellum, paper, cotton, wool, silk, moleskin, and wood could be effectively cleaned with lasers, while laser cleaning of stained glass, ceramics, paintings, and frescos showed both promise and problems. This study culminated in the development of an Nd:YAG laser system that was delivered to Venice in 1975 to conduct further study.8

Word of Asmus’ laser experiments spread among the conservation community, but at

the time the method failed to gain much research momentum as it was inefficient for practical use and the equipment was large, cumbersome, and expensive. The technique was slow—laser Overview / History

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pulses were seconds apart.9 As a comparison, the pulse frequency of modern equipment can be programmed in milli-, micro-, nano- and picoseconds.10

The development of another stone cleaning method may have overshadowed Asmus’ laser

treatment discovery.11 Modern abrasive cleaning techniques were developed in the 1970s, around the same time that John Asmus was in Venice. This method of surface cleaning lacked some of the hindrances of laser technology—namely, expense and mobility—and conservators readily adopted it. Laser cleaning was given limited attention in the 1980s; in fact, there is a dearth of documented research on laser cleaning during this period. Beginning in 1989, renewed interest and advances in technology reignited discussion about the method as a practical cleaning method. A groundswell in research initiatives emerged in Europe. Also at this time, a well-publicized project in China brought international attention to laser cleaning. Asmus, who had continued to develop the technology since its first application in Venice, brought his equipment across the Pacific to use on the terra cotta army of Emperor Qin Shi Huang. The collaboration with Chinese scientists and other conservators, including Italian Giancarlo Calcagno, showed great promise, but was unfortunately cut short due to political turmoil in China and the diversion of civic resources.12

From 1989 on, laser cleaning initiatives continued to appear and grow. The first major

European project to use laser cleaning on stone was at Amiens Cathedral in 1993.13 In 1995 the first conference devoted to laser applications in conservation was held in Crete. The organizing group called itself LACONA (Lasers in the Conservation of Artworks) and has met every two years since. Each conference yields a published volume of research papers (with the exception of the second meeting, the proceedings for which were never published). Subsequent to the inaugural meeting in Crete, conferences have taken place in Liverpool, U.K. (1997); Florence, Italy (1999); Paris, France (2001); Osnabrück, Germany (2003); Vienna, Austria (2005); Madrid, Spain (2007); and Sibiu, Romania (2009).

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2.2 Technology

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. La-

sers concentrate energy at a focused target—a process that can be used for cleaning stone. This ablation process that removes soiling from the surface is a function of applying intense, brief radiation in rapid bursts of a single wavelength, which causes the material to swell and pop off the surface in a small plasma cloud. The laser reaches the surface as a small round dot or, with newer models, as a wide line or fuzzy-dot grouping. The operator typically moves the beam source in small circular movements for even energy distribution and consistent cleaning results.14

Limestones, marbles, and sandstones are the most common building stones that require

cleaning.15 Typical soiling appears black, which can be attributed to soot and other environmental particulates. On a black pollution crust, most of the laser’s energy is absorbed by the surface encrustation and the rest is reflected. Light-colored stone is less affected by the laser energy than the dark soiled surface layer so it is not at risk of undergoing the same ablation reaction.

Dark colors absorb light more readily than light colors, and when the light is delivered as

pulsed energy from a laser, it is the greater absorption of the darker material that enables heat to accumulate. This is why using a laser to clean pollution crusts off many stones, such as limestone and marble, can be very effective—the soiling is dark and the stone is light in color. Conservators with laser cleaning experience emphasize that the ability of a laser pulse to remove soiling and surface material absent of physical impact is what set it apart from other cleaning methods. Microabrasives, for example, apply force against the surface, making it more difficult to distinguish between soiling and substrate, potentially abrading important surface material and decreasing depth of relief in the process of removing the soiling deposits.

The amount of energy delivered to a stone’s surface depends on several factors: the pulse Smith 8

duration and frequency, wavelength, and distance between the laser source and object surface.

A closer proximity results in higher energy delivery; the farther a laser is from the target, the weaker its impact will be. The pulse frequency, duration, and wavelength are programmed or selected by the operator. “By carefully selecting the wavelength, energy density and pulse duration of the laser radiation the desired ablation mechanism can be realized. From a conservator’s point of view, the choice of the laser is also based on the following arguments: the equipment must be available, affordable and easy to use.”16

A laser’s pulse duration also influences the surface energy impact; a longer pulse setting

results in more aggressive cleaning per beam output. Lasers classified as “Q-switched” emit pulses that are nanoseconds in duration, while “free-running” lasers can be set to pulse in microseconds and milliseconds. The Q-switch is more commonly used for laser cleaning of stone, however the technician must take care not to hold the beam too close to the target or on one area for too long or the mineral crystals at the surface may fracture.17

When in use, the laser ablation process produces a distinctive crackling sound. According

to Asmus, “It is a beautiful, self-limiting process, so precise you can actually do it with your eyes closed, by listening to the sound of the laser.”18 The sound is created by the interaction between the laser pulse and surface particles, which creates audible acoustic shock waves.19 The sound is louder with thicker, darker crusts and can be totally inaudible when the laser is applied to a light-colored surface. Generally speaking, the ablation process evaporates the surface deposits, however small particles can remain intact as they are expelled from the surface. The absence of harmful chemicals and abrasive materials that require cleanup and caution is another attribute of laser cleaning techniques. On certain projects, such as at a sacred tribal site, chemical cleaning is not allowable, and an alternative method such as laser cleaning may be used instead.

Several lasers are use for cleaning in conservation: Nd:YAG, Er:YAG, and Ytterbium, and

Overview / Technology

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excimer (excited dimer) KrF, ArF, and XeCl gas-halide lasers. The Nd:YAG is by far the most common laser used for cleaning stone, followed by the Er:YAG. The Nd refers to neodymium, and YAG is an acronym for yttrium-aluminum garnet.20 The infrared radiation of Nd:YAG lasers, at a 1064 nanometer frequency, is the most effective for cleaning stone or other materials with surface encrustations and is ideal for removing thick black crusts, but can cause damage if not operated properly. Examples of damage type include melting, modifications to the surface texture, and color changes,21 in addition to the mineral fracture previously mentioned. Er:YAG (erbium) lasers operate at a lower frequency (with longer wavelengths between 2940 and 2960nm) and interact with surface material differently; Er:YAG lasers are most suitable for use on thinner pollution encrustations and on biogrowth deposits, and for removing encrustations from painted surfaces.

Laser settings can vary for different stone types, soiling components, and depth of en-

crustation. This sensitivity has some limitations however, as polychromatic surfaces and dark stones are difficult—if not impossible—to clean with lasers. Older, weathered stone has been affected by so many conditions over time that no stone can share the same characteristics as another. Likewise, surface encrustations are also different for similar reasons. Therefore, laser pulse settings cannot be standardized—each laser cleaning treatment method will differ, however there is a substantial volume of published research available that can provide helpful guidance and insight during the process of establishing a testing protocol and treatment plan.

Historic and current research seeks to answer many of the unknown characteristics and

effects of laser cleaning of stone. Two of the most prominent research endeavors are focused on the yellow discoloration of some marbles that is revealed or produced by laser cleaning treatment22 and the effect of laser cleaning on different stone types. Yellowing is a significant concern as it is a seemingly irreversible condition and is not well understood, nor is its occurrence easy to predict (fig. 2.1). Many theories have arisen as to what is taking place and how; the more popular Overview / Technology

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claims include speculation that the yellow coloration is the uncovered patina, or that the stone has been chemically and structurally changed through the laser ablation process. Scientific investigations have made progress in reducing the intensity of the yellowing effect by adjusting the equipment settings, however the underlying cause is still not known. Other popular current laser cleaning research efforts include cleaning of consolidated stone, pulse frequency effects, polychromatic surface cleaning, and the removal of unusual coatings,23 among many other topics.

Fig. 2.1 Test cleaning on marble that shows discoloration. The patch at left was cleaned with a laser at 355 nm; the patch at left was cleaned with a laser at 1064 nm. Note the yellow hue.

Scientific and cultural heritage organizations in Europe are undertaking numerous re-

search studies to determine optimal parameters for different stone types, with many publications from Italy, France, the U.K., and Greece. At the Istituto di Elettronica Quantistica in Florence, Italy, investigations have looked extensively into the damage thresholds of stones cleaned with lasers24 as well as optimal laser cleaning methodologies.25 The Laboratoire de Recherche des Monuments Historiques in Champs-sur-Marne, France, has pioneered efforts in laser cleaning research since 1989. Notable studies include a comprehensive review of different laser parameters in use with different stones, soiling compositions, and surface conditions (i.e. wet or dry). These studies Overview / Technology

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found that the thickness of the surface encrustation and surface wetness were the determining factors in rate of cleaning.26

The Conservation Center at the National Museums and Galleries on Merseyside in Liv-

erpool (now known as National Museums Liverpool/NML) has conducted studies to analyze the composition of particulates emitted during the ablation process,27 as well as experimented with laser cleaning on many kinds of materials in addition to stone.28 In Greece, scientists at the Foundation for Research and Technology Hellas (FORTH) have studied the effects of laser cleaning on stone with different lasers and explored the use of infrared and ultraviolet lasers in tandem.30

Practitioner use has informed technological developments in laser cleaning equipment.

Over the past 16 years, the number of commercially available laser systems has increased from one to about 15.31 The increased variety of available equipment gives conservators applicationspecific options. For example, some lasers are equipped with an oscillating mirror that moves the cleaning spot from side to side at a high rate of speed (up to 150 times per second), which creates a line (fig. 2.2).32 This type of beam is ideal for use on large flat surfaces, as it covers surface area more quickly than a small round spot could. Another evolution informed by practice is the wetting of stone prior to laser cleaning; some studies have implied that a wet surface improves the cleaning effect while reducing color changes and other side effects.33

Fig. 2.2 Photo of beam emitted from laser system equipped with oscillating mirror during the Nickerson Mansion restoration project.

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A major technology advance is the reduction in size of some cleaning systems. CleanLaser

manufactures a backpack-sized system that can, in fact, be worn as a backpack, and Lynton Lasers’ Compact Phoenix—which is about the size of a large microwave oven—is a popular system currently in use on architectural restoration projects.

Despite the lack of dangerous chemicals and environmentally hazardous runoff, laser

cleaning techniques are not without risk. Precautions must be taken to ensure the safety of the laser operator and those around him/her. Goggles and enclosed spaces are mandatory; goggles protect the laser operator’s eyes, and walls eliminate the risk of danger to nearby workers from reflected or errant beams. For exterior architectural projects, walls must be erected around the operator, equipment, and area being cleaned. If the laser cleaning takes place in a laboratory, the room should be windowless and well ventilated, ideally with a vacuum system installed near the work surface. The laser beam itself can be harmful to bare skin and unprotected eyes, so extreme caution must be exercised when the laser is in use.34

2.3 Current Practice

It would be difficult to find an objects conservator practicing today that has no knowledge

of laser cleaning. The truth of this statement is a far cry from the laser landscape of 20 years ago.

However, the use of lasers in conservation is still small when compared to other cleaning methods. Within the specialized field of laser cleaning, it is well documented that European conservators are currently at the forefront of technological development and application, followed by practitioners in the United States.

An examination of published research and documented case studies shows that there are

fundamental differences between American and European applications for cleaning stone with Smith 13

lasers, not the least of which is the number of projects in each area. In Europe, laser cleaning is more commonly one of several cleaning methods used on architectural conservation projects. In the U.S., lasers are rarely used for architectural cleaning, but there are several noteworthy projects that have used the technology. The Nickerson Mansion in Chicago, for example, recently underwent a full interior and exterior restoration. Clad in light-colored Ohio sandstone that had a century’s worth of black soiling, the mansion’s exterior was cleaned entirely by laser ablation over 18 months (figs 2.3 and 2.4). The favorable results of the project, coupled with the unprecedented surface size to be cleaned using lasers, are a remarkable feat considering the limited use of laser cleaning in the U.S.

Figs. 2.3 and 2.4 Photos of the Nickerson Mansion in Chicago before laser cleaning (above) and after (below).

Overview / Current Practice

Smith 14

Another high-profile laser-cleaning project in the U.S. took place at the New York Public

Library as part of a major restoration that is still underway. Most of the building’s marble exterior—which is mostly flat—is being cleaned with steam and some mechanical scrubbing. However, two areas of the entry façade were cleaned with lasers on loan from Italy. The marble sculptures in the pediments on the library’s eastern elevation were so deteriorated that any other cleaning method would have resulted in a loss of material (figs. 2.5 and 2.6). Laser cleaning was the only viable option. Other similarly deteriorated sculptural details on the building, such as parts of the modillions beneath the cornice and pieces of the lion-head lintels, were largely replaced with replica pieces and therefore did not require such a delicate cleaning method.

Figs. 2.5 and 2.6 Photos of the pediment sculptures at the New York Public Library before laser cleaning. Note the fragile condition of the marble in the photo at left.

Despite these noteworthy examples, projects utilizing lasers to clean stone in the United

States take place more often in museums for art conservation, particularly on the East Coast, than on larger architectural projects. This differentiation is likely due to a variety of reasons including equipment availability and funding; smaller and more easily managed object size; controlled indoor environment; a higher likelihood of skilled training; and absence of client timeline.

Conservation firms, on the other hand, often contend with conditions that are more variable than in a museum: a more limited budget and tight schedule from private clients; much larger Overview / Current Practice

Smith 15

objects; uncontrollable temperature and humidity when outdoors; precarious work space when on scaffolding; and often untrained, less-skilled labor.

Of the eight known laser cleaning systems used in conservation in the U.S.,35 five are

owned by museums. Of the three others, two are owned by a private conservator and one is at the National Center for Preservation Technology and Training (NCPTT) in Louisiana, which currently runs the only laser cleaning research program in the U.S.

In Europe, in addition to frequent use in museums, laser cleaning methods are regularly

considered for architectural restoration projects. Equipment can be purchased or rented without great difficulty and—in the case of equipment rental— without great expense. Europe is home to several laser technology training centers, and the higher volume of laser cleaning projects provides opportunity for in-the-field training. The preservation of cultural patrimony is a focus of many organizations throughout Europe, which means sources for project funding are numerous.

As demonstrated with the New York Public Library restoration, laser cleaning is typically

just one of several cleaning methods to be used, depending on the needs of the project and the condition of the stone. For example, a project at the Mausoleo di Teodorico in Ravenna, Italy, used several cleaning methods to remove biogrowth and residual soiling. Biocides, hydro-jet washes, chemical compresses, microabrasion, and laser cleaning were used in a successful cleaning regiment.36

Figs. 2.7 a, b, c The denticulate feature of the Mausoleo di Teodorico during the cleaning process. Photo 2.7a, at left, was taken prior to cleaning. Photo 2.7b, at center, was taken after cleaning by ammonium bicarbonate. Photo 2.7c, at right, shows the results after laser cleaning.

Finally, funding and research are two areas that can be looked at to gauge the current

Overview / Current Practice

Smith 16

worldwide practice of laser cleaning in conservation. The majority of laser cleaning research and case study papers are published out of Europe. Robust cultural heritage preservation programs as well as inter-European organizations that coordinate funding and research projects further support laser cleaning activity abroad, whereas the U.S. lacks comparable organizational attention.

By all indications, American conservators (particularly those already working with lasers) are interested in seeing an increase in domestic laser cleaning research, collaboration, and training opportunities. To that end, the strength of the European program could provide valuable guidance for American conservators going forward.

1. Koss, A., and J. Marczak. “Implementation of laser technology in conservation—last decade.” Proceedings of the Eleventh International Congress on Deterioration and Conservation of Stone, September 15–20, 2008 (2008): 939–948.

2. Fotakis, Costas, Demetrios Anglos, Vassilis Zafiropulos, Savas Georgiou and Vivi Tornari. Lasers in the Preservation of Cultural Heritage: Principles and Applications. New York & London: Taylor & Francis Group (2007). p263 3. Cribb, Julian. “From Star Wars to fine art.” Rolex Award for Enterprise Journal 23 (2008). p20 4. Asmus, John. “Serendipity, punctuated.” LACONA VI Proceedings, September 21–25, 2005 (2007): p1 5. de Leschery, Karen. “Space age art restorer.” Rolex Award for Enterprise Journal 9 (2001).

6. Bordalo, Rui. “Interview with John Asmus: from Lasers to Art Conservation.” E-Conservation Magazine 3 (2008).

7. Op. cit., Cribb, p20 8. Op. cit., Bordalo 9. Cooper, Martin (editor). Laser Cleaning in Conservation: An Introduction. Oxford: Butterworth Heinemann (1998). Preface.

10. Torraca, Giorgio. Lectures on Materials Science for Architectural Conservation. Los Angeles: The Getty Conservation Institute (2009). p101 11. Op. cit., Cooper 12. Author interview with Giancarlo Calcagno in Bassano del Grappa, Italy, January 18, 2010.

13. Bromblet, Phillippe and Thomas Viewager. Laser Cleaning of Stones. Centre interrégional de conservation et restauration du patrimoine. Date unknown.

14. Circular motion method was explained by Tony Sigel at Harvard, who was trained in laser cleaning during a visit from Martin Cooper and John Larson of National Museums Liverpool in 1999. Cooper and Overview / Current Practice

Smith 17

Larson administered their two-day training course on site with participation from several Boston museum conservators.

15. Rodríguez-Navarro, Carlos, Kerstin Elert, Eduardo Sebastián, Rosa Maria Esbert, Carlota Maria Grossi, Araceli Rojo, Francisco Javier Alonso, Modesto Montoto and Jorge Ordaz. “Laser cleaning of stone materials: an overview of current research.” Reviews in Conservation (2003): 65–82.

16. Salimbeni, R., V. Zafiropulos, R. Radvan, V. Verges-Belmin, W. Kautek, A. Andreoni, G. Sliwinski, M. Castillejo and R. Ahmad. “The European community research concerning laser techniques in conservation: results and perspectives.” COST G7 affiliated paper—publisher unknown, date unknown. p3 17. Op. cit., Torraca, p101 18. Op. cit., Cribb, p20 19. Lee, Jong-Myoung and Ken G. Watkins. “Prediction system of surface damage.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 303–309.

20. Op. cit., Torraca, p101 21. Op. cit., Rodríguez-Navarro, p72 22. Vergès-Belmin, Véronique and Carole Dignard. “Laser yellowing: myth or reality?” Journal of Cultural Heritage, Vol. 4, Supplement 1 (2003): 238–244.

23. Pan, Aldera, Stefano Chiussi, Julia Serra, Pío González and Betty León. “Excimer laser removal of beeswax from galician granite monuments.” Journal of Cultural Heritage, Volume 10.1 (2009): 48–52.

24. Siano, Salvatore, F. Fabiani, D. Caruso, Roberto Pini and Renzo Salimbeni. “Laser cleaning of stones: assessment of operative parameters, damage thresholds, and associated optical diagnostics.” ALT ‘99 International Conference on Advanced Laser Technologies, Proceedings of SPIE Vol. 4070 (2000) 27–35.

25. Sabatini, Giuseppe, Marco Giamello, Roberto Pini, Salvatore Siano and Renzo Salimbeni. “Laser cleaning methodologies for stone façades and monuments: laboratory analyses on lithotypes of Siena architecture.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 9–19.

26. Labouré, Martin, Phillippe Bromblet, Geneviève Orial, Günter Wiedemann and Christophe SimonBoisson. “Assessment of laser cleaning rate on limestones and sandstones.” Journal of Cultural Heritage, Vol., Supplement 1 (2000): 21–27.

27. Feely, Jessica, Stephen Williams and P. Stephen Fowles. “An initial study into the particulates emitted during the laser ablation of sulphation crusts.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 65–70.

28. Sportun, Samantha, Martin Cooper, Ann Stewart, Marie Vest, Rene Larsen and Dorte V. Poulsen. “An investigation into the effect of wavelength in the laser cleaning of parchment.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 225–232.

29. Marakis, Giorgos, Pagona Maravelaki, Vassilis Zafirpulos, Stefan Klein, Jens Hildenhagen and Klaus Dickmann. “Investigations on cleaning of black crusted sandstone using different UV-pulsed lasers.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 61–64.

30. Marakis, Giorgios, Paraskevi Pouli Vassilis Zafiropulos and Pagona Maravelaki-Kalaitzaki. “Comparative study on the application of the 1st and the 3rd harmonic of a Q-switched Nd:YAG laser system to clean black encrustation on marble.” Journal of Cultural Heritage Vol. 4, Supplement 1 (2003): 83–91.

31. Thompson, Helen and Martin Cooper. “The use of laser cleaning in the conservation of public copperalloy monuments in the UK.” Journal of Architectural Conservation 16.1 (2010): 7–24.

Overview

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32. Dajnowski, Andrzej. “Laser cleaning of the Nickerson Mansion: The first building in the U.S. entirely cleaned using laser ablation.” LACONA VII Proceedings, September 17–21, 2007 (2008): 209–214.

33. Op. cit., Rodríguez-Navarro, p72 34. These precautions were experienced first-hand at the Straus Conservation Center in Boston Massachusetts when I visited in March 2010 to use the facility’s Lynton Phoenix laser, and are outlined in numerous articles and books on laser cleaning.

35. This number is based on confirmed accounts of laser use and ownership in the U.S. through direct correspondence with conservators. This number may change as correspondence continues and additional laser systems become known.

36. Pini, Roberto, Salvatore Siano, Renzo Salimbeni, Valter Piazza, Marco Giamello, Giuseppe Sabatini, Fabio Bevilacqua. “Application of a new laser cleaning procedure to the mausoleum of Theodoric.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 93–97.

Overview

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3. Research Methodology

Data for this thesis were largely collected through direct feedback from objects conserva-

tors, conservation firms, and laser manufacturers in the United States and abroad. This was accomplished through in-person interviews, online surveys, and email correspondence along with site visits and laboratory experimentation.

Two outreach endeavors took place online. The first, a survey, was distributed to more

than 40 international conservators with laser cleaning experience. Hosted by Survey Gizmo (www. surveygizmo.com), the questionnaire included more than 20 questions designed to cull information on the conservators’ training, professional experience, technical preferences, and opinions on laser cleaning techniques. The second online outreach method targeted objects conservators who had little or no experience with laser cleaning. The intent was to gain an understanding of contemporary perceptions of the technology and gauge interest in learning more about it. This query was posted to an online forum hosted by the American Institute for Conservation of Historic and Artistic Works (conservation-us.org), an organization with more than 3,500 members.

Primary research texts provided a comprehensive introduction to stone conservation

and the development of laser cleaning techniques and early case studies. These include Martin Cooper’s Laser Cleaning in Conservation: an Introduction (1998); Nicola Ashurst’s Cleaning Historic Buildings (1994); A Review of the State of the Art of Laser Cleaning, a report for the National Center for Preservation Technology and Training by Meg Abraham and John Twilley (1997); Lasers in the Preservation of Cultural Heritage: Principles and Applications by Costas Fotakis et. al. (2007); the proceedings of each LACONA conference (1995–2008); and Laser Cleaning of Stones by Phillippe Bromblet and Thomas Viewager for Centre Interrégional de Conservation et Restauration du Patrimoine. Secondary sources included numerous articles, reports, and books on stone conservation; Smith

20

scientific analysis of laser cleaning techniques; case studies; and reports on new technological developments.

In-person interviews took place in Venice, Italy; Boston, Massachusetts; and New York

City. I travelled to Venice—site of the first successful demonstration of laser cleaning of stone— in mid-January 2010 to meet with laser cleaning practitioners as well as visit several sites where architectural laser cleaning projects had taken place. In Venice I met with Jonathan Hoyte and Paolo Pagnin, two conservators with extensive laser cleaning experience. I also met with Pagnin’s colleague Elvira Boglione, a conservator who is currently developing techniques for cleaning polychromatic and dark stone surfaces with lasers. Finally, I spent an afternoon in Bassano del Grappa with Giancarlo Calcagno at his Altech company headquarters. Venetian site visits included the Porta della Carta at the Doge’s Palace and 18th-century Church of the Maddalena in Venice’s Cannaregio neighborhood.

Fig. 3.1 The Porta della Carta at Palazzo Ducale in Venice, Italy

Research Methodology

Fig. 3.2 The Church of the Maddalena in Venice, Italy

Smith

21

In mid-March 2010 I travelled to Boston to meet with Tony Sigel, conservator of objects

and sculpture at Harvard’s Straus Center for Conservation. The Straus Center owns a Lynton Phoenix laser, one of three lasers in Boston used for cleaning stone and other objects. I was able to use the equipment to clean test patches on several soiled stone samples, including two types of marble, glazed terra cotta, and sandstone. While in Boston I also met with Holly Salmon, a conservator at the Isabella Stewart Gardner Museum, who also has extensive laser cleaning experience. The Gardner Museum owns one of the other Boston lasers (the Museum of Fine Arts has the third); this one is also a Phoenix manufactured by Lynton, but is a newer, smaller model than the laser at Harvard.

In New York City I met with Kyle Normandin of Wiss Janney Elstner Associates, Inc.,

to discuss the laser cleaning component of the New York Public Library restoration project. The meeting included a site visit where I was able to see the project up close from the scaffolding.

Extensive email correspondence was conducted with museum conservators and conser-

vation firms throughout the U.S. to determine laser cleaning use, if any. Additional correspondence from international conservators provided further insight on contemporary perceptions of laser cleaning.

Fig. 3.3 Operating the Lynton Phoenix laser at Harvard’s Straus Center for Conservation

Research Methodology

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4. Published Literature 4.1 Historical Overview

Significant technological developments in laser cleaning began to emerge in the early

1990s, reflected in the volume of published research and literature about laser cleaning that grew shortly thereafter. Information about laser cleaning at this time was primarily published in scientific journals, although stone conservation books also began including brief mention of the new technology such as in Nicola Ashurst’s 1994 two-volume Cleaning Historic Buildings: “Laser radiation is proving successful in museums for the surface cleaning of sculptures, particularly those that are fragile and require cleaning without immersion in a liquid. Impressive rates of cleaning have been achieved with the laser method and it is probable that the method will soon be seen in use on building facades.”1

The increasing application of laser cleaning methods coupled with advances in technol-

ogy led to widespread European experimentation and research. Lasers in the Conservation of Artworks (LACONA) was formed in 1995 and first met in Crete. A slim volume of the proceedings were published and included 19 papers. The most recent volume of proceedings was published in 2008 from the 2007 LACONA VII meeting in Madrid, Spain; proceedings from the 2009 conference in Romania are expected to be published in summer 2010. This volume contains 77 articles, a more-than 400 percent increase from the edition published just 11 years earlier, and reflects the increase in research and application that is known to have taken place. LACONA remains the primary organization that enables new discoveries and research in conservation laser technology to be shared among conservators. In addition to laser cleaning, other conservation uses for lasers under the LACONA purview include laser-induced breakdown spectroscopy (LIBS), laser scanSmith 23

ning, fluorescence lidar imaging, 3D laser digitization, digital holographic interferometry, and shearography, among other conservation-oriented laser technologies.2

Proceedings are grouped according to discipline within the laser conservation field; the

most recent volume, LACONA VII, includes the following categories: innovative approaches in laser cleaning and analysis; analytical techniques; portable laser systems for remote and on-site applications; laser cleaning of monuments and sculptures; laser cleaning of paintings and polychromes; laser cleaning of metal objects; laser cleaning of documents and textiles; structural diagnosis and monitoring; imaging and documentation; and miscellaneous.

The proceedings from the third and fourth LACONA conferences, held in Florence, Italy,

and Paris, France, respectively, were published as supplements of the Journal of Cultural Heritage in 2000 and early 2003. The fifth, sixth, and seventh LACONA volumes (Osnabr端ck, Germany; Vienna, Austria; and Madrid, Spain) were published as independent titles by Springer Proceedings in Physics.

Fig. 4.1: Timeline marking growth in LACONA literature from 1995 to 2007. The 1997 conference did not yield a published volume.

The Journal of Cultural Heritage continues to publish articles on laser cleaning indepen-

dently of LACONA conference proceedings, as do other conservation and science journals and Published Literature / Historical Overview

Smith 24

newsletters such as the Journal of Architectural Conservation and the APT Bulletin.

Interestingly, two reports about laser cleaning were published by American authors with

American conservation organizations in the mid-1990s. The first, entitled Stone Cleaning: An Overview of Current Research, was published by the Getty Conservation Institute in 1996. The author, C.A. Price, devotes only two paragraphs to laser cleaning, but it is enough to indicate there was awareness and interest in the technology within the U.S. conservation community at the same time it was generating interest elsewhere:

The possibility of using lasers to clean stone is attracting increasing attention, and laser cleaning is now available commercially. Its great attraction is that it does not entail any physical contact with the stone and so lends itself ideally to the cleaning of very delicate surfaces. The principle is essentially simple: A laser beam impacts on the surface, and the energy of the beam is dissipated by vaporization of the dirt. While the stone is dirty, the light is absorbed and cleaning proceeds. Once the dirt has been removed, however, the light is reflected by the clean surface, and no more material is removed. The technique is described by a number of authors, including Cooper, Emmony, and Larson (1993); Maravelaki et al. (1992a); and Orial and Riboulet (1993).

At present, the speed of cleaning is comparable to that which can be achieved with a pencil-sized air-abrasive gun, and it is debatable whether the technique can reasonably, or safely, be scaled up for the cleaning of entire facades. Current research is aimed at selecting the optimal wavelength and pulse energy; at examining the effects on the stone, both physical and chemical; at comparing the performance of lasers with other cleaning techniques; and at identifying possible hazards to the operator (Cooper, Emmony, and Larson 1992; Vergès-Belmin, Pichot, and Orial 1993; D’Urbano et al. 1994; Larson 1994a). Further development of equipment is also sure to take place, identifying, for example, the best means of delivering the laser pulse to the surface of the stone. 3

Despite considerable resource allocations to stone conservation research, the Getty Con-

servation Institute has not explored laser cleaning techniques and does not appear to have intentions to do so.4

The second mid-1990s American publication is a comprehensive overview of laser clean-

Published Literature / Historical Overview

Smith 25

ing techniques and advocates for the formation of a dedicated laser cleaning research facility in the U.S. Written by Meg Abraham and John Twilley in 1997, the 55-page report, entitled A Review of the State of the Art of Laser Cleaning in Conservation, was produced in the Conservation Research Division of the Los Angeles County Museum of Art (LACMA) and submitted to the National Center for Preservation Technology and Training (NCPTT), which also funded the report. NCPTT currently runs a laser cleaning training and research program in Louisiana that was formerly instituted at LACMA.

The first (and thus far only) book on laser cleaning in cultural heritage was published in

1998 and edited by National Museums Liverpool’s Martin Cooper. Laser Cleaning in Conservation: An Introduction gives a comprehensive scope of the cleaning method and how it works, however the development of laser cleaning technology has evolved considerably since 1998 and may no longer adhere to limitations outlined in the Cooper book.

4.2 Publications

Titles that publish information about laser cleaning are listed below and grouped by for-

mat. All of them have been consulted for this thesis; there certainly may be other titles not included here. It should be noted that many of these sources are conference papers, which are not always diligently reviewed for accuracy prior to publication.

Publications listed under the ‘miscellany’ heading have published information about laser

cleaning but as an ancillary focus. For example, the Rolex Award for Enterprise Journal published two biographical articles about laser-cleaning pioneer John Asmus because he was an award recipient; this title does not otherwise have any content or focus on laser cleaning techniques.

Smith 26

Journals Accounts of Chemical Research Applied Surface Science Association of Preservation Technology Bulletin Atmospheric Environment Bulletin of the American Institute for Conservation of Historic and Artistic Works E-Conservation Magazine Environmental Science and Pollution Research Heron Journal of Cultural Heritage Journal of Architectural Conservation Laser Physics Laser-Surface Interactions for New Materials Production Optics & Laser Technology Pure and Applied Chemistry Reviews in Conservation Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy Studies in Conservation Conference Proceedings International Conference on Advanced Laser Technologies International Congress on Deterioration and Conservation of Stone (formerly: International Symposium on Deterioration of Building Stones) LACONA (Lasers in the Conservation of Artworks) Books Ashurst, Nicola. Cleaning Historic Buildings, Vols. 1 & 2. London: Donhead (1994).

Cooper, Martin (editor). Laser Cleaning in Conservation: An Introduction. Oxford: Butterworth Hei nemann (1998).

Fotakis, Costas, Demetrios Anglos, Vassilis Zafiropulos, Savas Georgiou and Vivi Tornari. Lasers in the Preservation of Cultural Heritage: Principles and Applications. New York & London: Taylor & Francis Group (2007).

Henry, Allison. Stone Conservation: Principles and Practice. Shaftesbury: Donhead (2006).

Torraca, Giorgio. Lectures on Materials Science for Architectural Conservation. Los Angeles: The Getty Conservation Institute (2009).

Reports Abraham, Margaret and John Twilley. A Review of the State of the Art of Laser Cleaning in Conservation. Los Angeles County Museum of Art submission, National Center for Preservation Technology and Training, Publication No. 1997-01 (1997).

Asmus, John. A proposal for technical support in the laser cleaning of inscriptions and frescoes at the GWU Etruscan excavation site. George Washington University College of General Studies submission for research funding (1973).

Bromblet, Phillippe and Thomas Viewager. Laser Cleaning of Stones. Centre interrĂŠgional de conservation et restauration du patrimoine.

Handbook on the Use of Lasers in Conservation and Conservation Science. European Cooperation in Science and Technology (2008).

Laser Stone Cleaning in Scotland. University of Southampton, Optoelectronics Research Centre.

Edinburgh: Historic Scotland (2005).

Published Literature / Publications

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Maxwell, Ingval. “Cleaning sandstone: risks and consequences.” Edinburgh: Technical Conservation, Research and Education Group, Historic Scotland (2007).

Price, C.A. Stone Conservation: An Overview of Current Research. The Getty Conservation Institute (1996).

Zhang, Jie. Laser Tube Bending and Laser Cleaning Of Metallic and Non-metallic Materials. PhD Dissertation: Columbia University (2006).

Miscellany The Building Conservation Directory Clem Labine’s Traditional Building Stulik, Dusan and Herant Khanjian. “New Projects of the Getty Conservation Institute: Laser Cleaning Research.” Getty Conservation Institute Newsletter 12.3 (1997).

Rolex Award for Enterprise Journal World Monuments Icon

4.3 Author Affiliations

In analyzing the geographic distribution of conservation professionals working with la-

sers, it is helpful to look at the professional affiliations of published authors. Not only does this information yield geographic data, it also indicates whether the author works in museums, science, academia, or private practice.

Authors from different countries and affiliations in Europe collaborated on many of the

articles consulted for this thesis. The COST organization (European Cooperation in Science and Technology), which coordinates funding and research efforts between European groups with the same project interests, has helped to facilitate many of these international collaborations. There are undoubtedly other cultural heritage and scientific organizations that provide similar coordination services.

Listed below are the professional affiliations of authors who published papers consulted

for this thesis, organized by country. This list is by no means comprehensive; the majority of consulted resources focused on laser cleaning of stone—there are many other research groups experimenting with laser cleaning of other surfaces, such as metal, paintings, and textiles.

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The most notable component of this organizational breakdown is the limited number of

contributing affiliations in the United States. Eight American organizations are included here, compared to Italy’s 27. In many cases there were multiple authors from the same organizations in Europe, so this list only paints a partial picture of the true disparity between American and European authorship. Because these authors are writing from first-hand laser cleaning experience and expertise, this list provides a map that is indicative of where things are taking place, and to what extent.

Austria Rossatz: Laser Tech Vienna: Central Institute for Restoration Bundesdenkmalamt Belgium Antwerp: Micro and Trace Analysis Centre, Department of Chemistry, University of Antwerp Canada Ottawa: Canadian Conservation Institute Denmark Copenhagen: School of Conservation France Champs-sur-Marne: Laboratoire de Recherche des Monuments Historiques Guyancourt: Thomson CSF Laser S.A. Lille: CETE-APAVE nord-ouest Marseille: Centre Interrégional de Conservation et Restauration du Patrimoine (CICRP) Pessac: Institut de Chimie de la Matière Condensée de Bordeaux Pessac: Maison de l’Archéologie, Université Michel de Montaigne Bordeaux Rueil-Malmaison: Société Quélin Strasbourg: Fondation de l’Oeuvre Notre-Dame Toulouse: Laboratoire de Physico-Chimie des Phosphates

Published Literature / Author Affiliations

Germany Berlin: Bundesanstalt für Materialforschung und -prüfung Dresden: Fraunhofer Institut Werkstoff und Strahltechnik Dresden: Institut für Diagnostik und Konservierung an Denkmalen Erlangen: Friedrich-Alexander Universität Erlangen-Nürnberg Hannover: Laser Zentrum Hanover Naumburg: Labor für Baudenkmalpflege Osnabrück: Deutsche Bundesstiftung Umwelt Steinfurt: Lasercenter Fachhochschule Münster Weimar: Bauhaus Universität Weimar: Ingenieurbüro für Bauwerkserhaltung Wertheim: Fraunhofer-Institut für Silicatforschung (ISC) Greece Athens: The Acropolis Restoration Service (YSMA) Athens: Committee for the Preservation of the Acropolis Monuments Chania: 25th Ephorate of Prehistoric and Classical Antiquities Crete: Institute of Electronic Structure and Lasers (IESL), Foundation for Research and Technology-Hellas (FORTH) Heraklion: Department of Physics, University of Crete India Orissa: INTACH Indian Conservation Institute

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Italy Aosta: Superintendence of Cultural Heritage Bologna: Centro Restauri Conservativi Calenzano: EL.EN SpA Capezzano Pianore: RestauroItalia s.r.l.

Como: Università agli Studi dell’Insubria Florence: Dipartimento allo Sviluppo Economico della Regione Toscana Florence: Dipartimento di Ingegneria Elettronica, Universita’ di Firenze Florence: Istituto di Elettronica Quantistica del Consiglio Nazionale delle Ricerche Florence: Istituto di Fisica Applicata “Nello Carrara” del Consiglio Nazionale delle Ricerche Florence: Istituto Nazionale di Ottica Florence: Opera do S. Maria Del Fiore Florence: Opificio delle Pietre Dure, Laboratori di Restauro Milan: Dipartimento do Fisica, Politecnico di Milano Milan: Istituto per la Conservazione e la Valorizzazione dei Beni Culturali Montegrotto Terme: Altech s.r.l.

Occhiobello: Altech s.r.l.

Padua: Altech, s.r.l.

Pisa: Dipartimento do Scienze della Terra, Universita di Pisa Pisa: Scuola di Specializzazione in Archeologia, Universita di Pisa Pisa: Soprintendenza Beni Ambientali, Architettonici, Artistici e Storici Ravenna: Soprintendenza per i Beni Ambientali e Architettonici di Ravenna Sesto Fiorentino: Istituto di Fisica Applicata, Consiglio Nazionale delle Ricerche Siena: Dipartimento di Scienze Ambientali, Universita degli Studi di Siena Siena: Istituto di Geochemica Ambientale e Conservazione del Patrimonio Culturale Lapideo, Universita di Siena Venice: Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali Venice: IUAV Istituto Universitario di Architettura di Venezia Venice: Soprintendenza per i Beni Ambientali e Architettonici di Venezia Netherlands Delft/Utrecht: TNO Built Environment & Published Literature / Author Affiliations

Geoscience Poland Gdansk: Department of Photophysics and Laser Technique, Polish Academy of Sciences Warsaw: Institute of Optoelectronics, Military University of Technology Warsaw: Inter-Academy Institute for Conservation and Restoration of Works of Art, Academy of Fine Arts Romania Bucarest: National Institute of R & D for Optoelectronics Russia St. Petersburg: St. Petersburg State Electrotechnical University Spain Barcelona: Materias Primas Abrasivas (MPA) Ferrol: Departamento de Enxeñaría Industrial II, Universidade da Coruña Granada: Departamento de Mineralogía y Petrología, Universidad de Granada Madrid: Instituto de Estructura de la Materia, CSIC Madrid: Instituto de Química Física Rocasolano, CSIC Madrid: Instituto del Patrimonio Histórico Español Oviedo: Facultad de Geología Sevilla: Instituto de Recursos Naturales y Agrobiología, CSIC Valencia: Department of Graphic Expression in Architecture, Institute for Heritage Restoration, Polytechnic University of Valencia Valladolid: Centro de Conservación y Restauración de BBCC de Castilla y León Vigo: Departamento de Física Aplicada, Universidad de Vigo South Africa Johannesburg: School of Process and Materials Engineering, University of the Witwatersrand United Kingdom Edinburgh: Historic Scotland Liverpool: Conservation Centre, National Smith 30

Museums Liverpool Liverpool: Department of Engineering, University of Liverpool Loughborough: Departments of Chemistry and Physics, Loughborough University of Technology Shrivenham: Centre for Applied Laser Spectroscopy, Cranfield University Surrey: The Textile Conservation Centre Watford: Building Research Establishment United States Durham, North Carolina: Duke University Forest Park, Illinois: CSOS Inc.

La Jolla, California: Institute for Pure and Applied Physical Sciences, University of California, San Diego La Jolla, California: Institute of Geophysics and Planetary Physics New York City, New York: Columbia University New York City, New York: Wiss, Janney, Elstner, Associates Inc. Philadelphia, Pennsylvania: Philadelphia Museum of Art Seattle, Washington: Seattle Art Museum

1. Ashurst, Nicola. Cleaning Historic Buildings, Vols. 1 & 2. London: Donhead (1994). Vol. 2, p87 2. These methods are includes in various LACONA journals in articles that include discussion of techniques for examining the impact of laser cleaning on stone.

3. Price, C.A. Stone Conservation: An Overview of Current Research. The Getty Conservation Institute (1996). p14–15 4. As indicated through private email correspondence with Jeanne Marie Teutonico, associate director of programs at the Getty Conservation Institute.

Published Literature

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5. Laser Systems: Manufacture & Expense

For use on his early laser cleaning experiments in Venice, Italy, John Asmus customized

commercially available laser systems that were marketed for civilian industries. In 1976, the lasers Asmus used cost between $10,000 and $30,000.1 The first commercially available machine for laser cleaning went into development in 1987. Manufactured by B.M. Industries out of France, this laser cleaning prototype was the only standardized model in use until the mid-1990s.2

Companies that provide laser equipment for the medical, cosmetic, science, engineering

and beauty industries manufacture the majority of laser equipment used for cleaning stone. The technology capable of effectively removing gypsum crusts from limestone is also ideal for tattoo removal, skin resurfacing, and surgical incisions. The market demand for these usage types far surpasses that of stone conservation; however, the supply of equipment is much greater as a result.

Companies that manufacture (or in some instances used to manufacture) laser systems

used for cleaning stone are profiled in this section. Their identities were made known through the online survey, research publications and other literature, online searches, and a database compiled by COST G7. Where possible, system pricing has been included.

Adapt Laser Systems

Adapt Laser Systems is an American company based out of Kansas City, Missouri that

sells CleanLaser systems for laser cleaning. See the CleanLaser section for product information.

B.M. Industries

B.M. Industries manufactured the first commercially available Nd:YAG laser used in con-

servation for cleaning stone. B.M. Industries was purchased by Thomson CSF, which later was Smith

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renamed Thales, which no longer manufactures lasers for use in conservation.3 The Foundation for Research and Technology—Hellas (FORTH) has adapted the B.M. system to create a modern prototype for use in its laser cleaning research endeavors. Fourteen percent of survey respondents have worked with a B.M. Industries laser system.

CleanLaser

Founded in 1997, CleanLaser is based in Herzogenrath, Germany and manufacturers

seven different laser systems that range in power from 20 to 1,000 watts. The portfolio includes a backpack laser, the CL 20 Backpack, that can be used for cleaning stone and operates using direct power or battery and features a two-meter-long fiber optic cable. Other systems range from lower power (CL 20 and CL 50), mid-range power (CL 150, CL 300, and CL 500), and high power (CL 1000). The laser name refers to the number of watts of power it operates on. CleanLaser has two sales offices in the U.S. that operate as Adapt Lasers, which have sold three lasers for use in conservation, and a location in Barcelona, Spain known as MPA Laser. The cost for each system (in USD) is: CL 20 Backpack, $72,000 and up; CL 150, $185,000 and up; CL 300, $285,000 and up; CL 500, $397,000 and up, and the CL 1000 pricing is undisclosed.4 Twenty-nine percent of survey respondents have worked with a CleanLaser laser system (this statistic factors in systems sold under Adapt and MPA).

Fig. 5.2 The CL 20/50 Laser

Fig. 5.1 The CL 20 Backpack Laser

Laser Systems

Fig. 5.3 The CL 150/300/500 Laser

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Coherent

Headquartered in Santa Clara, California, Coherent is a laser technology development

company that manufactures products for a variety of uses. Coherent used to manufacture a laser that is used among conservators for cleaning. Coherent’s Infinity line has been discontinued5 although the lasers are still in use, such as the one owned by the National Center for Preservation Technology and Training (NCPTT) in Louisiana, which is currently using the laser to conduct tests on graffiti cleaning of stone.6 Nineteen percent of survey respondents have worked with a Coherent laser system.

Fig. 5.4 Coherent Infinity laser at NCPTT

EKSPLA

This Lithuanian company manufactures laser systems

for a variety of applications, from solid-state lasers and optoelectronics to industrial lasers and material processing systems.7 EKSPLA’s NL200 series of three diode-pumped nanosecond Qswitched systems can be used for laser cleaning applications, however it is not a widely used system in conservation.

Laser Systems

Fig. 5.5 EKSPLA NL200 Laser

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El.En. Group

An Italian manufacturer of industrial and medical lasers, El.En. Group’s lasers are among

the most widely used in conservation when partner systems are factored in. Laser systems manufactured by El.En. include the Smart Clean II, EOS 1000 LQS, Thunder Art, and Bramante (developed with Quanta System). The Thunder Art, a 1064 nm wavelength model with articulated arm, replaces El.En.’s older Q-switched Short-pulse laser line and is available for £45,950 ($68,500 USD) and up.8 The EOS 1000 LQS costs £26,950 ($40,000 USD) and up. The Smart Clean II, which can be purchased for £33,950 ($50,000 USD), is specifically designed for use on stone and was developed in conjunction with the Italian National Research Council (CNR) laboratories. El.En. has established partnerships with Lynton Lasers and Quanta System. Nineteen percent of survey respondents have worked with a laser system fro El.En. Group.

Fig. 5.6 El.En.’s Smart Clean II

Laser Systems

Fig. 5.7 El.En.’s EOS 1000 LQS

Fig. 5.8 El.En.’s Thunder Art

Fig. 5.9 El.En.’s Bramante

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Lambda Scientifica

Lambda Scientifica S.p.A. is an Italian company that manufactures lasers specifically for

use in cultural heritage conservation. Located in Brendola, the company’s four Nd:YAG systems are known as: ArtLaser, ArtLight II, ArtDuo, and ArtINY. The ArtLaser is a Q-switched system and features a gun-delivered beam and is capable of cleaning large surfaces with a spot size that can be adjusted from one to twelve millimeters. The ArtLight II is a 1064 nm system that can be run as Q-switched and free-running, and is run through a three-meter-long optic fiber beamdelivery platform. Lambda Scientifica recommends the ArtLight II for laboratory use. The ArtDuo is an articulated-arm system that can be run on 1064 and 532 nm wavelengths, and the ArtINY laser system is a small portable machine with a three-millimeter spot size that can also run on 1064 and 532 nm wavelengths. Lambda Scientifica’s lasers have been used at the Louvre and Prado Museums, at the Alhambra, and the Vatican.9

Fig. 5.12 Lambda Scientifica’s ArtINY

Fig. 5.10 Lambda Scientifica’s ArtLaser

Fig. 5.11 Lambda Scientifica’s ArtLight II

Laser Systems

Fig. 5.13 Lambda Scientifica’s ArtDuo

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Litron Lasers

A new product from Litron Lasers has been designed for a variety of uses but was re-

cently marketed by the company specifically for its stone-cleaning abilities. The Nano TRL series includes 14 Nd:YAG laser systems programmable to 266, 355, 532, and 1064 nm wavelengths.10 The company is based in Rugby, England, and has a U.S. sales office in Bozeman, Montana.

Fig. 5.14 Litron Lasers’ Nano TRL laser series model

Lynton Lasers

Based out of the U.K., Lynton Lasers first began manufacturing lasers for cleaning in the

mid-1990s through a partnership with conservators in the Conservation Centre at the National Museums and Galleries on Merseyside, now known as National Museums Liverpool ( NML).

Lynton developed a commercial laser based on a prototype created by Martin Cooper, Stephen Fowles, and John Larson.11 Lynton’s newest laser system, the Compact Phoenix, delivers a 1064 nm wavelength beam through a handheld gun via optic fibre. The beam size can be adjusted from three to ten millimeters in diameter. Prices for the Compact Phoenix system begin at £15,950 Laser Systems

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($23,000 USD) and can be rented for £1,095 ($1,634 USD) per month. Lynton systems are used at the NML laser training research facility. In the U.S., three are located in Boston, Massachusetts at Harvard University (which has a larger, older Phoenix model), the Museum of Fine Arts Boston, and the Isabella Stewart Gardner Museum. Established in 1994, Lynton has recently formed a partnership with the El.En. Group and Quanta System. Thirty-eight percent of survey respondents have worked with a Lynton Lasers system.

Fig. 5.16 Lynton’s Compact Phonex laser system

Fig. 5.15 Lynton’s Phonex laser system

MPA Laser

MPA Laser is a Spanish company that sells CleanLaser systems for laser cleaning. See the

CleanLaser section for product information.

Laser Systems

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New Wave

New Wave is a California company that manufactures lasers for scientific and technology

applications. The Tempest laser line includes four models—the Tempest 10, 20, 30, and 300— that are used for laser ablation in conservation applications on stone and other materials. Each system can be customized for 1064, 532, 355, and 266 nm wavelength output.12 NCPTT used a Tempest 300 laser inherited from the Los Angeles County Museum of Art until recently; it is no longer functioning.13

Fig. 5.17 New Wave’s Tempest laser

Quanta System

Headquartered in Solbiate Olona, Italy, Quanta manufactures laser systems for a vari-

ety of industries including conservation, medicine, environmental monitoring, and government projects. The conservation lasers are among the most advanced currently in practice; they are known as Palladio, Michelangelo, Raffaello, Leonardo, Bramante, and Flexo. The Bramante, created with the El.En. Group, has a 12-millimeter beam size that make it among the most capable systems for cleaning large architectural surfaces. Two Quanta Michelangelo lasers were utilized in the laser cleaning portion of the New York Public Library exterior restoration project. Founded Laser Systems

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in 1985, Quanta entered into a partnership with the El.En. Group and Lynton Lasers in 2004.

Twenty-nine percent of survey respondents have worked with a Quanta laser system.

Quantel

Quantel, a French company, designs and manufactures laser equipment for use in the

semiconductor, industrial, scientific, military, aerospace, medical (dermatology and ophthalmology), and heritage conservation industries. Quantel’s LaserBlast line of lasers is popular among stone conservators. Four models are available: LaserBlast 60, 500, 1000, and 2000.14

Fig. 5.18 Quantel’s LaserBlast 1000 laser

Thales Laser

Thales is a French company that formerly manufactured several lasers for cleaning stone

(including the NL 20 and 220 models) and other materials. While Thales lasers may still be in use by conservators, the company stopped manufacturing this type of system in 2003.15 The company now focuses its resources in the aerospace industry.

Laser Systems

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1. Asmus, John. “The development of a laser statue cleaner.” Proceedings of the Second International Symposium on Deterioration Of Building Stones, Athens (1976): 137–141.

2. Bromblet, Phillippe and Thomas Viewager. Laser cleaning of stones. Centre interrégional de conservation et restauration du patrimoine.

3. According to a Thales products and system sales engineer via email correspondence.

4. Information provided by a representative with Adapt Laser Systems via email correspondence. Two of the three lasers are the CL 120 system and were purchased by conservator Andrzej Dajnowski in Chicago, and the third is the CL 20 backpack laser owned by the Philadelphia Museum of Art. 5. According to a Coherent sales representative via email correspondence. He is not aware of any new Coherent laser systems used for ablation on stone.

6. According to correspondence with Mary Striegel, head of the NCPTT Materials Research Program.

7. According to the company website: www.ekspla.com.

8. El.En. laser system pricing information available from Lynton Lasers: www.conservationlasers.com/#/ our-products/4533647511.

9. According to the company website: www.lambdascientifica.com.

10. Information available on the Litron Lasers website: www.litronlasers.com.

11. Cooper, Martin (editor). Laser Cleaning in Conservation: An Introduction. Oxford: Butterworth Heinemann (1998). Preface 12. Information available on the New Wave website: www.new-wave.com.

13. According to correspondence with Mary Striegel, head of the NCPTT Materials Research Program.

14. Information available on the Quantel website: www.quantel-laser.com/en.

15. According to a Thales products and system sales engineer via email correspondence.

Laser Systems

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6. Cleaning Stone: Lasers & Conventional Methods 6.1 Cleaning as a Conservation Tool

Stone decay is inevitable, however, the rate and extent of deterioration can be mitigated

with careful intervention and maintenance. Stone deterioration can occur in a variety of ways and create problems with differing levels of severity. In some cases, the weathering effect may be so severe that it has whittled the stone to practically nothing, which on a building may threaten the safety of passersby below. Another common deteriorating influence on stone is deposition soiling.

Soiling buildup can entrap water and salts and accelerate surface and substrate deterioration. Unlike weathering, soiling causes a starkly different aesthetic from the stone’s natural appearance, particularly if the stone is light in color. Stone conservation projects often incorporate cleaning treatments that are directed by concerns of appearance as much as they are of performance.

The black encrustation on the stone artworks has to be removed for the following reasons. First, the black appearance greatly decreases the aesthetic value of the artworks. Second, the encrustation accelerates the biodegradation of the stone since it can host the factors inducing the bio-decay, such as bacteria, fungi, algae, and lichen etc. Third, the ongoing degradation or damage from the previous restoration in the stone is obscured by the encrustation. The removal of encrustation is helpful to [find] and treat them, to locate and eliminate the cause, thereby arresting the decay and extending the life of the artworks.1

Soiling has become a significant concern as chemicals in the environment have grown

more complex, particularly in urban areas where pollution is more abundant. “Building stone is most affected by acid air pollution when it contains a high amount of calcite, like marbles, limestones, sandstones and mortars. The natural compounds and artificial pollutants most relevant Smith

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to stone decay are: carbon dioxide, sulphur oxides, nitrogen oxides, particulate matter, ammonia, ozone, hydrogen fluoride and hydrogen chloride.�2 Marble and limestone are particularly susceptible to pollution, as a buildup of atmospheric deposits can chemically transform the calcite to gypsum,3 rendering the surface layer more soluble and therefore vulnerable to water-based cleaning techniques. For these stones, the composition of pollution crusts generally consists of gypsum, soot and street dust that are responsible for the black coloration, iron oxides and iron oxide hydroxides, residual calcite, and low concentrations of other organic matter (fig. 6.1).4

Fig. 6.1 Example of black encrustation on stone (Palazzo Ducale, Venice, Italy)

6.2 Conventional Cleaning Methods

The desire to remove soiling has led to the development of numerous cleaning materials

and methods. These vary in cost, aggressiveness, time and training required, and environmental impact. When factored against variables such as stone types and characteristics, amount of soilSmith

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ing, solubility, and chemical composition, the cleaning methods must first be tested to determine the most effective treatment protocol. Other factors will also influence this process, such as availability of materials, historic designation limitations, and community impact.

Nicola Ashurst has identified four categories within which each conventional cleaning

technique falls: water-based methods, chemical methods, poultices and packs, and air abrasive and mechanical methods.5 Water-based methods include continuous or intermittent misting (with and without pressure), and steam. Chemical methods involve the application of acidic and alkaline solutions; similarly, poultices and packs utilize pastes made with acidic, alkaline, and neutral solutions. Air abrasive and mechanical methods require pressurized wet, dry, micro-abrasive or hand-applied systems. Unlike each of the traditional methods, laser ablation does not require contact with the stone.

In order to facilitate an effective discussion on the use of lasers to clean stone, it is impor-

tant to also highlight relevant aspects of all other common cleaning treatments, particularly the benefits and disadvantages of each.

Water-Based Cleaning Methods

Water and steam cleaning methods can result in deep penetration of the stone. If used in

freeze-thaw zones, this needs to be looked at carefully. Water will result in runoff, however the lack of chemicals in the wash means environmental concerns are limited to the chemical composition of the encrustations being removed.

1. Misting Continuous or intermittent spray is applied to surfaces with soiling that is water soluble.

This technique softens the soiling and is sometimes aided with gentle scrubbing with brushes or other light abrasives. This method is sometimes used in conjunction with Cleaning Stone / Conventional Methods

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other cleaning techniques such as poultices and abrasives. Water misting is relatively inexpensive to administer, results in little to no environmental impact. However, it must be closely monitored and may lead to efflorescence, staining, corrosion (if metals are present), and freeze/thaw damage.

2. Pressurized Water This water application can be used on its own as an independent cleaning technique, however it is commonly applied as the initial and final steps in cleaning processes that also utilize chemical washes or poultices. The risk of damage to the stone or mortar joints escalates as the water pressure is increased; friable surfaces are particularly susceptible to pressurized water application. Water at high pressure should not be used as a chemical rinse to avoid the potential for inadvertent chemical exposure to nearby people and objects. Pressurized water at temperatures up to 95 degrees Celsius can improve the solubility of soiling and effect of chemical cleaners, however this method should not be used in cold weather to avoid causing thermal shock damage to the stone (fig. 6.2).

Fig. 6.2 Cleaning with pressurized water.

Cleaning Stone / Conventional Methods

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3. Steam or Hot Water Vapor Applied through a pressurized “lance,”6 steam is infrequently used as a cleaning technique due to the extreme caution its application requires and the sometimes-uneven cleaning results. Most often used on limestone, steam can be effective for removing soiling in recessed areas. For projects where water runoff should be minimized or the water supply is low, steam can be an effective substitute for other water-based cleaning techniques. It is also often used with chemical cleaning methods.

Chemical Cleaning Methods

Acidic and alkaline solutions are effective agents for cleaning stone but are high-risk

methods. Their use must be closely supervised to avoid health and safety violation and risk. Stone surfaces must be tested for pH after every step in a chemical cleaning process to ensure as much residue has been removed as possible. Alkali-acid processes are commonly used on stone with significant soiling buildup and are accomplished in several stages, alternating between alkaline, acetic acid, or hydrofluoric acid washes.

Generally speaking, chemical methods can leave residues that may manifest in problematic

ways (such as accelerated weathering or efflorescence) long after the treatment application.7 Some case studies have shown that chemical cleaning can etch the stone’s surface, leading to increased absorption of water or cleaning that sometimes surpasses the desired effect and inflicts damage.8

Fig. 6.3 Chemical cleaning

Cleaning Stone / Conventional Methods

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1. Hydrofluoric (HF) Acid Best for use on sandstone and brick, hydrofluoric acid is applied to thoroughly wetted stone and can effectively clean the surface with short dwell times. Because application in liquid form can result in uneven cleaning, hydrofluoric acid is often applied to the stone in gel form and agitated during the dwell time. When used incorrectly or on the wrong stone types it can cause irreversible damage such as “over cleaning,” undesirable staining, or chemical alteration of the stone. Hydrofluoric acid should not be used on glazed or polished surfaces, limestone, or marble because it dissolves calcite minerals. Due to extreme hazards associated with its use, hydrofluoric acid application must meet rigid safety requirements. It must be thoroughly rinsed after application, and is commonly used after an alkaline degreaser has been applied to the stone.

2. Hydrochloric (HCl) Acid Primarily used for specific lime-based stain removal, hydrochloric acid is not advised for use in general cleaning. Like with hydrofluoric acid, stone cleaned with hydrochloric acid must be thoroughly wetted and rinsed before and after application and must be closely supervised.

3. Alkaline Solutions Ideal for “degreasing” chemical components in heavy soiling, alkali-based solutions are used on sandstone, brick, terra cotta, granite, and limestone. Considerable safety precautions must be taken to avoid health and safety risks.

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4. Acetic Acid Acetic acid is an effective neutralizer for materials treated with alkaline solutions. They are not generally used independently of other cleaning methods.

Poultices and Packs

Poultices and packs are chemical applications that see longer surface exposure and result

in shallower penetration. A poultice is made by mixing chemicals with powder to form a paste, which is applied to the surface of the stone and left to dry. The reaction that takes place is much like a facemask treatment: as the paste dries, it draws out impurities from the stone. The residue that remains after the dried paste falls away is cleaned with de-ionized water.9 Poultices and packs are relatively gentle cleaning techniques because they do not require pressurized application, however, as with chemical washes, the severity of the reaction cannot be determined until the agent is removed, and the method of application does require contact with the surface.

Fig. 6.4 Poultice paste application.

Air Abrasive and Mechanical Methods

Air abrasives require the propulsion of particulates using pressurized air and/or water,

which can be very effective in removing surface encrustations but often results in the removal of Cleaning Stone / Conventional Methods

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surface material as well. Mechanical methods include surface scraping, such as with a scalpel or brush, leaving the surface vulnerable to the skill and care of the conservator.

Fig. 6.5 Mechanical scrubbing

Fig. 6.6 Sponge jet wash.

6.3 Laser Cleaning

As mentioned in other sections, laser cleaning can be a very effective method for cleaning

stones with fragile surfaces. The non-contact application approach ensures that delicate, delaminated material will remain intact and adhered to the substrate. Furthermore, cleaning can expose the true condition of a stone, so if there is a question or concern as to what this condition may be, laser cleaning is often the best cleaning method as it will preserve as much surface material as possible. “Damage previously obscured by the encrustation can be treated and its cause located and eliminated, thereby arresting the decay and extending the life of the sculpture.”10

Lasers tend to be most effective on a light-colored stone with a dark surface encrustation

layer. “The best possible situation is a dark, highly absorbing soil on a white calcareous stone.”11 Marbles and limestones respond well to this treatment technique, although the level of effectiveness ultimately depends on the characteristics of the encrustation to be removed.

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As effective as it is for treating certain conditions, laser cleaning also has limitations. It

is generally ineffective on dark stones due to the absorptive quality that the stone and surface soiling share. For this reason, polychromatic stones can present similar problems. Also, the small spot size that allows for extreme precision and control also contributes to slow progress and operator fatigue. Equipment is susceptible to malfunction and breakdown, and certain safety precautions must be taken that other methods do not require. Studies have shown that lasers can produce yellowing on some marbles. Despite extensive research and experimentation, scientists are still unclear as to what causes this undesirable phenomenon, a risk that may preclude a property owner from considering the technique despite opportunities to test the material prior to application.

The success of early experiments with lasers to clean stone led to speculation about the

technology’s potential impact when used on other types of soiled objects. It wasn’t long before scientists and conservators began experimenting with laser technology to clean other architectural and artistic materials such as metal, glass, ceramics, paper, wood, textiles and plastics; even fossils and daguerreotypes have been cleaned with lasers. The layers targeted for removal do not exclusively comprise pollution and gypsum crusts—biogrowth and paint are often the focus of laser cleaning methods on these different materials.

6.4 Lasers Used with Other Cleaning Techniques

Laser cleaning methods are generally used because other methods are not appropriate.

For example, at the New York Public Library, the marble pediment sculptures were so sugared and fragile that any contact cleaning method would have caused irrevocable damage (although gentle pre-treatment manual cleaning with cotton and bristle brushes and light deionized water Smith

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washing were applied to certain areas13). Lasers were able to clean the surface while retaining the delicate surface material and sculptural detail. However this was the only area of the library’s exterior that required such careful cleaning treatment. Steam was used to clean the remainder of the library’s exterior because it is an efficient method that did not jeopardize the integrity of the stone, and takes less time at lower expense than laser cleaning does.

In general, more than one cleaning method will be utilized in an architectural restoration

project because different techniques accomplish different things. “It is very often the case that the best cleaning results are obtained by combining laser cleaning with more traditional methods such as poultice, chemical, and micro-air abrasive cleaning.”14 Lasers will usually be applied after the initial cleaning application to eliminate residual encrustations and surface soiling. Rarely will lasers be chosen as the only cleaning method on a large-scale architectural project, and cleaning tests should always be conducted prior to any application.

Fig. 6.7 Cleaning test at the Isabella Stewart Gardner Museum in Boston. Finials and capitals cleaned with lasers and calcium-based poultice. The brighter, second finial was cleaned with poultice; the third finial was cleaned with a laser.

Cleaning Stone / Lasers Used with Other Cleaning Techniques

Fig. 6.8 Cleaning tests prior to restoration of the Washington Square Arch in New York City. From top to bottom: gomage, laser, water. Note slight yellowing in laser band.

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1. Zhang, Jie. Laser Tube Bending and Laser Cleaning Of Metallic and Non-metallic Materials. PhD Dissertation: Columbia University (2006). p26–27 2. Van Grieken, R., F. Delalieux and K. Gysels. “Cultural heritage and the environment.” Pure and Applied Chemistry 70.12 (1998): 2327–2331.

3. Vergès-Belmin, Véronique. “Pseudomorphism of gypsum after calcite, a new textural feature accounting for the marble sulphation mechanism.” Atmospheric Environment 28.2 (1994): 295–304.

4. Maravelaki, P.V., V. Zafirpoulos, V. Kylikoglou, M. Kalaitzaki and C. Fotakis. “Diagnostic techniques for laser cleaning of marble.” Proceedings of the Eighth International Congress on Deterioration and Conservation of Stone, Berlin (1996): 1395–1401.

5. Ashurst, Nicola. Cleaning Historic Buildings, Vol. 2. London: Donhead (1994). p1 6. Cooper, Martin (editor). Laser Cleaning in Conservation: An Introduction. Oxford: Butterworth Heinemann (1998). p5 7. Ibid., p5 8. Ibid., p6 9. Ibid., p6 10. Ibid., p2 11. Included in survey questionnaire feedback from a U.S.-based laser cleaning conservator.

12. As outlined in a protocol treatment document for the NYPL project provided by Giancarlo Calcagno.

13. Op. cit., Fotakis, p265

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7. Case Studies

Lasers are used for cleaning stone in the field at historic sites, in museums, and in labo-

ratories for testing and experimentation. The case studies discussed in this chapter are limited to laser-based stone-cleaning methods implemented on architectural restoration projects. Many applications of laser radiation on stone are limited to the laboratory. These instances may be related to testing various cleaning methods for a given project or stone type, while other experiments serve to test the parameters of the technology on various stone types with different soiling patterns to expand the conservation canon. Laser cleaning applications limited to the laboratory will not be discussed, except where conclusions drawn from experimentation led to adjustments in the field. Case studies from museum collections will also be excluded from this chapter because the lack of published material on museum laser cleaning projects would result in a weak representation of an area where much of the technology is actually in use.

The case studies listed below are organized by country and represent laser cleaning applica-

tions on stone only. The majority of these projects were executed on portions of the architecture (rather than the entire structure) and many utilized multiple cleaning methods in addition to lasers.

Sites in italics are profiled in detail following this list.

Austria Vienna: Minoritenkirche Vienna: St. Stephen’s Cathedral Belgium Brussels: Hotel de Ville China Terra Cotta Warriors

Croatia Split: Diocletian Palace Peristyle Denmark Copenhagen: St. Frederick’s Church France Amiens: Amiens Cathedral Arles: St. Trophime Cloister Bordeaux: Church of St. Michel* Bourges: Bourges Cathedral Smith

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Chartres: Cathedral of Our Lady of Chartres Clamecy: Collégiale Saint-Martin Mantes-la-Jolie: Notre Dame Collegiate Church Paris: Cathedral of Notre Dame* Poitiers: Cathedral of Notre Dame* Saint-Benoît-sur-Loire: Saint-Benoît-sur-Loire Abbey Saint-Denis: Cathedral Basilica* Saintes: Cathédrale Saint-Pierre Tours: Tours Cathedral Versailles: Garden Façade Capitals Germany Berlin: Brandenburg Gate Dresden: Chapel of the Residence Castle Greece Athens: Caryatids’ Porch Athens: Parthenon West Frieze Italy Bologna: San Petronio Cathedral Brescia: Brescia Loggia Facade Cremona: Cathedral of Cremona Florence: Cathedral of Santa Maria del Fiore Florence: Pallazzo Rucellai Pisa: Cathedral of Pisa Pisa: Church of San Frediano Pisa: Tower of Pisa Ravenna: Mausoleum of Theodoric Siena: Town Hall Torino: Villa della Regina Torino: Palazzo Madama Venice: Church of the Maddalena Venice: Church of St. Helena Venice: Palazzo Ducale Venice: Venice Casino Capitals

Ventimiglia: Cathedral of Ventimiglia Viterbo: Church of S. Giovanni The Netherlands Rotterdam: City Hall Poland Krakow: King Sigismund’s Chapel at Wawel Castle* Tum: Arch-Collegiate Church Portugal Lisbon: Monastery Jeronimos Spain Granada: Granada Cathedral Oviedo: Cathedral of Oviedo Seville: Cathedral of Seville* Valencia: Palacio Ducal de Gandia Valladolid: Antiguo Convento de las Francesas United Kingdom Lincolnshire: Lincoln Cathedral Liverpool: St. George’s Hall Liverpool: Ince Blundell Oxford: Oxford University United States Chicago, Illinois: Nickerson Mansion/Driehaus Museum Joshua Tree National Park, California: graffiti removal from boulders New York City, New York: New York Public Library* Philadelphia, Pennsylvania: Philadelphia Museum of Art Charlottesville, Virginia: Pavilion II (University of Virginia)*

* Photos of laser cleaning treatment included at end of section.

Case Studies

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Amiens Cathedral (Portail de la Mère Dieu), France (1993–1995)

The restoration of Amiens Cathedral was the first major laser cleaning project to be

carried out on a large-scale surface.1 Made of limestone and built between 1225 and 1240, the Amiens Cathedral was named a UNESCO world heritage site in 1981. Between 1993 and 1995 laser cleaning was applied to the south portal on the cathedral’s west façade, with the central and north portals cleaned in later phases, between 1998 and 1999. Three ‘NL101’ Nd:YAG lasers manufactured by B.M. Industries were used to carry out the cleaning efforts. The south portal was cleaned exclusively using lasers, whereas the central portal was finished with lasers after first undergoing poultice and microsandblasting. The north portal’s statues were cleaned with lasers with microsandblasting applied to the face stones and moldings.2 The reason for the combined cleaning treatments on the subsequent portals has been attributed to criticism of too much yellowing and induced color uniformity from the treatment on the south portal.3

Figs 7.1 and 7.2 Amiens Cathedral in France before restoration (at left) and after restoration (at right).

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Arch-Collegiate Church of Tum, Poland (2001–2007)

Built more than 800 years ago, the Arch-Collegiate Church in Tum, Poland, is the larg-

est Romanesque structure in the country and an important national monument. Constructed primarily of limestone, the structure suffered from long-term degradation and damaging repairs over many years. Paint, for example, which had presumably been applied to protect the surface, closed pores and trapped moisture in the stone. A multi-year restoration project to clean the building was begun in 2001 and included the 12th-century main portal, sacristy portal, a limestone sculpture, a 14th-century epitaph, vault springers, and entrance portal on the Northern aisle.4 A Q-switched Nd:YAG laser system was used for the project; the success of the restoration was instrumental in the approval of a similar cleaning project later implemented at Sigismund Chapel at Wawel Castle in Krakow.

Fig. 7.3 Portal of the Arch-Collegiate Church in Tum, Poland, before and after laser cleaning.

Church of the Maddalena, Italy (1997)

The Church of the Maddalena in Venice, Italy, was designed by Tommaso Temanza and

built between 1763 and 1778 of fine-grain Istrian marble. A long-term restoration effort for the entire building was begun in 1988; the cleaning component was scheduled for the final phase. The church’s entry portal had darkened with a thick surface encrustation, particularly in the sculptural relief above the entry, and was cleaned in 1997 primarily by laser ablation. This cleaning Case Studies

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treatment was chosen after extensive analysis was conducted to examine a unique deterioration characteristic of the stone. The fine grain size of the Istrian marble featured a dense pitting pattern that manifested as a thick black crust with white sulphated cavities. Over 60 days during winter a laser was used to successfully clean 140 square meters of the church’s exterior; this area represents the amount of thick black encrustation present. Mechanical, solvent, and water cleaning techniques were used to clean other types of decay, such as animal droppings, graffiti, and cement residues.5

Figs. 7.4 and 7.5 Photos of the Church of the Maddalena in Venice, Italy, before (left) and after cleaning (right).

Ince Blundell, estate of Henry Blundell, UK (2000)

The estate included an 18th-century pantheon and garden temple, to which approximate-

ly 60 pieces of marble sculpture are attached. Laser cleaning of these sculptures was undertaken by conservators from National Museums Liverpool (then known as National Museums and Galleries on Merseyside) in the late 1990s using a Phoenix Nd:YAG laser manufactured by Lynton.

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The garden temple restoration was funded by a 1996 grant from the European Regional Development Fund, which was received and distributed by the Sefton Borough Council.6 In addition to laser cleaning, other restoration efforts included consolidation of deteriorated marble and sculptural panel replication with the aid of laser scanning and artificial patination.

Figs. 7.6 and 7.7 Sculpture panel at Ince Blundell before laser cleaning (left) and after (right).

Mausoleo di Teodorico, Italy (late 1990s)

The restoration of the Mausoleo di Teodorico monument in Ravenna, Italy, involved several

cleaning methods. Built between 520 and 526 A.D. of Aurisina limestone, the mausoleum’s large dome was covered with biogrowth and black encrustations. Biocides were used initially applied followed by a series of hydro-jet washes. Chemical compresses were then applied to residual black crusts. After this process, soiling residue remained around the base of the dome’s frieze, in the denticulate decoration. Microabrasion was attempted on this area, but the results showed limited success and conservators had concerns about the loss of surface material. After extensive laboratory testing to determine optimal laser cleaning parameters, a laser was applied to the denticulation with great success.7

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Figs. 7.8 and 7.9 Mausoleo di Teodorico in Ravenna, Italy during restoration with laser cleaning (left and right).

Nickerson Mansion/Driehaus Museum, US (2004–2005)

Designed by Edward Burling and clad in a light-colored Ohio sandstone, the Nickerson

Mansion was built in 1883 after the Great Chicago Fire of 1871. By 2004, when a major restoration effort began, the mansion appeared completely black due to a 122-year accumulation of environmental particulates on its surface. The thick black crust was trapping moisture, leading to degradation of the stone. Much of the pollution buildup could be attributed to the first 50 years of the mansion’s history, when emissions from coal, leaded fuel, and other pollutants were abundant. Because the entire building—25,000 square feet of surface—was black, the restoration called for a monumental cleaning task, and represents the first entire building to be cleaned using laser ablation. Many cleaning treatments were tested; the abundance of iron in the stone resulted in uneven cleaning in the tested areas, particularly with chemical applications. Chemicals were also problematic due to the enormous amount of surface area to be treated, which would Case Studies

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create great public and environmental hazards. Tests showed that the pollution crust had deeply penetrated the surface; sand and silica sandblasting tests resulted in the loss of 1–2 mm of surface material. Largely due to the aforementioned conflicts with conventional cleaning methods, lasers proved to be the only viable cleaning technique for this project. Several laser systems were tested; the CL120 Q-switched Nd:YAG system from Clean Laser with an oscillating mirror (which produces a beamed line) was determined to produce the most optimal results and was chosen for the project. Cleaning began in June 2004 and continued uninterrupted until November 2005, at a rate of between one and five square feet per hour. The majority of the cleaning was handled by one laser, but two additional systems were eventually acquired to meet the project deadlines.8 The Nickerson Mansion is now home to the Driehaus Museum and is open to the public.

Fig. 7.10 Nickerson Mansion cornice with thick black pollution encrustation before laser cleaning.

Fig. 7.11 Nickerson Mansion cornice during laser cleaning.

Fig. 7.12 Nickerson Mansion cornice after laser cleaning.

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Parthenon West Frieze, Greece (2002–2005)

Deteriorated marble blocks form the Parthenon’s West Frieze were removed for resto-

ration in 1993. Deterioration from mechanical, physical, chemical, and atmospheric influences manifested as disintegration, material loss, cracking, black pollution encrustations, and damage from prior interventions. Prior to cleaning, a restoration phase was completed that included surface consolidation, bronze dowel and mortar removal, and fragment reattachment. Cleaning was needed to address the black crusts and loose deposits present. Four cleaning methods were tested: poultices, microblasting, inversion of gypsum to calcite, and laser cleaning. Of these, only laser cleaning met the criteria previously established, which included preservation of the noble patina, gypsum layer, and monochromatic surface, while efficiently removing all types of encrustation with consistent results. The course of laser cleaning treatment implemented represented a new methodology—the application of laser ablation using two simultaneous wavelengths.

Fig. 7.14 Detail of marble block from the Parthenon West Frieze during laser cleaning.

Fig. 7.13 Laser cleaning relief on a marble block from the Parthenon West Frieze.

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A prototype Q-switched Nd:YAG laser capable of emitting wavelengths at 1064 nm (IR/

fundamental) and 355 nm (UV/third harmonic) with overlapping pulses was designed by researchers at the Institute of Electronic Structure and Lasers with the Foundation for Research and Technology, Hellas (IESL—FORTH) and employed for the Parthenon cleaning project. The different radiations could be used independently as well as in tandem, and the type of application depended on the composition of the encrustation being removed. Wetting the stone when the infrared wavelength ( 1064 nm) was in operation was found to be effective, however this method produced uneven results when the laser operated with combined wavelengths. Thus, the marble blocks were successfully cleaned using a variety of settings on different areas. They are now on display in the Acropolis Museum in Athens.9

St. George’s Hall Reliefs, UK (2005)

Located in Liverpool, St. George’s Hall is a mid-19th century neoclassic concert hall and

courthouse designed by Harvey Lonsdale Elmes. The building saw the installation of 12 carved Istrian limestone panels between pilasters on the east facade in 1894 and 1901 that symbolized Liverpool’s role as a commercial port. Over the ensuing century, black pollution crusts formed on the panels in areas of sculptural relief. In addition, high-relief areas were severely weathered and bleached, and had blistered in some places. The conservation program undertaken over three months in 2005 involved cleaning and consolidation. For the cleaning component, steam and lasers were the two methods tested. Steam cleaning was ruled out after it was determined that the amount of pressure needed for a successful cleaning outcome resulted in a loss of surface material. A Q-switched Nd:YAG Zenith laser manufactured by Lynton provided good results and was thus chosen. Water was misted onto the surface immediately prior to cleaning, which had been determined as the most effective regiment with consistent cleaning results. Each panel is approxCase Studies

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imately 30 square feet in size and took one week to clean, on average. Yellowing of the limestone, which had been apparent prior to cleaning (and was therefore not induced by the laser), was later reduced by the application of paper poultices to the cleaned surface. Consolidation took place after cleaning, and treated areas were sprayed with polyvinyl alcohol as a protective coating.10

Figs. 7.15 and 7.16 Relief panels at St. George’s Hall in Liverpool before (left) and after cleaning (right).

Fig. 7.17 St. George’s Hall in Liverpool after restoration.

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St. Stephen’s Cathedral, Austria (1998)

St. Stephen’s Cathedral in Vienna, Austria, represents a monumental laser cleaning proj-

ect. Built in the mid-14th century, the cathedral’s Albertino Chancel and Portal of the Riesentor were deteriorating from plaster and carbon deposits that manifested as thick black encrustations coating the sandstone. A large-scale cleaning effort was carefully planned after extensive materials testing. Thirty thousand hours of labor were dedicated to cleaning 30 gargoyles and statues, 82 meters of wine leaf ornament, parapets, 17 windows, and 14 pillars. Four different laser systems were used: the Quantel Laserblast 500/1000, Art Master from EV laser, the NL103 from Thales, and the Palladio from Quanta Systems.11 As many as seven lasers were used at a time.12

Figs. 7.18-7.22 St. Stephen’s Church in Vienna during laser cleaning treatment (top left and center; before cleaning (above); after cleaning (below); and exterior (left).

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Fig. 7. 23 Vault sculpture at the Cathedral of Notre Dame in Paris during cleaning. Photo at left is before treatment; at center, after poulticing; and at right, after laser cleaning.

Fig. 7. 24 Sculpture at Poitiers Cathedral. Photo at left shows preliminary cleaning to remove dust; at right, after laser cleaning.

Fig. 7.25 Portal sculptures at Cathedral Basilica in St. Denis, France, before cleaning (left) and after cleaning (right).

Fig. 7.26 Laser cleaning treatment of the capitals at Pavilion II at the University of Virginia in Charlottesville.

Fig. 7.27 Cleaning tests on the Church of St. Michel in Bordeaux.

Fig. 7.28 Sigismund Chapel detail during laser cleaning.

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Fig. 7.29 Terra cotta sculpture ornament at the Cathedral of Seville before (left) and after cleaning (right).

Fig. 7.30-32 Cleaning of the Vermont marble sculptures in the entry-facade pediments at the New York Public Library. After cleaning, above; before cleaning, above right; during cleaning, right.

1. Weeks, Christopher. “The Conservation of the Portail de la Mere Dieu, Amiens Cathedral.” LACONA I Proceedings, October 4–6, 1995 (1997): 25–29.

2. Bromblet, Philippe, Martin Labouré and Geneviève Orial. “Diversity of the cleaning procedures including laser for the restoration of carved portals in France over the last 10 years.” Journal of Cultural Heritage Vol. Case Studies

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4. Supplement 1 (2003): 17–26.

3. Ibid.

4. Koss, A., J. Marczak and M. Strzelec. “Arch-Collegiate Church in Tum: Laser renovation of priceless architectural decorations.” LACONA VII Proceedings, September 17–21, 2007 (2008): 203–207.

5. Armani, Emanuele, Giancarlo Calcagno, Claudio Menichelli and Marisol Rossetti. “The Church of the Maddalena in Venice: the use of laser in the cleaning of the façade.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 99–104.

6. Larson, John H., Claire Madden and Ian Sutherland. “Ince Blundell: the preservation of an important collection of classical sculpture.” Journal of Cultural Heritage, Vol. 1, Supplement 1, (2000): 79–87.

7. Pini, Roberto, Salvatore Siano, Renzo Salimbeni, Valter Piazza, Marco Giamello, Giuseppe Sabatini, Fabio Bevilacqua. “Application of a new laser cleaning procedure to the mausoleum of Theodoric.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 93–97.

8. Dajnowski, Andrzej, Adam Jenkins and Andrew Lins. “The Use of Lasers for Cleaning Large Architectural Structures.” APT Bulletin 40.1 (2009): 13–23.

9. Frantzikinaki, K., G. Marakis, A. Panou, C. Vasiliadis, E. Papakonstantinou, P. Pouli, T. Ditsa, Vassilis Zafiropulos and Costas Fotakis. “The Cleaning of the Parthenon West Frieze by Means of Combined IR- and UV- Radiation.” LACONA VI Proceedings, September 21–25, 2005 (2007): 97–104.

10. Cooper, Martin and Sam Sportun. “The Application of Laser Cleaning in the Conservation of Twelve Limestone Relief Panels on St. George’s Hall.” LACONA VI Proceedings, September 21–25, 2005 (2007): 55–64.

11. Pummer, Erich. “Exists a Demand for Nd:YAG Laser Technology in the Restoration of Stone Artworks and Architectural Surfaces?” LACONA VI Proceedings, September 21–25, 2005 (2007): 143–150.

12. Calcagno, Giancarlo, Erich Pummer and Manfred Koller. “St. Stephen’s Church in Vienna: criteria for Nd:YAG laser cleaning on an architectural scale.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 111–117.

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8. Research, Training Programs, Conferences, & Funding Support 8.1 Research

Numerous research facilities throughout Europe devote resources to experimentations

with laser cleaning for conservation, many of which are affiliated with universities. Some of the organizations that conduct substantial research are explored in detail here; a comprehensive outline of laser cleaning research locations can be viewed in section 4.3, which lists the professional affiliations of authors who have published articles on their work with laser cleaning experimentations and applications.

From the onset of mainstream laser cleaning research in the early 1990s, four organiza-

tions have been at the forefront of laser cleaning innovation, testing, and development: the Foundation for Research and Technology Hellas in Greece; National Museums Liverpool (formerly National Museums and Galleries on Merseyside) in the United Kingdom; the Consiglio Nazionale de Ricerche in Italy; and the Laboratoire de Recherche des Monuments Historiques in Champs-surMarne, France. Also in Europe, the German Federal Foundation for the Environment, the Centre for Architectural Conservation in Austria, and the Spanish High Council for Scientific Research have allocated considerable resources to laser cleaning research through distribution to a variety of research facilities and conservation agencies. In addition, there are notable organizations that have emerged over the past decade as centers for laser cleaning research, including in the U.S. at the National Center for Preservation Technology and Training in Natchitoches, Louisiana. The Institute for Pure and Applied Physical Sciences (IPAPS) at the University of California, San Diego, where John Asmus taught and conducted laser research beginning in 1973, was apparently dissolved in early 2010.1 Smith

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In addition to current laser cleaning work in Europe and the U.S., there are of course oth-

er regions experimenting with the technology. In Australia, for example, laser cleaning first appeared in 2006 during a workshop organized by researchers with the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO), which arranged for Martin Cooper and Helen Thompson of National Museums Liverpool to deliver a video-linked training broadcast.

The laser used for the workshop, a Compact Phoenix Nd:YAG from Lynton Lasers, was later purchased by an art gallery in New South Wales.2 Other laser cleaning research has been undertaken at the Australian National University and the Macquarie University in Sydney, and studies were conducted at the National War Memorial in Canberra. Despite these examples, one Australian conservator does not anticipate laser cleaning will gain much ground. “We have very little of this type of soiling on stonework in Australia (there is some, but not very much), so that there is limited applicability for lasers in cleaning stonework in Australia. Add to that the tyranny of distance and aversion to being the first to use new technology, as well as the cost, and there is limited scope for implementing lasers.”3

Outreach to other international conservation organizations found that laser cleaning of

stone and objects of cultural heritage is also taking place in Santiago, Chile at the Centro Nacional de Conservación y Restauración, which uses a LaserBlast 60 Nd:YAG system that was gifted to the facility in 2004 from the Government of Japan and is primarily used for cleaning marble sculptures.4 In Hong Kong, China, a laser was used to clean an outdoor mural,5 and in Argentina laser cleaning work has been underway since 1999 at the Laboratorio de Ablación, Limpieza y Restauración con Láser.6 A formal study could yield more complete information on the worldwide scope of laser cleaning.

In additional to European research organizations, there are professional organizations

devoted to linking groups of researchers and funding together on a project-by-project scale. The Research

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largest of these groups is known as COST G7, which includes membership from 35 European countries and has a dedicated laser cleaning working group that meets several times a year. The International Center for the Study of Preservation and Restoration of Cultural Property (ICCROM), the International Council on Monuments and Sites (ICOMOS), and the International Institute for Conservation of Historic and Artistic Works (IIC) also coordinate international collaborations in conservation efforts.

More detail about COST G7, ICCROM, ICOMOS, IIC, and the major research organizations

from Austria, France, Germany, Greece, Italy, Spain, the U.K. and the U.S. is included below.

Austria

The Centre for Architectural Conservation is a branch of the Austrian Federal Office for

the Care of Monuments (Bundesdenkmalamt). Much like the American Institute for Conservation of Historic and Artistic Works, the Austrian Centre seeks to provide an interface for a variety of preservation-related interest groups. High priorities include extensive research and testing of building materials and conservation methods; documentation and evaluation; building material archives and collections; and continuing education.7 Within the laser cleaning field, the Centre has supported numerous research initiatives as well as the restoration of St. Stephen’s Cathedral in Vienna, one of the largest architectural cleaning projects to be undertaken using lasers in the world.8

France

The Laboratoire de Recherche des Monuments Historiques (Historical Monuments Research

Laboratory/LRMH) is part of the French Ministry of Culture and Communication. As one of the first organizations to establish a laser cleaning research program, LRMH is frequently involved Research

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with restoration projects both within and outside France. Major early laser cleaning projects include the Cathedrals of Notre Dame in Paris and Poitiers and Amiens Cathedral, which was the first architectural laser cleaning project in Europe.9 With in situ conservation approaches, LRMH is active in the European conservation community and especially prominent among laser cleaning research facilities. Primary research focuses include building materials, causes of decay, restoration treatments, and environmental conditions. LRMH oversees Cercle des Partenaires du Patrimoine,10 a non-profit entity that facilitates materials research through public and private organizations that mobilize financial, technological, and scientific resources.11

Germany

Located in OsnabrĂźck, the Deutsche Bundesstiftung Umwelt (German Federal Founda-

tion for the Environment/DBU) provides research support and training for environmental technology, research, and communication; nature conservation; and cultural assets.12 DBU facilitates research cooperation between institutions, awarding funding based on the potential for reducing environmental impact. Its subsidy criteria include: 1. Innovation: the project has to be an advance to the state-of-the-art of current research and technology.

2. Exemplary and model character: the innovation should be of interest to a broad segment of the population (e.g. a complete industry). It should also be possible to implement the innovation under commercial conditions within a brief time-scale.

3. Environmentally beneficial: the innovation should lead to new, supplementary measures for the protection of the environment.13

DBU-supported research may include international collaborations, particularly among

bordering nations. LACONA V (2003) was held at DBU’s Osnabrßck facility.

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Greece

Supported by the General Secretariat for Research and Technology of the Hellenic Min-

istry of Development and the European Commission, the Ultraviolet Laser Facility–Foundation for Research and Technology Hellas (ULF-FORTH) is a scientific laboratory with a focus on laserbased technologies. Areas of research include: atomic and optical physics; molecular physics and chemical dynamics; interaction of lasers with materials; fundamentals and applications; laser applications in biomedicine; and laser techniques in cultural heritage.14 The organization’s robust research program hosts international scientists for cross-disciplinary laser technology research, including many focused on laser cleaning methods in conservation. In addition to publishing a large number of research papers on laser cleaning, ULF-FORTH is an active member of LaserlabEurope, an international scientific organization that facilitates research in laser technology (although it does not yet seem to have extended its research efforts to include laser cleaning in cultural heritage).

Italy

Founded in 1923, the Consiglio Nazionale de Ricerche (National Research Council/CNR)

is a primary support agency designed to facilitate scientific, economic, technological and social development in throughout Italy and Europe. More than 4,000 researchers are affiliated with CNR, which has partnerships with more than 100 institutes.15 Much of the substantial work in laser-based cleaning methods in Italy is based in Florence at the Istituto di Fisica Applicata “Nello Carrara” (Institute of Applied Physics/IFAC), which is a research branch of CNR. IFAC focuses on research development in the areas of optoelectronics, spectroscopy, and information and communication technologies. Scientists at IFAC have provided critical research to the laser cleaning field including experimentations with damage thresholds of stone,16 compatibility of laser cleaning Research

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with conventional cleaning techniques,17 and the impact of variations in wavelength and pulse duration.18

Spain

The Consejo Superior De Investigaciones Científicas (Spanish High Council for Scientific Re-

search/CSIC) coordinates research, funding, education, training, and collaboration among scientists, universities, and professional organizations in Spain and throughout Europe. Comprising eight scientific research areas, CSIC supports laser cleaning research through its materials science and technology arm. Among the research facilities supported by CSIC that conduct research in laser-based cleaning methods are Instituto de Estructura de la Materia and Instituto de Química Física Rocasolano in Madrid; and Instituto de Recursos Naturales y Agrobiología in Sevilla.

United Kingdom

Until recently known as National Museums and Galleries on Merseyside, National Muse-

ums Liverpool (NML) is a museum complex that includes eight facilities: World Museum Liverpool; Walker Art Gallery; Lady Lever Art Gallery; Sudley House; Merseyside Maritime Museum; the National Conservation Centre; and the International Slavery Museum. The National Conservation Centre was established in 199619 but its sculpture conservation section has existed since 1991,20 in tandem with emerging research in laser cleaning technology. In addition to sculptural conservation efforts due to its substantial museum affiliation, the National Conservation Centre is involved with numerous architectural restoration projects that incorporate laser cleaning methods, such as at Lincoln Cathedral in Lincolnshire and St. George’s Hall21 in Liverpool. As a major conservation research facility, NML is also home to the most robust laser cleaning training center in the world.

More information on the Centre’s training program can be found in section 8.2.

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United States

Despite several promising developments in the 1990s, little research is currently taking

place in the U.S. A 1997 Getty Conservation Institute (GCI) newsletter alludes to the pending establishment of an internal laser cleaning project,22 but no further information can be found. A comprehensive “Review of the State of the Art of Laser Cleaning in Conservation” was written by Margaret Abraham and John Twilley of the Los Angeles County Museum of Art (LACMA) Conservation Research department in 1997 and submitted to the National Center for Preservation Technology and Training (NCPTT).23 The report advocated for the formation of an American laser cleaning research facility and included recommended budget outlines. A research program was eventually established at LACMA and later transferred to the NCPTT, which is the only conservation center in the United States with a research facility for laser cleaning.

NCPTT’s mission is to advance historic preservation initiatives through training, educa-

tion, research, technology transfer, and partnerships.24 Many research endeavors take place at the Louisiana facility, however NCPTT supports preservation technology research and testing nationwide. Through a partnership with Northwestern State University of Louisiana (NSU) in Natchitoches, NCPTT has established a joint laser research and program on the NSU campus. The onsite laser, a Coherent Infinity Nd:YAG system, is currently being used to experiment with graffiti removal from Colorado Yule Marble.25 The center plans to acquire another laser system and eventually implement a training program.26

COST G7

The European Cooperation in Science and Technology (COST) is an intergovernmental

entity that organizes national funding efforts to synthesize research and experimentation in science across Europe. Its goal, to “ensure that Europe holds a strong position in the field of Research

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scientific and technical research for peaceful purposes,”27 is realized by combining the research efforts of universities, foundations, institutes, and private industries across Europe. COST does not provide funding, but coordinates the collaboration of national groups to work together on a project, called an ‘action.’28 An ‘action’ must be relevant to at least five COST countries. The 35 COST member countries are: Austria, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey, the United Kingdom, Serbia, and the Former Yugoslav Republic of Macedonia. Israel also is affiliated with COST as a ‘cooperating’ state. In addition, COST maintains reciprocal agreements with Australia, New Zealand, and South Africa.28

The nine COST research groups are: Biomedicine and Molecular Biosciences (BMBS) Chemistry and Molecular Sciences and Technologies (CMST) Earth System Science and Environmental Management (ESSEM) Food and Agriculture (FA) Forests, their Products and Services (FPS) Individuals, Societies, Cultures and Health (ISCH) Information and Communication Technologies (ICT) Materials, Physical and Nanosciences (MPNS) Transport and Urban Development (TUD)

Within the Materials, Physical and Nanosciences group is an organization identified as

MPNS Action G7: Artwork Conservation by Laser. This action group, self-identified as COST G7, is a networking hub for conservators working with lasers in three areas: laser cleaning, laser monitoring of environmental pollution, and laser systems for investigations and diagnosis.

Comprising 20 of COST’s member countries, the sector of COST G7 focused on laser

cleaning provides invaluable resources for practitioners, such as an online database of laser cleaning equipment manufacturers and a directory of firms providing equipment rental and training.

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The 20 member countries are: Austria, Belgium, Cyprus, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Malta, Norway, Poland, Portugal, Romania, Russian Federation, Slovenia, Spain, the Netherlands, and the United Kingdom.

Known as “Working Group 1,” the laser cleaning sector holds four tenets as its mission: 1. Gather bibliographical references on artworks cleaning, to be part of an input in the general literature database of the COST G7 project.

2. Collect information about all laser cleaning systems for conservation in Europe. These tools include commercially available systems (manufacturers), and systems in institutions like museums or universities, where access to equipment and expertise is available. The information is to be available on the COST G7 website. It is aimed at helping conservators interested in purchasing/renting a system, or in collaborating in research project.

3. Contribute to the general task “demand for new research in the field of artwork conservation by laser.”

4. Establish user guidelines for laser cleaning. Foreseen guidelines include recommendations on the way to perform and to report on laser cleaning, on the methods available to examine laser impacts on artifacts, and on safety issues.30

Working Group 1 is made up of professionals with expertise in science and engineering,

conservation, and laser equipment manufacture. The group meets for several days two or three times a year at different locations throughout Europe. Past meeting sites include Restauration Centre in Ljubljana, Slovenia; the National Library of Malta in Valletta, Malta; the Federal Institute for Materials Research and Testing in Berlin, Germany; Museu Nacional de Arte Antiga in Lisbon, Portugal; and the EVTEK Institute of Art and Design in Vantaa, Finland.31

In addition to the equipment and training database, COST G7 maintains a catalog of re-

search projects and a database of individual participants organized by country, along with a downloadable handbook published in 2008 entitled Handbook on the Use of Lasers in Conservation and Conservation Science. The handbook, a collaboration between COST and the European Science Foundation (the governing body that oversees COST’s legal obligations), includes trends in curResearch

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rent research and European laser cleaning case studies, in addition to chapters on documentation, diagnosis, and analysis.

ICCROM, ICOMOS, & IIC ICCROM

The International Centre for the Study of the Preservation and Restoration of Cultural

Property (ICCROM) is a great sponsor of international preservation efforts. Governed by a general assembly, the organization coordinates heritage-related training and research collaborations and has a strong public advocacy foundation. ICCROM offers educational and training course such as the International Course on Stone Conservation, which was founded in Italy in 1976 and is co-sponsored by UNESCO, the Getty Conservation Institute, the University IUAV of Venice, and the Italian Ministry of Culture. The International Course on Stone Conservation, which has met 16 times, has incorporated modern restoration treatments into its curriculum, which includes laser cleaning.32

ICOMOS

The International Council on Monuments and Sites (ICOMOS) is an association that co-

ordinates discourse between its 9,500 members and provides substantial support to research, education, and training initiatives for the preservation of cultural heritage worldwide. Despite its emphasis on preserving cultural heritage, laser cleaning does not appear to be a subject of focus aside from the promotion of the National Museums Liverpool training program.33

IIC

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Works (IIC) is a professional organization based out of London that facilitates conferences, workshops, research, and training. IIC provides scholarships and funding to students, IIC fellows, professionals, and member organizations from developing countries, and publishes several journals including Studies in Conservation, Reviews in Conservation, and News in Conservation, as well as the proceedings of IIC Congresses that are held every two years.34 Papers on laser cleaning have been published in Studies in Conservation35 and Reviews in Conservation, but there does not appear to be much exploration in laser cleaning beyond this.

8.2 Training Programs

There are few options for formal training in laser cleaning. Two facilities—one in England

and one in Italy—provide standalone training programs, and occasionally a one-time training session will be organized, as one was in Slovenia in 2005.36 It appears that the majority of conservators familiar with laser technology became so through onsite training during restoration projects that utilized lasers for cleaning. According to information provided by some laser system manufacturers, training on equipment use may be provided upon purchase.

The foremost active laser cleaning training program is in Liverpool, England. Established

in 1991, the training center is operated through National Museums Liverpool in the National Conservation Centre. Training courses are offered four times a year in two-day sessions and are open to everyone—no prior laser cleaning experience is required. The enrollment is small—only five people per course—to ensure optimum hands-on activity. The two-day training workshop currently costs £395 plus VAT. Participants attend lectures, examine case studies, and experience the technology first-hand.37 Of the survey respondents, 24 percent either trained at NML or hired Martin Cooper for onsite training at their own facility. As of April 2010, a total of 173 people have Smith

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participated in the NML training program over 15 years.38

In Venice, Italy, architect Giancarlo Calcagno administers a formal laser cleaning training

program affiliated with the Università Ca’ Foscari Venezia. This course includes 126 hours of training that are completed through four phases over 21 days of laboratory and classroom learning.

Calcagno teaches the course and utilizes an Nd:YAG prototype laser he developed through his company, Altech (Applied Laser Technology). Students who enroll in all four modules learn about the history of laser cleaning, all relevant aspects of safety for equipment use and working with a laser, basic principles of conservation and reversibility of interventions, examine the relationship between science and conservation, the theory of patina and time, and theoretical considerations, among other items in the curriculum. Scientific study incorporates characteristics of stone materials through petrography and mineralogical identification training. Other cleaning techniques are taught, but the emphasis is on laser cleaning with particular focus on stone. Upon completion of the course, students may apprentice for an additional 200 hours in order to receive full training certification.39

Also in Italy, the Opificio delle Pietre Dure in Florence has a laboratory for laser cleaning

and it is likely that conservators are trained to use the equipment there, however a formal training program does not appear to exist currently.

8.3 Conferences

It is probable that laser cleaning is a topic at many preservation- and cultural heritage-relat-

ed conferences around the world, however there are three in particular that have singular focuses on the technologies and/or materials related to laser cleaning of stone: the International Conference on Advanced Laser Technologies; the International Congress on Deterioration and Conservation of Smith

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Stone; and, most importantly, LACONA (Lasers in the Conservation of Artworks).

The International Conference on Advanced Laser Technologies (ALT)

This annual conference was founded in 1993 by Alexander Prokhorov, a Nobel Prize win-

ner and director of the General Physics Institute of Russian Academy of Sciences. ALT conferences focus on “the recent developments and advances in laser technologies and their applications,”40 and topics of discussion include photonics, material science, photoacoustics, LIBS and spectroscopy, and lasers in cultural heritage, among others. Each year the conference is held in a different European country; the 2010 conference will take place in Egmond aan Zee, outside of Amsterdam, from September 11 to 16, 2010. Each conference yields a published volume of proceedings, including papers on laser cleaning in cultural heritage.

The International Congress on Deterioration and Conservation of Stone

Founded by Vsevelode Romanovsky in 1972 as the International Symposium on the Dete-

rioration of Stone, the International Congress on Deterioration and Conservation of Stone meets every four years (at first meeting every two years). The most recent conference—the eleventh— met in 2008 in Torun, Poland.41 A multi-volume set of proceedings is published after each conference, and in recent years they have included a number of papers on laser cleaning. While most of the conferences take place in Europe, the fourth was held in 1982 in Louisville, Kentucky.

LACONA (Lasers in the Conservation of Artworks)

Without question, LACONA is the primary vehicle by which conservation professionals

working with lasers are able to share and expand their experience and knowledge of the technology and its applications. As noted in sections 2.1 and 4.1, the first LACONA conference was held in Conferences

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1995 in Crete, and has met every other year since in different European cities. In addition to cleaning applications, equal focus is devoted to laser scanning and analysis, along with other relevant topics such as safety and new technological developments. Stone is just one of the materials under the laser cleaning purview; because LACONA has a focus on art conservation, other common materials discussed include paintings, wood, metal, paper, and more. With one exception, each conference has resulted in a volume of proceedings, generally published the year following each conference. According to one American laser cleaning professional, an attempt to hold a LACONA conference in the U.S. was made several years ago but the prevailing ideology was that the distance would be too far to travel for most potential participants and attendance would be low.42

8.4 Funding Support

As many accounts indicate, the cost of laser cleaning equipment often exceeds what a

conservation firm, university, individual, or research facility is willing to pay to acquire a system for use. It is therefore imperative for funding to come from outside sources, and indeed there are many foundations and cultural heritage organizations that grant resources for laser cleaning research and use. In addition to some of the funding organizations previously discussed, there are other agencies and private foundations allocate funding for research projects and restoration efforts that may be applied to laser cleaning. A list of recent projects, as identified by Salimbeni et. al.,43 is included below.

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Fig 8.1: List of funded laser cleaning projects.

Most research papers include a section for acknowledgments. In an effort to create a list

of laser cleaning funding sources, these sections were mined for mention of financial support.

The resulting list, included below, represents only a fraction of funding sources that have aided in the increase of laser cleaning applications, testing, and information. In truth, a comprehensive list would be impossible to establish due to requests for anonymity, lack of acknowledgment, unspecified recipients in broad distributions of funding allocations, etc.

The purpose of including this list is to show geographic origins of funding given for laser

cleaning research. Section 4 of this research paper includes a geographic breakdown of author affiliations for papers published on laser cleaning research initiatives, which demonstrates a poor representation of American literature contributions. The low volume of American-published papers therefore will result in a low volume of acknowledged financial supporters but, again, may represent an accurate scope of laser cleaning use in the U.S. Fewer funding resources will result in fewer opportunities for research, which will result in fewer projects undertaken and therefore fewer published studies. Conversely, a robust funding program will yield more research and projects, generating more publicity and enticement for future funding. And in Europe, the international collaborative research programs enabled through organizations such as COST G7 amplify the number of resources available, increasing the number of projects and therefore strengthening the focused discipline.

Funding Support

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Sources of funding and assistance acknowledged in resources consulted for this thesis

that have contributed to research and practice of laser-based cleaning methods in conservation include: Worldwide International Institute for Conservation of Historic and Artistic Works (IIC) Europe European Commission European Cooperation in Science and Technology (COST) European Regional Development Fund Austria Centre for Architectural Conservation Belgium University of Antwerp France French Ministry of Culture Historical Monuments Research Laboratory ( LRMH) Germany German Federal Foundation for the Environment (DBU) Thuringia Ministry for Science, Research and Art (TMWFK) Greece Committee for the Preservation of the Acropolis Monuments Ephorate of Pre-historical and Classical Antiquities Greek Institute of Geology and Mineral Exploration (IGME)

Ultraviolet Laser Facility窶認oundation for Research and Technology Hellas Italy Italian National Research Council (CNR) High Technology Regional Network of Tuscany The Netherlands Rijksgebouwendienst Poland Polish Ministry of Science and Information Society Technologies State Committee for Scientific Research Spain City Council of Valencia Comision Interministerial de Ciencia y Tecnologia Heritage Conservation Institute of the Polytechnic University of Valencia Junta de Andalucia Junta de Galicia Spanish High Council for Scientific Research (CNIC) United Kingdom British Council Division National Museums Liverpool United States Mary Duke Biddle Foundation National Center for Preservation Technology and Training (NCPTT) Ottmar Foundation

It is important to note that the author affiliations listed in chapter 4 also provided fund-

ing support or other resources for the project on which the affiliation was listed. The organizations listed here are entities that have provided outside support.

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1. The IPAPS website (http://ipaps.ucsd.edu) states the following: “IPAPS has been shutdown as of 2010.

Please go to the Physics department at UC San Diego for your needs.”

2. Information provided through email correspondence with various Australian conservators including a representative from CSIRO and the art gallery in New South Wales that purchased the Lynton laser.

3. Quote from personal email correspondence with an Australian objects conservator with laser cleaning experience.

4. Correspondence with a Chilean objects conservator provided this information.

5. According to a conservator with the Central Conservation Section of the Leisure and Cultural Services Department.

6. Correspondence with an Argentinean conservator led me to the website for the Laboratorio de Ablación, Limpieza y Restauración con Láser (www.ciop.unlp.edu.ar) which provided this information.

7. According to the Bundesdenkmalamt website: http://bda.at.

8. Calcagno, Giancarlo, Erich Pummer and Manfred Koller. “St. Stephen’s Church in Vienna: criteria for Nd:YAG laser cleaning on an architectural scale.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 111–117.

9. Weeks, Christopher. “The Conservation of the Portail de la Mere Dieu, Amiens Cathedral.” LACONA I Proceedings, October 4–6, 1995 (1997): 25–29.

10. According to Cultural Heritage Advanced Research Infrastructures (www.charismaproject.eu).

11. According to www.lrmh.fr (English translation).

12. According to the German Federal Foundation for the Environment website (www.dbu.de).

13. Ibid.

14. According to the FORTH website: www.iesl.forth.gr.

15. According to the CNR website: www.cnr.it.

16. Siano, Salvatore, Francesca Fabiani, Roberto Pini, Renzo Salimbeni, Marco Giamello and Giuseppe Sabatini. “Determination of damage thresholds to prevent side effects in laser cleaning of pliocene sandstone of Siena.” Journal of Cultural Heritage, Vol. 1, Supplement 1 (2000): 47–53.

17. Siano, S., R. Pini, R. Salimbeni, M. Giamelloa, A. Scala, F. Fabiani and P. Bianchini. “Integration of laser with conventional techniques in marble restoration.” Proceedings of the Ninth International Congress on Deterioration and Conservation of Stone, Vol. 2, Venice (2000): 569–576.

18. Siano, S., M. Giamello, L. Bartoli, A. Mencaglia, V. Parfenov and R. Salimbeni. “Laser cleaning of stone by different laser pulse duration and wavelength.” Laser Physics 18.1 (2008): 27–36.

19. According to NML website: www.liverpoolmuseums.org.uk/about.

20. Cooper, Martin (editor). Laser Cleaning in Conservation: An Introduction. Oxford: Butterworth Heinemann (1998). Preface Research, Training Programs, Conferences, & Funding Support

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21. Cooper, Martin and Sam Sportun. “The Application of Laser Cleaning in the Conservation of Twelve Limestone Relief Panels on St. George’s Hall.” LACONA VI Proceedings, September 21–25, 2005 (2007): 55–64.

22. Stulik, Dusan and Herant Khanjian. “New Projects of the Getty Conservation Institute: Laser Cleaning Research.” Getty Conservation Institute Newsletter 12.3 (1997).

23. Abraham, Margaret and John Twilley. A Review of the State of the Art of Laser Cleaning in Conservation.

Los Angeles County Museum of Art submission, National Center for Preservation Technology and Training, Publication No. 1997-01 (1997).

24. According to the NCPTT website: www.ncptt.nps.gov/about-us.

25. According to correspondence with Mary Striegel, head of the NCPTT Materials Research Program, and the NCPTT website: www.ncptt.nps.gov.

26. According to Jason Church, a materials conservator at NCPTT. 27. According to the COST website: www.cost.esf.org/about_cost 28. According to the COST website: www.cost.esf.org/about_cost/how_does_cost_work 29. According to the COST website: www.cost.esf.org/about_cost/reciprocal_agreements 30. According to the COST G7 website: alpha1.infim.ro/cost/pagini/Wg1.htm.

31. COST G7 meeting dates and sites are listed online at alpha1.infim.ro/cost/pagini/calendar.html.

32. According to the ICCROM training courses website: www.iccrom.org/eng/01train_en/announce_ en/2009_04StoneVenice_en.shtml.

33. The NML laser training course appears on both websites (ICCROM and ICOMOS) through a keyword search, however this is most likely attributable to the marketing efforts of NML, which have been evident elsewhere (such as on the AIC Conservation Dist List).

34. According to the IIC publications website: www.iiconservation.org/publications/pubs_sale.php.

35. Studies in Conservation articles include; Asmus, John, G. Guattari, L. Lazzarini, G. Musumeci and R. F. Wuerker. “Holography in the Conservation of Statuary.” Studies in Conservation 18.2 (1973): 49–63; Fekrsanati, F., J. Hildenhagen, K. Dickmann, P. Mottner and U. Drewello. “Feasibility Studies on Applying UV-Lasers for the Removal of Superficial Deposits from Historic Glass.” Studies in Conservation 46.3 (2001): 196–210; and Weeks, Christopher. “The ‘Portail de la Mere Dieu’ of Amiens Cathedral: Its Polychromy and Conservation.” Studies in Conservation 43.2 (1998): 101–108.

36. The 2005 laser cleaning workshop was held November 17–18 in Ljubljana, Slovenia. Cosponsored by COST G7 and the European Science Foundation, registration was capped at 80 participants. Part of the workshop included hands-on laser cleaning training for those that were interested. www.science4heritage.org/COSTG7.

37. According to NML conservation training course description: www.liverpoolmuseums.org.uk/conservation/technologies/courses.aspx.

38. According to Martin Cooper, via email.

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39. Giancarlo Calcagno provided this information to me during my visit to his Altech office in Bassano del Grappa on January 18, 2010.

40. According to the ALT ’09 website: alt09.kocaeli.edu.tr.

41. Lukazewicz, Jadwiga W. and Piotr Niemcewicz, editors. Proceedings of the Eleventh International Congress on Deterioration of Stone, September 15–20, 2008 (2008).

42. According to conservator Adam Jenkins, in a discussion related to research for this thesis.

43. Salimbeni, R., V. Zafiropulos, R. Radvan, V. Verges-Belmin, W. Kautek, A. Andreoni, G. Sliwinski, M. Castillejo and R. Ahmad. “The European community research concerning laser techniques in conservation: results and perspectives.” COST G7 affiliated paper—publisher unknown, date unknown.

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9. Practitioners

The conservators who answered the research questionnaire created for this thesis repre-

sent only a small percentage of laser cleaning practitioners worldwide. Of the 41 professionals contacted, more than half responded. This chapter synthesizes the information they provided to represent the scope of training, education, preferred equipment, experience, and experimentation of the contemporary field of laser cleaning in conservation.

9.1 Professional Backgrounds

Stone cleaned in museum settings is generally related to sculpture conservation and usu-

ally is applied to marble objects. This is not to say that museum conservators who use lasers on stone do not possess the same general knowledge about weathered and pollution-impacted masonry as their architectural counterparts; many statues in museum collections were outdoors for centuries, or were housed in churches or castles.1 The primary difference between a museum conservator and architectural conservator is the size of the objects on which they work.

Fortunately, there is a lot of interaction between museum conservators and architectural

conservators, and there are many professionals who practice in both environments. Of the survey respondents, 19 percent are employed by a museum, and 29 percent have worked in both museum and architectural conservation settings.

9.2 Nationalities For a newer conservation treatment approach with narrow application potential such as laser Smith

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cleaning, an international support system is crucial for it to gain favor. Chapters 4 and 8 outlined the strength of Europe’s cooperative programs through the publication of jointly authored research papers and organizations facilitating international collaborations. One obvious advantage to international collaboration is the sharing of knowledge and experience. All of the conservators who completed the survey questionnaire have worked internationally.

Nationalities of survey respondents: 12 American 2 British 2 Canadian 1 Chinese

2 Greek 1 Spanish 1 Italian

Location of Practice: Canada 1 Ottawa, Canada

United Kingdom 1 Liverpool

United States 2 Philadelphia, Pennsylvania 3 Boston, Massachusetts 1 Washington, DC 1 Kansas City, Missouri 1 Natchitoches, Louisiana 1 Los Angeles, California 1 Eastchester, New York 1 Chicago, Illinois 1 San Francisco, California

Europe 1 Venice, Italy 1 Iraklion, Greece 1 Nikaia, Greece 1 Milan, Italy 1 Valencia, Spain Asia 1 Hong Kong

9.3 Training Conservation Training Academic: In the Field:

67% 33%

Laser Cleaning Training Formal Program: In the Field:

24% 76%

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9.4 Feedback

The survey included questions about the benefits and detriments of laser cleaning of

stone. Respondents were invited to answer as many or few questions as they liked; the section on advantages and disadvantages yielded a nearly 100 percent response rate. Generally, regardless of geographic background, all conservators cited similar positive and negative attributes of laser-based cleaning methods. Among the most common benefits named are the non-contact aspect; retention of relief; minimal waste; and controllability. Consensus of the negatives aspects includes yellowing, expense, need for training, and slow speed. A complete list of these responses is included below, ordered first by positive attributes and followed by negative and arranged by geographic background.

Benefits of Cleaning Stone with lasers

United States “Cleaning of stone surface with the Er:YAG enables the removal of sulfur deposits, soluble salts and organic materials with the minimum penetration of the original patent surface. The use of tools that can abrade the patent surface are minimized.”

“The alternatives are frequently strong acids or bases that can both break down the stone and leave residues that can be harmful in the long term. It is also very clean—requiring no disposal of hazardous materials, etc.”

“When comparing laser costs to other cleaning methods, factor in set-up, protection and clean up costs because in that light, laser cleaning is often competitive or minimally more expensive on historic preservation projects.”

“Removal of dirt, accretions and restoration paint without altering the stone surface, driving dirt into porous stone or leaving behind residues.”

“Promise of lower risk of damage to delicate surfaces (e.g. polished, fine tool marks present, etc.), compared with chemical and mechanical cleaning methods. Avoidance of toxic, hazardous chemicals, etc. is a plus.”

“With laser cleaning no contact is needed. Friable elements can be cleaned, consolidated, and saved. The threats posed by water, chemicals, high pressure, and abrasives are not present. Stone Smith

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structures cleaned by laser do not appear too clean.”

“No waste or hazardous byproducts and no excessive water byproduct that has to contained and/ or go into sewers.”

“The main benefit is as an adjunct to other forms of cleaning. The lack of mechanical force that is possible with laser divestment is probably its main benefit.”

“It removes damaging over-layers from very fragile stone (and other material) substrates leaving more of the ‘original’ surface then other treatments usually are able to do. “ “It is less environmentally destructive then solvents.”

“It decreases solvent exposure levels for the conservator.”

“It may meet the needs of some cultural groups who do not like the use of solvents on culturally sensitive objects.”

“No need for use of chemicals, safety of a conservator, quality of the final result.”

“Non-contact, non-abrasive, speed.”

“Extremely delicate and low impact on substrate, time consuming but readily limited and controllable cleaning method.”

“Lasers are useful for problems that cannot be addressed by other means. Occasionally, laser treatment may be used as a precursor, and more traditional methods used to complete treatment.

We have recently used other traditional methods to clean ancient marble, and used the laser sparingly on the rest.”

“Lasers are essentially non-contact. With short pulses, there is negligible thermal penetration” “On some materials (white marble), within appropriate energy density parameters, the cleaning is self-limiting.”

“Once staging, clean-up, total materials costs, etc. are factored in, laser cleaning can be very costeffective on a per-square-foot basis.”

Elsewhere China “Some stains are too stubborn to be removed by chemical or mechanical means, but can be removed with the use of laser.”

Italy “No direct contact with the surface and gradation of cleaning.”

Practitioners / Feedback

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“Laser cleaning allows for the cleaning of deteriorated areas without the need for pre-consolidation. The level of cleaning is very controllable and one can set the parameters and know that the cleaning will reach that level and not go further.”

United Kingdom “When used appropriately, the selectivity and control.”

Greece “Lasers hold an important role in conservation as they are a unique tool which enables high control and accuracy, material selectivity and immediate feedback.”

“It efficiently removes homogeneously all types of encrustation, it is fully controllable, leaves no by-products and offers unique features such as high selectivity and precision.”

Spain “When working with the appropriate parameters lasers are the most technically advanced, as no other technique discriminates between the soiling and original material. Such discrimination is very difficult with other traditional techniques.”

“Allows stone to be cleaned without preconsolidation.”

“It is impossible to recover polychrome with other cleaning techniques.”

“Cleaning is immediate. You can see the end result at the time of irradiation.”

Limitations of Laser Technology

United States “Because of the shallow penetration of the Er:YAG laser thick crusts are not easily removed.”

“Most of the equipment is cumbersome to move and deploy (a few exceptions). Most of the equipment requires high voltage (many 480V 3 phase power)—there are a few exceptions here too. It always requires blocking off an area in which to work where the general public or other contractors won’t risk eye damage.”

“Occasionally discoloration of the stone or the appearance of spotting in a cleaned area.”

“Cost.”

“It’s very time consuming to clean stone with lasers. The speed coupled with the cost of the equipment makes the technology difficult to sell. Access can be an issue as well. Depending on the laser, you need to work within reach of the fiber optic cable or, if the laser has an arm, you need to position the laser in front of the area being cleaned and the chiller needs to be located in near proximity.”

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“With some older laser systems, reliability was an issue.”

“We are concerned about the volatile materials released upon laser cleaning and the potential health effects these might have on conservators. We recommend at a minimum a HEPA filtration system to be used near the cleaning area.”

“If the pulse and duration are not properly established, surface loss can be unacceptable. We have seen patterns or textures created when the fluence is too high.”

“The equipment needs to be maintained and operator fatigue needs to be monitored.”

“Residual white ash from lichen that is unresponsive.”

“Cost and to a lesser extent laser reliability (this is really a cost issue as well and occurs when old lasers stay in the field too long).”

“Reliability of the equipment.”

“The laser “yellowing” phenomena, and polychromy color change, or loss.”

“Time consuming, expensive, and delicacy of cleaning can exceed the treatment level of other interventions, i.e.: after cleaning, detracted crusts and flaking disaggregated skins may still be intact, where they would not be if cleaned with other means but which cannot be effectively preserved or stabilized after cleaning. In other words, it is necessary to determine what material is to be saved and what potentially is beyond that point before committing to cleaning it. With other techniques, this is self-determining in that the cleaning typically removed these fragile sections.

Water washing or micro-abrasion would simply wash these areas away, lasers do not. As a result, it becomes necessary to limit application in these areas to avoid wasted effort.”

“Lasers are not a magic bullet. Some degraded stones will scorch, some inclusions within the stone will change color (light gray veining in with white marble can turn darker gray), sometimes the laser will just not touch the soils in question. Sometimes you need to use water or some other energy absorbing material in order to get the laser to remove the soil. Sometimes that material cannot be used on the substrate.”

Elsewhere Italy “It can be slow on thick crusts, produces much dust that then needs to be cleaned, is a very delicate machine for work site use.”

United Kingdom “Discoloration of pigments and damage of veining in marble.”

Practitioners / Feedback

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Greece “Among the most important limitations are: discoloration (yellowing) of (mainly marble) stone substrates; blackening of pigments; cleaning control in cases of very thick and hard (i.e. alluminosilicate excavation) encrustations from fragile substrates (given that their removal laser energy density thresholds may be very close).”

“The problem I’ve encountered is that you can’t work for a long time since the user should stop every 15 to 20 minutes and work no more than two hours. The other problem is that the goggles used for protection of the eyes distort the real colors of the surface that is treated.”

Spain “Cleaning system is a slow, but not as much as most people think.”

“Restorers do not know how to clean with the laser and must be taught.”

“The cost of laser cleaning is still much higher compared with other cleaning techniques and, although the technique generally offers the best results, often for economic reasons and time it is not used as often as it should be used.”

1. Larson, John H., Claire Madden and Ian Sutherland. Ince Blundell: the preservation of an important collection of classical sculpture.” Journal of Cultural Heritage, Vol. 1, Supplement 1, (2000): 79–87.

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10. Conclusions

The information presented in the preceding chapters claims that laser cleaning research

and practical applications on stone are far more numerous in Europe than in the U.S. It is the details of this evidence that illuminate disparities between European and American experiences, interest, and support. The fundamental aim of this thesis is to explain why the disparity of laser cleaning applications between Europe and the U.S. is so profound. This question can be answered through the summation of parts previously examined.

Correspondence from international conservation practitioners has yielded some valuable

insight. For one, enrollment in a laser cleaning training workshop is not necessary to become adept at laser cleaning; only 24 percent of conservators polled had been formally trained. However, a training program is more likely to teach important aspects that someone trained on laser cleaning in the field may miss, such as safety precautions, optimal parameter settings for a particular stone type, and proper technique. What a training program also accomplishes is gathering groups of similarly interested conservators who are most likely from different geographic regions and have a variety of professional backgrounds. These professional connections could lead to later collaborations. Because the training programs are in Europe, the majority of participants are European.

It is clear that familiarity with laser-based cleaning methods can lead to more applica-

tions of the technique and greater familiarity will exist where projects are more numerous. There are more sites in Europe that have used lasers, and therefore, there have been more opportunities for practitioners to be trained in the field. By this notion, increased laser cleaning use in the U.S. would contribute to greater training opportunities.

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ing. The budget must take all components into account, and the cost of laser cleaning may be disproportionate to the full scope of the project and available funding. “One of the concerns of the conservation community is the high cost of a laser intervention. Cleaning is just one part of the conservation process, and in many cases the high cost of laser cleaning cannot be justified. The use of lasers in conservation has undoubtedly grown markedly during recent years, but it remains a major challenge for scientists and laser companies to make laser systems more accessible to the conservation community.”1

Traditional wisdom suggests that if a new technology sticks around long enough, resis-

tance and skepticism give way to adoption and understanding. Costs drop as popularity escalates and production increases. In a January 2008 interview, John Asmus said, “The bottom line is that much of the entire laser industry is awaiting a breakthrough much as took place in the semiconductor/computer industry some decades ago. What everyone awaits is a low cost, high power, versatile, semiconductor laser that can be mass-produced.”2

Historically, the most compelling factor influencing the disparity in the geography of

the use of lasers in conservation is cost. Laser equipment is expensive, but some systems, such as the Lynton Compact Phoenix (priced at $23,000), now cost less than some scientific equipment owned by conservation firms or used in museum conservation labs. The price escalates with maintenance, add-ons, testing, and training. However the same can be said for most new equipment purchases.

Another major difference between laser use in Europe and the U.S. is the availability of

equipment. In Europe, it is fairly easy to obtain a laser on a project-by-project basis and arrange for training. A German conservator, for example, recently rented a Quantel Laserblast 50 laser from a local university to clean encrustations on green sandstone. He paid 200 Euro for insurance and the rental rate was 10 Euro per hour.3 In the U.S., the limited number of lasers in action and lack Conclusions

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of a pay-per-use system prohibits widespread experiential dissemination of laser cleaning technology. Increased equipment availability—through greater distributor presence and the availability of rentable systems—would greatly expand the use of lasers for cleaning stone in the U.S. More distributors are located in Europe than the United States, and because it is important for these laser manufacturers to provide demonstrations of their technology, these facilities provide an additional opportunity for training and testing.

Funding figures prominently in the question of laser cleaning. As chapter 8 shows, Eu-

rope’s infrastructure supports far more research projects than what is taking place in the U.S. Geography is partly to blame—certainly it is easier to combine scientific teams from neighboring countries than it is from an ocean away—but so is interest. European conservators seem excited about laser cleaning. And while there are pockets of similar intrigue in the U.S., the prevailing attitude appears to straddle curiosity and indifference. Correspondence with museum conservators throughout the U.S. yielded comments such as: “We have had no reason to start pursuing this since what we do is working so well”4 and “We have other priorities.”5 Outside of the museum environment the U.S. potential for increased laser cleaning remains uncertain—until laser-based cleaning methods are more widely adopted among architectural conservators, the technique will remain relatively underused.

Ultimately, the limited use of lasers for conservation projects in the U.S. can largely be

explained by conservators’ lack of familiarity with the technology and familiarity with other effective cleaning methods. Conservators routinely employ methods with which they can demonstrate a fair amount of skill and familiarity, particularly on projects that are historically significant. Attempting a new method on a significant project without proper training and comfort with the procedure is a professional risk. This aspect—acquaintance—might be the biggest hurdle laser cleaning must overcome within U.S. conservation realms.

Conclusions

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A training program similar to the four-times-a-year workshop in Liverpool would provide

an accessible introduction to laser cleaning. The implementation of research programs to study problems and questions pertaining to laser cleaning techniques at existing conservation labs, and equipment availability on a project-by-project basis would ensure broader considerations and use of the cleaning method for conservation projects.

If the resources devoted to supporting the laser cleaning industry in Europe were avail-

able to some extent in the U.S., it is feasible to expect broader acceptance and use of lasers. Based on the professional feedback, the general assumption among American conservators that do not use lasers is that laser cleaning is not relevant to conservation needs in the U.S. This is easily disproved by looking at the restorations of the Nickerson Mansion in Chicago and the New York Public library, which determined that cleaning with lasers was the most effective method for the unique deterioration problems present.

Finally, the U.S. could pursue partnerships with training, research, and heritage organiza-

tions that are active in the European laser cleaning community, such as with COST G7. Despite their distance from the European continent, Australia, South Africa, and New Zealand all maintain reciprocal agreements with COST. More active involvement with future LACONA conferences would also guarantee laser cleaning development for the U.S.

Laser-based cleaning methods have already proven to be successful on many soiled stones.

Current research continues to find new applications for laser cleaning methods, and old problems continue to get solved. The technology has existed for nearly 40 years, but was only integrated into mainstream conservation cleaning projects 20 years ago. It is still a young technique, with great opportunities for innovation. Once the primary hindrances to widespread acceptance— cost, training, and equipment portability and availability—are definitively conquered, there is every reason to believe the geographic disparity evident today will be greatly reduced.

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1. Fotakis, Costas, Demetrios Anglos, Vassilis Zafiropulos, Savas Georgiou and Vivi Tornari. Lasers in the Preservation of Cultural Heritage: Principles and Applications. New York & London: Taylor & Francis Group (2007). p265 2. Bordalo, Rui. “Interview with John Asmus: from Lasers to Art Conservation.� E-Conservation Magazine 3 (2008).

3. Private email correspondence with German conservator who requested anonymity for professional discretion.

4. Quoting a museum objects conservator in Chicago.

5. Quoting a museum objects conservator in San Francisco.

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Bibliography Abraham, Margaret and John Twilley. A Review of the State of the Art of Laser Cleaning in Conservation. Los Angeles County Museum of Art submission, National Center for Preservation Technology and Training, Publication No. 1997-01 (1997).

Aldrovandi, Alfredo, Carlo Lalli, Giancarlo Lanterna and Mauro Metteini. “Laser cleaning: a study on greyish alteration induced on non-patinated marbles.” Journal of Cultural Heritage, Vol. 1, Supplement 1, (2000): 55–60.

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APPENDIX Questionnaire distributed via Survey Gizmo (surveygizmo.com) 1. Name 2. Email Address 3. Nationality 4. Country of Residence 5. Job Title 6. Briefly describe your professional training and work.

7. Do you use lasers in conservation projects? If your answer is no, please explain why and submit your questionnaire. If yes, please continue.

8. What were the circumstances surrounding your first use of a laser? When was it and where were you? Were you formally trained or did you learn in the field?

9. Please list the names and locations of projects you have personally worked on that utilized lasers for the cleaning of stone.

10. Within the laser technology field, is there an area in which you have unique expertise?

Please describe.

11. Which laser manufacturers have you used in professional cleaning applications?

12. Which laser types have you used? (ex: QS Nd:YAG, Nd:YAP, UV, etc.) Of the types you have worked with, is one preferable? Why? 13. If applicable, what is your preferred wavelength and/or pulse duration?

14. If applicable, what alterations have you made to the mechanical equipment to better suit your needs?

15. Do you typically wet the surface of the stone? Why or why not?

16. Do you keep your equipment and/or training updated with the latest technological developments? Please elaborate.

17. Please describe any instances wherein multiple cleaning techniques were tested (chemical cleaning, microabrasives, etc.) along with laser ablation. Which treatment(s) was chosen and why?

18. Based on your experience, which stones are cleaned most effectively with lasers? Least effectively?

19. From your personal experience, what are the benefits of cleaning stone with lasers?

20. What problems or obstacles have you encountered when using lasers to clean stone?

21. Where has the majority of your laser cleaning projects taken place—In Europe, the US, or elsewhere?

22. Please provide any additional thoughts these questions may not have adequately addressed.

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