September 2000 Volume 21 Number 3
Lasers, as a tool for conservation of works in stone and other media, have been studied and employed for over 25 years. In general, lasers offer some distinct advantages to conservators, including an increased ability to control removal rates for materials being cleaned, the ability to focus down to spot sizes on the order of micrometers (this coupled with micro-positioning equipment can allow for very precise spatial control during treatment), and the option to select wavelengths of light which augment the removal of specific layers of dirt or other surfaces while leaving behind various patinas.
These intriguing properties of lasers, coupled with the recent reduction in the cost and size of many laser systems has led to increased interest and research in the subject, over the past ten years. Today, various laser systems are being employed as tools for cleaning of art (primarily objects and architectural elements) at a number of institutions and sites across Europe. And, as is demonstrated by the adjoining article, investigation into the use of lasers on other materials is underway both in the United States and abroad. In fact, the whole field of lasers as a tool for art conservation may well revolutionize how conservators clean and treat art in the future. Already the field has its own conferences, specialist literature and adherents who are using lasers as a standard part of their tool kit.
But as the interest in utilizing this new tool grows it is important that the community keeps in mind one disadvantage that lasers currently have when compared with traditional methods of treating objects. That is the fact that, with the possible exception of works in stone, there is little evidence as to the long-term effects of laser cleaning. The laser itself has only been available since the early 1960's. Therefore, it is indeed remarkable that studies and monitoring of works in stone, laser cleaned in Italy in the cities of Cremona and Padova over 10 years ago, are offering us our first glimpse of how successful the technique can be. In addition, materials based research on the cleaning of stone at such renowned institutions as the Foundation for Research and Technology in Heraklion, Opificio delle Pietre Dure e Laboratori di Restauro Firenze, and the Laboratoire De Reacherche Des Monuments Historiques (LRMH), in Champ-Sur-Marne have indicated that the main problem for conservators using lasers as a tool for cleaning stone sculpture exposed to environmental pollutants, may well be how to preserve the fragile patina which would have been lost using other more aggressive cleaning methods.
Still, it is almost intuitive that stone should be fairly resistant to the effects of photon energy, after all, one seldom sees rapid deterioration of stone sculptures under the influence of the sunlight alone. Thus one would expect that any damage caused by a laser would be the result of secondary effects such as localizes heating or plasma formation rather then by exposure to light. With careful control over the process, many of these secondary effects can be significantly moderated. But the potential of photons to directly damage or alter other materials, especially organic materials, is well known. The susceptibility to damage under the influence of light irradiation is a function of many factors including the light wavelength, dosage, the material, and the environment. And while light damage is often seen in the immediate effects of bleaching, fading, yellowing, etc. of materials, the long term effects of drying, cracking, and polymerizing, or breaking of chemical bonds may not be easy to detect initially. It is clear that careful study of these effects is essential to build confidence in the feasibility of using lasers in conservation
In fairness, it needs to be recognized that cleaning by laser so lends itself to precision, and the subsequent precise analysis of the work, that laser cleaning is often subjected to a high level of scrutiny. Thus, conservators using lasers have sometimes expressed the opinion that they are being held to a very high standard, one, which might not be achievable using other techniques. Still comparative research could shed light on this question as well.
Recognizing that the laser can be used to achieve high quality conservation in some areas and may be unsuitable as a tool in other areas (or at best needs a tremendous amount of development before the tool is generally useful), the Los Angeles County Museum of Art, with funding from the National Center for Preservation Training and Technology and the Samuel H. Kress Foundation as well as corporate donors is establishing a center for the study of lasers in conservation.
Scheduled to open in the beginning of 2000, the center will be dedicated to the study of both the short and long-term effects of laser cleaning of art and archaeological objects. The center will also function as a educational resource for conservators interested in learning more about lasers, and may aid in the development of processes for special applications. In so doing, the center hopes to add to the information available to the conservation community at large, on the relative merits of using lasers for various cleaning and analytical operations.