The author is a physicist from the University of Milan, Italy. She has been working at the Central Research Laboratory (Gabriël Metsustraat 8, 1071 EA Amsterdam, The Netherlands) for the past year. The full report of this investigation is available at the Central Research Laboratory for Dfl. 50 (about $27).
The aim of this project was to investigate the state of the art of laser use on art objects.
In order to achieve this goal I have reviewed the basic ideas about lasers, listed the properties of the laser beam and studied the mechanism of light amplification. In my report I describe technical features such as the main pumping systems and the Q-Switched Operation Mode (a procedure using very short and intense pulses), as well as the working of optical fibers and the lasers commonly used in conservation: the ND:YAG and the excimer lasers.
I reviewed the means by which a laser beam is able to remove surface layers from a target material, an action called ablation, and saw that ablation is the result of the combined action of thermal, photochemical and mechanical processes.
First I focused my attention on the thermal model for laser ablation with infrared lasers used in the normal operation mode on stone objects. In this case the phenomenon is fully understood: The laser light vaporizes a black crust but does not damage the underlying stone because it is almost completely reflected. When the laser is used in the Q-Switched Operation Mode, with very short and intense pulses, it is necessary to take into account effects such as the blocking of the laser light by the dust plume and the mechanical effects generated by the expanding plasma and the rapid heating of the target material.
I then reviewed current ideas about UV laser ablation. In this case, photochemical effects have to be taken into account because the energetic UV photons are able to break molecular bonds in the target material. In recent years, researchers in the field of laser cleaning of art objects have been turning to the use of UV lasers because the thickness of the layer removed by a single pulse is very small.
Workers in this field have used a variety of analytic techniques to characterize the state of the artifact before and after laser action, and to monitor the ablation mechanism online. These techniques are described in the last chapter of the project report. I tried to focus attention on which kind of information these techniques can provide.
An extensive literature review was carried out for this project. The different papers are summarized in a series of tables in the report. The use of infrared lasers on stone objects has been extensively studied: eleven papers were found on this subject. However, only three papers on the use of UV lasers were found, apart from the abstracts of the papers presented at the LACONA conference (LAsers in the CONservation of Artworks) held in Crete in October 1995.
Even if the use of lasers on stone objects seems efficient for cleaning purposes, and even if the use of UV lasers seems promising on a number of different substrates, two main issues are still waiting to be addressed: the discoloration effect of the material beneath the layer removed, and the long-term effects subsequent to laser action.
A discoloration of the substrate has been noticed by a number of authors whether the substrate is a plain stone or a pigment layer on stone or on canvas. In this last case it has been verified that discoloration occurs both with infrared and UV lasers for almost all the commonly used pigments (cinnabar hematite, red ocher, massicot, vermillion, azurite, etc.) and that it occurs regardless of the media. So far the researchers have not been able to determine whether discoloration is a photothermal or a photochemical effect. More research is necessary to determine the exact causes of this phenomenon and to investigate which laser parameters (type of laser, operation mode, energy density, etc.) are most important in the control of discoloration.
The second issue that must be addressed is the long-term effect of laser action. The condition of the substrate has been studied mainly to check the effectiveness of the cleaning or the occurrence of immediate damage such as melting of stone or modification of its calcite structure. It is now important to determine whether the effects on the substrate (rapid heating of the substrate, shock wave pressure generated by the expanding plasma, breaking of chemical bonds due to UV radiation) could have long-term consequences on the substrate even if there is no immediate damage visible.
The reason for this investigation was the mounting interest of conservation in the possibilities of laser technology and the eagerness of the laser application industry to supply the equipment for application tests.
The main concern of the CRL obviously was for the safety of the objects, particularly in relation to the effects of laser radiation on the aging of materials.
As a result of this investigation we have come to the conclusion that on the basis of existing knowledge and evidence it may-after careful testing-be justified to use laser radiation on stone and related materials under certain conditions.
The CRL does not recommend experimentation with laser cleaning of paint or organic materials without further research into the above-mentioned effects.