Lasers: a preliminary study of their potential for the cleaning and uncovering of wall paintings
A pilot study was conducted which set out to observe and record the mechanisms by which radiation from three different laser types interacted both with coatings on polychromed surfaces and with the pigment layers themselves. To provide a framework for evaluating the potential of lasers for removal of coatings in a conservation context, the basic principles governing laser operation were first outlined. The various means by which laser radiation acts upon surfaces were also described, and the principles and problems of establishing an approach to cleaning and uncovering were also briefly discussed.
Three terracotta panels, each prepared using a different medium (fresco, tempera and oil) were coated with three types of material which are commonly found as surface contaminants (limewash, a wax-based, and a soot-based coating). A wide range of typical wall painting stratigraphies was thus provided on which to conduct laser trials. In order to differentiate between the effects of laser radiation on pigments, media and coating materials, two ceramic tiles were also prepared, one with a limited palette of pigments applied without binding media; the second with two types of medium (egg tempera and linseed oil) and one type of coating material (wax).
Trials were conducted using three different types of laser (Nd-YAG, excimer and dye) operating respectively in the infrared, ultraviolet and visible wavelength ranges. By increasing energy densities or number of pulses, four threshold levels÷minimum cleaning, optimum cleaning, damage, and loss of pigment layers÷were established.
The Nd-YAG and dye lasers achieved limited layer by layer uncovering of limewash from the fresco panel, but not without some disruption and loss of the paint layer. The Nd-YAG removed most of the wax coating from the fresco panel with much less physical damage to the paint layer, but some pigment alteration was observed. None of the lasers removed either wax or limewash coatings from the oil panel without pigment alteration, disruption and penetration of the paint layers. All three lasers removed the soot-based coating from the tempera panel, particularly from lime white, however, many of the pigments were again observed to discolour, and loss of the paint layer occurred with some pigments before optimum cleaning levels could be reached.
Though separation of coatings from the paint layer was observed during some of the trials, success was often compromised by the problem of pigment alteration. Alteration (usually darkening) of many of the pigments was observed using all three laser types on all three panels. By subjecting the pigment-only tile to laser radiation it was possible to demonstrate that the same types of pigment alteration occurred, and were therefore independent of the media. Following the laser irradiation of the media-only tile it was observed that egg tempera, linseed oil and wax discoloured at higher energy densities that those required to induce alteration in most of the pigments examined on the pigment-only tile.
The results of these trials provided useful indicators of some of the potential hazards and advantages of developing lasers as conservation tools. The conclusions which can be drawn from these preliminary studies, however, are limited and should be regarded primarily as the basis for further research.