Laser removal of contaminants from painted surfaces

An analysis of surface ablation by laser exposure of art objects as part of the conservation process indicates that heat diffusion from the site of laser exposure may be minimized by proper selection of wavelength and exposure duration. A model for unwanted material removal with a laser has been developed taking account of the threshold phenomenon of ablation as a function of wavelength, and exposures at 2.94 μm by an Er:YAG laser with short duration pulses is compared with those from a Nd:YAG (1 064 and 532 nm), CO2 (10.6 μm) and the ultraviolet excimer laser at 193 nm. Thermal diffusion is minimized by taking advantage of the large amount of heat removed by the phase change of water into steam. This model suggests that for bulk removal at strongly absorbed wavelengths, many short pulses are better than continuous exposures. The selection of the Er:YAG laser allows the use of hollow glass waveguides of high flexibility, which are commercially available, as delivery systems. Examples of successful removal are given for contaminants overlying a Madonna's gold leaf halo and the heavy dark accumulation of soot on an unvarnished oil painting, as well as for thick synthetic adhesive on canvas.