4 UNDERSTANDING THE MECHANISM OF LASER STAIN REMOVAL

Based on the work carried out to date, it appears that the light from the laser is absorbed by the stain to produce intense local heating within the stain, ink, or pigment and vaporize them from the paper fibers. If all the heat is absorbed in the vaporization process, then no damage occurs to the paper fibers. Burning of the paper seems to occur when the rate of heat dissipation is not rapid enough to offset the rate of energy absorption from the laser. Slowing the repetition rate to less than two or three pulses per second for the power levels we employed seems to solve the problem, but one may have to consider additional factors, such as the color of the stain and the wavelength of the laser light, in determining optimal conditions.

This finding suggests that one might be able to optimize the effectiveness of stain removal by selecting a laser wavelength that is strongly absorbed by the stain or by choosing a wavelength that is more strongly absorbed by the stain than the ink to be protected in the artwork. The proposed mechanism is also consistent with the observation that two of the stains, purple and brown, were not removed by the laser light. Neither stain effectively absorbs the green light of the laser. Future work could involve some wavelength-dependent studies using both the YAG laser and the dye laser to determine optimal wavelengths for particular stains.

The disappearance of the fungal bodies may also be occurring by instantaneous heating by the laser as well. On occasion, looking through an appropriate eye-protecting optical filter that blocked the laser light, one could actually observe small clouds of fine particles, which might be the fungus, leaving the surface of the paper. The completeness with which the fungus is removed and the lack of damage to the paper is truly remarkable (fig. 4). Removal is far superior to any mechanical or solvent methods that have been tried to date.