Abstract

Current mechanical and chemical techniques for the conservation and cleaning of stone, although successful in many applications, are not always kind to underlying surfaces. This is a major consideration when cleaning delicate artwork. These techniques are often abrasive, caustic, onerous, and time-consuming. Many surfaces can be cleaned by laser treatment. Previous laser cleaning techniques involved Q-switched pulsed lasers. The use of cw lasers has been neglected for this application. Pulsed lasers divest encrustations by vaporization. The underlying surfaces can be damaged if also subjected to these high-power densities. This is almost inevitable unless peak output powers from Q-switched lasers are accurately controlled. In this paper, a novel two-stage technique using a cw mode-locked Nd:YAG laser (1.06 μm) is presented for removing gypsum encrustations. In the first stage, gypsum is dehydrated by cw laser irradiation. This anhydrite gypsum is then easily removed by brushing. The growth of the anhydrite area is theoretically and quantitatively investigated as a function of laser intensities and exposure times. Optimum laser scanning speed and the maximum volume of encrustation removal per day are determined from these data. A comparison is made with pulsed laser cleaning systems.

© 1985 Optical Society of America