The interaction of laser radiation at 3 microns with azurite and malachite pigments

We present a detailed analysis of the behavior of azurite and malachite pigments exposed to the laser radiation. A pulsed, free-running, erbium:YAG laser (2.94 μm) has been used. Pure pigments, laboratory models and original wall painting fragments have been exposed to the laser in order to establish the conditions at which the alteration of the color or binder occurs. Energy density variation, consecutive pulses and various surface wetting agents have been considered. MicroRaman spectroscopy, chromatic measurements and optical microscopy are employed to evaluate the interaction of the laser radiation with the pigments; GC/MS follows the changes in the organic substances. Malachite, as a pure pigment, is more sensitive than azurite although partial darkening of both pigments, due to the formation of CuO, is observed in the absence of surface wetting agents. However, in the presence of surface wetting agents no color alteration occurs at the tested fluences (7-10 J/cm2).