Discoloration of marble during laser cleaning by Nd:YAG laser wavelengths

The present article focuses on the color change of marble during the removal of superficial deposits by laser radiation. A comparison between cleaning results obtained by the first three harmonic wavelengths of a Q-switched Nd:YAG laser is presented. Experiments were carried out on two sample types. In order to simulate a black encrustation, a model sample was developed based on former studies. In the second case a naturally microbial encrusted marble sample was used. The effects of Nd:YAG laser irradiation at lambda = 1064 nm (omega), lambda =532 nm (2 omega) and lambda =355 nm (3 omega) were investigated by laser-induced breakdown spectroscopy (LIBS), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and color measurements using the Commission Internationale de I'Eclairge (CIE) Lab color system with respect to the development and the absolute surface color-change. The results show that the surface color remaining after the removal of the artificial encrustation by the fundamental wavelength, turned from originally white to yellow. It was possible to identify an Iron content located in nanometer-sized spheres on the surface to be the source of the yellowing. To remove a superficial biogenic encrustation the second and third harmonic were observed to be the most efficient wavelengths, supporting the best result. (C) 2001 Elsevier Science B.V. All rights reserved.