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 super®cial deposits by laser radiation. A comparison between cleaning results obtained by the ®rst 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 l ˆ 1064 nm (o), lˆ532 nm (2o) and lˆ355 nm (3o) 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 lʹE clairge (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 arti®cial 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 super®cial biogenic encrustation the second and third harmonic were observed to be the most ef®cient wavelengths, supporting the best result. # 2001 Elsevier Science B.V. All rights reserved.