This article reports on a project that comparatively examined the effect of fluence level on the discoloration of marble surfaces after the removal of the encrustation by 355 nm laser pulses.
Considering the thermochemical reaction possibly occurring in the encrustation during laser irradiation, the mechanism responsible for the discoloration of the cleaned marble surface was analyzed. The reduction of iron oxides by graphite played a key role in determining the final color of the cleaned marble surface. A two-dimensional laser ablative cleaning model, including the reaction heat, was applied to calculate the temperature distribution during laser heating. The kinetics of the thermochemical reaction was estimated based on the simulated temperature field. The occurrence of the thermochemical reaction was also verified indirectly with experiments. The marble surfaces before and after laser irradiation were characterized in terms of the chemical components through surface enhanced Raman spectroscopy. The surface color was measured with a chromameter using a 1976 CIE L*a*b* color system. The proposed mechanism was also applied to numerically analyze the severe discoloration of marble cleaned with laser pulses at 1064 nm. (publisher abstract modified)