Online Monitoring of the Laser Cleaning of Marbles by LIBS Sulphur Detection

Abstract

In the present work, the feasibility of sulphur detection was demonstrated during laser ablation of the gypsum layers on the polluted surface of aged marbles. The laser used for cleaning and generation of the LIBS signal was a Q-Switched Nd:YAG emitting dual pulses at 1,064 nm. Different sulphur emission lines were identified through comparative LIBS measurements on gypsum encrustation and K2SO4 and MgSO4 reference materials, but only few, not weak, sulphur lines resulted suitable for monitoring the cleaning of marble, due to the overlap of most features with emissions from bulk marble constituents. Sulphur vertical distribution in the encrustation was then monitored by LIBS during the cleaning. Results of LIBS sulphur stratigraphy were comparable with those obtained by SEM–EDX measurements. In the present study it was possible to establish by LIBS the boundary between bulk marble and its encrustation containing gypsum and, consequently, to determine the optimal point for the interruption of the laser cleaning at the end of the heavily sulphated crust.