Laser cleaning of historical sandstone and the surface discoloration due to gas shielding

For the Gotlandic sandstone samples extracted from historical monuments and covered with model encrustation the effect of pulsed, ablative laser cleaning at 1064 nm on the surface discoloration was investigated for the irradiation in ambient air and in the N2 flow at various velocities. The colorimetric data of a sufficiently large sample population were measured. A slightly stronger decrease in the surface lightness for the artificially coated and also pure substrate samples irradiated in nitrogen compared to the ambient air case were observed. This difference was ascribed to material oxidation and partial combustion of surface remnants due to presence of O2 contributing to the final effect of laser cleaning.