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.
The effect of laser cleaning on historical sandstone samples investigated by means of surface techniques SEM and EDX reveals surface damages such as scaling off, cracking of the grains, and even melting of the crust. Also differences in the binder quantities contained in the upper layers are observed. The presence of compounds containing C, Al and F in the dark coverage is confirmed by the EDX data and indicates that the encrustation originates from environmental pollution.
The laser cleaning of historical sandstone was investigated using air and also pure N2 for shielding of the laser interaction region. The yellowing measured in the colorimetric L*a*b* space was lower by more than 20% for samples cleaned in nitrogen. The effect was ascribed to the dirt combustion products on the surface due to ablation plume and confirmed by the surface inspection.
For the laser cleaning of Gotlandic sandstone monuments by means of surface ablation the advantages are experimentally confirmed and the expense factors of this laser application are compared with their counterparts of the conventional restoration technique. Results indicate for the case of commercially available, Nd:YAG, Q-switched laser systems that the laser cleaning represents an efficient, fully controllable and selective tool for conservator’s practice. Depending on the system applied the expenses in the range of 0.17-0.45∈per dm2 of the laser-cleaned surface are lower by an order of magnitude than these of chemical and mechanical methods.