Colour changes of a historical Gotland sandstone caused by laser surface cleaning in ambient air and N₂ flow

Abstract

The surface discoloration due to laser cleaning was investigated for a historical Gotland sandstone. The difference in discoloration for cleaning performed in air and in the shielding environment of N₂ flowing at low velocities was studied by means of colorimetry and scanning electron microscopy and energy dispersive X-ray spectroscopy techniques. For ablative removal of the natural as well as artificially applied encrustation a pulsed 1064-nm laser operated at a fluence of 0.5 J/cm² was applied. It was observed that the natural colour variations (ΔL^*=21; Δb^*=23) of the stone can completely screen the laser-induced changes. Under conditions of shielding with nitrogen, darkening and yellowing slightly stronger than those occurring in the ambient air were revealed for the laser-cleaned, artificially crusted samples and the effect was independent of the gas-flow velocity. The observed difference confirmed the contribution of iron oxidation to the laser-induced yellowing and showed that the presence of oxygen in the ambient air affects favourably the cleaning by supporting removal of a variety of combustible surface remnants and crust components of organic as well as inorganic origin.