Laser cleaning of Cu-based artefacts: laser/corrosion products interaction

This study aims to develop a low invasive and selective laser cleaning procedure for the removal of reactive corrosion products on Cu-based artefacts without damage the substrate. In a preliminary step, laser cleaning was performed on two typologies of artificially corroded copper reference samples. The effect of the variation of laser parameters as pulse duration and output power, was thus evaluated on an oxide layer, simulating a protective patina, and a hydroxychloride layer, simulating a reactive corrosion products layer to be removed. The optimized cleaning procedure was validated on an archaeological artefact, a bronze coin. Morphological, microchemical and microstructural characterizations were performed by means of optical microscopy, confocal microscopy, field emission scanning electron microscopy, X-Ray diffraction and Raman spectroscopy, before and after laser cleaning. The experimental findings show that laser cleaning, in optimized conditions, can reduce the thickness of the hydroxychloride layers slightly affecting the oxide layers. The difference in the interaction with laser radiation of these two layers seems to be mainly related to the difference in grain size and porosity. Notwithstanding these encouraging results, in order to define the real feasibility of the laser cleaning procedure, a further validation on real artefacts is mandatory due to the variation in thickness and composition of the corrosion products formed during long-lasting uncontrolled degradation processes.