Selective laser removal of corrosion products from metallic artefacts

Laser cleaning is an attested method to clean surfaces. It is a selective and precise procedure and it is considered a safe technique for the conservators/restorers and for the environment. For these reasons, the use of laser cleaning on cultural heritage has been grown in the last decades. Setting the parameters conveniently, it is possible to remove the corrosion products respecting the original surfaces: selectivity and precision make laser cleaning an important instrument for cultural heritage safeguard. Unfortunately the interaction among laser parameters and corrosion products it is not yet clear. The aim of this study is to optimise tailored laser cleaning procedures for archaeological metals artefacts and to evaluate its real feasibility as a standard conservation procedure. With the purpose of doing this, the research analyses the combination and the interaction of different parameters and different work modalities to remove dangerous corrosion encrustations without affecting both the protective patina and the metallic surface. A Q-switched Yb:YAG fiber laser has been used on a set of artificially-aged Cu-based reference samples and on some real ancient bronze coins. Several irradiance, scanning speed and repetition rate values have been evaluated to optimise a possible standard procedure. XRD and SEM-EDS analyses have been performed in order to detect the chemical composition of the corrosion products and to assess the substrate and the corrosion products ablation thresholds. This allowed to perform a focused analysis and cleaning procedure. From the experimental findings, it seems clear that the number of passes and the irradiance values used have a key role in removing chlorides and other surface contaminants without affecting the original protective patina. It is possible to conclude that laser cleaning is a powerful technique due to its characteristics of selectivity and precision: it improves the legibility of the bronze artefact surface and, as a consequence, the knowledge of the artefact itself. Nevertheless, more studies are needed in order to verify the set parameters on other type of materials and artefacts.