Study of laser cleaning of corrosion layers on bronze archaeological artefacts

Cleaning is one of the first action during restoration of archaeological artefacts and in the last years several new methods for cleaning and stabilising metallic artefacts have been developed (1, 2). In this context, laser treatments present several advantages in comparison to conventional mechanical and chemical methods and could be proposed to optimise highly accurate, selective and self-limiting cleaning treatments for removing corrosion products without affecting the metallic substrates. In the case of bronze artefacts, laser cleaning could be employed for removing in a selective way the dangerous corrosion products mainly composed of hydroxy-chlorides compounds, leaving on the surface the protective patina, remarkable also from the aesthetical point of view. In order to reach this goal, a deeper knowledge of the laser-material interaction is needed and therefore the effect of the different experimental parameters as a function of the chemical composition and morphology of the corrosion layers has to be assessed. In this study some Cu-based reference specimens artificially aged through different treatments have been cleaned by using a Q-switched Yb:YAG fibre laser operating in the near-IR. Laser cleaning has been performed both in controlled reactive and non-reactive atmospheres and in O18 rich atmosphere. Several microstructural and morphological techniques (SEM-EDS, XRD, μRaman and ToF-SIMS) have been used to evaluate the behaviour of the corrosion layer toward the laser parameters during the ablation process. The same procedure has been applied on some ancient bronze coins too, in order to study the laser’s effects on real patinas. First characterisations, on non-treated and treated samples, have shown a reduction of the patinas’ thickness and, at the same time, no changes in the composition of the corrosion layer have been noticed, due to the laser cleaning treatment. These preliminary results confirm that laser cleaning is a powerful technique to be employed in the Cultural heritage field, due to its selectivity in the removal of the corrosion products on metallic archaeological artefacts. The EUROCORR Young Scientist Grant has supported this study. (1). R. Pini, S. Siano, R. Salimbeni, M. Pasquinucci, M. Miccio, 2000. “Tests of laser cleaning on archaeological metal artefacts”, Journal of Cultural Heritage, 1(1): S129–S137. (2). H. Lee, N. Cho, J. Lee, 2013. “Study on surface properties of gilt-bronze artifacts, after Nd:YAG laser cleaning”, Applied Surface Science, 284: 235-241.