In this study we evaluate the laser cleaning of tarnished pure and sterling silver substrates using a nanosecond Q-switched Nd:YAG laser at 1064, 532 and 266 nm. To assess the effects associated with cyclic laser cleaning treatments, several cycles of tarnishing followed by laser cleaning were applied on silver coupons that were characterized by gravimetry, colorimetry, scanning electron microscopy, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. According to the obtained results, none of the three wavelengths is recommended for laser cleaning of pure silver objects, while for sterling silver artifacts, the visible laser wavelength of 532 nm seems the most appropriate.Peer reviewe

Tarnishing is produced by reaction of silver with gaseous reduced sulphur compounds from atmospheric pollution. It induces the formation of AgS crystals which produce a dark appearance. To remove sulphur tarnishing, different mechanical, chemical and electrochemical methods have been traditionally used. To assess the effect of different cleaning treatments on tarnished silver, coupons of pure and sterling silver (92% Ag/8% Cu) were subjected to six cycles of induced tarnishing and cleaning. The cleaning methods evaluated were mechanical (soft abrasives and rubber point mounted on a rotary tool); chemical (chelating and acid solutions) and electrochemical (potentiostatic reductions). Surface morphology, composition, weight, color and luminosity were evaluated by optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and colorimetry. The cleaning impact on tarnished silver depended on the cleaning procedure and the composition of silver. Mechanical treatments restored the original visual appearance of silver although they produced a significant mass loss and a fast re-tarnishing. Chemical cleaning methods were fast; nevertheless the surface appeared completely attacked. Electrochemical cleaning did not yield good result for sterling silver, but was an effective cleaning method for pure silver coupons.This work has been partially funded by project CREMEL Ref. HAR2011-22402 and the pre-doctoral FPI grant BES-2012-052716 from the Spanish Ministry of Economy and Competitiveness (MINECO), and GEOMATERIALES 2-CM Program Ref. S2013/MIT-2914 from the Community of Madrid. Finally, the authors want to mention the professional support of the Spanish Network on Science and Technology for the Conservation of Cultural Heritage (TechnoHeritage).Peer Reviewe