Characterisation and assessment of two green aging methods for silver mock-up systems using multi-analytical techniques

Due to the high values of historical silver artefacts, artificially aged mock-ups are often used for the development of conservation concepts and treatments in the field of conservation and heritage study. However, some chemicals used in recent aging methods have the risks of being toxic and environmentally harmful. We have tested and improved green aging protocols based respectively on boiled egg white and albumin solution on both sterling silver and pure silver coupons. The artificially created tarnish layers on the coupons showed high homogeneity and reproducibility. We identified the successive phases formed on the sterling silver coupons during the aging process with albumin solution through X-ray diffraction analysis: (1) stromeyerite (AgCuS), (2) coexistence of stromeyerite, jalpaite (Ag 3 CuS 2 ) and sub-sulphide (Ag 8 S), (3) intermediate phases progressing towards Ag and Cu sulphides. Raman spectroscopy confirmed that adapting the quantity of reagents and exposure time during the aging process resulted in tarnish composition close to that observed on naturally tarnished reference objects. Changes in the surface morphology of the coupons were monitored by optical microscopy and scanning electron microscopy. While sulphidation dominated the tarnishing process, we observed that albumin solution aged samples showed an initial predominance of oxidation. Surface microstructures consisting of islands or rings of various sizes were observed on several albumin solution aged coupons. Measurements via Kelvin probe force microscopy and atomic force microscopy showed that the formation of these structures was linked to the accumulation of corrosion products.