Phase Evolution of Tin Amalgam Degradation and Implications for the Preservation of Qing Dynasty Reverse Glass Paintings

The corrosion of tin amalgam reflective coatings on Qing Dynasty reverse glass paintings has traditionally been attributed to oxidation pathways. No other corrosion types have been reported for this artifact class to date. This study investigated two reverse glass paintings using Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and micro-Raman spectroscopy (Raman). In one sample, the corrosion products are romarchite (SnO), tin tetroxide (Sn3O4), and cassiterite (SnO2), confirming the conventional oxidation pathway. In the other sample, abhurite (Sn21Cl16(OH)14O6) and hydroromarchite (Sn3O2(OH)2) are identified alongside tin oxides. This is the first report of chlorine-bearing corrosion products in Qing Dynasty reverse glass paintings. This finding indicates that the coating was exposed to a high-humidity, chloride-containing environment. The coexistence of chloride-mediated and oxidation-pathway products reflects microenvironmental heterogeneity on the coating surface. Preventive conservation for similar objects should prioritize maintaining a stable relative humidity at approximately 50%, excluding exogenous chloride sources, and avoiding acidic pollutants.