Hg/Sn amalgam degradation of ancient glass mirrors

Tin amalgam, which is obtained by pouring mercury onto a sheet of tin, has been used in the production of reflective coatings for mirrors. The corrosion processes of the amalgam layer were investigated in several mirrors from historical buildings located in southern Spain using SEM/EDS, XPS, and GID. Mercury and Sn4+ are present as spheres on the amalgam surface due to the evaporation process (∼5 nm). The profile shows a mixture of Sn2+ and Sn4+. The original amalgam was composed of a binary alloy of tin and mercury (Hg0.1Sn0.9) and metallic tin. In this paper the tin oxidation mechanism of the amalgam is described. Liquid mercury is volatile and evaporates slowly, leaving fine tin particles that oxidize easily, forming tin monoxide (SnO) and tin dioxide (SnO2). The mercury-rich phase accelerates the corrosion of the tin-rich phase.