EDS and μ-XRF mapping of amalgam degradation products in ancient mirrors

An amalgam mirror is a mirror type, used from the 15th century until the end of the 19th century, where the reflective layer is constituted by a tin amalgam layer adhered to a glass sheet. In this work two amalgam mirrors samples were studied by Scanning Electron Microscopy with an Energy Dispersive Spectrometer and by micro-X-ray Fluorescence elemental mapping to go deeply into the understanding of the degradation mechanism of the amalgam layer of ancient mirrors. The investigation has been focused for the first time on the reflective surface of the amalgam layer adherent to the glass sheet to better understand the processes of amalgam corrosion. The two amalgam degradation compounds romarchite and cassiterite has been spatially differentiated by SEM and EDS maps. SEM images and μ-XRF and EDS maps showed that the amalgam degradation products grow up to form hemispherical stratified calottes. This structure is probably due to a mechanism involves cyclic phases and oxygen radial diffusion from a superficial oxidation nucleus