Investigation on corrosion morphology and products of ancient tin amalgam mirrors by AFM, SEM–EDS and micro-Raman spectroscopies

This study detected and investigated the degradation effects and dynamics of the tin amalgam reflective layer of ancient mirrors. The experimental results have been obtained by different spectroscopic techniques: Light Microscopy, Scanning Electron Microscopy with Energy Dispersive X-ray Microanalysis (SEM-EDS), Atomic Force Microscopy (AFM) and p-Raman Spectroscopy. The latter two were applied for the first time to characterize the amalgam of ancient mirrors. Amalgam alteration products in form of concentric rings with alternate different compositions of tin oxides have been studied on both the amalgam reflective layer inner surface and on its cross-sections. For the first time SEM images of micro-crosssections of different amalgam degradation products, such as amalgam "drops" and craters, have been obtained. The size of the nano-particles of tin oxides, as amalgam degradation products, was estimated around 3-8 nm by AFM and micro-Raman, as already previously detected through TEM. Micro-Raman showed the presence of both cassiterite and romarkite nano-crystals forming the concentric rings, as only previously hypothesized. This investigation allowed to obtain further experimental evidence and a more advanced comprehension about the structure and formation phases of the amalgam decay products, giving further support to some statements by previous authors and suggesting some assertions different from the ones of previous studies.