Tin amalgam mirrors: investigation by XRF, SEM-EDS, XRD and EPMA-WDS mapping

Abstract

Ancient mirrors were constituted by a tin-mercury amalgam layer superimposed to a glass sheet. This was the only one method used until the nineteenth century, when the wet silvering process was invented. The tin amalgam is a binary alloy of tin and mercury constituted by two different phases: a mercury-rich liquid phase and a tin-rich solid phase. The amalgam alteration produces mercury loss and a general growth of the solid crystalline phase. In addition, tin dioxide and monoxide are formed with a consequent decrease of the amalgam adhesion to the glass. These degradation phenomena led to reduction or disappearance of the mirror reflective power.The aim of this study was the characterization of the amalgam layers of eight mirror samples dating during the seventeenth and nineteenth centuries. The samples were analyzed by X-ray diffraction and by a Scanning Electron Microscope with an Energy Dispersive Spectrometer (SEM-EDS), and for the first time on this type of alloy by X-ray Fluorescence and EPMA-WDS (Electron Probe Micro Analysis with Wavelength Dispersive Spectrometry) elemental mapping.The contents of tin, mercury, and some trace elements in the amalgam layers have been determined. The investigation of the superficial patterns of the amalgam by SEM, EPMA-WDS mapping, and SEM-EDS allowed a first understanding of some morphologies and processes of the degradation of the amalgam layer

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

“…5). Drops are present also on the top of some well pronounced craters, as previously detected [4][5][6][7][10][11][12]. SEM pictures in Fig.…”

“…In these images it is possible to recognize the different steps of the corrosion processes, already presented in a previous work on surface images [11].…”

“…The majority of studies on amalgam ancient mirrors were performed in the last decade and concerned the characterization of the tin-mercury amalgam layer, whose interface with glass constitutes the reflective surface of the mirror [1][2][3][4][5][6][7][8][9][10][11][12]. They indicated that this reflective layer contains about 25 wt% mercury and 75 wt% tin and that it consists of two different phases: a solid one, distributed as "islands", composed by about 20 wt% mercury and 80 wt% tin, surrounded by a liquid phase composed by about 95-100 wt% mercury and 0-5 wt% tin [1][2][3][4][5][6][7][8][9][10][11][12]. The amalgam alteration involves mercury evaporation and formation of tin monoxide (SnO, romarkite) and dioxide (SnO 2 , cassiterite).…”

“…This causes the formation of amalgam drops and tin oxides craters [4-7, 11, 12]. In particular, the results of a recent study [11] suggested a possible mechanism of transformation from amalgam drops to craters.…”

“…Several studies among those cited above concern the alteration processes of the amalgam layer on the back side of ancient mirrors (interface amalgam-air) [1,[4][5][6][7][8][10][11][12], while a recent study was based for the first time on the investigation of the amalgam side in contact with the glass (reflective surface) [12]. It highlighted the formation of a superficial morphology of the amalgam alteration products in form of concentric rings, with two alternate different compositions.…”