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

Abstract

An amalgam mirror is a mirror type, used from the fifteenth century until the end of the nineteenth 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 scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) maps. SEM images and micro-X-ray fluorescence 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.

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.…”

“…7a) may correspond to a hemispherical calotte center at the surface. As hypothesized in previous studies on the basis of experimental evidence, the corrosion should start at the back free surface of the amalgam layer and grow as a hemispherical calotte into the bulk of the amalgam layer [3,11,12]. When the growth of a calotte reaches the reflective surface adherent to the glass sheet, the calotte can expand only two-dimensionally and it is detected as superficial ring structure on the reflective surface.…”

“…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).…”

“…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.…”

“…These rings represent the superficial section of a system of hemispherical concentrically stratified calottes. They seem to have grown from an initial corrosion point on the air-interfaced surface towards the bulk, to reach the opposite reflective surface [12].…”