Manganese-containing inclusions in late-antique glass mosaic tesserae: A new technological marker?

The present study focuses on manganese-containing inclusions identified in late-Antiqueglass tesserae, light brown/amber and purple in colour, from Padova (Italy), in order to clarify thenature of these inclusions, never identified in glass mosaics until now, and provide new insightson the production technologies of such kinds of tesserae. Multi-methodological investigations onmanganese-containing inclusions were carried out in this work by means of optical microscopy (OM),scanning electron microscopy (SEM), micro-X-ray di_raction (micro-XRD), electron backscattereddi_raction (EBSD), electron microprobe (EMPA), and micro-Raman spectroscopy. The combinationof analytical results shows that inclusions are crystalline, new-formed phases, mainly composed ofmanganese, silica and calcium, and are mineralogically ascribed as a member of the braunite-neltneriteseries, with unit-cell parameters closer to those of neltnerite. However, the low Ca content makes suchcrystalline compounds more similar to braunite, in more detail, they could be described as Ca-richbraunite. The occurrence of such crystalline phase allows us to constrain melting temperaturesbetween 1000 and 1150 oC, and to hypothesize pyrolusite, MnO2, as the source of manganese.In addition, it is worth underlining that the same phase is identified in tesserae characterisedby di_erent colours (light brown/amber vs purple due to di_erent manganese/iron ratios), glassymatrices (soda-lime-lead vs soda-lime) and opacifiers (cassiterite vs no opacifier). This suggests thatits occurrence is not influenced by the "chemical environment", revealing these manganese-containinginclusions as a new potential technological marker.