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

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