Evidences of glass-ceramic white opaque tesserae from Roman age: a thermo-analytical approach

Abstract Mosaic white tesserae of the Roman Age were analyzed from compositional and structural viewpoints. Interestingly, the tesserae exhibited an internal silicate glass core surrounded by a glass-ceramic layer constituted by CaSiO 3 and Na 2 Ca (Si 6 O 12 ) crystals embedded in the glass matrix. Opaque white colour of the tesserae was due to the presence of these crystalline phases developed during a thermal treatment above ~ 850 °C, as assessed from thermal analysis of the reproduced starting parent glass of the tesserae . Devitrification through an appropriate thermal treatment was used as a deliberate technique by Roman glassmakers to impart an opaque (white) colour and superior mechanical properties to the tesserae .