Preliminary Thermal Investigations of Calcium Antimonate Opacified White Glass Tesserae

Abstract

Calcium antimonate (in the hexagonal or cubic form) dispersed in the glass matrix is an artificially synthesized phase commonly documented as opacifier for white glasses during the Roman period. Glasses of this type occasionally contain variable amounts of lead oxide. There is no consensus about the origin and role of the lead component in white glasses, whether it was functional to modify the workability of the glass and/or to help the precipitation of the particles, or whether it was an unintentional pollutant introduced with the raw materials. A group of lead and lead-free white mosaic tesserae from the fourth-century CE villa of Noheda in Spain were analyzed by differential scanning calorimetry (DSC) and high temperature X-ray diffraction (HTXRD) to evaluate the impact of variable amounts of lead oxide in the precipitation of calcium antimonate on the viscosity of the glass. The analyzed glasses show thermal events that have been related to the composition of the glass via multicomponent linear regression model. CaSb2O6 and Ca2Sb2O7 formed in the glass during the cooling phase and the glass was not reheated. Lead oxide influences the thermal behavior of the glass, lowering the onset temperatures of all the events, implying a more cost-effective production process. We propose that lead was added intentionally or that lead-bearing raw materials were selected specifically by the ancient glass artisans.

Production or Consumption? Glass Beads from the Roman Villa of Aiano, Tuscany

“…The identification of the colouring technology underlying the Aiano beads can serve as an additional chronological marker (Figure 4). The majority of the Roman glasses are opacified with calcium and lead antimonate that were commonly used until the fourth century ad (Boschetti et al, 2020b, with references). The transition to tin-based compounds is documented systematically from the fourth century, and this change emerged first in the eastern Mediterranean (Tite et al, 2008).…”

Glass in Rome during the transition from late antiquity to the early Middle Ages: materials from the Forum of Caesar

“…Except for sample CF 03, 10, 026 and 046 that are not opacified, all tesserae show high concentrations of antimony, suggesting that they have been opacified with antimony compounds [59]. The elevated tin in the CF 024 tessera that is otherwise opacified by antimony compounds, was most likely unintentionally introduced with a tin-rich copper source (e.g.…”