New insight on the in situ crystallization of calcium antimonate opacified glass during the Roman period

Glass is usually opacified by small crystalline particles, called opacifiers, dispersed in the translucent vitreous matrix. To understand the glassmaking conditions used to produce calcium antimonate opacified glass, the in situ crystallization process has been studied through synthetic glasses made in the laboratory. The effects of the nature and the concentration of the antimony source, the temperature and the duration of heat-treatments on the calcium antimonates crystallization have been tested. The physico-chemical characteristics of these glasses were compared to Roman mosaic tesserae from Aquilea and Rome (1st cent. B.C.-6th cent. A.D.). We show that the glass composition (EDX), the microstructure (SEM-BSE, imaging treatment), the oxidation state of antimony in the vitreous matrices (mu-XANES) and the proportion of the crystalline phases (XRD with Rietveld refinement) are suitable parameters to assess glassmaking conditions used in ancient times. We demonstrate that opaque Roman glasses were obtained by in situ crystallization, probably using roasted stibnite Sb(2)O(4) and by doing heat-treatment between 1 or 2 days.