Since Antiquity, the coloring and opacification of glass are properties sought by glassmakers for the manufacture of decoration or glass objects such as glazes or mosaic tesserae. These properties can be brought by the presence of crystals within the glass matrix inducing the diffusion of light within the material. Calcium antimonates CaSb2O6 and Ca2Sb2O7 are crystallized compounds that were used in the manufacture of glass in Roman times and provide both an opacification and a white coloring to the glass. Also, they can be added together with a chromophore during the manufacturing process to bring a new color to the glass depending on the chromophore used.

In the case of turquoise opaque glasses, the color is brought by copper, possibly added in the form of metallic copper (bronze) during the manufacturing process. For antimony, it is added in the form of antimony sulfide (Sb2S3) or antimony oxide (Sb2O3, Sb2O4). Antimony and copper are multivalent elements that can be present in different oxidation states once present in the glass matrix (Sb3+/Sb5+ and Cu0/Cu+/Cu2+) or in calcium antimonate crystals (Sb5+ only) and can interact with each other depending on the synthesis conditions used. This redox coupling can then be a factor influencing the crystallization of calcium antimonates. The presence of multivalent elements can be a determining factor in the choice of the crystallization process (in-situ crystallization or crystallization by addition of previously synthesized crystals) as well as the synthesis conditions (temperature and working time, furnace atmosphere, annealing, ...). Understanding the interaction between antimony and copper is crucial to trace the manufacturing process of colored ancient glasses opacified by calcium antimonates and in the control of glass coloring.

The aim of this work is to study the impact of copper in the manufacturing process of glasses opacified by calcium antimonate crystals through a comparison between ancient glasses and glasses reproduced in laboratory. The ancient glasses studied are turquoise mosaic tesserae dating from the 4th century AD and are from the archaeological site of Villa Noheda, Spain. The glasses reproduced in the laboratory are synthesized from the composition of the Roman turquoise mosaic tesserae by varying the conditions of synthesis (nature and concentration of Sb and Cu sources, type of crystallization process, temperature and annealing time). The analyses carried out by optical spectroscopy in reflection, XRD, SEM-EDS and EPR will make it possible to determine the composition and the proportion of the crystalline phases, the microstructure, the rate of crystallization and to quantify the degree of redox of the elements of transition as well as the color of glass.