Geochemical study of Roman glass mosaic tesserae from the archeological site of "Villa dei Quintili" (Rome, Italy)

Here we report on a chemical study carried out on a set of 21 coloured vitreous mosaic tesserae, dating back to the 2nd century CE, collected from the Roman archaeological site of Villa dei Quintili in Rome (Italy),. The chemical analyses were performed on all vitreous tesserae by a Field Emission Scanning Electron Microscopy equipped with an Energy Dispersive X-ray Spectrometer (FE-SEM-EDS), for major elements, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), for trace and Rare Earth elements. Micro-Raman spectroscopy with excitation at 632.8 and 473.1 nm was used to characterize the colouring and opacifier agents. The main goal was to document the compositional features, in order to determine which raw materials were used to make the glass and the production technology adopted to obtain the different colors. The results indicate that the tesserae were produced through processes of high technological level. The majority of the tesserae show the characteristic composition of the natron glass, typical of the Imperial age. As vitrifying agent a coastal sand was used, high-purity oxide stabilizers (CaO) being present in the sand for the phenomena of bio-accumulation of marine organisms. The red and orange tesserae represent two outliers, because of their lead-alkali mixed composition. The higher levels of K2O, MgO and P2O5 suggest the use of a different flux: for the red glass plant ash was likely used, while the orange glass displays an intermediate composition between natron and plant ash glass. It is worth noting that the trace element composition of red and orange seems to indicate the use of copper smelting product or brass, respectively, following the high content of copper and tin in the red tesserae and copper and zinc in the orange tesserae. The color and the opacity of the tesserae were obtained by the combination of chromophores ions and/or opacifiers and coloring crystals. For example, the dark blue color was derived from the combination of cobalt and less frequent Ca-antimonates, if compared to the light blue tesserae. The two shades of green depend on the combination of Copper and Ca-antimonates (emerald green) and Pb-Sn-antimonates (bluegreen). The red opaque color is due to nanoparticles of metal copper scattered in the glassy matrix, while nanometric cuprite is responsible for the orange opaque color.