Micro-Raman on Roman glass mosaic tesserae

The results of a Micro-Raman spectroscopy investigation performed on twenty-one Roman glass mosaic tesserae (II century A.D.), from the “Villa dei Quintili” excavation in Rome, are reported. The set of tesserae was retrieved in the thermal baths and covers the majority of the colour palette of that time. The aim was to identify the raw materials, the colouring agents and the opacifiers as well as the production technology used during the Roman Imperial Age. FESEM-EDS and LA-ICP-MS were also employed for a detailed spectroscopic characterization of both the glass matrix and the crystalline inclusions. The tesserae are made by soda-lime glass produced with natron as flux. For some red samples higher levels of MgO e K2O suggest the use of plant-ashes as a source of alkali. Fig.1 - Raman spectra of calcium antimonates: (a) CaSb2O6, (b) Ca2Sb2O7 Sn-Pb antimonates (yellow), Ca-antimonates (white), a mixture of Cu2+ ions and Sn-Pb antimonates (green), a mixture of Ca-antimonates and Cu2+ or Co2+ ions (blue and blue-green) are the colouring and opacifying agents used. Cu0 metal nanoparticles and Cu2O nanocrystals are found in the red and orange lead-containing tesserae. The results confirm the high technological level of Imperial Age glassmakers and emphasize the importance of complementary micro-analytical (SEM-EDS and LA-ICP-MS) and spectroscopic (µ-Raman) techniques.