Study of mosaic glass tesserae from Delos, Greece using a combination of portable μ-Raman and X-ray fluorescence spectrometry

Abstract

The present work discusses the results of an archaeometrical investigation on glass tesserae present in the mosaics from Delos (Greece) that was conducted in situ using two complementary non-invasive techniques, namely μ-Raman and portable X-ray Fluorescence spectrometry. The main objective of this work was to provide a compositional and technical characterisation of the glass tesserae. Since they are generally coloured and opaque or translucent, the study mainly focused on the identification of the employed colouring and opacifying agents.The three hundred and fifty four mosaics from the Cycladic island of Delos (130 to 88 BCE) form one of the most relevant mosaic corpus of the Hellenistic period. These numerous mosaics are often of high quality and stand out by the large use of artificial materials, in particular glass and faience, which is remarkable for the period.During three in situ analytical campaigns, around 270 glass tesserae were analysed by pXRF and nearly 40 tesserae using portable μ-Raman spectroscopy. This integrated approach allowed to gain an overview of the raw materials and the techniques employed in the production of these glasses. In particular, for all the tesserae the use of natron as a flux could be hypothesised. The majority of red glass tesserae were identified as a high-copper high-lead glass generally referred to as sealing-wax. The colourings agents used in the other tesserae (Fe, Co and Cu ions, Pb2Sb2O7) are the common ones used in Antiquity and the antimony-based opacifiers are typical for the considered period and geographic area. Blue and turquoise tesserae remarkably show a highly variable lead concentration which appears correlated with antimony. This correlation raised the hypothesis that an antimony ore rich in lead was used for the opacification of these glasses.