Opaque red glass tesserae from Roman and early-Byzantine sites of north-eastern Italy: new light on production technologies.

Opaque red glass tantalizes modern scientists as much as it fascinated ancient people. Its gem-like appearance, its symbolic value, and the challenge of its production are at the basis of this interest. Despite the apparent chromatichomogeneity, opaque red glasses so far analysed show a wide range of chemical and textural features, indicating a large variety of production technologies. It is generally accepted that copper-based phases are responsible for the red colour and its variants (mainly orange and brown), and that rigorous reducing conditions must have been established and maintained throughout the glassmelting, but there is no clear consensus on the mechanism of colour generation and the parameters influencing it. In this context the present work, focusing on opaque red tesserae dated from Roman to Early- Byzantine time (2nd-6th century AD), aims to investigate and compare the different production technologies employed. In particular, 16 Roman tesserae, ranging from orange to brown in colour, excavated in the sites Domus delle Bestie Ferite (Aquileia, dated to the mid-4th century), Domus of Torre di Pordenone (Pordenone, dated between the 1st and 5th century) and Santa Maria Maggiore Cathedral (Trento, dated between the 2nd and 4th century) are here considered. Strict comparisons are made with 26 Byzantine opaque red glass tesserae from Padova and Vicenza, dated to the 6th century AD and previously analysed with comparable methodological approach. The study was carried out using scanning electron microscopy coupled with energydispersive X-ray spectrometry (SEM-EDS) for textural and qualitative chemical analyses, electron microprobe (EMPA) to determine quantitative chemical compositions of the glassy matrix, X-ray diffraction (XRD) and Raman Spectroscopy to define the crystalline phases of pigments, providing valuable insights into the complexity of opaque red glass technology from Roman to Early-Byzantine period in northeastern Italy.