Vitreous tesserae from the calidarium mosaics of the Villa dei Quintili, Rome. Chemical composition and production technology

The archaeological excavation of the Villa dei Quintili in Rome (2nd century AD) brought to light one of the most impressive residential complexes of the Roman Empire. Among the astonishing number of findings, inside and outside the buildings, a large amount of glass mosaic tesserae were gathered from the thermal bath environments. This work reports the results of a microtextural and microchemical characterization of 29 colored opaque glass tesserae, by using an analytical approach that included the use of scanning electron microscopy with energy-dispersive X-ray spectroscopy, laser ablation-inductively coupled plasma-mass spectrometry. The glass is usually a soda-lime-silicate glass, with the exception of red and orange glass tesserae, which showed the typical composition of lead glasses mixed with plant ashes. Red tesserae were likely produced with a natron-based glass mixed with plant ashes, acting as flux and reducing agent. The opacifiers were all antimony-based with different natures according to the chemical composition: calcium antimonates (white in color) and lead antimonates (yellow in color). In the 29 tesserae studied in this research, the colors of glass were strictly correlated to the concentrations of coloring agents, the two main coloring ions identified were copper, introduced in the majority of samples as bronze scrap, and cobalt. Finally, the levels of trace elements indicate the use of mature sand, rich in quartz and poor in heavy metals and clay minerals, for the production of all samples.