Comparative Investigation of Red and Orange Roman Tesserae: Role of Cu and Pb in Colour Formation

This study aims at the characterisation of red and orange glass tesserae from the 4th-century Roman villa of Noheda (Spain). Due to the limited number of analyses available for such ancient materials, many questions remain unanswered about the production processes in the Roman period. Six samples were chosen for their hue variations, including two samples showing banded patterns of red and orange. Differences in copper speciation were investigated by X-ray absorption spectroscopy and compared with colour and compositional variations obtained by EPMA. The shapes and sizes of colouring crystals could be investigated using scanning and transmission electron microscope imaging. The brown-red colour is due to metallic copper nano-particles and corresponds to a low-copper and low-lead group usually described in the literature. The orange samples and bands are coloured by copper oxide Cu2O nanoparticles with remaining Cu+ in the glass and have greater contents of Cu. Compositional analyses reveal that the same base glass is used in the red and orange bands with additions of Cu, Sn, Pb and probable Fe. Furthermore, based on our results and on the literature review, a high-copper low-lead group of glasses highlights the variability of compositions observed in cuprite colours.