Identifying regional contamination patterns in glass recycling at medieval Tusculum (Italy)

Glass is made for recycling, yet ancient glass compositions survived and remain identifiable after more than five centuries. 144 glass fragments excavated at Tusculum (Italy) from the Roman and medieval periods were analysed by LA-ICP-MS and optical absorption spectroscopy. The results confirm the presence of 1st - to 4th -century Roman glass types, while recycled late antique Foy 2 glass predominates in the medieval contexts. Recycling was clearly intensive in medieval Italy, and Foy 2 was the main substrate available at the time. If enrichment in transition metals is characteristic of Italian glass assemblages and distinguishes them from other European regions, a subgroup of the recycled material exhibits an unexpected shift in the Th/Zr ratios. No systematic colour pattern is evident in the recycled glass, except for a small subset produced from cullet in which antimony retained its oxidation potential, pointing to the incorporation of opacified mosaic tesserae during recycling.