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Compositional investigations were performed on 81 Roman and medieval glass fragments (first to 14th centuries AD ) from four Italian archaeological sites. The samples were sodalime-silica in composition, with natron as flux for the Roman and early medieval glass samples, and with plant ash as flux for the late medieval ones. The varying colours are due to the differing FeO, Fe 2 O 3 , MnO and Sb 2 O 3 contents. Hierarchical cluster analysis identified six compositional groups related to age, which were compared with those found in the literature. In this way, technological continuity from the Roman to the early medieval period and the appearance of plant ash technology in the ninth century, 200 years in advance of the period previously believed, are demonstrated.

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