To characterise the manufacturing technologies and circulation of glass in the Byzantine world, we studied a substantial collection of 6th- to 7th-century Byzantine glass weights, revealing the existence of a novel glass group (Magby) as well as a new cobalt source. No high boron glass was identified, indicating that this type of glass was not yet produced on a significant scale. The first Byzantine high boron glasses outside Asia Minor were found in southern Italy, and especially among the 10th-century tesserae from the Great Mosque in Córdoba and in the palace of Madīnat al-Zahrā'. These mosaics represent the largest collections of medieval boron glass outside modern Turkey, and provide the first insights into the Byzantine traditions of primary and secondary glass production and how they relate to socio-economic and commercial developments on a Mediterranean scale.
By the 10th century, soda-rich plant ash glass was produced across the Islamic world. To classify Islamic plant ash glass, we adopted a large-scale comparative approach and analysed numerous early Islamic glass assemblages, including Samarra (Iraq) and Merv (Turkmenistan), more than 1,000 tesserae from the Great Umayyad Mosque in Damascus (Syria) and Khirbat al-Minya (Palestine), a large number of glass weights from Egypt, and several glass assemblages from al-Andalus and Sicily. We were able to establish a chronological and geographical model of early Islamic glass production in Egypt. The compositional features of the mosaic tesserae from Greater Syria have brought to light the extensive trade of glass between the Levantine coast and Egypt and shown that the switch from natron glassmaking to a soda ash recipe in Syria did not occur until the turn of the 9th century. This change in glassmaking was preceded by the import of considerable quantities of Egyptian glass in the 8th century, and the relocation of the Abbasid capital to Baghdad.
Another major goal of the project was to determine glass consumption in al-Andalus and the advent of Iberian primary glassmaking. Chemical analyses of numerous glass assemblages reveal a diachronic sequence of technological and cultural developments and changes in the connectivity of the Iberian Peninsula. The glass compositions of the Roman and late antique periods are in line with the general trends observed across the Roman Empire in that the assemblages are characterised by the presence of Levantine and Egyptian natron glass. Glass becomes increasingly rare in the wake of the Muslim conquest of the Iberian Peninsula (711-714 CE). In a recent study of the glass from Šaqunda/Cordoba (756 – 818 CE), published in PNAS, we identified different stages in the transformation of the archaeo-vitreous record of the Iberian Peninsula. The glass assemblage as a whole shows a reduction in the absolute quantities of glass and an increase in recycling, indicating a disruption of supplies from the eastern Mediterranean. The lack of imports may have been the cause for experimentation and the development of a special type of lead glass production, using local resources in the form of vitreous slag from lead and/or silver mining. This recipe was further refined to produce a quintessentially Iberian soda-ash lead glass that dominates the archaeo-vitreous record of Cordoba in the 10th century.
