Mapping the First Millennium Glass Economy

Periodic Reporting for period 4 - GlassRoutes (Mapping the First Millennium Glass Economy)

Période du rapport: 2020-04-01 au 2021-03-31

«GlassRoutes» was a five-year ERC Consolidator project led by Nadine Schibille at IRAMAT-CEB/CNRS (Orléans, France) exploring the developments in the production, circulation and use of vitreous materials in the Mediterranean region between the 4th and 12th century CE based on compositional data. The ultimate aim was to use chemical analyses of archaeological glass to infer the multiple technological, economic and cultural relationships that existed across the Mediterranean, and to reveal continuities and changes in the connectivity of the medieval world. We analysed more than 5,000 glass finds from well-dated archaeological sites by LA-ICP-MS to significantly expand the available database of major, minor and trace element compositions. Mapping compositional glass groups across time and space detected commonalities and differences, identified mechanisms of technological transformations, innovation and transfer, and ultimately human behaviour, by integrating the analytical data into social and historical contexts.

Major lines of research were concerned with the transformation of the medieval glass economy, the range of Byzantine high boron glass, the spread of Islamic plant ash glassmaking, and the identification of compositional discriminants to distinguish regional production groups. Islamic plant ash glass can be differentiated by multidimensional comparisons of a combination of trace elements and a large-scale approach based on high-resolution compositional data. The approach revealed substantial regional differences in the organisation and scope of the glass industry alongside more global developments in the economic connectivity and interregional exchange. Many of these transformations already began shortly after the disintegration of the Western Roman Empire, but gathered pace after the Arab conquest as commercial activities shifted to Damascus and Greater Syria in the late seventh century CE, and again after the establishment of Baghdad as the Abbasid capital. GlassRoutes demonstrated how geopolitical trends have consistently had an impact on the production and trade of glass in the medieval Mediterranean.

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.

The project has considerably advanced the field beyond the state of the art and achieved several breakthroughs. (1) We have established the emergence of an Iberian glassmaking tradition at the turn of the ninth century. No evidence of earlier glass production was found in the Iberian Peninsula despite textual sources to the contrary. (2) We have put Sicily on the map of Islamic glass production as early as the 10th century. Commonalities with al-Andalus underscore that the developments of local glassmaking in the western Mediterranean regions were related to the broader processes of Islamization and conditioned by the geopolitical transformations of the Mediterranean and Europe. (3) We have discovered the largest collection of Byzantine high boron glass in the form of mosaic tesserae in 10th-century Cordoba, illustrating the range of the circulation of Byzantine glass and its exchange across cultural and religious boundaries. These findings lend veracity to Islamic textual sources claiming the Byzantine origins of the material. (4) In contrast, the tesserae from the Great Umayyad Mosque in Damascus were found to be overwhelmingly made from Egyptian glass, offering crucial new insights into the making of mosaic decorations at the time. (5) We also published the first comprehensive set of trace element data of Islamic glass from Egypt, filling an important gap in our understanding of the developments of the Egyptian glass industry, one of the principal glass producers of the first millennium CE. We were thus able to establish the first clear classification of Egyptian, Levantine and Mesopotamian plant ash glass groups.

In summary, we substantially expanded the corpus of first millennium glass compositions. From these data we have reconstructed the transformations of the Islamic glass industry and the multifarious relationships between technological changes and wider socio-cultural and geopolitical developments in the medieval Mediterranean.

De Juan Ares & Schibille (forthcoming) Late Roman and early Islamic glass in Spain

Periodic Reporting for period 4 - GlassRoutes (Mapping the First Millennium Glass Economy)

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