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Analysis of Late Antique and Byzantine Glass Mosaics

Final Report Summary - ALABGM08 (Analysis of Late Antique and Byzantine Glass Mosaics)

The project set out to shed light on the manufacture of Byzantine glass mosaic tesserae about which little is known despite the widespread use of glass mosaics in Byzantine monumental decoration. The main aim was to determine the compositional characteristics of mosaic assemblages from different archaeological contexts in order to elucidate the chemical variability among sets of samples and to identify geographical and/or chronological patterns in the chemical make-up of tesserae. Since the elemental composition of the tesserae may reveal the specific source of the raw glass and/or the raw materials by way of comparison with known primary glass production groups, the provenance of ancient glasses can be indirectly ascertained. With this it was hoped to gain substantial insights into artistic practices during the Byzantine period and into the cultural and economic exchange within the Byzantine Empire and beyond. To explore how mosaic making related to contemporaneous Byzantine glass-making traditions, other glass artefacts (i.e. vessels and windows) from Byzantine contexts were also included in this project.

The archaeological evidence for the primary production of glass during the Byzantine period other than the glass making installations along the Levantine coast is relatively scant. This is particularly intriguing in view of the extensive glass finds from throughout the Byzantine domain and the fact that the primary glass production in Palestine cannot account for some of the glass found, for instance, in Byzantine Butrint, Pergamon and Sardis (Brill, 1999; Schibille 2011a & 2011b). Based on the seminal paper by Sayre and Smith from 1961, ancient soda-lime-silica glass is typically divided into two distinct types: on the one hand, glasses with low potassium and low magnesium contents that were produced using a relatively pure mineral soda from Egypt; on the other hand, glasses with elevated levels of magnesium and potassium that are associated with the use of plant ash. In the period of interest (4th to 12th century), mineral soda glass prevailed until about the eight/ninth century, when mineral soda was increasingly substituted with plant ash fluxing agents. These are the two glass types commonly found in the eastern Mediterranean as well as Mesopotamia. New analytical evidence obtained within the scope of the Marie Curie IEF project puts into doubt the limitation in the production and consumption of glass at least as regards the middle Byzantine period (eight to thirteenth centuries) and suggest that additional, as yet unknown compositional types of raw glass were in circulation and possibly even manufactured in what is now Turkey. The study of the glass finds from sixth- to fourteenth-century Pergamon (Schibille 2011a) established the existence of a glass group among the eight- to thirteenth-century samples with high levels of alumina (≥ 5%, see Fig. 1a) that is highly unusual for European and Middle Eastern glass of this period. Judging from the trace element patterns and elevated boron and lithium concentrations (Fig. 1b), these glasses were produced with a mineral soda different to the Egyptian natron from the Wadi Natrun, suggesting a possible regional Byzantine primary glass production in Asia Minor. The Pergamon assemblage thus demonstrates that there is yet a lot to be explored as regards the developments in the manufacture of Byzantine and early Islamic glass.

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