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Mapping the First Millennium Glass Economy

Periodic Reporting for period 3 - GlassRoutes (Mapping the First Millennium Glass Economy)

Reporting period: 2018-10-01 to 2020-03-31

The production, trade and consumption of glass in the Mediterranean underwent fundamental changes between the 4th and 12th centuries CE that are reflected in the chemical make-up of glass assemblages from archaeological sites. The compositional investigation of glass has emerged as a key tool in elucidating these technological, economic and cultural developments and by extension yielding insights into the connectivity of the Mediterranean. The study of well-dated assemblages combined with the application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has significantly advanced our understanding of the compositional characteristics, variability and chronological dimension of primary glass groups, due to its high accuracy and sensitivity as regards trace and rare earth elements.

GlassRoutes addresses three major gaps in our knowledge of the medieval glass economy by chemically analysing selected glass assemblages from Mediterranean and Near Eastern sites. The main focus is on Byzantine glassmaking, the spread of Islamic plant ash glass and how glass from the Iberian Peninsula features within the wider Mediterranean context. The action purposefully takes a large-scale perspective, exploring fundamental developments over a wide chronological and geographical range. It thus aims to establish a socio-cultural and geopolitical framework for the history and archaeology of glass in the medieval Mediterranean.
IWe initially embarked on a comprehensive analytical and typological study of Byzantine glass weights, in order to delineate the beginnings of Byzantine glassmaking as distinct from the Roman tradition and to examine its distribution patterns. LA-ICP-MS allowed us to identify the principal glass groups produced and distributed during the early Byzantine period, their provenance and relative prevalence. The study also revealed the existence of a sub-group of glass produced with a plant-ash component that pre-dates the general onset of soda-rich plant ash glasses in the eastern Mediterranean by at least two centuries (Schibille et al. 2016).

Our investigations of the trace element patterns of the glass weights furthermore identified a new cobalt pigment associated with elevated nickel contents that reflects a secondary glass-working that is likewise different from the Roman tradition. We have since identified this novel cobalt colorant among mosaics in situ at Durres, Albania (Neri et al. 2017), in late antique tesserae recovered from the catacombs of San Gennaro in Naples (Italy) as well as in a seventh-century glass workshop in Beirut (Lebanon). Our data provide strong evidence that this new cobalt pigment began to be exploited in the Near East between the late fourth and the early sixth century CE from where it spread and increasingly replaced the earlier Roman cobalt source (Gratuze, Pactat & Schibille forthcoming). This late antique / Byzantine cobalt pigment was in turn replaced in the eighth century by an Islamic zinc-rich variant.

Based on our experimental results, we can now begin to reconstruct distinctly Byzantine primary and secondary glass production traditions and how these relate to wider socio-economic and technological developments. For example, the investigation of a set of glass beads from medieval Albania exposed clear differences between the Slavic and Byzantine occupations in relation to its connectivity insofar as long-distance trade re-emerged only after the Byzantine re-conquest. This is reflected in the import of predominantly Islamic glass beads. This study has been published with open access in Archaeological and Anthropological Sciences (Neri et al. 2018).

The systematic characterisation of Islamic plant ash glasses requires a large-scale approach due to their greater variability. We have therefore analysed 265 glass finds from ninth-century Samarra, now housed in the Museum für islamische Kunst in Berlin, the British Museum and Victoria & Albert Museum in London. We identified both the possible regional primary production as well as the selective import of glass objects based on the detailed evaluation of the trace elements and typological considerations. Variations in the mineralogical characteristics of the raw ingredients clearly indicate different technological strategies. The assemblage reflects the deliberate use of a particularly pure quartz-rich silica source for the manufacture of a special type of colourless architectural glass, highlighting that aesthetic qualities were evidently appreciated in ninth-century Samarra. Due to its robust sampling strategy and statistically valid approach, this study provides an essential tool to categorise and provenance early Islamic plant ash glasses more generally (Schibille et al. forthcoming).

The gradual development of an Iberian glassmaking tradition was demonstrated on the basis of the chemical make-up of the glass assemblage from Ciudad de Vascos (near Toledo, Spain). We analysed virtually the entire glass assemblage from Vascos, part of which is very well dated, giving us a good temporal resolution of the developments (de Juan Ares & Schibille 2017). We identified four distinct Islamic soda-rich plant ash glass groups, of which the earlier glasses are compositionally related to contemporary plant ash glasses from the eastern Mediterranean, whereas the chemical characteristics of the later, eleventh-century glass groups suggest a regional glass production, possibly on the Iberian Peninsula itself. A handful of slightly earlier high lead glasses suggest that the earliest primary production of glass in al-Andalus dates possibly to the tenth century.

No signs of an earlier local glass production in Iberia were found thus far. The LA-ICP-MS analyses of 200 samples from the Visigothic site of Recópolis testify to the large-scale import of Levantine glass and to a lesser extent Egyptian glass during the late sixth and early seventh century CE (article in preparation).
At the start of GlassRoutes, only very limited reliable analytical data were available about the type of glass that was used and possibly produced on the Iberian Peninsula between the fourth and twelfth century CE (reviewed in: de Juan Ares & Schibille 2017). By now, we have already analysed over 1,100 glass samples from different sites and of different dates, and this data can be compared and integrated to data from other regions around the Mediterranean. Geographical and chronological patterns slowly emerge. Our research into Iberian glass has already generated substantial interest among archaeologists and glass scientists alike, which we hope will lead to the emergence of a dynamic research environment in Spain and Portugal.

One of the major objectives of GlassRoutes is to classify Islamic plant-ash glasses into distinct compositional groups according to the silica sources as well as the plant ash component. Plant ash glasses are more complex than natron-type glasses because they are produced from two highly variable components, the silica source on the one hand and the plant ash component on the other hand. Hence, a systematic large-scale approach and high-resolution LA-ICP-MS are needed to draw meaningful conclusions. Our study of a comprehensive glass assemblage from Samarra provides an essential tool to classify early Islamic plant ash glasses more generally due to the high number of analysed samples and the statistical comparison of the group data.