Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - GlassRoutes (Mapping the First Millennium Glass Economy)

Teaser

The production, trade and consumption of glass in the Mediterranean underwent some fundamental changes between the 4th and 12th centuries CE, reflected in the chemical make-up of glass assemblages from archaeological sites. The compositional investigation of glass has emerged...

Summary

The production, trade and consumption of glass in the Mediterranean underwent some fundamental changes between the 4th and 12th centuries CE, 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. Particularly 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.

Work performed

In an initial step we 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. We further identified a new cobalt colorant not previously recognised. This study has been published in PLoS One (Schibille et al. 2016).

To elucidate Byzantine secondary glass working practices and colouring techniques, we investigated the use of base glasses, colorants and opacifiers in different mosaic assemblages, one from Kilise Tepe (Turkey) the other from an in situ mosaic at Durres (Albania). Our analytical studies have provided evidence for a degree of diversification of secondary production techniques during the late antique period and the emergence of new colouring and opacifying recipes. The novel cobalt colorant identified in the context of the Byzantine glass weights has been also found for the first time in some mosaic tesserae, attesting its widespread use. Two articles have resulted from this research (Neri et al. 2017 a & b).
To date, only very limited reliable analytical data are available about the type of glass that was used and possibly produced on the Iberian Peninsula between the fourth and twelfth century CE. An overview of the state-of-the-art of archaeological glass studies in Spain was synthesised in a review “La Hispania antigua y medieval a través del vidrio: la aportación de la arqueometría” in the Boletin de la Sociedad Española de Cerámica y Vidrio (de Juan Ares & Schibille 2017). This synthesis highlights distribution patterns comparable to other Mediterranean sites as regards late antique and early medieval glass assemblages, whereas a significant break in the supply chain appears to have occurred in the eighth or ninth century CE.

Final results

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. The first compositional patterns have become evident in our analyses of Islamic glasses from Ciudad de Vascos, but more analytical data from other sites are clearly required to define the variability of plant-ash glasses in more detail. The Vascos study has also provided first insights into the developments of a specifically Iberian glassmaking industry that is independent of the developments of Carolingian Europe and related to the glassmaking tradition of the eastern Mediterranean.

More generally, our research on Iberian glass has 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.