Les routes du verre: systèmes d’échange dans le monde médiéval/ Glass routes: systems of exchange in the medieval world

Séminaire organisé par Nadine Schibille du 9 au 14 octobre 2023


Ana Clara CADENA IRIZAR (VICARTE – NOVA School of Science and Technology – Lisbon, France), Claudio CAPELLI (DISTAV, Università di Genova – Genova, Italie), Francesca COLANGELI (Università degli studi di Roma Tor Vergata – Rome, Italie), Ian FREESTONE (University College London – Londres, Royaume-Uni), Caroline GOODSON (University of Cambridge – Cambridge, Royaume-Uni), Bernard GRATUZE (CNRS-IRAMAT/CEB – Orléans, France), Elizabeth JAMES (University of Sussex – Brighton, Royaume-Uni), Eduardo MANZANO MORENO (CCHS-CSIC – Madrid, Espagne), Marie-Dominique NENNA (Centre Camille Jullian – Aix-en-Provence, France), Veronica OCCARI (UCL Institute of Archaeology – Londres, Royaume-Uni), Paul REYNOLDS (Universitat de Barcelona – Barcelone, Espagne), Nadine SCHIBILLE (IRAMAT-CEB – Orléans, France), Frans THEUWS (Leiden University, Faculty of Archaeology – Leiden, Pays-Bas).


Image: Mosaics in the Basilica di Sant’Apollinare Nuovo in Ravenna. © N. Schibille

Le séminaire interdisciplinaire organisé par Nadine Schibille (IRAMAT, CNRS UMR7065) qui s’est déroulé à la Fondation des Treilles du 09 au 14 octobre 2023, a réuni 13 chercheurs internationaux – archéologues, archéomètres, géologues, historiens et historiens de l’art, dont trois chercheurs en début de carrière – et a été consacré à l’étude des “GlassRoutes : Systèmes d’échange dans le monde médiéval”. L’atelier s’était initialement inscrit dans le cadre d’un projet de l’ERC explorant la connectivité de la région Méditerranéenne entre le 4e et le 12e siècle sur la base des modèles de distribution du verre. L’objectif du séminaire était de croiser et d’intégrer différentes approches et perspectives méthodologiques. Le verre a été choisi comme indicateur pour éclairer l’histoire sociale et économique de la Méditerranée médiévale, car il s’agit d’un matériau relativement peu étudié et parce que sa production a été complètement réorganisée vers la fin du premier millénaire de notre ère. L’objectif du séminaire était donc de réexaminer les questions de continuité et de changement en demandant quel rôle le verre a joué dans l’économie médiévale et ce que l’étude du verre peut nous apprendre sur l’histoire sociale et économique de la Méditerranée médiévale que d’autres matériaux et sources ne peuvent pas révéler.


The interdisciplinary seminar organised by Nadine Schibille (IRAMAT, CNRS UMR7065) which took place at the Fondation des Treilles from 09 to 14 October 2023, brought together 13 international researchers – archaeologists, archaeological scientists, geologists, historians and art historians, including three early career researchers – and was devoted to the study of “GlassRoutes: Systems of exchange in the medieval world”. The workshop was originally planned in the context of an ERC project that explored the connectivity of the Mediterranean region between the 4th and the 12th century based on the distribution patterns of glass. The aim of the seminar was to cross-reference and integrate different methodological approaches and perspectives. The choice of glass as core material to shed new light on the social and economic history of the medieval Mediterranean was motivated by the fact that glass is a relatively under-researched material and because the production and the trade of glass were completely reorganised towards the end of the first millennium CE. The aim of the seminar was therefore to re-examine questions of continuity and change asking what role glass played in the medieval economy and what the study of glass can tell us about the social and economic history of the medieval world that other materials and sources may not.


In a general introduction, Nadine Schibille outlined some of the main findings of her ERC project. She highlighted the transformations in the supply of glass in the western Mediterranean during the late antique and early medieval periods, and traced the main developments in glassmaking between the 7th and 12thcenturies CE across the Muslim world, foregrounding the changes with respect to earlier periods. This provided the impetus for lively discussions on some of key concepts for the entire seminar and the current models of glass production and distribution. A simplistic application of the laws of supply and demand cannot account for the changes in the origins of primary glass and transformations in glassmaking in the second half of the first millennium CE. It transpired that glass research lags significantly behind the study of ceramics in terms of temporal resolution and quantification of finds. A quantitative approach is needed if we are to understand the transformations and mechanisms underlying the economic patterns of medieval glass. A factor complicating the quantification of fresh glass in circulation at any given time is the phenomenon of recycling, which has been generally underestimated. The evidence of continuous recycling since Roman times raises the fundamental question of whether a quantitative assessment similar to what has been done in relation to ceramics is feasible and how it can be achieved.

The evolution of recipes and technologies has to be seen in the context of the primary production system established during the Roman period. The identification of primary production sites in Syria-Palestine has enabled the detailed characterisation of raw materials and manufacturing processes. The latest findings from fourth-century Jalame (Israel), presented by the archaeological scientist Ian Freestone, exemplified the complexity and ambiguity of the evidence. Jalame was originally seen as a secondary glassmaking site (a place where glass was made into objects). However, recent research has brought to light evidence of primary glass production (the fusion of glass from its raw materials). The heterogeneous nature of the primary glass melt strongly suggests that the fused product was subsequently broken up and the material sorted by colour. This new understanding of the production system explains that glass compositions from a single primary production event can be highly variable, which must be taken into account when interpreting glass finds from consumer sites and/or secondary workshops. The extent to which a similar model can be assumed for Egypt, another main glass production region during the first millennium CE, remains to be seen. Egyptian glassmaking unfortunately remains little explored, and not many analytical data of well-contextualised glass products from Egypt are currently available due to the complete export ban on archaeological materials imposed by Egypt. This introduces a considerable chronological bias in the dating of Egyptian glass types, as emphasised by the archaeologist Marie-Dominique Nenna. The dearth of data is critical because the archaeo-vitreous record in Egypt is distinct from other parts of the Roman Empire. Casting, for example, was practiced as late as the fourth and fifth century CE, architectural glass and lamps flourished during the late antique period (fifth to seventh centuries), and there was a long-standing unique tradition of mosaic glass and bead-making in Egypt.

Egypt and the Levant were the sole producers of natron-type glass until the eighth or ninth century CE. Egyptian and Levantine glass was used throughout the Mediterranean region and Europe, for example, for ninth- to tenth-century window glass and glass beads found in Scandinavia. Bernard Gratuze presented the analytical evidence from Viking Scandinavia, where many of the natron glasses were either reused or recycled, while some fragments show surprising little recycling markers. The role of recycling increased in the western Mediterranean and Europe during the seventh and eighth centuries. By the end of the eighth century, wood ash glassmaking was introduced in north-western Europe, while the simultaneous invention of lead glass in the Iberian Peninsula may also have been triggered by bottlenecks in the glass supply, and seems to have preceded the start of soda-rich plant ash glassmaking here. Ana Cadena Irizar (doctoral student) discussed fundamental changes in glassmaking in Iberia that coincided with the transformation of the glass industry in the eastern Mediterranean. At the end of the ninth century, an exceptional technological transfer from lead glass to ceramic glaze took place in al-Andalus, which has thus far not been observed elsewhere.

Another case study, glass production in medieval Venice, followed developments that differ from other parts of Europe. The archival records in Venice are among the earliest written sources that provide information about glass production, the raw materials and their exchange throughout the Mediterranean. In her contribution, Veronica Occari, a postdoctoral researcher, traced the development of the medieval glassmaking industry in Venice prior to its maturation in the fifteenth and sixteenth centuries, based on chemical and isotopic data of glass assemblages from different sites in the Veneto. She was able to demonstrate that early medieval Venetian glass is characterised by a high degree of variability and the existence of different recipes using raw materials (silica sources and plant ash) of different origins. The standardisation and specialisation of Venetian glass production appear to have gone hand in hand with the emancipation and recognition of the profession of the glassmaker.

After having discussed concrete analytical and archaeological evidence of the production and movement of glass and new regional trends, the focus of the seminar shifted to the wider mechanisms and motivations driving technological innovation as well as cultural and economic exchange in a globalised medieval Mediterranean. The archaeologist Paul Reynolds presented his comprehensive findings on the changing maritime networks and the circulation of pottery and other goods (e.g. oil, wine, fish, textiles) between the fourth and seventh centuries CE. This illustrated the great historical potential of a meticulous study of distribution patterns, something that does not yet exist in glass research. The advantages of ceramics lie in the fact that its exploration has a much longer history and that the material cannot be recycled. Nonetheless, parallel trends between glass and ceramics can be observed in terms of circulation. One of the themes central to discussions throughout the workshop was how it might be possible to reconcile and integrate ceramic and glass data in order to obtain a more complete picture of trade and exchange in the medieval Mediterranean.

A comparison of ceramics and glass on a regional level was attempted in context of medieval Sicily. As shown by the geologist Claudio Capelli, glazed pottery from Palermo dating to the ninth and tenth centuries shows close links with Tunisian ceramics and Fustat ware, while at the same time indicating a new local ceramic production with their own colouring and opacifying specificities. It is also during this early Islamic period that we find the first signs of local glass production alongside imported goods. These parallel developments are probably not entirely coincidental, as the postdoctoral researcher, Francesca Colangeli, pointed out. In the discussion it became clear that the study of both ceramic and glass in Sicily suffers from the lack of comparative data from North Africa, especially from Tunisia and Egypt, which prevents a more conclusive assessment of matters such as technological transfer, the exchange of recipes or the possible movement of skilled craftsmen.

In the same way, as a result of a limited differentiation of the archaeo-vitreous record, analysis of the various drivers of economic and cultural exchange remain currently underdeveloped. From an historical perspective, Caroline Goodson argued for the importance of urban centres and the state or its equivalents in creating elite demand and in controlling exchange networks during late antiquity. It became clear in conversation that this, however, does not tell the whole story, and even though the production and distribution of glass is often linked to state structures, there are consumer patterns that follow different trajectories of exchange. Such entrepreneurial ventures in the trade of glass must be considered especially after the disintegration of the Mediterranean networks. Related to this discussion were Eduardo Manzano’s observations that regular and irregular exchange networks need to be distinguished. Whereas Roman and late antique natron glass epitomises regular long-distance exchange, the multiplication of soda-rich plant ash glass productions in the early Islamic period suggest a different set of conditions. Urban centres still provided the melting pot for glassmaking and innovations, which may have led to the first local production of primary glass in the area around Córdoba (Spain). The production and consumption of glass thus reflect enduring patterns over the first millennium CE, for which its portability and material value must have been instrumental.

Moving from the specificity of glass production and trade, Frans Theuws dissected the various dimensions that make up the value of an object, including material value, object value, social and imagined value. These different layers together form the ‘absolute value’, which heavily depends on context and is crucial for the object’s/material’s capacity to be recycled. Bringing this back to glass, it was argued that recycling or reuse may often require the disenchantment of the object by, for example, fragmentation. Similarly, alien appropriation can re-define the value of an item in its new context, potentially serving a new purpose. Grave goods are certainly to be understood in this way. Sixth-century burials in north-western Europe, for instance, typically yield more glass lamps than tableware, presumably reflecting a symbolic and ritual value of the objects.

Recycling, reuse and later interventions often impede the (art) historical assessment of glass assemblages. Medieval stained glass windows and mosaic decorations are no exception. Ian Freestone elaborated on the various constraints in examining medieval stained glass, including alterations caused by deterioration of the glass and restorations where the integrity of the original window was often not a priority. The difficulties experienced in the framework of his project brought to light the necessity to work in multi-disciplinary teams to ensure informed sampling, as well as systematic analysis and the cross referencing of portable methods with laboratory-based techniques to ascertain reliable data.

On the final day, Liz James rounded off our seminar with an art-historical look at Byzantine mosaic decoration and the juxtaposition of issues of availability and choice in relation to colours and types of glass. Linking to Frans Theuws’ comments on value, she argued that there was no static colour symbolism in the medieval world, but that the meaning of colours, like the value of an object or material, is dictated by context. Colours are therefore open to different interpretations, but this does not mean that colours were not selected deliberately. Our seminar came full circle with the question of the quantities required for monumental mosaic decorations such as Hagia Sophia in Constantinople (Istanbul) or the Great Mosque in Damascus. This, in turn, brought our discussion back to questions of trade, trade networks and their control.

The aim of our workshop at Les Treilles was to move beyond traditional disciplinary perspectives and open up new discussions for an integrated view of the medieval glass economy. The benefits of bringing the different skills together were obvious from the initial conversations. By the end of the week, all participants had broadened their horizons and we identified the scope of future research projects. The final discussion encapsulated the central objective of this meeting, namely the confrontation between archaeologists, archaeological scientists, geologists, historians and art historians. Each discipline has a contribution to make to understanding the medieval world, and intriguingly, glass is at the centre of it all. Because glass was a material that travelled, it serves to trace long distance exchange; it can be used to contrast urban and rural economic behaviour; glass combines technological and material properties common to other materials such as ceramics and metals; it is part of everyday life but also of monumental art, architecture and archaeology. Our discussions thus confirmed that glass potentially erodes existing divisions in medieval history. Terminology became crucial and we agreed that we should perhaps move away from descriptive terms for periods and cultures and use dates and places instead, when discussing technologies and typologies. The research seminar at Les Treilles provided an ideal platform for exchange, both formal and focused, as well as more casual over lunch/dinner. The format of a week-long residence in a relatively remote location where everything is taken care of allows for intensive but refreshingly informal debates across disciplinary boundaries, which is critical for historical research and difficult to achieve in a more formal setting.

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