Regional patterns in medieval European glass composition as a provenancing tool
Introduction
Glass production in the medieval period was a large industry; in central Europe alone, it has been estimated that about 40,000 tonnes of glass were produced between 1250 and 1500 (Wedepohl, 2003). The demand for glass was in large part due to a dramatic increase in church building during the Gothic period, an architectural style that was characterised by large openings for windows to maximise light (Bony, 1983; Nussbaum, 2000; Philippe, 1998; Scott, 2003; von Simpson, 1962; Wilson, 1990). The sources of window glass and other artefacts are often not evidenced in documentary sources. An understanding of the compositions of medieval window glass and other glass artefacts can lead to important information about technology and trade during the medieval period.
Medieval treatises describe the recipe of sand and the ashes of trees and terrestrial plants such as ferns (Theophilus in Hawthorne and Smith, 1979; Eraclius in Merrifield, 1967), which results in a glass characterised by high potassium and calcium, relatively low silica, and moderate concentrations of sodium, magnesium, phosphorus, manganese, alumina, and iron (Freestone, 1992). Compositions of this “forest glass” are highly variable relative to the variability of the soda-lime-silica glasses of the Mediterranean and Middle East, reflecting the high variability in the composition of wood ash.
Numerous factors affect the ash composition; one of the foremost of these is the underlying substratum upon which the plant grew (Drobner and Tyler, 1998; Meiwes and Beese, 1988; Sanderson and Hunter, 1981; Stern and Gerber, 2004; Turner, 1956). Other factors also impact the composition of the wood ash, including the plant species (such as different types of tree, or the use of fern or bracken), as well as time of harvest (seasonally and year to year) and the part of the tree/plant used (for example, trunk versus bark; Jackson et al., 2005; Jackson and Smedley, 2008, 2004; Stern, 2017; Turner, 1956; Wedepohl, 1998; Wedepohl and Simon, 2010). Regional variations in glassmaking technology also affect the glass composition; for example, Cílová and Woitsch (2012) argue that addition of refined potash to the batch was practiced in Bohemian glass production.
In light of this, it seems likely that regional geology, regional availability of different plant species (requiring variations on the basic recipe) and regional technological traditions may result in identifiable regional patterns in forest glass composition. Characterisation of such trends would be a powerful tool for provenancing glass; this is particularly the case for a region such as England where the absence of evidence for local production of coloured glass, coupled with documentary evidence for glass importation, has led to the view that all coloured glass was imported from Europe (Marks, 1993). Furthermore, there is likely to have been inter-regional exchange of glass in continental Europe; potash rich glasses of northern character are found in southern regions (e.g. Alonso et al., 2007). Therefore, an elucidation of regional trends in glass composition is likely to have wide application. Some observations on regional variations have been made previously: most notably, Barrera and Velde (1989) published a detailed study of French compositions, looking at both regional and chronological trends; Wedepohl (2003) observed differences between French and German glass; and Brill and Pongracz (2004) noted similarities between glass from French contexts and glass found in England. In spite of these important contributions, no formal synthesis of European glass compositions exists to our knowledge, and the constraints of modern national borders on research agendas may have confused current understanding as the situation in the medieval period was very different.
This paper provides a synthesis of data from a range of sources to form a picture of regional characteristics of the potash-rich forest glass typical of production north of the Alps during the medieval period. As a demonstration of the application of this approach, three previously unpublished case studies involving different types of medieval glass are presented, comprising two English stained glass window campaigns and a glass mirror found in Egypt.
Section snippets
Meta-analysis of legacy data
The making of forest glass marks a change in glass-making technology that occurred in northern Europe around the end of the first millennium, with technological changes that saw not only a shift in raw materials from soda-rich natron to potash-rich wood or fern ashes, but also changes in production location, furnace design and types of products (Charleston, 1978; Jackson and Smedley, 2008; Thorpe, 1949). The present meta-analysis of legacy data focuses on forest glass of the 12th through the
Analytical methods
The three case studies we explore are based upon standardised energy dispersive X-ray analysis in the scanning electron microscope. Although currently less favoured in archaeometric studies because of its high detection limits and the consequent narrow range of elements measurable, SEM-EDS is easily accessed, inexpensive, rapid and can yield accurate major element data suitable for regional studies such as this one. Small fragments were removed from each glass piece, mounted in epoxy resin and
Regional variations
The two western groups we have identified correspond more-or-less to the regions likely to have been supplied by the well-known sources of medieval glass in Normandy (NW France) and somewhere in the Rhenish area, while the East appears related to the traditional glassmaking area centred on Bohemia. Archaeological evidence for glass-making sites in early medieval Europe is rare until about the 12th century, which could be due in part to the operation of production models other than the
Conclusions
This survey of the compositions of medieval forest glass has allowed the discrimination between three compositional groups of glass that correspond to the three main known regions of medieval glass production, NW France and England, the Rhine and its environs, and Central Europe. These groups may be differentiated by the major components associated with vegetal ash: the oxides of Ca, K, Mg, P and Na.
Three case studies have demonstrated the likely dependence of glazing c.1300 upon coloured glass
Acknowledgements
This work was funded by a UCL Graduate/Overseas Research Scholarship to L W Adlington and a Leverhulme Trust Project grant to I C Freestone and T Ayers. We thank the British Museum for permission to publish the analysis of the mirror. We are grateful to the Dean and Chapter of York Minster for permission to analyse the York Minster glass, and Mr Nick Teed and the staff of the York Glaziers Trust for their help in sampling. The referees are thanked for their helpful comments, which allowed us to
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