The Origins and Use of Medieval Glazing in England

About 900 religious houses were built in England during the medieval period; from the late 12th century these were vast, stone-built Gothic constructions with pointed arches and the structural strength to carry expansive glazing schemes. Up until the 13th century all window glass was imported into England from the continent, especially from northern France and Germany.

Archaeological evidence and documentary sources show that during the 13th century a glassmaking industry developed in England in the Weald on the Surrey-Sussex border, and in the 14th century in Staffordshire. These areas had a local source of the main raw materials used in medieval glassmaking, basically, sand to form the glass and wood both for the flux to reduce the melting temperature and for the huge amounts of fuel needed for the furnaces. However, very little is known about what stimulated the market for English-made window glass, how the industry grew, or who was consuming the glass.

The collections covered in the study

The sites studied are four Benedictine and three Cistercian abbeys in England:

• Whitby and Rievaulx in the north
• Alcester and Bordesley in the midlands
• Bardney and Louth Park in the east
• and Hyde in the south.

The distribution allows an examination of spatial and temporal variation in window glass use, so a key part of the project is to date the glass as accurately as possible to provide a chronology of glass consumption at each site.

The glasses come from dissolution layers – material dumped during or soon after the Dissolution of the Monasteries in the 16th century and excavated over the last 200 years. Given the sometimes limited contextual information about the sites, one of the challenges of the project has been to develop ways of interpreting the small, often corroded, fragments of excavated glass.

The glasses have been loaned to the project from a number of places: Historic England, England Heritage, local museums and archives, the University of Reading’s Bordesley Abbey Project and the Hyde900 community archaeology group. It is very fortuitous not just to have access to so much material but also to have curators and archaeologists interested in the project and keen to learn more about their collections of medieval window glass.

The research is based in the Department of Archaeology at the University of Sheffield, supervised by Professor Caroline Jackson and Colin Merrony, and at the Historic English Materials Science laboratories at Fort Cumberland, supervised by Dr Sarah Paynter and Dr Francesca Gherardi.

How will I use this material?

The first stage of the project has been to characterise each assemblage through a stylistic assessment which allows dating of the glasses, followed by a technical assessment to understand how the glasses were made. Chemical analysis is then used to obtain a chemical composition to group the glasses and try to provenance them to an English or continental origin.

Stylistic

Dating each assemblage is achieved by matching the painted decorative motifs on the excavated fragments with similar motifs on extant, in-situ windows of known dates. These are called dated parallels. Some motifs have been found to have a long date range, such as gothic lettering and depictions of micro-architecture. Others, such as heraldic and naturalistic motifs, are shorter lived. A framework has been devised that helps refine the dates of the excavated glass and establish glazing episodes by combining several types of evidence together in a visual graphic, here called an Evidence Informed Dating Framework - EIDF.

This works in the following way, using the Louth Park Abbey assemblage as an example:

Technical

This consists of a macro and microscopic examination of the fragments to determine how the glass was made. In the medieval period all window glass was hand-made using the crown or cylinder methods. Crown glass was spun on the end of a pontil rod and centrifugal force was used to widen a disc. This technique can sometimes be recognised by the concentric rings and bubbles in the glass orientated in a circular pattern. Cylinder, or broad, glass was created by blowing a long, thin tube that was cut, opened up and flattened and can be recognised by a fire-rounded edge where the hot glass slumped and by elongated air bubbles. A high resolution imaging microscope is used to observe bubble orientation and help determine forming techniques.

Chemical

Compositional data is obtained using both portable X-ray fluorescence and also micro X-ray fluorescence (Micro XRF) to determine the major and some minor elements in the glass composition. For each assemblage, 30-50 representative samples are analysed in more detail using both methods. Scatterplots of the ratios of different chemical elements are plotted so that compositional groups as determined by the recipe can be seen. These compositional groups will then be tested to see if they can be linked to the chemistry of glasses from known production sites and to indicate which glass is English-made and which is imported from the continent.

One of the advantages of using excavated material is it fits easily into the micro X-ray fluorescence chamber. In addition, both analytical techniques are non-destructive, which is a key factor that will hopefully encourage museum curators to offer their window glasses in future projects.

After the analytical stage

The analytical work is now near completion and the data will be used alongside historical research to help understand the development of the English industry and try to elucidate if Benedictine and Cistercian abbeys were consumers of window glass made in England.

Transport networks, trade routes and the procurement process in the medieval period will be examined, alongside the role of the guilds and immigrant workers from the continent as drivers of change.

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