What’s on : Lectures

The Science of Medieval Stained Glass

9 Nov 2021
Start time
7:30 PM
Professor Ian Freestone, UCL
The Science of Medieval Stained Glass

Event Information

“The Science of Medieval Stained Glass”

Ian C Freestone PhD, FSA
Professor of Archaeological Materials and Technology
UCL Institute of Archaeology

Part of a Mini series on “Art and Science”.

The Great East Window of York Minster is one of the largest medieval windows in Britain and was glazed by John Thornton of Coventry between 1405-1408. A major conservation programme, York Minster Revealed (2008-2018), allowed unprecedented access for scientific examination of the glass. How was the glass made? Who made it? Why are some colours so vulnerable to atmospheric decay? Beginning with an introduction to stained glass and the way in which it was made, I go on to explain how scientific methods are providing answers to some of these questions, not only for the East Window but for stained glass elsewhere, such as Canterbury Cathedral.
Ian Freestone trained as a geochemist before joining the British Museum in 1979, where he undertook the scientific investigation of ancient artefacts. He joined Cardiff University as Professor of Archaeological Science in 2004, then moved to UCL in 2011 where he is Professor of Archaeological Materials and Technology. He specialises in the investigation of ancient artefacts using scientific techniques and in recent years has focused increasingly upon glass. A past president of the Association for the History of Glass, he is a recipient of the the Archaeological Institute of America’s Pomerance Medal for scientific contributions to archaeology.

This lecture will be held on Zoom and invitations will be sent to YPS members and the general mailing list two days before the event. This is a free event but non members can help to cover our lecture programme costs by donating here:

Donate to YPS here

Member’s report

Ian Freestone’s stimulating lecture centred on the differing compositions of mediaeval glass their consequential effect and source, referencing his work on the Great East Window of York Minster during the recent conservation process. A major problem for the conservators is that glass from this period is not robust and tends to corrode, through ingress of water, flaking paint or general weathering. The 14th-century glass in York Minster provides a significant example of this. Freestone explained that the composition of mediaeval glass was primarily silica and ash, but crucially with a lower proportion of silica than in either Roman or modern glass, and it is this composition which led to greater corrosion.

During the recent conservation of the Great East Window, Freestone was invited by Tim Ayers of the History of Art department at the University of York to conduct an analysis of both the coloured and ‘white’ (ie clear) glass. As the window was to be dismantled this provided a unique opportunity to analyse portions of glass normally hidden beneath the lead ‘cames’ or ‘calms’ and to establish both composition and source. The question arose as to whether the sources for coloured and white glass were the same.

There is no evidence of coloured glass being manufactured in England until at least 1449 when the first patent was granted, and probably not even then. Blue glass which, after the 12th century contained cobalt, was imported from Saxony and possibly Bohemia. Some came from including Roman blue mosaic tesserae in the mix. Red glass was notoriously difficult, initially produced by nanoparticles of copper, followed by mixed Roman mosaic producing a streaky effect before arriving at a process known as ‘flashing’. This consisted of very thin layers, formed by molten red glass mixed with white and then blown, a process tending to possible corrosion through flaking. Up until c1350 coloured glass in York is believed to have come from Northern France, but on analysing the composition of the Great East Window (1405-08) it was shown that this coloured glass was Rhenish in origin, a factor which may account for the high level of corrosion.

Was this the same source for the white glass or could that be more local in origin? To decide this, it was necessary to look at trace elements and at the different types of sand used. It was known that there were two areas in England producing white glass, the Weald and Staffordshire, but whose products were noticeably different. An initial analysis indicated that the source for the York glass was Little Birches in Staffordshire, a view which was subsequently confirmed by the use of isotopes. Further work is being carried out on the way in which glass could be analysed without the need to dismantle complete windows by using an instrument Freestone likened to a hairdryer, so we can expect close interpretations of mediaeval glass elsewhere in the Minster and further afield in due course.

This was a well-received lecture as evidenced by the many and varied questions and complimentary comments on chat.

Dorothy Nott