Composition, Corrosion and Origins of Medieval Window Glass
Window glass is one of our most important sources of medieval imagery, but our understanding of its production is limited relative to other media such as wall-paintings and sculpture. Furthermore, it represents the majority of the glass material used in the medieval period in northwestern Europe. This project aims to determine
(1) the variability in composition of English medieval window glass and its relationship to date and location
(2) if the glass used by a single medieval glazier over a series of commissions was constant, reflecting a preferred source
(3) the likely sources of English medieval window glass
(4) the relationship between these characteristics and the tendency to corrode
Chemical compositions of some 860 samples of medieval window glass spanning the mid-twelfth to early sixteenth century, mainly from sites in England were determined using SEM-EDXA and trace elements measured in a subset of 300 using LA-ICP-MS. They fall into two main groups: forest glass, the predominant type from the 12th-14th centuries, and a new calcium-rich glass type(HLLA), from c.1400. This unexpectedly early occurrence of HLLA in stained glass windows strongly suggests that it is was imported from the continent. Analytical work in support of this two-source model has not previously been presented.
A particular focus on the windows of John Thornton of Coventry and his contemporaries confirmed a consistent relationship between maker, date and material. A gradual change in composition between 1150 and 1400 probably reflects the progressive replacement of fern ash by the ash of harder woods.
The intensive sampling of panels from the east window at York allowed us to map the use of different batches of glass as work had progressed, with benefits for conservation in identifying displaced pieces. Evidence that twelfth-century glass makers coloured their blue panels by adding old Roman mosaic tesserae to contemporary potash glass is more convincing than has been previously found, while red glasses appear to have been made using a completely unanticipated method before 1400 AD, effectively revealing a lost technology.
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