This is heritage science…medieval window glass

In this post, David Dungworth of Historic England explores new ways of looking at decorated medieval window glass. He explains the use of two X-ray based techniques to improve the visibility of the decoration on the glass, traditional X-radiography and a new technique, scanning micro X-ray fluorescence (µXRF).


Coloured, painted and sometimes stained window glass decoration has been a feature of English churches for over a millennium. The ‘atmosphere’ inside a church often depends on the lighting and the decorated windows give daylight rich and varied hues.

While the earliest decorated windows were simple ‘mosaics’ of coloured glass, most later windows were also painted to illustrate scenes from the bible, the lives of saints, royalty, heraldry or benefactors. Some medieval window glass remains in its original windows but most has been moved at least once and much of it has been destroyed.

Medieval window glass has also been found by archaeologists. Although the decoration on some medieval window glass survives well, the glass itself tends to corrode which can make the glass ‘unreadable’. The decoration on a significant proportion of decorated medieval window glass from archaeological excavations is simply not visible.

Hugh Threlfall recently donated a collection of medieval window glass to Historic England which is being used to test a new approach which should improve the ways in which we can view its decoration. The glass (over 1000 fragments) is generally in rather poor condition. Some examples have corroded so much that they are now opaque and the decoration all but invisible (in some cases a layer of mortar appears to have been applied to the decorated surface of the glass).

Imaging the Decorated Glass

We have explored the use of two x-ray based techniques to improve the visibility of this decoration. Traditional X-radiography works well in some cases and we have also begun to use a new technique – scanning micro X-Ray Fluorescence (µXRF). X-ray fluorescence is widely used to discover the chemical composition of materials, by micro-focussing the X-ray beam it can be directed on a tiny spot (0.02mm). By scanning (using a motorised platform) it is possible to scan across the surface of an object. Importantly the paint has a different composition to the underlying glass: it contains high levels of iron and lead. By looking at scans of these two elements we get a picture of the original decoration.

Types of Decoration on the Glass

So far just over 200 fragments have been scanned using µXRF and it is clear that the painted designs on many are closely related. Most were part of the same decorative scheme and so were probably made at the same time and came from the same building (and possibly even the same window). There are a few fragments which have quite different designs. These designs suggest later manufacture (in most cases these fragments are also far less corroded).

Threlfall Glass #0001. A devil from the Torments of Hell? 

Note the yellow was achieved using silver staining which suggests to us that it was manufactured after the beginning of the 14th century.

Most of the glass is painted to produce simple shapes based on leaves or curvilinear geometric shapes (called grisaille). Commonly, some fields are left plain while others, in contrast, are painted with much thinner lines to produce cross-hatching.

A common border comprises a wavy line and dots but this is clearly related to the cross-hatching as at least one fragment of glass has elements of both. This type of grisaille decoration was popular in the first half of the 13th century. Numerous parallels can be found among glass from York Minster, Salisbury Cathedral and Battle Abbey.


A few fragments of glass are decorated with letters, mostly in a font usually described as Lombardic which was commonly used in decorated glass (and manuscripts) until the later 13th century. Careful examination of the lettering suggests that these were produced by two different glass painters. While TG#0010 is well executed, TG#0019 clearly shows how the letter was produced by scraping away paint (before it was fired). TG#0020 contains the letters REX which possibly refer to a king.

Figurative representations are rare but include part of a seated figure (TG#0022) whose right hand is extended with all four fingers straight (ie not a gesture of benediction) suggesting a secular figure (possibly a king). One fragment (TG#0051) comprises part of a face and another (TG#0109) shows the battlements of a castle from a canopy.

The Ian Threlfall Collection and Alcester Abbey

Hugh’s father (R Ian Threlfall) had been a barrister who, in the middle of the 20th century, helped archaeologically excavate a number of medieval sites (usually with Martyn Jope). Ian found numerous archaeological artefacts when sorting out the family home. He managed to work out where most of them had come from and donate them to the appropriate museum; however, there was a large collection of medieval window glass with no information about its origin. A thorough search through Historic England Archives revealed that R Threlfall was the director of archaeological excavations by the Birmingham Archaeological Society at a Benedictine monastery at Alcester Abbey in 1938. The abbey was founded in the late 1130s but went into decline in the 14th century, had become a cell of Evesham Abbey in 1465 and was dissolved in 1540.

R I threlfall QC
Ian Threlfall