Conserving a Canterbury ‘Eye’
Canterbury Cathedral’s two ‘eyes’ are the oculus windows set in the north wall of the north eastern transept and the south wall of the south eastern transept. Unlike rose windows, oculi are not divided into segments by stone tracery. The Canterbury eyes are large and extremely rare examples of Romanesque circular windows with iron armatures. Indeed, the combination of an early date (c.1180), their large scale (4.47m in diameter), and the elaborate design of the original ferramenta makes them unique. The oculi were painted with figures from the Old Testament (north oculus, fig. 1) and the New Testament (south oculus, fig. 2). The north oculus retains a substantial amount of its original scheme, with the central scene of Moses and Synagoga largely intact. In the south oculus, however, only twelve of the original ornamental panels survive, while the rest of the design is a convincing reconstruction of the iconographic scheme of Christ and Ecclesia made by George Austin Jnr, the cathedral glazier from 1848 to 1862.
All of the panels in the south oculus are now buckled to various degrees, the result of exposure to high winds, progressive creep in the lead matrix, and pressure from the distorted ferramenta (despite the innovative construction). Their exposed position makes it likely that some areas may have also become dangerously unstable. Moreover, the medieval glass is deeply corroded and shows an alarming degree of paint loss (figs 3–4). While the Victorian glass seems to be in sound condition itself, the painted decoration is also beginning to show signs of decay. A major programme for the conservation of the south oculus has now begun. As a first step, sample panels from the window have been removed prior to being refitted in a temporary sub-frame behind protective glazing. Once installed, comparative environmental monitoring will be carried out on both the protected and the unprotected glass over the next twelve months as conservators decide how to save the glass.
To date, the most effective method of protecting vulnerable glass has been the installation of isothermal glazing. The north oculus window was set into such a system in 1992 without any adverse effect, but before the same scheme is installed on the south side, the Cathedral’s stained glass conservation studio is carrying out the lengthy monitoring process described. Studio head, Leonie Seliger, explains. ‘Although we believe that isothermal glazing systems offer the best protection and have used them extensively elsewhere in the cathedral, we have some particular concerns regarding the introduction of protective glazing in south-facing windows. We need more and better information. For example, the effects of heat radiation onto the historic glass could lead to higher thermal stresses, which, in turn, might distort the lead matrix and, by extension, increase the bending strain on the individual glass pieces. In extreme cases fractures might occur both in the lead and in the glass. ‘It is impossible to predict in advance how a stained glass window will react to an increase in thermal stress – the location and size of the window, the design of the protective glazing, the shape, age and section of the lead matrix, the size and thickness of the individual glass pieces, even the colour of the glass can influence how it will behave. ‘We need to collect as much data as we can about any likely changes to the historic glass before finalising the design of its protection. This can only be done by an extended period of monitoring of a protected sample section of the south oculus which can then be compared with data taken from the equivalent unprotected section of the same window [fig. 5]. The data logging must record temperatures on the inside surface of the stained glass, as well as the rate of movement and distortion in the entire panels. This monitoring will be done over at least a twelve-month period before we make any final decisions how to proceed.’
In addition to the importance of the south oculus glazing for our knowledge of the original glazing of the cathedral, the window is perhaps even better known for the survival of its remarkable twelfth-century ferramenta. Filling large openings with stained glass at high levels posed significant structural challenges to medieval builders. At Canterbury the architects overcame the problem of lateral stability by introducing what was, in essence, a space frame construction around the window. The ornamental ferramenta of the south oculus were attached to a second layer of iron armature, consisting of a simple grille of horizontal and vertical bars on the outside of the window; the two are connected by over fifty metal rods (fig. 6). This system is almost certainly the original construction, as scars on the outside of the north oculus ferramenta correspond with the rod attachments on the south oculus, indicating that a similar reinforcement was originally in place on the north side.
For further information about isothermal protective glazing, visit the Conservation section of the CVMA website, and for further images of glass from the cathedral, visit our Picture Archive.
Further reading includes M. H. Caviness, The Windows of Christ Church Cathedral, Canterbury, Corpus Vitrearum Great Britain II, London, 1981, and M. A. Michael, The Stained Glass of Canterbury Cathedral, London, 2004. You can order a copy of the latter by visiting the Canterbury Cathedral Shop.
Painting with Light
A major exhibition of medieval glass owned by the Museum of the Middle Ages in Paris opened on 18 October. Thirty of the museum’s most beautiful panels are shown opposite thirty-five works on parchment, paper or canvas highlighting the close links between stained glass artists and illuminators, draughtsmen and engravers of the twelfth to sixteenth centuries. Panels from Rouen, Saint Denis and Troyes are among the exhibits. The exhibition will run until 15 January 2007, and the Museum is open every day (except Tuesdays), 9.15 a.m. to 5.45pm. Admission: 7.70€.
For further information about the Musée National du Moyen Age visit its website, and for further information on the exhibition, visit the Réunion des Musées Nationaux’s website.