Canterbury Cathedral: The Jesse Tree Window in the Corona Chapel The conservation of the medieval stained glass 2010-11

© Dean and Chapter of Canterbury. Author: Leonie Seliger, Head of Stained Glass, Canterbury Cathedral 2/14/2012

The Jesse Tree window

in the Corona Chapel

in 2008, with a close-

up showing the internal

surface

The conservation of the Jesse Tree window in the Corona Chapel of

Canterbury Cathedral

1. History of the glazing

The Corona Chapel at the easternmost end of Canterbury Cathedral contains

five tall lancet windows. Of the original glazing little remains; only the east

window retains most of its stained glass. Two figures of a Jesse tree which

showed the genealogy of Christ are currently housed in the north-facing

window nIII. The two surviving panels of the Jesse Tree window (the Virgin

and Josias) are an extremely rare and artistically as well as technically

sophisticated treasure. Images of the Virgin in particular are rare survivors in

England, as they were routinely targeted for defacement and destruction

during the iconoclasm events of the Reformation and Puritanism. Indeed,

this is the only intact medieval representation of the Virgin in Canterbury

Cathedral.

In the absence of contrary evidence it can be assumed that the two figure

panels originated in the chapel, albeit not necessarily in this particular

window. The seven ornamental border panels that surround them most

likely come from window nIX in the north-east transept of the cathedral. The

upper portion of the window is glazed in plain diamond quarries.

The Corona Chapel was built to house the relic of the skull fragment (the

‘corona’) of Thomas Becket. Offerings received suggest that this relic was

translated there by 1198, more than twenty years before the translation of

the saint’s body from the crypt into the Trinity Chapel.

The history of the building as well as stylistic analysis suggest a date for the

Jesse Tree of ca 1200. The tree would have without doubt originally filled an

entire lancet window. The earliest mention of the glass dates to 1841, when

the two figure panels were recorded by Joyce in the east window of the

Corona Chapel, the ‘Redemption Window’. They may have been installed

into that window in 1661, when repairs were carried out, to fill gaps left by

the iconoclasm of the 1640s. In 1848 Charles Winston referred to them as

‘part of a Jesse tree’ and placed them amongst the earliest glass in England.

In 1853, George Austin Junior restored the Redemption window, and

supplied new glass for the areas that had been previously ‘patched’ with the

figures from the Jesse Tree. In 1861 he created a new Jesse Tree, based upon

the two surviving figures, and placed the new stained glass into the window

adjacent to the East Window. The two medieval figure panels remained in

store until they were sold to Philip Nelson in 1908 for £26. The panels were

returned to the cathedral in 1953. The glass was dismantled from the

medieval lead matrix, cleaned, and re-leaded. The remnants of the medieval

lead were mounted on card and archived. The stained glass panels were

New corrosion products,

fractures, decaying paint

installed into window nIII of the Corona Chapel without any protective cover. They remained fully

exposed to the elements until their removal for conservation in August 2010.

2. Condition

The glass and its painted decoration were in remarkably good

condition when they returned to Canterbury Cathedral in 1953,

but it was obvious that they had suffered serious new decay since

the panels were re-installed in the Corona Chapel. Fresh

corrosion products both internally and externally indicated active

deterioration, as did the evidence of loose and flaking glass paint.

Fractured pieces were in danger of falling out of the lead matrix.

In several places backing plates had been inserted to provide

support for badly fractured glass. As the sealant between original

glass and backing plate had failed, water had penetrated into the

interspace, causing new and rapid corrosion.

The internal surface of all panels was covered in dirt and dust

deposits, which provide a basis for microbial attack.

The 1953 lead matrix was in sound condition, as was the

medieval iron ferramenta frame, although its paint finish was

failing. The ferramenta are held in place by an oak frame which is

wedged into the stone rebate of the window. Condensation run-

off and leaks were beginning to affect the lower parts of the oak

frame and the stonework at the window’s sill.

3. The conservation and protection of the medieval glass

All nine medieval panels were removed from the ferramenta

frame in August 2010.

The glass was taken to the stained glass conservation studio at

Canterbury Cathedral and recorded photographically, before

each panel was inspected in detail on the bench with the aid of

microscopes.

A comprehensive condition report was compiled for each panel,

including information on previous restoration and on the current

interventions.

Photographic recording

Compiling the condition report and cleaning of surfaces under the microscope

Cleaning, edge-bonding, securing glass with coiled lead wool

Manufacture of protective glazing

The panels were carefully cleaned under the microscope, using only tiny sable brushes and the tips

of scalpels. No chemical or wet cleaning was undertaken, and neither was the glass dismantled from

its lead matrix. The old backing plates, which had created such detrimental conditions for the glass

they were meant to protect, were removed, and the fractured pieces were edge-bonded and

stabilised with the addition of copper wire supports. This resulted in some gaps between lead and

glass. Those gaps were filled with lead wool to prevent the oily residue that traditional linseed oil

putty would bring to the medieval glass.

Once the panels were stabilised, a rubbing was taken from each to provide a basis for the design of

the outer protective glazing. The protective glazing was made to resemble the stained glass from

Cleaning the iron wedges and bars of the support structure

Support bars and installation of protective glazing into the medieval ferramenta frame

the outside, in order to reduce the visual disturbance to the building as a whole. Each protective

glazing panel is essentially a clear colourless version of the historic stained glass panel, with a

simplified lead matrix based upon the original.

Meanwhile, a very precise template was made of the medieval iron ferramenta frame which had

held the stained glass panels in place. This template was the basis for a new replica ferramenta

frame that would hold the historic stained glass in the future.

The original ferramenta frame was stripped of its 1950s paint and repainted, while the small wedges

and support bars that hold the panels in place were stripped and then oiled.

After the ferramenta was treated, a new wooden frame was installed on the inside, which not only

held the aging oak frame in place, but which would eventually receive the new replica ferramenta,

made by a local traditional blacksmith. A new condensation tray and lead sill was also installed to

prevent further water damage to the sill.

Once all these elements were in place, the protective glazing panels could be installed into the

original medieval ferramenta. They will provide the external weather shield for the historic stained

glass from now on.

The replica ferramenta frame is being installed on the protected interior of the window

The medieval stained glass is installed into the new ferramenta frame

The cleaned and consolidated medieval glass panels were then installed into the new replica

ferramenta frame on the inside of the window, with a distance of ca 60mm or slightly over 2 inches

between the outer protective glazing and the inner stained glass. Ventilation gaps at the top and the

bottom pull air from the inside of the Corona Chapel into the 60mm interspace between the outer

and inner glazing. This ensures that the precious medieval glass is protected not only from rain and

wind, but also from the danger of developing condensation on its corroded and fragile surfaces.

The installation of the medieval glass behind its new protective glazing took place in October 2011.

The corrosion which had been so clearly active and rapid will now progress at a very much slower

rate, which is expected to be below measureable. With regular minimal maintenance to ensure that

the protective glazing remains sound and the internal surface is dusted once every few decades, the

precious early glass is now preserved for many more centuries to come.

The nine medieval stained glass panels after conservation in situ in the Corona Chapel, 2012