Conserving Canterbury Cathedral's Great South Window

A falling piece of masonry was a sign that Canterbury Cathedral’s Great South Window and its medieval glass, including the 12th-century Ancestors of Christ, needed to be saved.

Canterbury Cathedral Great South Window.jpg

Canterbury Cathedral’s Great South Window. Some of the glass predates the great fire of 1174. Photo by Richard Chivers.

The perpendicular gothic Great South Window, dominating the crossing beneath Bell Harry Tower, is the cathedral’s showpiece. Standing at 16.76 metres high and 7.56 metres wide, it floods the crossing with daylight. The circa-1420s Caen stone window holds precious medieval stained glass, including the Ancestors of Christ, thought to be the world’s most extensive genealogy of Christ in any art form. Some of that glass predates the great fire of 1174, making the panels by far the oldest painted windows in Britain.

On a hot summer’s day in 2009, a piece of masonry fell to the ground immediately outside the principal south door. With a full programme of conservation repair works underway elsewhere, attention was immediately diverted to safeguard the precious window. Later to become affectionately nick-named ‘The Punctured Tyre’ in recognition of the unplanned but urgent nature of the work, this project exemplified how extraordinary feats can be achieved in times of adversity.

From the outset, it was clear that Canterbury’s conservation ethos of ‘as little as possible but as much as necessary’ would be severely tested. The window was structurally failing and to save it, something would have to give. The question that quickly rose to the surface was, at face value, a simple one: what was more important, the stone frame or the stained glass?

Reaching a conclusion required a heritage-value-based understanding of where significance lay in the fabric. This was developed in partnership with project stakeholders, including experts in the field of stained glass and medieval gothic structures, alongside the statutory authorities and consultees. While not always in agreement, all parties contributed to ensuring that the team’s developing proposals were well-reasoned, thoroughly tested and buildable.

While the window was stabilised and cordoned off to visitors and the cathedral community below, a four-year research and development phase saw exhaustive investigations to ascertain why the failure had occurred. The team concluded that multiple factors had impacted its structural stability. These included failing medieval drains and unstable ground water levels, corroding ironwork and the impact of previous campaigns of building work. Just as important, the team was able to rule out the possible influences of second world war ordnance damage and the resonance of the fabric from regular bell ringing.

Understanding how the window’s history contributed to present day characteristics was essential to developing the conservation strategy. It was discovered that the window had settled over time, bowing and leaning in two directions, and that major repairs had been needed every century to keep the window upright. The compound effects of previous repairs, including the use of ill-matched stone types, meant that its structural elements were not working together. Originally built of Caen stone, the window also had elements of Portland, Doulting, Clipsham and Bath stone that were previously used for repairs. The window was essentially a structure of two facing halves, amazingly glued together with lime, cements and adhesives. While the majority of original stone survived in the tracery, only two original full-depth stones survived in the mullions and transoms.

The structural issues were somewhat inherent to the historic 15th-century stone frame, but due to its relative condition and the dearth of original fabric, it was judged that the greater value now lay within the 18th-century-devised assemblage of 12th-century glass. It became clear that the only means of securing this valuable structure as the home of the cathedral’s most precious glass was to fully dismantle it, salvaging and repairing as much stone as possible, supplemented with new Caen stone as required to reconstruct the window.

Sustainability is at the heart of Canterbury’s spirit of conservation: the implicit understanding that decisions made will have a long-lasting impact on the longevity of repair. Much can be learnt from the actions of our predecessors as they grappled with their own financial, climatic and ideological context. Beyond conservation decisions on the fabric as an artefact then, prevailing awareness of 1,400 years of continuous change and adaptation to the cathedral fabric, inspired the team to confidently tackle the structural and environmental issues that threatened to undermine the window’s future. Preceding work on the window itself, repairs to the monastic drain and to the vaults of the transept had to be put in hand.

With support scaffold restricting access, the cleaning, setting out, measuring, protecting, dismantling, repairs and reinstallation all had to be undertaken by hand. Many of the stones weighed more than 250 kg, some were half a tonne. The re-build tolerance had to be millimetre perfect, with all historic deformities reconstructed exactly, to allow the historic glass to be re-fitted unaltered. Bespoke tools for setting out were developed and manufactured, the success of which is evidenced by not a single glass panel having needed to be adjusted on reinstallation.

Two beds of natural stone were employed for renewals. The mined Ferme bed Caen limestone used for piecing repairs to the tracery and jambs was tested to match the characteristics of surviving medieval fabric. The available bed depths could not exceed around 300 mm, so an alternative stone was required for the mullions and transoms of the lower windows. For this, Chauvigny A’grain, a limestone sourced from an opencast quarry near Poitier, has been employed.

All stone was sawn on site, and banked and carved by hand. Stone chippings and dust were re-used by the mason’s shop in the formation of traditional lime mortars. Poured molten lead joints were necessary to allow the window to be rebuilt with its characteristic deformations and to allow it to move and settle over time. The relearning of this long-lost skill led to later collaborations and skill-sharing with York Minster and Durham Cathedral in their own major conservation projects.

With a lineage traceable back to 1819 and the appointment of George Austin (Senior), the conservation of stained glass at Canterbury Cathedral has been prolific, with four generations of the Austin family working on the glass for the next 130 years. Building on that legacy, The Cathedral Studios of today, founded in 1972, still undertakes all of the cathedral’s glass conservation in house and on site, alongside a growing portfolio of conservation and new glazing design both nationally and internationally.

The window’s glazing, which had been extensively conserved in the 1970s, was removed and placed under the care of the studio. This was the first time in two generations that specialist glaziers were able to inspect and assess the stained glass. As a known assemblage of the best of Canterbury Cathedral’s glass, the team was able to establish the provenance of some panels which, alongside the known provenance of Ancestors of Christ from the Trinity Chapel, further included architectural lancet heads and quatrefoil lights from the now lost glazing of the nave.

Future care and repair methodologies are now embedded in the design. Significant improvements to the 1970s protective glazing system now provide the historic glass with enhanced protection from atmospheric pollutants, rain and condensation, and thermal buffering performance for the church interior. Ventilated to the interior and supported on new phosphor-bronze glazing bars, the historic glass now exists within the comparative stability of the internal environment.

The three new eight-metre-long stainless steel tie bars that restrain the window can move through precision manufactured milled stainless steel slip joints in the mullions. The metalwork can withstand temperature changes over 100 degrees Celsius without exerting pressure on the masonry. These modifications to traditional glazing techniques mean that the window retains an authentic appearance, while being better equipped to deal with its immediate environment and to meet future requirements for access and maintenance.

It is more than just the window that has been preserved. Over the course of the project, six stone masonry apprentices have honed their conservation and carving skills and qualified as stonemasons, continuing a centuries old tradition of training on site. The conservation teams have duly received multiple accolades for their work, including a RICS Conservation Award and a Natural Stone Award.

A sense of collective ownership and responsibility for cultural heritage has blossomed through the life of this project. The temporary removal of the glass created a unique opportunity to see the dazzling medieval art displayed at ground level. In 2013–14, six of the ancestor figures travelled to the J Paul Getty Museum in Los Angeles and to the Metropolitan Museum of Art in New York before returning to the cathedral. There they and another 11 figures were exhibited in the Chapter House, giving the local community a once-in-a-lifetime opportunity to ‘meet the ancestors’ face-to-face.

The Canterbury exhibition was accompanied by evening lectures throughout 2015. And while every conservable stone has been reused, those that could not be reinstated were rehomed through a public stone auction that raised £210,000 towards future projects, training and development.

The £2.5 million project was funded through the generosity of private sources. Taking 24 months to complete, the window was dedicated by the Archbishop of Canterbury in October 2016 and completed in March 2017.

This article originally appeared as ‘Conserving the Great South Window’ in the Institute of Historic Building Conservation’s (IHBC’s) Context 177, published in September 2023. It was written by Jonathan Deeming, a partner at Purcell and surveyor to the fabric of Canterbury Cathedral.

--Institute of Historic Building Conservation