Microscopy makes amazing glazing

The York Glaziers Trust, Britain’s oldest and largest specialist stained glass conservation studio, has recently embarked upon the conservation of York Minster’s Great East window, Britain’s largest expanse of medieval stained glass. We sent John Waite along to find out more about the role of microscopy in their work

I cannot be alone in not having given much thought to stained glass windows. Pushed to describe them, it would be along the lines of: “Small shaped pieces of coloured glass bound together with lead to be viewed from a distance in historic buildings, preferably in bright sunshine.”

So a visit to The York Glaziers Trust (YGT) workshops – situated in the shadow of the vast York Minster – was a revelation. I was there to meet Sarah Brown, the Trust’s director, and learn more about the use of microscopy in their work of conserving historic stained glass from around the UK. But it was clear from a very early stage that first I needed a little education – about the Trust and about stained glass.

Although modern usage includes domestic leadlights and objets d'art, for more than a thousand years the term ‘stained glass’ has been applied almost exclusively to the decorative windows of churches and other significant buildings. Small pieces of glass held together by strips of lead are arranged to form pictures or patterns but a substantial part of the process, and one that I had given no thought to, is the application of paint to provide detailing and subtlety.

[caption id="attachment_28595" align="alignright" width="200" caption="Figure 2: One of the individual panels before conservation showing the obscuring effect of the large leads used in previous restoration efforts"][/caption]

The individual glass pieces may be clear but are usually coloured, either by the addition of metallic salts during manufacture or by painting a colour onto the clear glass surface and fusing in a kiln. The paint, usually black or dark brown, is formed by mixing finely ground iron or copper with powdered glass in water or solvent and is applied in a series of washes, beginning with the detail. Usually just the internal face of the glass is painted and the paint layers fixed to the glass by ‘firing’ in a kiln.

Medieval stained glass was the principal pictorial art form in England, Germany and France from the 10th to the 16th century, culminating in the giant windows that survive to this day. Their creation blends the artistic skill necessary to conceive the huge artwork with the engineering prowess to assemble something that is structurally capable of supporting its own weight and withstanding the elements.

York is the most important centre for medieval glass in the UK with 128 significant pieces in the Minster alone – the country’s largest collection of medieval stained glass – and comparable to the collections at Nurnberg in Germany and Rouen in France.

YGT is a charitable trust dedicated to the care and conservation of historic stained glass in York Minster and throughout the UK. It is Britain’s oldest and largest specialist stained glass conservation studio and builds on centuries of craftsmanship associated with the building and preservation of York Minster. The 14-strong team of conservators and glaziers is led by Sarah Brown, a national expert on medieval ecclesiastical architecture, stained glass history and conservation. Sarah is president of the British Corpus Vitrearum Medii Aevi and a member of the Church Building Council's stained glass committee. From the beginning the Trust has been committed to education and research and has always had a close working relationship with the University of York, the centre of stained glass scholarship in Great Britain. Sarah is the Course Director of the University’s MA in Stained Glass Conservation and the University provides art historical support to the YGT.

YGT’s expertise was put to the test in 1984 when a fire in the Minster's south transept caused terrible damage and endangered the famous Rose Window. This was followed by the conservation of the St William Window, achieved between 1997 and 2008 and, more recently, it has undertaken the conservation and protection of three of the Minster's oldest windows, in the Chapter House vestibule.

Outside York, the Trust has worked on many of Britain's most important windows. Clients have included four Oxford Colleges (Balliol, Lincoln, New College and Trinity) and scores of parish churches throughout England and Wales. Nor is its work confined to glass of the Middle Ages. Projects have also included Karl Parsons' much-admired east window at All Saints, Porthcawl (1927-28) and panels from the west window of Beverley Minster (1859-65) by John Hardman of Birmingham.

The reason for my visit is YGT’s latest conservation project, the evidence-based restoration of the largest window in York Minster – the Great East Window.

[caption id="attachment_28596" align="alignleft" width="200" caption="Figure 3: The same panel after conservation in YGT’s studio"][/caption]

Dominating the eastern face of the Minster comprising over 300 individual panels and covering an area greater than a tennis court, the Great East Window was commissioned in 1405 from the Coventry glazier John Thornton and completed in just three years. According to Sarah: “Thornton not only managed the large and talented team he assembled for the project, but also designed each of the narrative panels that depict the beginning and the end of all things. Under a figure of God holding a book with the words ‘Ego sum alpha et omega’ (‘I am the beginning and the end’), the main lights tell the story of Creation and the events of the Apocalypse (the Biblical Book of Revelation).”

Sarah is keen to emphasise that the window has not survived untouched for over 600 years. “The window was fully restored and releaded in the 1820’s and a further refurbishment project ran from 1946 to 1953, both of which introduced thicker 10-12mm leading both for the main nets and repairs to cracking in individual glass pieces. Now, with funding from the Heritage Lottery Fund, our five-year project will see the complete restoration of the window using the original 5mm leading net to separate the glass pieces and modern edge-bonding techniques to repair cracked pieces. On its return in 2016, the window should once again be seen with its original clarity and, with a new state-of-the-art environmental protection system, ensure its preservation for generations to come.”

“Modern stained glass conservation is crucially dependent upon both highly skilled craftsmen and women and an intimate understanding of the chemistry of glass composition, materials science, environmental conditions and conservation ethics. When the fabric of any window is restored, our artists and glaziers are balancing the need to address the consequences of the deterioration and corrosion of the stained glass and the leading net with the absolute necessity to retain as much as possible of the original and unique glass. Therefore, every complete panel is examined and photographed in detail before any work commences – first with a Hasselblad medium format camera and then microscopically using a ZEISS Stemi DV4 SPOT zoom stereomicroscope equipped with a ZEISS AxioCam digital camera.

“The protracted microscopic examination of the entire surface of the large irreplaceable panes demands an exceptionally robust, stable and smooth long-reach arm mechanism allied to an ergonomically-sound stereo optical head and well-directed illumination. The exceptional optics and the superb build quality of the ZEISS microscopes are ideal for the task, allowing precision scrutiny of the fragile panels with the minimum of effort. And, with a fibre optic reflected light spot illuminator integrated into the body of the DV4 SPOT, specimens are always well illuminated whatever the position and direction of observation.”

The results of an early conservation trial informed the development of a conservation methodology that required the approval of the Cathedral Fabric Commission for England. Only after their agreement could the painstaking conservation work begin – each panel consuming 450–600 man hours!

After careful pre-conservation examination of each panel and the compilation of a detailed condition report, the conservators begin by gently dismantling the glass from the very thick post-war lead, which in places disfigures and obscures the exceptional medieval painting. The base glass is generally very well preserved, although almost all show the consequences of deterioration and corrosion that results in surface pitting, reduction in glass thickness, loss of the surface decoration and the accumulation of dirt and corrosion products. Some paint has also been lost thanks to the over-zealous cleaning of earlier restorers. Cleaning and repair of the glass surfaces is a process in which the high powered zoom stereomicroscope is essential, with constant monitoring to ensure that neither the delicate protective gel layer of the base glass nor the potentially vulnerable paint layers are damaged by the cleaning process.

Loose surface dirt may be removed with a small soft brush but cleaning itself is with a mixture of deionised water and ethanol, applied with cotton buds which are gently ‘rolled’ over the glass surface. Where cracks have occurred or small pieces of clear glass lost, edge bonding and filling with epoxy resins, such as Araldite and Hxtal, is used to restore the visual coherence and legibility of the medieval designs. And, where glass is lost, replacement pieces are painted and fired by YGT’s skilled glass artists just as their predecessors did in the early 15^th century. Finally, all of the pieces are re-glazed in leads that more closely resemble the lost medieval originals.

[caption id="attachment_28597" align="alignleft" width="200" caption="Figure 4: Anna Milsom, one of the Trust’s conservators, works on one of the first panels to be restored in the five-year project"][/caption]

Interestingly, the microscopic examination has also in a number of cases managed to re-unite some of the original panel designs. During the course of the pre-conservation examination, YGT’s team have occasionally found original 15^th century glass pieces that seemed somehow at odds with the rest of the design. Subsequent microscopic examination of the edges and surfaces has enabled them to be ‘matched-up’ with glass pieces from other panels. “This is the result of previous restoration work on the window, when cracked pieces have been replaced and the fragments used to repair other smaller areas within the same panel or in another panel entirely. Our microscopic image library is enabling us to conclusively settle the argument as to whether these fragments were once part of a larger piece of stained glass and, thanks to modern repair techniques, is enabling us to return them to their original position.”

The Stemi DV4 is also proving invaluable in the investigation of corroded glass surfaces. “In all glass, silicon ions migrate to the surface to form a ‘Gel Layer’”, says Sarah. “The concentration of silica means that this layer is hard but susceptible to micro fissuring and water ingress to the fissures, together with oxalic acid secreted by bacteria growing on the glass surface, corrodes the glass. Laminate glasses, especially the ‘flashed’ reds used in medieval windows, are especially vulnerable to corrosion.

“Our microscopes and AxioCam imaging systems are enabling us to record both distinctive and characteristic corrosion phenomena and document them for future reference. We have also discovered a small number of deliberately abraded areas of ruby glass that have been of particular interest as the flashed surfaces have been deliberately modified to create the medieval equivalent of ‘special effects’, required to convey the ferocious beasts described in the Book of Revelation. The corrosion patterns, affected by the application of paint to both surfaces, have created unusual and distinctive corrosion phenomena which may reflect methods of manufacture and even place of origin of the base glasses, something which we are keen to investigate further with our colleagues in the scientific community.”

I was lucky to be invited to visit the Trust’s workshop and talk with the craftsmen and craftswomen who are carrying out this delicate and historically significant work. But you can also see first-hand how these irreplaceable glass windows are being preserved.

Thanks to a generous grant from the Heritage Lottery Fund, the Trust has converted a disused medieval chapel close by York Minster into the Bedern Glaziers Studio. For the first time in over 50 years, there is now the chance for the public to get up close to York Minster's Great East Window, to see real conservation in action and to meet the experts entrusted with this ambitious conservation project. Led by expert guides who are familiar with the history of York Minster and the work of the York Glaziers Trust, you can meet the conservers and find out about the unique blend of ancient craft skills and 21st-century scientific understanding that is modern stained glass conservation.

Acknowledgement: All images are printed with the kind permission of the Dean & Chapter of York