The Great East Window of York Minster
Andrew Arrol and Sarah Brown
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Above: God the Father from the apex of the Great East Window. Below: general view of the Great East Window and
the east front of York Minster |
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The repair of York
Minster’s Great East
Window was a key
part of a larger programme
of repair and conservation
to the Minster’s east front,
which commenced in 2006.
It was completed under the
subsequent York Minster
Revealed project, which
received generous HLF
funding of £10 million.
The cost of fabric repair
(masonry and glass)
amounted to approximately
half of the overall
£20 million project cost.
York Minster Revealed
was a five year project,
allowing sufficient time for the
remainder of the east front
masonry to be repaired and
conserved and for the Great
East Window glass (and the
windows of the adjoining
fenestration) to be removed,
conserved and then reinstated
incorporating a new protective
glazing system. Other parts
of the project focussed on
improving visitor facilities,
creating a new below-ground
exhibition space in the undercroft
and forming a new approach piazza
outside the south side of the Minster.
THE GLAZING
The Great East Window of York Minster,
made between 1405 and 1408, is the
largest expanse of medieval stained glass
in England and one of the most ambitious
glazing projects ever undertaken.
Depicting the beginning and the end of
the Christian cosmos, from the Creation
in the Book of Genesis to the Apocalypse
and the Second Coming of Christ, it
summarises the medieval perception
of human history, which unfolds under
the feet of God the Father (top) and the
company of heaven.
While the Apocalypse had been a popular subject in illuminated
manuscripts, to depict it
in glass and on this scale
was extremely daring. The
Chapter of York looked
outside the city for someone
with design abilities and
entrepreneurial skills equal
to the task and in 1405
contracted with the Coventry glazier John Thornton. While
Thornton was required
to paint only some of the
glass himself, the contract
stipulated that he was
to ‘cartoon’ every single
one of over 300 panels
entirely with his own hand
which, in the Middle Ages,
meant marking up full-size
working drawings on the
whitened glazier’s table.
The complexity of the
Apocalypse subject matter,
in which each scene is a
unique narrative, made this
a particularly challenging
undertaking, and while recent
research by Professor Nigel
Morgan has demonstrated
Thornton’s familiarity
with other Apocalypse
imagery, the window is characterised
by the originality and freshness of
Thornton’s approach to the subject.
CONSERVATION CHALLENGES
The window was last restored between
1946 and 1953 under the direction of
Dean Eric Milner-White. With scant
regard for the efforts of earlier restorers,
Milner-White set about reordering glass
within individual panels, removing many
earlier restoration insertions.
Inadequate art historical research
meant that some scenes were
misidentified so that they were incorrectly
restored ( see illustrations of Panel 5b below) and returned to the window
in the wrong location. The many holes
opened up by the removal of 18th- and early 19th-century stopgaps were filled
with pieces of unpainted coloured
glass, an honest repair which was easily
distinguished but visually very disturbing.
The most aesthetically destructive
aspect of the restoration was the use
of thick (10-12mm) leads throughout
the window, darkening the panels and
disguising the delicate relationship
between glass and lead achieved by
Thornton and his collaborators.
Exploratory conservation trials
undertaken by the York Glaziers Trust
(YGT) between 2005 and 2008 reviewed
a number of conservation options,
ranging from a light overall clean, to
the dismantling and full conservation
of the panels, with the recovery of John
Thornton’s cutline as one of several
objectives. However, one over-riding
priority was to install state-of-the-art
ventilated protective (isothermal) glazing.
In step with the guidelines of the
International Corpus Vitrearum (see
Recommended Reading), no dismantling,
reordering or restoration could be
justified without thorough research. As
a result, the forensic examination and
recording of every piece of glass has been
complemented throughout the project by
in-depth exploration of the antiquarian
and art historical context of the window.
The discovery of drawings and
photographs dating from the 1730s,
1880s and c1939 has been essential to
understanding the restoration history of
the window. Above all, the meticulous
description compiled in the 1690s by
antiquary James Torre has not only
shed light on individual panels, but has
confirmed the original panel order of this
immense biblical narrative.
After long and detailed consultation
with the Cathedral Fabric Commission for
England and other statutory consultees,
the Minster decided to proceed with
the dismantling, conservation and
reglazing of the window. This process was
supported throughout by the guidance of
Chapter’s East Window Advisory Group.
After careful cleaning to remove
hygroscopic dirt, the dismantled glass
pieces were closely examined for evidence
of their original location in the panel.
Clues provided by edges which had been
‘grozed’ by the medieval glaziers (nibbled
away with a hooked tool to fit snugly
into the lead) were always invaluable
evidence of authenticity and relationship
to adjoining pieces. Also, indications of
glass structure, corrosion patterns and
traces of lost paint, observable through a
binocular microscope, often confirmed
the evidence of surviving painted detail,
allowing multi-fractured and heavily
corroded pieces to be reunited. Mending
leads in obtrusive or lightly coloured areas
have been removed whenever possible.
The epoxy resins Araldite 2020 and Hxtal
NYL-1 have both been used for edge-bonding,
depending on the condition of
the glass and the nature of the fracture.
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Panel 5b following incorrect restoration by Dean Milner-White, who inserted
a second beast in the centre of the panel constructed from miscellaneous
fragments |
The panel following conservation in 2013: ‘And they adored the beast, saying
‘Who is like to the beast? And who shall be able to fight against him?’
(Revelation 13: 4-6). |
Detailed criteria have been used
to determine whether later stopgaps
should be retained or removed, and
those removed from the window have
been recorded and retained as part
of the project archive. Every process
and decision has been meticulously
recorded and new methods of digital
documentation have been developed
specifically for this project.
PROTECTIVE GLAZING
From the outset the East Window
Advisory Group was clear in its
view that the provision of protective
glazing was the single most important
contribution that modern conservation
could make to the preservation of John
Thornton’s medieval masterpiece.
In 1861 the Great East Window was
provided with crude exterior glazing, first
in the form of single sheets of glass and
later by diamond panes or ‘quarries’. Two
new glazing grooves were cut into the
window mullions of the main lights. The
exterior glazing was mortared into the
outer groove. The original glazing position
was abandoned and the stained glass
was set into the new inner groove. The
window was effectively double-glazed,
with no ventilation between the outer
and inner glazing. Advances in protective
glazing design have demonstrated the
importance of ventilation and this project
provided the opportunity to significantly
improve the system.
The new protection of the Great
East Window will take advantage of the
additional exterior glazing groove, but
the system is governed by the principles
of modern isothermal protective glazing
(see diagram opposite). An interspace of c50mm and gaps at the top and bottom
of each light allow a constant air flow
between the protective glass and the
historic stained glass, ensuring that both
surfaces of the medieval glass are kept
dry. Although the efficacy of this kind
of protective glazing has been tested in
projects all over Europe (Bernardi et al,
see Recommended Reading), the design
of the east window installation was also
informed by a year of environmental
monitoring undertaken by Ivo Rauch.
A major development has been the
decision to use Glashütte Lambert’s
Restauro UV glass for the exterior
glazing, a mouth-blown cylinder glass
that incorporates effective UV protection
up to 405nm. Restauro UV handles
like any other mouth-blown antique
glass but also protects light-sensitive
materials, notably the epoxy resins used
in edge-bonding, against light-induced
alteration or discolouration. York Minster
is the first building in the UK to use this
extraordinary new glass.
The new protective glazing system,
designed by YGT’s senior conservator
Nick Teed and surveyor of the fabric
Andrew Arrol, has eschewed the ferrous
materials which can cause damage
to stone, working with compatible
bronze alloys that minimise the risk
of bi-metallic corrosion. McKechnie
Brass’s bronze composition Alloy 300
is being used for the manufacture
of frames for the medieval glass, the
integral support bars, T-bars and the
rods from which the glazing pins are
made. New T-bars will support both
the exterior glazing and the medieval
panels, each nearly one metre square.
The exterior glazing is seated
on a T-bar and is mortared into the
stone in the traditional manner. Each
medieval panel will sit on an inner
row of T-bars, and will be held firmly
in place on the ledge of the bar with
bronze pins.
For safety, exhibition or
future conservation, panels can therefore
be removed easily and safely from an
interior scaffold, leaving the exterior
glazing in place as an effective weathershield.
The medieval panels are framed
in bronze Alloy 300 U-channel.
YGT’s Nick Teed has developed a new
framing method which has moved away
from the hot-working methods commonly
used to connect components of the panel
frames. Only traditional tin solder can be
worked at a sufficiently low temperature
to be used in proximity to medieval glass
but YGT was seeking a stronger bond. The
simple screw-fixed frame that Teed has
developed can be assembled around the
panel of historic glass, allowing maximum
precision, greatly increased strength and
an entirely reversible and recyclable use of
expensive metal components.
STONEWORK
The masonry repair project began with
a meticulous stone-by-stone assessment
of the fabric leading to the compilation
of detailed proposals for repair,
replacement and conservation which had
to be submitted to the Cathedrals Fabric
Commission for England for approval.
From the outset the project was seen
as a holistic design exercise, incorporating
design considerations for the protective
glazing and the integration of the conserved
glass panels into the masonry work.
A very careful assessment was made
of the overall geometry of the east front
and in particular the distorted geometry
of the window masonry, which leans out
at an angle of approximately 2° from the
vertical in an easterly direction. North-south
movement (across the window)
has also resulted in a 35mm wide crack at
the head of the window. This, however, is
relatively small, particularly compared to
the overall outward tilt of the east front,
which would have amounted to almost
1 metre had the upper parts of the facade
not been reconstructed during the 18th
and 19th centuries.
The four principal challenges arising
from the repair and conservation of the
east window masonry were as follows:
- to devise an appropriate methodology
for the introduction and indenting of
new tracery elements into the window
- to carry out extensive plastic repair,
pinning and conservation to those
tracery elements which were being
retained
- to conserve the intrados sculpture, which
was all in poor condition as a result of
19th-century applications of linseed oil
- to design and carve a new seated
St Peter figure, which required a great
deal of consultation and discussion.
STONE SELECTION
The basic building stone of the Minster
is magnesian limestone which, over the
years, has been sourced from several
different local quarries. The original
quarry source was Thevesdale, not
far from Tadcaster where the current
quarry source for the majority of Minster
stone is located. Other historic quarry
sources of magnesian limestone were the
Huddleston group of quarries and the
Bramham group of quarries. Stone is also
taken from Warmsworth and has in the
past also been supplied from Cadeby.
After detailed testing of previous
medieval stone types and an assessment
of contemporary quarry sources, the
stone from Tadcaster has been selected as
the preferred replacement stone with the
best match to medieval stone.
MORTAR
The pointing and bedding mix is made
up of naturally hydraulic lime (NHL 3.5,
usually St Astier). The standard mix for
general pointing uses one part sieved
washed river sand (Nosterfield), one part
Leighton Buzzard sand, one part South
Cave sand and one part NHL 3.5 St Astier.
This mix provides an extremely well-graded
aggregate proportion which bears
a very close resemblance to the colour
and technical performance of the historic
mortars used at the Minster. Where
necessary wide joints are ‘galleted’ using
shards of oyster shells to fill the gaps and
reduce the area of mortar exposed to the
weather, closely following earlier examples
found on the Minster, some of which date
from the medieval period.
REPAIR WORK
The full size drawings and templating
for every individual piece of stone were
prepared by the Minster’s master mason
and geometry and carving details were
agreed with the surveyor of the fabric. The
work was carried out in accordance with
the ‘Current Stone Practice’ document,
prepared by the surveyor of the fabric.
The document sets out a detailed
methodology and specification for both
conservation and new work.
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View from scaffold of tracery section showing poor condition of stonework
and
diamond paned protective
glazing installed c1925 |
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Left: detail of a new tracery section ready for indenting (note grozed chisel finish to all surfaces). Right: Upper part of the Great East Window with masonry
complete and protective glass in place. |
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The new tracery indents were
secured in place using lead-poured
joints, a technique used in the Middle
Ages to create a rigid joint. These were
then pointed up with the standard
mortar. The surface tooling of all
new stonework followed established
medieval practice and all surfaces
were carefully worked using a ‘grozed
edge’ chisel, a form of claw chisel.
Almost every surface of the building was covered externally with linseed oil
during the 19th century and this has led to
a surface consolidation process which can
trap salts and sulphates in the outer zone
of all external masonry elements. After a
while this develops into a kind of ‘potato
crisp’ which then snaps and breaks off.
Such surfaces were carefully cleaned with
distilled water then stabilised with up to
six applications of nano lime followed
by shelter coating using limewashes
emulsified with casein and a small amount
of ochre pigment.
The design of individual tracery
elements involves a great deal of lobe and
cusp work and many of the cusps and
tracery profiles were missing as a result of
weather and fire damage over the years.
Although traditional lime-based mortars
are used for weathering and filling, where
profiles need to be restored and/or rebuilt,
a repair mortar with a greater degree of
slump resistance is needed.
Keim’s Restauro
mortar system met this requirement well
and the mortar can be chiselled and dressed
when it has hardened. The product’s
porosity and breathability also closely
match those of the host stone. Lost profiles
have therefore been rebuilt using these
mortars with careful colour matching, and
reinforced with either stainless steel wire
or hollow ceramic dowels.
A HISTORIC ACHIEVEMENT
John Thornton’s 300-panel representation
of the biblical Apocalypse in medieval
stained glass was a ground-breaking
achievement.
The task of meticulously recording,
conserving and reinstating the Great East
Window’s stained glass and stonework
has brought together art historians,
archaeologists, conservators and
craftspeople. One of Europe’s largest and
most complex conservation projects, the
scale and success of this collaboration
echoes that of Thornton and those who
worked alongside him. And the results
speak for themselves, as the quality
and sophistication of John Thornton’s
monumental design have re-emerged from
centuries of obscurity, no longer ‘a glorious
wreck’ but a magnificent work of art.
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Recommended Reading
A Bernardi et al, ‘Conservation of Stained
Glass Windows with Protective Glazing:
Main Results of the European VIDRIO
Research Programme’, Journal of Cultural
Heritage, vol 14/6, 2013
S Brown, Apocalypse: The Great East
Window of York Minster, Third
Millennium Publishing, London, 2014
Corpus Vitrearum, Guidelines for the
Conservation and Restoration of Stained
Glass, 2nd ed, Nuremberg, 2004
N Teed, ‘Bronze Framing for Historic
Stained Glass: A New Case Study from
the York Glaziers Trust’, Vidimus 88,
Feb 2015
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