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14
BCD SPECIAL REPORT ON
HISTORIC CHURCHES
22
ND ANNUAL EDITION
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.
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
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.
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 (www.
cvma.ac.uk/conserv/guidelines.html)
N Teed, ‘Bronze Framing for Historic
Stained Glass: A New Case Study from
the York Glaziers Trust’,
Vidimus
88,
Feb 2015 (vidimus.org/issues/issue-88/
feature-ygt)
ANDREW ARROL
is surveyor of the fabric
for York Minster and a partner of Arrol &
Snell Ltd (see page 45).
SARAH BROWN
is director of the York
Glaziers Trust (see page 10).
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.
View from scaffold of tracery section showing poor condition of stonework and diamond paned protective
glazing installed c1925
Detail of a new tracery section ready for indenting:
note grozed chisel finish to all surfaces
Upper part of the Great East Window with masonry
complete and protective glass in place