The Garrison Chapel Windows
||The foundry pit at Hargreaves Foundry, Halifax
The processes and precision required
in cast iron production and patternmaking
have changed very little over
the last century. Nevertheless, the task of
repairing and replacing historic 19th century
cast iron windows often requires the skills of a
master detective as much as those of a master
Loss, erosion and damage over many
years have transformed some historic cast
iron windows into heavyweight jigsaw
puzzles. When no original patterns and no
record of the original dimensions survive,
there is rarely a straightforward formula
for 21st century replication of cast iron
windows on important restoration projects.
The Garrison Chapel in Pembroke Dock
is a case in point. Thought to be the last of its
kind in South Wales, this Georgian classical
chapel was built in around 1830. It was
designed by the Royal Navy’s chief architect,
George Ledwell Taylor, and forms the focal
point of a busy dockyard ‘producing vessels
that ranged from battleships to royal yachts’.(1)
After the military abandoned the
dockyard, the chapel survived in a variety
of uses before closing in 1975, a year after it
was listed. It then fell into a steady decline
and by the time it was sold to a private
developer in 1986, the lead had been stripped
from the dome, parapet gutter and roof.
In 2003, when the building became the
subject of a compulsory purchase order by
Pembrokeshire County Council, the roof had
partially collapsed and the structure was
shrouded with a roofed scaffolding frame.
Sections of the dressed and rendered rubble
stone walls were badly damaged and the
five cast iron windows on the south side of
the building had been replaced in steel.
Ironically, the original cast iron windows
on the northern side were in much better
condition than the steel replacements,
which were badly rusted and in very poor
condition. The replacements also lacked the
unique hopper-style window openings of
their surviving cast iron counterparts.
Funding for a major restoration
programme came through the Heritage Lottery
Fund’s Pembroke Dock Townscape Heritage
Initiative, a project that was to win the Award
for the Reuse of a Georgian Building in the
2008 Georgian Group Architectural Awards.
It was clear from the poor state of the
building’s south side that all of the rusted steel
windows would have to be replaced. Welsh
Heritage Construction, the main contractor,
repaired around half of the surviving cast
iron windows in situ, along with some of the damaged stone masonry and glazing.
The remaining windows were removed by
hand in sections and the broken fragments
sent to Hargreaves Foundry in Halifax.
Hargreaves were tasked with casting 11
new windows with a finished height of
4,240mm and a width of 1,677mm, featuring
80 individual window lights to be cast in
grey iron Grade 250 BS EN 1561:1997.
To achieve this, Hargreaves’ pattern
makers had first to piece together the original
design from existing fragments and from
photographs of the original hopper openings.
Detailed measurements were taken and the
dimensions of every detail of the windows
were collated to the millimetre to produce
a complex technical drawing. With the
help of the pattern maker’s comprehensive
training (apprenticeships often last five
years), highly-developed skill and detailed
understanding of the foundry process, a
three dimensional pattern design is created.
|Window patterns for the Garrison Chapel, Pembroke Dock
Once the design has been approved, a
manufacturing drawing is produced and
the pattern is made by hand, usually in
FSC-sourced yellow pine, birch plywood or
mahogany, since these are among the least
likely to warp or crack under the stresses
imposed by the moulding process and storage.
Pattern making must also allow for
shrinkage of the metal as it cools. In
the case of grey cast iron, one per cent
increments are applied to each dimension.
The main tools used to achieve this
precision work are chisels and gauges, many
of which are more than a century old and
have been passed from one generation of
pattern maker to the next. The process is so
complex and detailed that a project on the
scale of the Garrison Chapel can require two
pattern makers working for up to six weeks.
Once complete, patterns can be reused
to create hundreds or even thousands
of moulds. But with zero tolerance of
warping, it is imperative that patterns are
stored in a dry environment before use.
The completed pattern is placed inside a
box in preparation for sand moulding. The
interior of the box is coated with micronised
talcum powder or micronised aluminium to
aid mould release and the box is then packed
with recyclable silica sand, which is then
compacted, often by hand, in another process
that requires expert knowledge.
||The new cast iron windows of the Garrison Chapel
Once the sand-resin mix has set, usually
in half an hour, the pattern is removed
from the box. Since the Garrison Chapel
windows featured profiles to both the
exterior and interior, two moulds had to be
created which could be brought together
to form two halves of a single casting.
The iron founder must create a runner
system: a critical element of the process which
ensures that the molten metal can be poured
into the mould at just the right speed. The
runner system has three elements, a down
peg, a run bar and an in-gate, all created from
the bonded sand and designed to exclude slag
from the final casting. This stage demands
great care and expertise. If the molten iron
flows too slowly it will solidify before it reaches
the casting shape, if it flows too quickly or too violently it could damage the mould,
resulting in expensive scrap castings.
Since the size, weight and depth of
decorative windows vary, the founder must
also plan for the metal contracting at different
rates. To compensate, the founder builds
one or more ‘risers’, to hold a reservoir of
molten metal which can then feed back
into the mould as the cast iron contracts.
Iron pouring is the most spectacular and
dangerous part of the process, but also the
least complex. The metal is a combination
of carbon, silicone, sulphur, manganese and
phosphorous, the proportions of which are
crucial to determining the grade of cast iron
produced. The windows were cast in flake
graphite cast iron with a tensile strength of
250 newtons per square millimetre. The metal
is heated in a furnace to a temperature of
1,500°c before the molten iron is poured into
Once cooled, the solid window castings must
be removed from the mould. The sand is then
recycled to make more moulds.
A ‘fettler’ breaks the sand mould with a
sledgehammer and chips off the runner and
riser systems with power tools. The fettler
also removes the flashes, the slivers of metal
which seep into the spaces between the two
mould sections. The casting, weighing up to
250kg, is then carefully removed by hand.
The final stage in the production process
involved shot blasting the window casting
to base metal, creating a sound surface for
painting before delivery back to Wales.
SERVICE AND SUSTAINABILITY
Now in situ and performing well in the
restored church buildings, the cast iron
windows can be expected to provide up to
a century of service, after which the entire
castings can be recycled.
(1) Jason Evans, ‘New Uses for Old Buildings’,
Civic Trust for Wales conference paper, 2005
(available online at www.civictrustwales.org/conf_nov05/pdf/evans.pdf)