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Cold
Metal Stitching of Historic Metalwork
The
repair of cast iron poses a problem. Compared with steel and wrought
iron, cast iron cannot be welded easily because it is relatively brittle
and can be damaged by heat. One alternative for the repair of historic
originals is 'cold metal stitching'.
Terry
Sims
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 The
Repair of a Village Pump
The
village pump in the Nottinghamshire village of Stanton-by-Dale
was erected in 1897 to commemorate 'the beneficent reign of
Queen Victoria'. It originally had a large horse trough and
a smaller trough at ground level to satisfy the thirst of passing
dogs!
At some point in its history, the horse trough was filled with
concrete. This absorbed rain water which froze and expanded
producing numerous cracks along the base of the trough as well
as breaking open one of the sides and fracturing the corner
of a panel. The upper part of the decorative pump column also
had large horizontal cracks which detracted from its appearance
and made it structurally unsafe.
In this case, repairs involved dismantling the pump to remove
all of the concrete, then employing a combination of metal stitching
and 'insert' techniques to repair fractures and replace sections
of missing material. In addition, the pump column was made structurally
safe with new internal steelwork. During the rebuilding phase,
the pump handle and spout were refitted and the decorative sections
restored to their former appearance. Now resplendent in its
finished coat of green and gold paint, the pump will continue
to delight residents and visitors to Stanton-by-Dale.
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As
an iron alloy with a high carbon content, cast iron is an ancient
material which was valued for its great strength in compression. With
major improvements in furnace technology in the mid 18th century,
the cost of manufacture fell, and production began on a mass scale.
Being
strong in compression, the material was well suited for the casting
of columns supporting heavy industrial floor loads in the burgeoning
factories and warehouses of the early industrial revolution. Being
cast, it was also ideally suited to the mass production of repetitive
details: columns, for example, could be embellished with the classical
orders at little extra cost. The growth of the railways saw it used
for the sumptuous decoration of platform roofs, and allowed the distribution
of heavy cast iron goods for every conceivable use nation-wide. The
manufacturers exploited its potential to the full.
Many
of Britain's finest buildings originate from the Victorian era when
cast iron production was at its height, both for structural and decorative
building purposes. This legacy of craftsmanship can be seen today
in a rich variety of outdoor structures from ornate Victorian bandstands
to village pumps.
However,
cast ironwork is inherently brittle, so it is not unusual for some
damage to have occurred with the passing of time. In the interior
of buildings, sections of banisters are often found to be broken or
cracked as a result of 'mechanical' accidental damage. In the case
of outdoor structures, ironwork is also exposed to the elements and
the risk of cracking through frost damage.
The Alternatives
Repairing
cast ironwork is not quite as straight forward as many people imagine.
In order to weld or braze cast iron, special heat treatment procedures
are required, and even then there is some risk of damage, due to thermal
stresses created in the material which might cause it to distort,
embrittle, or even shatter, and the material may recrystalise if too
much heat is applied. Successful welding usually involves dismantling
the ironwork, taking it to a workshop and preheating it before welding
to ensure a gradual temperature change. It must then be reassembled
on site, all of which can be time consuming and expensive.
Where
historic ironwork is concerned, repair methods should always be chosen
to minimise risk, and to retain as much of the original as possible.
Cold metal stitching overcomes much of the disruption caused by welding
and is far more economical. However, this too is a specialist process
which is carried out by only a handful of British companies.
The
main advantage of this method of repair is that it can usually be
carried out insitu so there is minimal disturbance to the fabric of
the building. This also ensures that as much as possible of the original
character of the ironwork is preserved.
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 The
Restoration of The Doncaster Corn Exchange
A fractured spandrel casting before repair. Repairs to
the Doncaster Corn Exchange were required following a major
fire which caused extensive damage to structural and decorative
cast and wrought ironwork.
Built in the 1870s, the Corn Exchange has an elaborate roof
design consisting of wrought iron curved beams with a span of
18 metres which are supported on either side by cast iron columns.
Above this is a lantern support feature incorporating decorative
cast ironwork. There are also gallery areas running along the
length of the building with many fine examples of cast iron
decoration including scenes from farming life such as shire
horses, plough shares and lambs. The intensity of the fire resulted
in severe cracking and distortion in the roof beams, whilst
whole areas of decorative ironwork were detached and shattered
by the heat. Although special welding methods were used for
the repair of the wrought iron beams, cold metal stitching was
ideally suited to the repair of the decorative cast ironwork.
The main advantage was that all work could be carried out on
site without the need for hundreds of sections of ironwork to
be dismantled and transported to a workshop, and without the
risk of further damage that this might entail. Also, because
the cold metal stitching process does not involve the application
of heat, there was no risk of causing further damage by distorting
or embrittling the metal. The aim of the project was to preserve
as much of the original fabric and character of the building
as possible.
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The
Cold Metal Stitching Technique
Cold
metal stitching has been used for repairing equipment and machinery
in the engineering industry for more than 100 years, but it is only
during the last few decades that it has been more widely applied to
the repair of architectural castings.
The
process begins by drilling a line of holes at right angles to the
crack in the casting and then converting them into a slot. To achieve
the correct spacing for the holes a special drilling jig is used with
centres at 1/8", 5/32", 3/16" or 1/4" depending
on the thickness of the material which is being repaired.
Preformed
locks are then fitted into the slots to create a bridge across the
broken sections. The locks are made from a high nickel steel with
the same co-efficient of expansion as the cast iron. This material
is specifically chosen because it is strong enough to take shear loads,
but sufficiently ductile to provide the necessary elasticity.
Most
repairs will have a series of locks and stitches spaced at regular
intervals along the crack. When this is completed, holes are drilled
along the line of the fracture between each stitch. These are tapped
to receive special screws which fill the crack and ensure that it
is completely watertight.
Finally,
the area which has been stitched is ground level to the surface of
the original material to create a seamless repair. In this way, when
the crack has been repaired the area of metal is often stronger than
the original cast iron material, and when it has been primed and painted
the repair can be invisible.
In
some cases involving heavy loading stresses, such as the repair of
cast iron columns, there is a need to provide extra reinforcement.
In these circumstances, a 'master key' is inserted in addition to
the normal 'stitches'. The 'master key' is a larger section of metal
which can be varied in shape or size to suit the particular requirements
of the repair, but basically it involves a larger bridging section
which is fitted using the same technique as conventional stitching.
In
cases where whole sections of material are missing due to the effects
of corrosion or mechanical damage, a patch of metal, known as an 'insert',
can be made to fill the gap and stitched into position. Inserts of
this type can be anything from a few inches wide to several feet.
Applications
In
all its variety, Victorian cast ironwork is a fascinating and extremely
worthy part of our architectural heritage. Because of its strength,
cold metal stitching enables original structural elements to remain
functional, carrying the same loads and operating in the same manner
as originally intended (provided that the original design was not,
in itself, inadequate for the purpose). Decorative and historic designs
can be preserved in their authentic richness and detail, usually at
a cost far less than replacing them with something more modern, but
perhaps lacking in the same amount of character.
The
process can be applied to a multitude of applications ranging from
the replacement of finials and sections of railings to the repair
of major structural elements such as columns and spandrel brackets.
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© Cathedral Communications Limited 2005 |
