Page 140 - The Building Conservation Directory 2013

1 4 0 t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 T w e n t i e t h a N N i v e r s a r y e d i t i o n
3.4
Structure & Fabric : External WorksCast and wrought iron tend to resist
corrosion far better than mild steel due to
impurities in the metal and the formation
of a protective surface skin in the casting or
forging process. Nevertheless, all iron relies
on the integrity of its paint layer to prevent
corrosion, particularly when used externally,
and wrought iron is more vulnerable than cast
iron. It is vital to maintain protective coatings,
particularly on wrought iron.
Maintenance
Maintaining good drainage and weathering is
essential. Care should be taken to ensure that
brickwork, copings and lead flashings are kept
in good order and that gutters and drainage
pipes are running freely.
Glass should be kept clean and, as
with timber, free from lichen and mould.
Putty work should be regularly inspected.
Replacement glazing should be laid on
putty but not face puttied, with the panes
secured by galvanised or stainless steel
glazing sprigs (flat metal spikes) as pins
tend to split the delicate glazing bars.
Beaver tailed panes can be hand cut using
a hardboard template. When fitted, their
overlap should be no more than half an inch.
Ironwork
In the interior of a glasshouse cast iron
needs relatively little maintenance and is
more susceptible to physical damage than
corrosion as old cast iron can be quite
brittle. However, it is susceptible to frost
damage from trapped water, particularly
in rainwater pipes, and rust-jacking (where
rust expands between components),
both of which can cause fractures.
Broken castings can be welded by skilled
craftsmen, but as it can fracture when heated,
mechanical repairs such as pinning and
stitching are often preferred. Alternatively,
new components may be cast using
existing ones as patterns if only one or two
replacements are needed. The co-efficient of
shrinkage in iron is only 1 in 96, so the original
pattern would have been only fractionally
larger than the finished component, and
copies made in this way generally suffice.
Brittle castings can have their structure
modified to make them less brittle through
regulated heating (normalising).
Wrought iron is more susceptible to
corrosion, particularly in glazing bars and
in those areas that are difficult to paint.
The fibrous structure makes it extremely
difficult to weld, and although mechanical
reinforcement may be possible in some
instances, a high degree of replacement is
often unavoidable.
Paint and rust may be removed from cast
iron by the selective and careful use of grit
blasting, although there are risks attached to
the process, not least of which is damage to
the protective skin formed in the casting or
forging process. If this method is to be used,
it is important to select a grit suitable for cast
iron as some types contain chemicals that
impregnate the porous iron and promote
corrosion. It is important to warm cast iron
before blasting to drive out residual moisture.
Rust is best removed by careful heating
until it can be removed by brushing.
Joinery
Timber should be kept clean and well
protected. It is best to take the paintwork right
back to the timber if using modern paints
because they do not always adhere to old paint
types, however well prepared they may be.
Be careful to use adequate protection when
removing lead-based paints.
Micro-porous paints are not suitable
for use in glasshouses. While moisture can
be drawn into the timber in the normal
way, glasshouses heat up very quickly in the
morning sunlight and the moisture cannot be
expelled through the paint quickly enough,
resulting in blistering. Linseed oil paints offer
satisfactory alternatives to modern systems.
Modern hand-applied paints are capable of
giving a service life in excess of ten years.
New timber decays much more rapidly
than old timber because the Victorians
could use ancient, slow-grown timber,
usually from virgin forest in the Baltic. This
timber contained a far smaller percentage of
sapwood (white wood) and tighter growth
rings. Modern plantations are sited in the
most favourable conditions and are thinned
to maximise growth, allowing the trees to be
felled earlier. The timber therefore has wider
growth rings and a greater proportion of
sapwood, making it much more susceptible
to decay. Douglas fir is the best substitute for
the original red deal, and iroko is an excellent
substitute for teak. Timber should always
be ordered in the longest lengths available
because it will be found that these timbers are
generally taken from straighter, older trees.
If splicing timbers together always try to
obtain a good fit, use an appropriate glue and
stainless steel or brass screws. Joints should
always be pre-painted prior to assembly.
Recommended Reading
GF Chadwick, The Works of Sir Joseph
Paxton 1803–1865, Architectural Press,
London, 1961
J Hix, The Glasshouse, Phaidon, London, 1974
WJ May, Greenhouse Management for
Amateurs, L Upcott Gill, London, 1885
MWoods & AS Warren, Glass Houses: A
History of Greenhouses, Orangeries and
Conservatories, Rizzoli, New York, 1988
English Heritage Practical Building
Conservation: Glass and Glazing,
Ashgate 2011
English Heritage Practical Building
Conservation: Metals, Ashgate 2011
English Heritage Practical Building
Conservation: Timber, Ashgate 2011
Robert Jameson studied agricultural
engineering and rural estate management.
His interest in the restoration of historic oak-
framed buildings subsequently led to the
formation of Jameson Joinery Limited and
the re-establishment of Foster and Pearson
Limited (see page 141). Both companies
employ traditional joinery and metalwork
skills. His role includes advising clients and
producing technical drawings.
Hot zinc spraying followed by etch priming
to allow the paint to key, gives a very durable
base for decorating.
Wrought iron should not be grit
blasted because this removes the protective
layer of mill scale formed during its
manufacture. Paint may be removed
where necessary using chemicals such as
dichloromethane (methylene chloride).
Simple seize-proof hinges for a top-hung opening light
Abandoned glasshouses at a stately home in Devon,
showing fine cast iron spandrel brackets
An Edwardian seed safe