Series editors' preface - Wood Tools

materials, used extensively by sculptors and
craftspeople, are highly variable both in chemistry
and properties. Detailed general information
on their chemistry and properties is
available in a variety of published sources.
However, new materials are developed and old
formulations become unavailable with such
regularity that current manufacturer’s literature
may be most useful and will generally yield an
appropriate polymer for virtually any purpose.
It may be necessary to pre-test to ensure good
results. Generally, slight shrinkage occurs upon
curing and all of these polymers are toxic to
some degree by inhalation, ingestion or skin
contact. Their successful use requires careful
measuring and mixing of ingredients. Heat is
generated by many of these materials as they
cure. Slow curing polymers can dissipate this
heat over a long period of time but faster
curing ones may generate enough heat when
used in large quantities to destroy both the
polymers and the pattern. Epoxies and polyesters
are particularly prone to such catastrophic
‘meltdown’. A slow setting resin
should be used for large pours. Alternatively,
faster setting resins may be used in multiple
thin layers or even chilled while they undergo
initial cure to help dissipate the heat. Barrier
layers and/or release agents must be used with
thermosetting polymers to prevent damage to
patterns and artefacts and to ensure clean
release.
Most thermoset materials have limited shelf
lives that may be prolonged by refrigeration.
The cured polymers generally maintain their
mechanical properties. Polysulphide rubbers
are the least permanent of the elastomeric
compounds and become sticky and unusable
after a few years. In contrast, silicone rubbers,
particularly those that utilize platinum catalysts,
are essentially permanent. All these materials
are several times more expensive than traditional
materials such as clay, glue and plaster.
Cost varies according to the specific compound
and the quantity in which it is purchased.
Silicone rubbers have been used extensively
by conservators for mould making because
they are easy to use, reproduce very fine detail
such as wood grain or fabric texture and can
be reused for a large number of multiple casts.
Silicone rubbers release easily from many
surfaces, thus manufacturers’ literature often
suggests that the use of a release agent is
Principles of conserving and repairing wooden furniture 487
unnecessary. Silicone rubbers release low
molecular weight silicone compounds, which
will alter the appearance of porous or unsealed
surfaces (Maish, 1994). In addition, residues of
these compounds may prevent accurate material
analysis in the future. Good conservation
practice therefore requires the use of a barrier
layer, such as wax, and in addition it may be
necessary to seal porous materials. Brückle et
al. (1999) have discussed the use of cyclododecane
in combination with a layer of methyl
cellulose or gum arabic for protecting sensitive
surfaces during mould-making. Silicone
rubbers will not cure in contact with sulphurcontaining
materials such as many non-hardening
clays. They are expensive but the cost is
often justified by their performance. The mould
may be reinforced if desired by an open weave
cloth such as mutton cloth or scrim. If necessary,
silicone rubber can be coloured using
acrylic/PVAC polymers such as the Maestro
range, although adding fillers will reduce
mechanical strength. Some flexible silicone
rubbers may be appropriate for making moulds
where the original has deep undercuts. Silicone
rubbers with the consistency of a firm modelling
clay are available from dental suppliers
and are very useful for flexible press moulds.
A variety of silicone rubbers are available
from suppliers to meet particular moulding
needs (see, for example, Table 10.5). Although
cheaper than platinum catalysts, tin catalysts
are generally less archivally stable. RTV-11 is a
good example of an easy to use, general
purpose silicone rubber for mould-making. It is
white, free-flowing and may be used for originals
where there is no deep undercut. It is suitable
for casting low melt metals, including
pewter. Increasing the proportion of catalyst
will shorten pot life and cure time. Additives
are available that allow manipulation of the
working properties of the rubber. One of
these, called ‘Thixo additive’, may be used with
T20 and T28 to give a rubber that may be
applied with a brush, does not trap air and
does not slump.
Polyester resins are often used as a casting
material in conjunction with a silicone rubber
mould. They can also be used as a moulding
material that may be reinforced with glass
fibre. There are usually three components: the
resin, a catalyst such as methyl ethyl ketone
peroxide, and an accelerator such as cobalt