Series editors' preface - Wood Tools

ures involved in preparing a copy of an original
via an intermediate mould will require the
following steps: selection of moulding material,
selection of casting material, preparation of
object or pattern from which the replica is to
be prepared, taking of mould impression,
preparation of the mould for casting the
replica, casting and finishing.
10.5.2 Selection of materials
Selection of materials for moulding and casting
requires consideration of several inter-related
factors. First the material of the original object
– which materials would be compatible for
casting a replacement? Secondly what type of
mould is necessary – is the piece to be cast
large or small, simple or complex, undercut or
detailed? Will it be necessary to reproduce
large flat areas, to use piece moulds or to cast
replacements in situ? Finally, of the materials
that are compatible with the original object,
which are also compatible with the mould
making material best suited to the job?
The method that is to be used to adhere or
attach the replacement part to the original may
also affect the selection of the casting material.
There should be no deleterious chemical reaction
between the moulding material and the
artefact, pattern or casting material. Materials
with a high water content, such as wet plaster,
should not be used against moisture-sensitive
surfaces unless an adequate moisture barrier
can be provided. The variety of potential
chemical or physical interactions between
materials is great and it may be necessary to
test procedures before using them on original
artefacts or laboriously produced patterns.
Moulding and casting materials should
conform well to, and retain shape and detail of,
the surface whilst releasing easily from both
the artefact and mould. The form in which the
material is used influences these properties.
Materials in liquid form are more difficult to
confine in use than plastic solids such as dental
impression compounds, oven-hardening PVC
gels such as Fimo and Sculpey, Plasticene, or
composition. The main advantage of semi-solid
impression compounds is the short duration of
exposure to an original surface. The graininess
or particle size within the material will affect
detail. Most thermoplastics shrink somewhat as
they cool. In a given application it will be clear
Principles of conserving and repairing wooden furniture 483
whether the use of a liquid or semi-solid paste
will be more convenient.
Moulding and casting materials should be
usable at temperatures that do not damage the
artefact, pattern or mould and should not
generate excessive heat during cure, which
might cause damage. Some thermoplastics
polymers, including waxes and PVC, have to
be melted and used hot. Clearly these cannot
be used where the pattern cannot tolerate high
temperatures, as is the case with many artefacts.
Plaster, epoxies and unsaturated polyester
resins may generate significant heat
during cure depending on the exact formulation
used, thickness, ambient temperature and
other circumstances of use. If the final cast is
required in metal then the moulding material
must be chosen to withstand the pouring
temperature of the metal.
Moulding and casting materials may present
some health and safety hazards. These include
the toxicity of thermosetting polymers, their
catalysts and hardeners, risk of skin burns from
exothermic reactions or hot melt materials, risk
of fire from waxes, and risk of dust and fumes
arising during processing. It is the responsibility
of the conservator to be fully aware of the
hazards presented and to take adequate
precautionary measures.
There is a wide range of both thermoplastic
and thermosetting materials available, some of
which are listed in Table 10.4. The letters M
and C in the table indicate that the material is
used for moulding or casting respectively. No
material is ideal in all respects and so one must
be carefully chosen to suit the requirements of
a particular job. Cost, for example, may be a
factor for large volume jobs. In many cases the
same material might be used to create a
pattern, make a mould or cast a replica.
Wax has been used since antiquity for
modelling, moulding and casting and various
more or less complex recipes using natural
waxes, resins and oils exist in historical literature.
Synthetic microcrystalline waxes have
largely replaced natural ingredients because
they are available in hardness and melting
ranges that will copy virtually any historic
formula while being cheaper and more stable.
Although waxes can be warmed and made
pliable both for modelling and press-moulding
purposes, they may also be used in fluid form.
They can be painted or poured over a surface