Series editors' preface - Wood Tools

and capture a high degree of detail. Waxes
release well from oiled surfaces, wet water clay
and thoroughly soaked gypsum plaster. If a
plaster mould is to be used for a wax cast, it is
best soaked in hot water before application of
wax. The water will act as a release agent and
heat will eliminate chill wrinkles. Waxes, especially
microcrystalline types, are chemically
inert but cannot be used in liquid form against
temperature sensitive surfaces. Slight to moderate
shrinkage occurs upon cooling.
Permanence is very good if stored below softening
temperatures and cost is moderate. They
present few health hazards but melting waxes
do pose a potential fire hazard.
Water clays are another ancient category of
inexpensive moulding and casting materials.
They are useful in modelling patterns and in
taking quick squeeze impression moulds from
patterns. They become progressively firmer as
water is lost, which may be desirable when
refining models to a high degree. The shrinkage
of roughly 10–20% on drying can be
exploited to scale down a model in size. Dried
green clay is very fragile and water-soluble but
otherwise inert. Clays cannot be used against
water-sensitive surfaces but when thoroughly
dry they will tolerate high temperatures, so
molten metals and polymers can be poured
against them. Health hazards resulting from
breathing of clay and glaze dusts and breathing
or ingestion of toxic metals in glazes have
been reported but these problems are unlikely
to be encountered in small-scale moulding and
casting.
Non-hardening clays, also known as plastilene
and Plasticene, have been mixed with oils,
waxes and other non-drying materials to keep
them permanently pliable. They are used in the
same way as water clays for modelling patterns
and taking impressions, but they do not dry or
shrink. Various firmness grades are available
and firmer clays should be chosen for more
detailed work. These clays can be made softer
or harder by manipulation of temperature.
Some of them contain sulphur, which will
interact with metal objects and inhibit the cure
of silicone rubber, but sulphur-free varieties
are also available. Oils present in these clays
may stain porous objects. Though they have
poor permanence, there are generally no
known health risks associated with these materials.
Cost is low to moderate.
Principles of conserving and repairing wooden furniture 485
Alginates are polysaccharides extracted from
various species of marine algae (seaweed).
They form hydro-colloidal gels in water that
are similar to those formed by animal glues.
Agar-agar is one such alginate and is also
sometimes called ‘Japanese isinglass’. Alginates
have been used in moulding since about the
mid-twentieth century, particularly in the
formulation of dental moulding compounds.
Most alginates are available as powders that gel
rapidly when mixed with water. Agar is
normally dissolved in hot water and allowed to
cool and gel. It can then be reheated and will
liquefy at around 42 °C. It is the principal
ingredient in ‘moulages’, which are used to
mould human flesh, because of its low gelling
temperature. Alginates may be difficult to use
because of rapid gelling and associated air
bubbles and wrinkles. Alginate gels have a
high water content and cannot be used against
water-sensitive materials. Only plaster and low
melting temperature waxes can be cast in
alginate moulds. Dimensions do not change on
gelling but alginate moulds shrink and warp
radically as they dry. Alginate moulds must
therefore be used immediately and cannot be
kept for any length of time. They do not retain
detail well and are not suited for multiple castings.
Alginates are non toxic and their cost is
moderate.
Animal/hide glues and gelatines were the
first materials used to produce flexible moulds
that could be stretched and released from
undercuts. They were used for this purpose in
the nineteenth and early twentieth centuries
until largely replaced by synthetic elastomers.
The properties of concentrated viscous glue
solutions could be manipulated by the additions
of materials such as glycerine to retard
drying/shrinkage and sugar to plasticize the
mixture. Glue moulds were poured at roughly
body temperature. They cooled rapidly
(depending on volume) and were prone to
form chill wrinkles unless poured very
smoothly. Glue moulds do not shrink appreciably
upon gelling but do so as they dry out.
They cannot be used in contact with water
sensitive surfaces but are non-toxic and safe in
all respects. Gelatine moulds were normally
surface hardened with alum, formaldehyde
(formalin) or tannic acid to prolong their usefulness
that in any case was rather short due to
surface blurring, contraction and distortion as