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The origins of the lost wax process are shrouded in antiquity,
but it has been used for thousands of years to produce objects in
metal which could not be produced any other way, due to the complexity
of their form. It permits anything that can be modelled in wax to
be faithfully transmuted into metal, and is still used today for
certain industrial parts, dental restorations, fine jewelry, and
sculpture.
While wax patterns were originally modelled by hand, and this can
still be done, it is now possible to cast wax into molds as well,
so that multiple copies may be made even though the wax pattern
is lost in the process. Modern synthetic rubbers have been developed
which capture very fine detail and can flex to release undercut
areas of a model, greatly reducing the number of mold parts and
the number of parting lines necessary, when compared to the earlier
technique of using wet plaster molds. Waxes can be cast either solid
or hollow, as the wax will coat the inside of a mold after it is
filled and poured out. This process is repeated to build up the
desired thickness of wax. The process is slightly different for
jewelry and sculpture; the smaller quantities involved when making
jewelry-sized pieces necessitate some adaptations to overcome the
effects of surface tension, such as wax injection instead of simple
pouring, and the use of a vacuum table or centrifugal casting machine
to force metal into molds.
After it is made and touched-up, the wax model is attached to a
"pour-cup", which is funnel-shaped to channel the metal into the
mold from the outside, using "gates"or "sprues" made from rods of
wax , and a venting system is made the same way to convey air and
other gasses out of the mold when it is filled with hot metal. Once
the model is set up with its gates and vents, it is surrounded with
a material that will cover it smoothly when wet and withstand high
temperatures when baked. In some places an ancient technique involving
cow-dung and clay is still used; a very fine mixture is painted
on to capture the detail, then coarser layers are added to build
up a mold that can be handled, baked, and poured into.
A modern variation on this method is called "ceramic shell"; instead
of the cow-dung and clay, a silicaceous slurry is used to cover
the model by dipping and/or pouring. Special dry aggregate is then
applied to the wet pattern, either by hand or by using a "fluidized
bed", which blows the dry particles around in a confined space,
covering the wet areas until no more will adhere. The coated pattern
is then left to dry a while and another layer of wet and dry material
is applied. This is repeated, using coarser aggregate on the outermost
layers, until a sufficient thickness has built up so the mold will
hold together through the burn-out and pouring.
The other method commonly used is called "investment" casting. The
pattern is set up in much the same way as for ceramic shell, except
that more venting is necessary due to the decreased porosity of
the mold material, which is made using gypsum plaster (plaster of
Paris) as a binder for sand, silica flour, or another refractory
aggregate. There are many proprietary investment mixtures available,
or one may choose to mix one's own. The dry ingredients are mixed
with water and poured into a container or "flask" surrounding the
gated model, which is either waxed down to a board or attached to
a commercially available rubber device which holds the pattern and
flask. Jewelry flasks are generally placed in a vacuum chamber while
this mixure is still fluid, where they are boiled at room temperature
to remove air-bubbles clinging to the models. It is helpful to de-air
the investment mixture before pouring it over the wax models, in
order to reduce boiling-over when vacuuming.
Once the plaster mixture has set hard, the flasks are placed in
a kiln, slowly heated to between 1000 and 1250 degrees F, and held
at that temperature until all wax residues have disappeared. The
place where the wax was is now a void- hence the "lost wax" designation
for this process. The metal is then melted and the molds are filled
with liquid bronze or whatever metal is being used. The flask is
then allowed to cool, then the plaster mold is broken away, revealing
the metal part, which faithfully reproduces every detail of the
original wax, plus the gates and vents, which must be cut off. Silver,
gold, aluminum, brass, or bronze may be used interchangably with
this method. Special investment materials and melting equipment
are necessary to deal with the higher temperatures of platinum,
iron or stainless steel, but the process is much the same. Lower-melting
metals such as tin, lead, and zinc may also be cast by this method,
but there are other easier methods of casting them, so it is not
often done. A variation on the lost-wax method is also used for
casting objects in glass.
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