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7. Painting wax into a rubber mould
To make a wax cast, molten wax is painted
into the rubber mould, taking care not to trap any air pockets in
the surface and is gradually built up to an even thickness. After
trimming any excess from the edges, the sections of the mould are
assembled and through an aperture made for the purpose, cooling
molten wax is poured in and then out of the mould. This leaves a
thin deposit of wax three to six millimetres thick which unites
the previously separate sections of wax. Once cool, the mould is
opened and the flexible rubber peeled off the wax to reveal a perfect
reproduction of the original sculpture. The thickens of the wax
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8. Applying runners, risers and core pins to wax
Joints and scams where different sections of
the mould join, need to be filled and the texture on them restored.
When this modelling is completed as carefully as the original
sculpture was made, the finished wax is ready for the next process
and can be moulded in a refractory mould. A system of wax rods
(runners) are welded to the wax with a hot knife. These will allow
metal to run into the sculpture. Smaller rods (risers) allow the
air to escape as the metal runs in. Core pins, previously pushed
through the wax at strategic points, will ensure that the ceramic
walls will not collapse when the wax is 'lost'.
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9.Investing the wax:
The wax needs to be encased in a mould able
to withstand the high temperature required to melt out the wax
from inside. It also needs to reproduce in detail the surface
of the original sculpture. The traditional material, still used,
is a mixture of plaster and grog (crushed and fired clay). This
liquid mixture is painted onto the surface of the wax.
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10. Pouring the core into the wax
The plaster and grog mix sets rapidly and the
hard mixture is formed into a block around the wax. The same mixture
is poured inside the wax to form the core.
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11. Removing the fired investment moulds from the kiln
The finished block mould is then placed upside
down in a kiln and heated up to 700 C. This melts the wax out
of the mould and burns away any residual gases left by the melting
wax. After cooling down to about 100 C the moulds are carefully
removed from the kiln and are strengthened with a coat of scrim
soaked in plaster.
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12. Pouring the bronze
When the mould is suitably strengthened it is filled with molten
bronze, an alloy of copper and tin. The metal should fill all
the space left by the melted wax and the sculpture, complete with
its runners and risers, becomes bronze.
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