Also known as cire-perdue
and precision or investment casting,
is the traditional method for the faithful reproduction of intricate designs in
carat gold jewellery. One of the most ancient
of goldsmiths’ techniques, it has been used at least since the Egyptians in 1500
BC.
Left-hand column, top to
bottom: master pattern set in a rubber
mould; removing the wax from the rubber mould; assembling
waxes on the tree; heating the tree in the investment can
Right-hand
column, top to bottom: placing the hot mould into the
casting machine prior to casting; stripping ornaments off the
tree; the tree with ornaments removed; hand finishing
(Credit: World Gold Council)
To begin with, a master pattern
is made by hand, usually in nickel silver or silver and rhodium plated (to prevent
attack by sulphur from the rubber mould during vulcanizing). A feeder channel,
known as a gate, is added to enable the molten gold to feed in during
casting.
The
master pattern is embedded in rubber, placed in a heated press and vulcanised.
The rubber mould formed around the pattern is carefully cut away into two halves
and the master pattern removed. The rubber mould is then injected with hot wax
to make replicas of the master model. When set, the rubber mould is opened and
the wax models removed.
Many wax models can be made
from this rubber mould. These wax models are then assembled on a central wax ‘sprue’
to form a ‘wax tree’. This is placed into a metal cylinder or ‘flask’ and invested
with a liquid slurry of a refractory powder (‘Investment plaster’) and allowed
to set after vacuum degassing to remove air bubbles.
This flask is then inverted
and heated in air or, preferably, steam to remove the bulk of the wax. It is then
placed in a ‘burn out’ furnace and carefully heated slowly to a maximum of 750°C
to fire the mould.
The
molten gold is then poured into the mould cavity and mould filling encouraged
by either centrifugal force or by gravity with vacuum assistance. Once cool,
the mould is broken away to reveal a tree of golden articles, which are cut
off, cleaned and polished to produce jewellery pieces.