Lost Wax Bronze Casting
Lost-wax casting sometimes
called by the French name of cire perdue (from the Latin cera perduta) is the
process by which a bronze or brass is cast from an artist's sculpture; in
industrial uses, the modern process is called investment casting. An ancient
practice, the process today varies from foundry to foundry, but the steps that
are usually used in casting small bronze sculptures in a modern bronze foundry
are generally quite standardized.
Other names for the process
include "lost mould," which recognizes that other materials besides
wax can be used, including tallow, resin, tar, and textile; and "waste
wax process" or "waste mould casting", because the mould is
destroyed to unveil the cast item. Other methods of casting include open
casting, bivalve mould, and piece mould. Lost-wax casting was widespread
in Europe until 18th century, when a piece-mold process came to predominate.
Casts can be made of the wax
model itself, the direct method; or of a wax copy of a model that need not be
of wax, the indirect method. These are the steps for the indirect process:
Model-making. An artist or mold-maker
creates an original model from wax, clay, or another material. Wax and
oil-based clay are often preferred because these materials retain their
Mold making. A mold is made of the
original model or sculpture. The rigid outer molds contain the softer inner
mold, which is the exact negative of the original model. Inner molds are
usually made of latex, polyurethane rubber or silicone, which is supported by
the outer mold. The outer mold can be made from plaster, but can also be made
of fiberglass or other materials. Most molds are at least two pieces, and a
shim with keys is placed between the two halves during construction so that the
mold can be put back together accurately. In case there are long, thin pieces
sticking out of the model, these are often cut off of the original and molded
separately. Sometimes many molds are needed to recreate the original model,
especially large ones.
Wax. Once the mold is finished, molten
wax is poured into it and swished around until an even coating, usually about
1/8 inch or 3 mm thick, covers the inner surface of the mold. This is repeated
until the desired thickness is reached. Another method is filling the entire
mold with molten wax, and let it cool, until a desired thickness has set on the
surface of the mold. After this rest of the wax is poured out again, the mold
is turned upside down and the wax layer is left to cool and harden. With this
method it is more difficult to control the overall thickness of the wax layer.
Removal of wax. This hollow wax copy of
the original model is removed from the mold. The model-maker may reuse the mold
to make multiple copies, limited only by the durability of the mold.
Chasing. Each hollow wax copy is then
"chased": a heated metal tool is used to rub out the marks that show
the parting line or flashing where the pieces of the mold came together. The
wax is dressed to hide any imperfections. The wax now looks like the finished
piece. Wax pieces that were molded separately can be heated and attached;
foundries often use registration marks to indicate exactly where they go.
Spruing. The wax copy is sprued with a
treelike structure of wax that will eventually provide paths for molten casting
material to flow and air to escape. The carefully planned spruing usually
begins at the top with a wax "cup," which is attached by wax
cylinders to various points on the wax copy. This spruing doesn't have to be
hollow, as it will be melted out later in the process.
Slurry. A sprued wax copy is dipped into
a slurry of silica, then into a sand-like stucco, or dry crystalline silica of
a controlled grain size. The slurry and grit combination is called ceramic
shell mold material, although it is not literally made of ceramic. This shell
is allowed to dry, and the process is repeated until at least a half-inch
coating covers the entire piece. The bigger the piece, the thicker the shell
needs to be. Only the inside of the cup is not coated, and the cup's flattop
serves as the base upon which the piece stands during this process. Prior to
silica, a mixture of plaster and fireproof material such as chamotte was used.
8. Burnout. The ceramic shell-coated piece is placed cup-down in a
kiln, whose heat hardens the silica coatings into a shell, and the wax melts
and runs out. The melted wax can be recovered and reused, although often it is
simply burned up. Now all that remains of the original artwork is the negative
space, formerly occupied by the wax, inside the hardened ceramic shell. The
feeder and vent tubes and cup are also hollow.
9. Testing. The ceramic shell is allowed to cool, then is tested to
see if water will flow through the feeder and vent tubes as necessary. Cracks
or leaks can be patched with thick refractory paste. To test the thickness,
holes can be drilled into the shell, and then patched.
Pouring. The shell is reheated in the
kiln to harden the patches and remove all traces of moisture, and then placed
cup-upwards into a tub filled with sand. Metal is melted in a crucible in a
furnace, and then poured carefully into the shell. If the shell were not hot,
the temperature difference would shatter it. The filled shells are allowed to
Release. The shell is hammered or
sandblasted away, releasing the rough casting. The spruing, which are also
faithfully recreated in metal, are cut-off, to be reused in another casting.
Metal-chasing. Just as the
wax copies were chased, the casting is worked until the telltale signs of the casting
process are removed, and the casting now looks like the original model. Pits
left by air bubbles in the casting, and the stubs of spruing are filed down and