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History
Metal casting is one of the earliest of all metal
working methods. Prehistoric casting was completed
by carving an impression of the implement into the
flat surface of a large stone. A groove was cut
from the edge of the stone to the carved form,
serving as a sprue, or vent. Another flat stone was
placed on top forming a container, and the molten
bronze or copper was poured in through the sprue.
Lost wax casting, also known as circ perdue was
possibly used as early as 5000 BC in India. Many
civilizations seem to have developed the same basic
methods from the Greeks and Romans, to the
Egyptians and Chinese. With the Industrial
Revolution came improvements in materials and
methods which improved the certainty of a
successful casting. The work made by Rodin for
example, reflect this new confidence in his free
style of modeling form and surface texture from
clay as compared to the intricately detailed work
of earlier sculptors who probably modeled directly
in wax, casting the master original, and then
followed up the sharp detail with hours of metal
tooling. Though methods are becoming more and more
refined, and innovative new approaches are being
discovered, the basic techniques of lost wax
casting remain much the same as centuries ago. Like
Rodin, whose work would be sent from his studio to
the foundry and returned as an accurate
reproduction in bronze, today’s artist can be
assured of the same accuracy through the use of a
professional art foundry.
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Making the
Mold
The original sculpture should be of a hard material
such as hardened clay, plaster, stone, or wood. The
mold maker examines the sculpture and decides how
to divide the master into smaller components for
casting. The number of divisions depends on the
complexity of the design, the volume capacity of
the crucible, and the manageability of the invested
mold. (Sculptures directly modeled in wax may be
cast without having a mold made, however, the
artist must realize the risk in the possibility of
casting mishaps. This practice is greatly
discouraged.) The mold most often used involves
applying a flexible molding compound such as latex
rubber or silicone rubber onto the original,
building to a ¼" thickness, then covering the
flexible mold with a rigid material called a
"mother mold" made of plaster or fiberglass. Once
the mold is prepared the original sculpture is
removed and a wax original is poured. Thicknesses
of less than one inch are poured solid. Greater
thicknesses are poured with a hollow core to allow
for shrinkage when cast, as well as conserving on
the amount of metal used. Preparing the Wax The wax
original is prepared following de-molding by
chasing seam lines, touching up and smoothing the
surface. Since the wax will serve as the pattern
for the finished bronze, it is important to check
it carefully before moving on to the next step. At
this point the casting is etched with an edition
number shown over the total edition number (e.g.
1/5). Having an edition allows the artist to spread
the casting and mold costs over the whole series.
Then the pouring gate is fitted, which is the
system of channels through which the molten metal
flows to ensure a quick, smooth flow without
chilling, and to allow gases and air to escape.
This is done by melting wax rods, or sprues, to the
wax original with a hot metal tool and attaching a
wax pouring funnel.
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Ceramic
Shell
Next the ceramic shell is applied. This process has
nothing to do with ceramics, but is a material that
allows an investment mold wall thickness to be as
little as ¼" like a ceramic piece. The
prepared wax, complete with the pouring gate and
cup, is evenly coated with a slury made of
colloidal silica and silica flour, then left to dry
and harden in a dry atmosphere. The first layer
picks up all the detail to provide an accurately
cast surface. Subsequent layers of ceramic shell
are then applied, including large particles of
fused silica which eventually build up to a
stucco-like appearance. Each application must be
allowed to dry and harden before another coat is
applied. When the desired wall thickness has been
achieved, the entire piece must be left to
harden.
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De-Waxing
Removing the wax is the next most important step in
the process. The prepared ceramic shell is placed
in a kiln pre-heated to about 1100 degrees (f). The
wax is melted from its outer surfaces inward to
prevent the expansion of the wax and cracking of
the shell. The melted wax is collected in a chamber
below the kiln and may be used again. Once the mold
is completely free of wax, the shell is heated to
over 1400 degrees (f) and held at that temperature
to burn off any carbon.
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The Molten
Bronze
Bronze is an alloy, or metallic compound formed by
blending copper and tin sometimes combined with
small amounts of zinc, lead, or nickel. Silicon
bronze is commonly used in art casting and is an
alloy of 92% copper 4% silicon and 4% zinc. The
melting point is 2190 degrees (f). Bronze ingots
are packed into the crucible which is placed in the
center of the furnace. Once the metal is molten,
the crucible is lifted from the furnace using
special tongs and placed into the pouring shank.
The slag and impurities which have floated to the
surface are skimmed off with an iron bar before the
molten metal is poured. The burned out ceramic
shell is removed from the hot kiln and placed
securely in a pit of sand. The molten metal is then
poured quickly and steadily to prevent any
chilling. Once the pouring is complete the casting
is allowed to cool naturally.
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The Big
Moment
Once the casting has cooled, the ceramic shell is
broken off using a hammer to reveal the metal
casting complete with the pouring gate and cup.
Remaining shell chips are removed by sandblasting
the piece. The foundry staff checks for chill marks
and other imperfections to make sure the casting is
in good shape for the finish work.
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Finishing
Initially, the bronze casting appears dull,
mottled. The pour gate must be removed and the
points of attachment to the sculpture must be
chased clean. Separately cast parts are welded
together using bronze welding rod, and the surfaces
are ground to match the original form. Minor
casting imperfections may be repaired at this time.
The final finish, is patination, or application of
color and texture. A wide variety of chemicals,
pigments and paint can be used to create various
effects. Often heat is applied to the sculpture
with a propane torch while simultaneously applying
chemicals such as iron, cobalt, and silver nitrate
through a sprayer or with a paint brush. Once the
patina is complete, paste wax is applied to the hot
sculpture with a brush, which is buffed to a soft
shine when cooled. The artist may then choose to
have the piece placed on a stone or wood base, or
perhaps left free-standing.
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Phone (360) 427-3857