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BRONZE SCULPTURE – THE ART OF LOST WAX
In the third millennium BC, somewhere between the Black Sea and
Persian Gulf, an artist crafted a vision in beeswax, covered it
in liquid clay and cooked it in a fire. In the flames the wax was
lost, replaced by empty space. Tin and copper –alloys of bronze–
were gathered and heated. Once melted, the metal was poured into
the cavity of the fire hardened clay. The metal cooled and the sculptor
knocked the clay from the metal. The first bronze was cast.
Ancient “Lost Wax” bronze castings have withstood the
centuries, visually telling the tale of past cultures, their religion
and their social structures. For example, Chinese bronzes depicted
ceremonial images; Indian and Egyptian castings symbolized deities;
Africans cast images of nature; and the Greeks re-created the human
form. Many of the cultures have grown obsolete, religions have evolved
and societies have changed. Elements of the “Lost Wax”
process have been refined. Yet today, bronze casting is essentially
the same as it was in 2,000 BC during the Akkadian period.
Modern sculptors who want their pieces cast in bronze depend upon
a Foundry. There, artisans skillfully apply the “Lost Wax”
method to wood, stone, clay, plaster or any other kind of Sculpture
to transform it into bronze.
THE RUBBER MOLD – THE WAX
"POSITIVE"
The metamorphosis of a sculpture from the original medium into bronze
begins with a rubber mold.
The original sculpture must remain stationary during the mold making
process. To accomplish this, half of the sculpture is nestled into
a base of soft plasticine clay, the other exposed half is painted
evenly with a clear, viscous rubber. (Polyurethane rubber is best
for single or small editions while larger editions require silicone
rubber.) When the half painted with rubber dries, a protective and
rock hard "mother mold" made of reinforced plaster is
built around the pliable rubber. The sculpture is then turned over,
and the process repeated. When the second side is complete, the
mold is opened and the original removed from within. The rubber
is rejoined with the other half, rendering an exact "negative"
rubber mold, a wax "positive" is created.
WAX CHASING
"Wax chasing” is the delicate process of joining the wax
pieces, removing seams and repairing imperfections with heated customized
soldering irons or tools – dental tools are ideal.
After the wax is chased and approved by the artist, the piece is
then advanced to “Spruing” or “Gating.” The
gates and sprues are also made of wax. They form the channels through
which the melted bronze will give travel to the artwork.
“Vents” (thin wax sticks) and “Gates” (thicker
wax sticks) are affixed to the wax reproduction with heated tools.
Later in the casting process, the space occupied by sprues or gates
become runaways through which the metal flows and trapped gas escapes.
Distribution of the bronze, low turbulence, ventilation and shrinkage
are important considerations in the science of gating and spruing.
INVESTING
“Investment” is the process of building a rock-hard
like shell around the wax sculpture. Later in the process, when
the wax has been melted out, the investment will serve as a mold
for the molten bronze. For most of history, an investment consisting
of plaster, sand and water was used to accomplish this task. In
the last 15 years, a new technology called ceramic shell has become
the industry standard.
The ceramic shell technique begins by dipping the gated wax into
vats of slurry followed immediately by a bath of sand. This process
builds a very thin wall of silica around the wax. When repeated
approximately 9 times, allowing for dry times in between dips, a
hard shell about 1/2"thick forms around the wax.
Prior to the invention of ceramic shell, sold plaster investment
was used. To invest by the solid plaster method, tar paper is loosely
wrapped around the wax reproduction in the shape of a cylinder.
The enclosed space surrounding the wax is then filled with a wet
plaster/sand mixture. When the plaster hardens, the tar paper is
removed and a solid plaster investment is ready for “de-wax.”
Whether ceramic shell or plaster is used to make the shell, the
wax is a “positive” which must disappear in order to create
a cavity or "negative" for the bronze to fill. Thus the
phrase “lost wax casting” comes from the process of the
wax being melted or “lost” from the shell. Ceramic built
shells are “de-waxed” in a high pressure steam chamber
known as an autoclave; plaster invested shells are de-waxed in a
kiln.
THE POUR
During the pour the ceramic shell mold receives the bronze. A huge
graphite crucible, fired by a furnace, is filled with bronze ingots
that are melted. The metal begins to melt at 1700ºF. Bronze
“seizes” (stops flowing) when confronted with cold, which
might occur if molten bronze was poured into a room temperature
shell; therefore at the same time the bronze is being blasted by
a natural gas furnace, the ceramic shell is heated in a kiln to
approximately 1100ºF.
When the Dance of the Pour begins, the crucible is lifted by a crane
out of the gas furnace. At the same time, the glowing ceramic shells
are brought out of the kiln to the pour area. Two artisans operate
the crane which holds the crucible in a “jacket.” The
artisan with the controls is the “lead pour,” the artisan
maintaining the crucible balance is known as the "deadman."
A third member of the pour team pushes away dross and slag on the
surface of the molten bronze.
The entire pour is very fast and very precise; one crucible of bronze
holds 400 lbs and can fill one or two large shells or ten or more
small shells. The first pieces poured are those with thin walls
and intricate details; requiring hot, fluid bronze to move throughout
the channel system.
BRONZE INGOT
Silicon Bronze is an alloy metal made up of the following elements:
Copper 94.0%
Manganese 1.1%
Silicon 3.9%
Trace Elements 1.0%
Silicon is an additive which helps the “flowability” of
the bronze. It achieved widespread use during World War II when
lead and tin were in short supply.
DEVESTING
"Devesting” is the process during which the investment
is removed from the metal.
Approximately one hour after the pour, the piece is cool enough
to handle. Skill and strength are combined with hammers and power
chisels to knock the investment off the freshly solidified metal.
The gates and sprues must also be removed with a high intensity
electric arc that can cut through the bronze like butter.
The final step is to sandblast the fine investment from the bronze.
When clean, the sculpture can advance to the metal shop.
METAL CHASING & FINISHING
Like wax chasing, bronze must also be chased or cleaned to address
the slight imperfections that may result from the casting or shell
building process. On larger sculptures, where assembly of cast sections
is required, chasing is essential to take down weld line formed
by the joining of two planes.
Metal chasing usually starts with large electric or pneumatic grinders
to remove the bulk of the unwanted metal. Then, more refined and
smaller tools such as die and pencil grinders are used to re-create
the artist's subtle surface texture.
Much as a house needs a wood frame to stand, many monumental bronzes
require a stainless steel internal structure that can withstand
earthquakes and high winds.
PATINATION
Patination is enhancement of bronze by the chemical application
of color. Three water soluble compounds form the basis for patinas:
Ferric Nitrate produces reds and browns, Cupric Nitrate creates
the greens and blues and Sulphurated Potash produces black.
Each foundry develops its own proprietary (and carefully guarded)
patinas that result from a carefully orchestrated blend of different
chemicals, pigments and application technique. A wide range of colors,
both transparent and opaque are available to the experienced patineur.
The final step is putting a thin coat of clear wax over the bronze
to enhance and preserve the patina.
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