BRONZE SCULPTURE: THE ART OF LOST WAX
In the third millennium B.C., somewhere between the Black
Sea and the 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 religions and
their social structures. For example:
Chinese bronzes often depicted ceremonial imagery, Indian and Egyptian castings
frequently represented deities, the Africans cast images of nature, and the
Greeks re-created the human Form. Many
of these cultures have grown obsolete, religions have evolved and societies
have changed, but an intriguing visual history survives through the surviving
bronze works. Certain elements of the
"Lost Wax" process have indeed been refined, yet today bronze casting
remains 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
and essentially any other form of sculpture to transform the artist’s vision
THE RUBBER MOLD
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" image of the sculpture in
rubber. The mold is often done in
several sections to facilitate proper and even flow during the actual bronze
THE WAX POSITIVE
The original sculpture is now used exclusively as a
reference point. From the "negative" rubber mold, a wax
"positive" must be created.
Wax is melted to about 210°F, poured into the mold and evenly coated or
"slushed" inside. Slushing is repeated three times using cooler wax
each time to avoid melting the previous coat. Under ideal conditions, the wax
wall will be about 3/16" thick --- any less might create flow problems for
the bronze; any more will result in a heavier than necessary sculpture. When
the mold is opened and the rubber peeled away, an almost perfect wax
reproduction is removed.
WAX CHASING · SPRUING & GATING
"Wax chasing" is the delicate process of joining
the wax pieces back together to form a complete “positive” of the sculpture
(including removing seams and repairing imperfections with heated customized
soldering irons or tools: dental tools being ideal). Artists are very involved
at this juncture, checking the integrity of the wax and, after approving it,
signing the piece.
After the wax is chased and approved by the artist, the
piece is then advanced to "Spruing" or "Gating." This is where channels, through which the
molten bronze will travel to the artwork, are added to the wax version. These channels are also made of wax.
"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
runways 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
"Investment" is the process of building a
rock-hard 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 drying time in between
dips, a hard ceramic shell, about ˝" thick, forms around the wax.
Prior to the invention of ceramic shell, solid plaster
investment was used. To invest by the solid plaster method: tarpaper was
loosely wrapped around the wax reproduction in the shape of a cylinder. The
enclosed space surrounding the wax was then filled with a wet plaster/sand
mixture. When the plaster hardened, the tarpaper was 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. Plaster built shells are "de-waxed"
in a high-pressure steam chamber known as an autoclave; ceramic invested shells
are de-waxed in a kiln.
A huge graphite crucible, fired by a furnace, is filled with
bronze ingots that are melted. The metal begins to melt at 1700°F. 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 1100°F.
When the "Dance of the Pour" begins, the crucible
is lifted by 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 that 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. The
alloy cast at Artworks is known as Silicon Bronze. The metal is made up of the following elements: COPPER 94.0%,
MANGANESE 1.1%, SILICON 3.9%, TRACE ELEMENTS 1.0%. Silicon is an additive that helps the "flowability" of
the bronze. It achieved widespread use during World War II when lead and tin
were in short supply.
"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
advances 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 to support
the bronze "skin." Many larger than life-size sculptures are analyzed
by a structural engineer who recommends an inner support structure that can
reinforce the piece to withstand earthquakes and high winds.
Patination is enhancement of bronze by the chemical
application of color. Three water soluble compounds form the basis for most
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. Wide ranges 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.
We do NOT do bronze
casting. Cantact your local art foundry or
Special thanks to
Piero Mussi, Mavis McClure and Artworks.