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 essentially
any form of sculpture to transform the
artist’s vision into bronze.
THE WAX POSITIVE
The original sculpture that your young
artist makes is a wax “positive”.
WAX SPRUING & GATING
After the wax is created 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 sculpture. These
channels are also made of wax.
“Vents” (thin wax sticks) and “Gates”
(thicker wax sticks) are affixed to the
wax sculpture 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
and spruing.
INVESTING
“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 ½”
thick, forms around the wax.
The wax is a “positive” which now 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
invested shells are “de-waxed” in a kiln.
THE POUR
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. 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
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 the foundry 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
“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
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.
PATINA
Patination is enhancement of bronze
by the chemical application of color.
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. Your
young artist’s patina will be burnished
so that it will reflect the true color of
bronze. Over time, it will age and
oxidize naturally to the environment to
which it is located.
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 essentially
any form of sculpture to transform the
artist’s vision into bronze.
THE WAX POSITIVE
The original sculpture that your young
artist makes is a wax “positive”.
WAX SPRUING & GATING
After the wax is created 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 sculpture. These
channels are also made of wax.
“Vents” (thin wax sticks) and “Gates”
(thicker wax sticks) are affixed to the
wax sculpture 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
and spruing.
INVESTING
“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 ½”
thick, forms around the wax.
The wax is a “positive” which now 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
invested shells are “de-waxed” in a kiln.
THE POUR
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. 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
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 the foundry 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
“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
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.
PATINA
Patination is enhancement of bronze
by the chemical application of color.
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. Your
young artist’s patina will be burnished
so that it will reflect the true color of
bronze. Over time, it will age and
oxidize naturally to the environment to
which it is located.
