The Art of Lost Wax Bronze Casting
History
In the third millennium B.C., 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 melted and was replaced by an 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.
For most of history, solid investment consisting of plaster, sand and water was used to cast bronze. To use the solid investment method a form 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 form is removed and the solid plaster investment mold is ready for firing. After the mold is fired molten bronze is poured into it.
A new technology called “Ceramic Shell” has become the industry standard for casting bronze sculptures because it can hold better detail. Below is a description of how Artworks Foundry uses the “Lost Wax Ceramic Shell Casting Method”.
Rubber Mold
The original is divided into sections with clay or shims and a mold release is applied to the surface. A two part rubber is mixed and applied to the surface by spraying or with a brush. After the rubber is built up to the desired thickness and cured, a release agent is applied.
Mother Mold
A supporting mold (mother mold) is constructed over the rubber with FGR plaster and fiberglass mat. The mold is then removed, rendering an exact "negative" image of the sculpture in rubber. The rubber mold is now ready for wax.
Wax Positive
From the "negative" rubber mold, a wax "positive" is created. Wax is melted to about 200° F, poured or painted into the mold and evenly coated. This is repeated three times using cooler wax each time to avoid melting the previous coat, until the wax is approximately 3/16" thick. When the mold is opened and the rubber peeled away, an almost perfect wax reproduction is removed.
Wax Chasing
“Wax chasing" is the delicate process of removing seams and repairing imperfections with heated customized soldering irons or tools. Artists are encouraged to visit the foundry at this point to sign and check the integrity of the wax.
Gating or Spruing
The “Gates” and “Sprues” are also made of wax. They form the channels through which the wax will exit the mold and molten bronze will 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 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.
Ceramic Shell
The ceramic shell technique begins by dipping the gated wax into vats of ceramic slurry followed immediately by a bath of sand. This process builds a very thin wall of silica around the wax. When repeated several times, allowing for drying between dips, a hard shell about ¼” to ½" thick forms around the wax
Dewax
The wax 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. The ceramic shell molds are "de-waxed" in a large kiln and fired at 1600° F.
The Pour
While the bronze ingots are melting in the melt furnace between 2000° F and 2100° F the cermic shell molds are being heated in a kiln to about 1300° F.
When the "Dance of the Pour" begins, the crucible is lifted by a crane out of the 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 alloy cast at Artworks is known as Everdure Silicon Bronze. The metal is made up of the following elements: copper 95.0%, manganese 1.0%, silicon 3.9%, trace elements 0.1%. 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 advances to the metal shop.
Metal 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". Most larger than life-size bronzes are analyzed by a structural engineer who recommends a support structure that can withstand earthquakes and high winds.
Patina
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 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.