M a r y S a n d S t u d i o

Bronze Casting Process


Hot bronze casting using the lost wax method vs. cold cast or bonded bronze.

Cold cast bronze, also referred to as bonded bronze, resin bronze, or faux bronze is achieved by casting with a resin plastic, adding a metallic powder to create the appearance of bronze. The resin is cast directly into the rubber mold, taken from the original clay figure. This eliminates labor intensive steps associated with "hot bronze casting" using the lost wax casting process as described below. In my work, I strive to achieve the highest quality possible and unfortunately I've never been satisfied with the end result that is achieved with cold cast bronze.

Below is a detailed description of the lost wax casting process that is used for casting each of my sculptures. With each individual casting, I pour and chase the wax sculpture, removing seams from the moldmaking process and imperfections that may have arisen from pulling the wax out of the mold. The wax is then shipped to Artworks Foundry in California where they proceed with spruing, gating and investing. Once the ceramic shell is complete, the bronze is poured and devested. I then fly out to California, inspect and chase the metal and supervise the patina.


Graciously provided by ARTWORKS FOUNDRY

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 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. Artworks Foundry, in Berkeley, CA, is a world renowned foundry that casts many prominent artists' work.

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.)

The mold making process

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.

Image of Wax

The original sculpture is now used exclusively as a reference point. From the "negative" rubber mold, a wax "positive" is 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" 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. Artists are encouraged to visit the foundry at this point to sign and check the integrity of the wax.
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 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.      

"Investment" is the process of building a rock-hard shell around the wax sculpture. Later, 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. Artworks Foundry converted to a 100% ceramic shell system in 1995.

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 ½" thick forms around the wax.

Prior to the invention of ceramic shell, solid 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.


Pouring the bronze


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.
Pouring the bronze
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.
Pouring bronze

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 which 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

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.


Applying the 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 (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.

At Artworks, we have successfully done our job if we translate your clay sculpture to bronze while preserving the integrity of the original.  To help ensure this, we strongly encourage your participation throughout the entire process. We realize that quality cannot be delivered unless we are listening to the Artist before the mold is made and then again in wax, metal and patina stages. rtworks is committed to making your project as interactive as you desire.

Artworks' exclusively uses the lost wax casting technique using ceramic shell technology. Our services include enlargement, rubber molds, casting, metal finishing, fabrication and patina. Our patineurs can create magical patinas while our metal finishers can make bronze defy gravity with complex internal structures. We are fully capable of undertaking the design, plan and implementation of monumental works including shipping and installation worldwide.


Copyright © 1997 - 2015 Sand Seelmann Sculptures, Mary Sand Studio - All rights reserved