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Bronze Foundry: Jock Hildebrand Arts Portal
The Process
BRONZE
Bronze is a metal compound containing copper and other elements. The term bronze was originally applied to an alloy of copper containing tin, but the term is now used to describe a variety of copper-rich material, including aluminum bronze, manganese bronze, and silicon bronze.
THE POSITIVE
The starting place for a bronze sculpture starts with a positive or original sculpture made from virtually any material (I generally use plaster or wax) from which a mold is taken. The mold will duplicate exactly and in a negative form the original positive.
THE FLEXIBLE MOLD
Most often, I will use a silicon or rubber mold. The positive will be coated with silicone or rubber mold which itself will be then coated with a hard "mother" mold which will help the soft flexible mold keep its shape. Then it is removed from the positive. This mold allows many waxes, which comprises an "edition" of bronzes to be made.
THE WAX POSITIVE
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. Under ideal conditions, the wax wall will be about 3/16" thick . 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, 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.
INVESTING
"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. 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.
The wax is a "positive" which must disappear in order to create a cavity or "negative" for the bronze to fill. The term "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 kiln.
THE POUR
A graphite crucible, fired by a furnace, is filled with bronze ingots that are melted. The metal melts to 2050 °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 pour begins, the crucible is out of the gas furnace. At the same time, the glowing ceramic shells are brought out of the kiln to the pour area.
The entire pour is very fast and very precise; one crucible of bronze holds 200 lbs and can fill one or two large shells. The first pieces poured are those with thin walls and intricate details; requiring hot, fluid bronze to move throughout the channel system.
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 Plasma Cutter that can cut through the bronze like butter. The final step is to sandblast the fine investment from the bronze.
CHASING THE BRONZE
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 lines 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.
Many monumental bronzes require an internal structure to support the bronze "skin".
PATINA
Patination is the coloration of bronze by the chemical and heat application of color. The last step is the application of a coat of clear wax over the bronze to enhance and preserve the patina.
How to Order
If you are interested in having something cast at my foundry, please contact me and I would be happy to provide you with a quote.
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