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The Bronze Casting Process

Lost wax bronze casting is an ancient process, first used by the Egyptians over 2000 years ago. While the basic process is still the same, many refinements have been made over the centuries.

Mold making is the first step to creating a fine art bronze casting. The original sculpture, typically in either oil or water based clay or in wax, is molded in rubber before being recreated in wax. This rubber mold is necessary for editions and because you cannot directly cast anything other than wax. It is at this point that the original is often cut up into smaller sections to make the molding and casting process more accurate. Our mold making department is well-know for it's meticulous attention to detail and quality.

original clay model rubber mold

Recently, we have added polyurethane with plastic shell mother molds to our department. painting on mother mold This type of mold making is especially useful for preserving the detail on small, very detailed originals and to create high quality and long lasting molds, capable of making large editions. finished mother mold

Once the rubber molds are finished and the original is removed, the molds are reassembled and hot wax is poured into the resulting cavity. When the proper thickness is achieved, the extra hot wax is poured out. After the wax has cooled it is removed from the rubber mold and the surface of the wax is finished by hand until it matches that of the original sculpture. sprued wax Sprues are then added and the wax copy is ready to go the the ceramic shell room.

We currently use ceramic shell molds rather than the traditional investment molds because ceramic shell offers a much finer quality and detail on the casting with a minimum of defects. Once the wax copies are sprued, they are dipped in a layer of a liquid ceramic material, then covered in fine sand. This process is repeated over and over until a layer of ceramic shell about 1/4" thick is built up around the wax.
ceramic shell molds removing ceramic shell molds from kiln burned out ceramic shell molds

After drying completely, the ceramic molds are placed in a kiln and heated to 1650 degrees F. The wax melts out of the ceramic molds and the ceramic is fired to a hard shell. The resulting cavity is then ready to fill with molten bronze.

Carolina Bronze uses only silicon bronze for all of our castings. The resulting sculpture is free from many of the defects found in castings of just a few decades ago. After the ceramic molds have been fired and while they are still hot, molten bronze is poured into the cavity. The bronze must be melted to 2100 degrees F. to make sure the bronze remains liquid as it courses through the intricate parts of the ceramic mold. The bronze cools quickly and the ceramic shell material can soon be knocked off of the cool bronze. Any remaining ceramic material is sandblasted away to reveal the new bronze sculpture.
pouring bronze bronze castings welded bronze castings

After the sculpture has been cast, the metal finishing process begins. Grinders remove sprues and sanders start to finish the surface of the piece. Most sculptures are cast in several sections which now need to be welded back together. Core pin holes and other small defects are also welded over and the surface is then sanded and chased by hand until all evidence of seams and holes is removed and the surface texture is matched to the original sculpture.

Once the metal work is finished, the patina is added through the use of a variety of chemicals and heat. Many different colors are possible through this process. An application of wax adds the final touch to the finished bronze.