 THE PROCESS: LOST-WAX CASTING
Many sculptors choose to create in clay because of the ease with which an artist can shape and move the material. Since clay is not a durable, permanent material, the sculptor will likely have a translation made into a lasting medium such as bronze. This bronze may be unique or part of an edition. The following is a simple outline of the process Sioux Falls' BronzeAge Art Casting foundry uses to translate a sculptors work into bronze. An artist's wood, stone, plaster, clay or any other sculptural medium can be replicated (cast) in bronze. MAKE A RUBBER MOLD The moldmaker creates a rubber mold of the original sculpture by applying layers of liquid rubber to surface of the artwork. After the rubber cures, this flexible mold is reinforced or supported by a plaster "mother mold." A simple sculpture like a portrait might have a mold in two halves. A sculpture with more complex shapes or of a larger scale would have multiple sections of mold. CREATE WAX "POSITIVE" OF SCULPTURE Next, a wax reproduction of the original sculpture is created by pouring hot wax in and out of the rubber mold to build up a thin layer of wax inside the rubber mold. If the mold is of a larger or more complex sculpture, wax may be poured over flat or unusually shaped panels to build up a thickness of wax. After the necessary thickness of wax is built up, the wax is carefully removed from the rubber molds. These hollow wax reproductions, with exact likeness to the original sculpture, now need to essentially be "turned into bronze." CHASE THE WAX SCULPTURE Next, any seamlines or imperfections in the wax sculpture need to be "chased." If a sculpture is large or complex, it may consist of separate pieces of wax that will be cast separately and then welded together later in the bronze. SPRUING THE WAX SCULPTURE A series of wax channels (sprues, gates, runners and vents) and a funnel shape (pour cup) need to be attached to the wax sculpture before it is encased (invested) in another type of mold material. Later in the casting process, the space occupied by the wax funnel and channels become runways through which the molten bronze flows and trapped gas escapes. MAKE THE CERAMIC-SHELL MOLD: INVESTING "Investment" is the process of building another kind of mold around the wax sculpture. Later in the process, when the wax has been melted out, the investment (in this case, ceramic shell) will serve as a mold for the molten bronze. The ceramic shell technique begins by dipping the wax sculpture (with all the wax channels and the funnel attached) into a tank of liquid colloidial silica (liquid ceramic). This is followed by the application of silica sand (stucco) to the wet surface. This process builds a very thin wall of silica around the wax. When repeated eight or more times, allowing for drying time in between dips, a ceramic shell forms around the wax. DEWAX THE CERAMIC-SHELL MOLD The wax sculptures, now encased (invested) in ceramic shell, are put upside-down in a kiln to allow the wax to melt out. The high heat in the dewax kiln melts all of the wax from inside the ceramic shell, leaving an empty cavity wher the wax had been. Hence the name "lost-wax casting" to refer to this casting method. This hollow cavity is where the molten bronze will be poured. POUR THE BRONZE Bronze ingots begin 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 ceramic shell; therefore at the same time the bronze is being blasted by a natural gas furnace, the ceramic shells are pre-heated in a kiln. The ceramic shell must also be vitrified at high heat to gain its full strength. The shells are removed from the kiln and the 2150 degree Fahrenheit metal is carefully and quickly poured into the ceramic shells. Bronze is mainly composed of copper 94%, followed by silicon 4%, manganese 1%, and trace elements 1%. The addition of silicon helps the bronze flow more readily. BREAK THE CERAMIC-SHELL MOLD: DEVESTING "Devesting" is the process during which the investment (ceramic shell) is removed from the metal. Approximately one hour after the pour, the piece is cool enough to handle. Using hand and air tools, the now weakened ceramic-shell is knocked off the freshly solidified metal. The final step is to sandblast the any remaining shell still stuck to the bronze. When clean, the now bronze sculpture needs to be "chased." METAL CHASING & FINISHING The sculpture now has bronze channels and a bronze pour cup attached. These will be cut away with a plasma cutter. Like wax chasing, the 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, any welds made in the bronze will need to be "chased" with grinders, chisels, etc. Monumental bronzes may require a stainless steel internal structure to support the bronze. APPLY A PATINA A patina may by added to the surface of the bronze to create different types of coloration on the surface of the bronze. Patinas are chemicals that oxidize the surface of the metal. The beautiful green on the Statue of Liberty or a copper penny is an example of a patina. Bronze does not rust, only the surface changes with exposure to different elements. THE HISTORY OF LOST-WAX CASTING 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 faded, 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.
|
