The Rubber Mold – The Wax “Positive”
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 silicone rubber. 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” rubber mold. A wax “positive” is then created.
“Wax chasing” is the process of joining the wax pieces, removing seams, and repairing imperfections with heated customized soldering irons or tools – dental tools are ideal. 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 give 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 like shell around the wax sculpture. Later in the process, 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. 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 nine times, allowing for dry times in between dips, a hard shell about 1/2″ thick forms around the 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, or in a kiln.
During the pour, the ceramic shell mold receives the bronze. A huge graphite crucible, fired by a furnace, is filled with bronze ingots that are melted. The metal begins to melt at 1700ºF. 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 1100ºF.
When 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 that holds the crucible in a “jacket.” The artisan with the controls is the “lead pour” and 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 400lbs 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 which require hot, fluid bronze to move throughout the channel system.
“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 can advance to the metal shop.
Metal Chasing and 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 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.
Much as a house needs a wood frame to stand, many monumental bronzes require a stainless steel internal structure that can withstand earthquakes and high winds.
Patination is enhancement of bronze by the chemical application of color. Three water-soluble compounds form the basis for 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.
The sculpture used as an example of all the stages of the casting process is “Avalon’s Legacy” by Tanya Ragir. To see the piece after installation, please click here. To see more of Tanya Ragir’s work click here, or here for her personal website.