Cire-perdue (Lost wax) Process
Derek Stuart uses the “cire-perdue” (lost wax) process to create his cast sculptures in bronze and glass.
The process entails a number of steps:
- The sculptural piece must first be created in wax. This can be achieved by modeling the wax with hand tools or by taking a plaster or silicone rubber mold from an existing object and subsequently casting a wax replica of the chosen object.
- The wax piece to be cast in glass or bronze is then encased with a coating of a refractory mixture (often Plaster of Paris, silica sand and other materials). This process is referred to as “investing”.
- The investment (wax + mold coating) is then placed in an autoclave, a pressure vessel similar to a “pressure cooker” where pressurized steam at 250 to 300 degrees F, melts the wax, leaving a hollow refractory shell with a negative void replicating the initial wax sculptural object.
- The investment mold contains up to 50% water by weight. Any excess water must be eliminated from the mold by slowly drying, sometimes for many weeks, depending on the size of the mold.
- The water not removed by room temperature drying is eliminated by slow controlled heating of the mold in a kiln. The final drying temperature can be as high as 1000 degrees F
- The next stage of production to cast bronze entails the melting of bronze, pouring molten metal into the refractory mold. The bronze cools in a few hours. The mold material is removed and the metal surface is finished using hand and power tools. Often a chemical “patination” is used to colour the bronze surface.
- The next stage of production to cast glass is to provide molten glass with which to fill the mold. A glass kiln, similar to a pottery kiln, is used to provide the high temperature environment.
- The mold is positioned in the kiln and often a flower pot is used as a refractory vessel. The vessel is positioned above the mold entrance into which the molten glass will flow. Glass, either ingots weighing about 1 kilogram each or smaller pieces of fractured glass (frit) are placed in the flower pot. An alternative method is to fill the mold with finely powdered glass of the desired colours.
- The kiln is turned on and as the temperature reaches approximately 1450 degrees F, the glass material melts. The molten glass flows out the hole at bottom of the pot into the heated refractory mold. The powdered glass simply melts and fills the mold.
- The mold and glass are then cooled under very controlled conditions called annealing. This process can take from 8 hours for ½ inch thick glass, up to months and years for thicker pieces. (The 18 inch thick, 200 inch diameter glass mirror blank for the Mt. Palomar telescope took 2 years to cool). A typical sculpture of 3 inches thickness requires a casting/annealing cycle of 100 hours.
- When the sculpture containing mold has cooled to room temperature, the encasing mold structure is very carefully removed, by chipping, from the fragile glass sculpture.
- Often in the casting process small cracks or shifts develop in the mold material resulting in thin “fins” of glass projecting from the surface of the sculpture. The removal of the excess glass on the cast surface is accomplished with diamond coated files and grinding bits, very similar to those used by your Dentist. Remnants of such surface features are indicative of a “hand tooled” original glass piece.
- The surface of the glass may then require further treatment with an acid etchant to bring about the final textural appearance desired by the Artist.
- A final surface treatment with a polishing/finishing wax, similar to furniture wax brings out the intrinsic beauty of the cast glass surface.