The Process: Working in Bronze

Lost Wax Process (Cire Perdue)

Clay to wax to bronze, the lost wax process is a method for changing a sculpture made of soft clay into a harder material such as bronze. Changing the clay to bronze involves two molds for each sculpture. The first mold (made of plaster and rubber) yields a wax cast of the sculpture. The hand pouring of the wax determines the thickness, and therefore the weight, of the final bronze.

Immersing the wax cast in a liquid ceramic material forms the basis of the second mold. When hardened, the material can withstand the high temperature of the molten bronze. Next, the mold is heated until the wax melts out or disappears. Because the wax disappears, it becomes "lost wax" as the name of the process implies. The resulting empty space left by the lost wax in the second mold thus forms a "bucket", shaped just like the sculpture, which is later filled with molten bronze.

The first mold is called the master mold because it is taken from the clay original. It is made of special rubber supported by a plaster shell. The rubber registers the details sculpted in the clay. When the soft clay sculpture is removed from the master mold (becoming ruined in the process) the resulting space is filled with hot wax.

When the reworked wax is perfect, it is encased in the second ceramic mold (figure 6). This mold is made of ceramic material because it must withstand the high temperature of molten bronze (about 2,200°F).

Next, the ceramic mold with the wax cast inside is placed in an industrial oven to melt out the wax. Thus, the wax becomes "lost wax" as it melts out and disappears. Molten bronze is then poured into the space left by the melted wax. After cooling, the mold is carefully hammered apart to expose the bronze casting. The pouring gates must be ground off, and the bronze perfected by careful welding, filing, grinding, and sanding.

At this stage, Gesso again visits the foundry to add the finishing touches that will make the bronze meet his exacting standards. When she considers the bronze perfect, the patina, or color, is added. This is hand-applied by heating the bronze and applying combinations of chemicals that control the color of the natural oxidation process. Careful attention to the patina results in deep, rich colors which become a permanent part of the sculpture. Finally, the edition number and the foundry mark are hammered into the bronze and the sculpture is waxed to protect the patina.

The laborious casting process requires a wax mold to make the bronze must be repeated for each casting or sculpture part. Gesso Cocteau would not do otherwise. She is convinced that the casting times and energies involved (usually about eight weeks for a small sculpture) are well worth the effort. For once in bronze, the sculpture attains a permanency, a kind of immortality that spans the generations and centuries. Perhaps, one day in the distant future, it may help explain to people yet unborn what it was like to live a life today.