The lost wax method is an ancient method of casting bronze. Few changes have been made over the centuries. It is still considered to be the only acceptable method to cast sculptural art.
The process begins as the artist creates his sculpture in clay or wax. When complete, the artist makes a mold of the sculpture. Today this mold is generally made of polyurethane rubber. The rubber is painted on the sculpture in layers and each coat is allowed to “set up” before another is applied. A plaster or fiberglass “mother mold” is then applied over the rubber by adding thin coats until the desired thickness is achieved.
The artist has taken care to plan where the “parting lines” of the mold will be prior to applying the rubber and plaster in order to remove the original clay sculpture, and later the wax positive duplicate of his sculpture.
After the clay original has been removed, the rubber mold is cleaned thoroughly. The mold is then assembled and held tightly together by bolts. Melted wax is poured in the negative cavity of the mold and swirled gently to cover the interior surface. The wax is then poured out. This is repeated until the desired thickness is achieved.
The wax is removed from the mold. The parting lines are removed and the “wax positive” is made as perfect as the original clay sculpture. The wax positive is then dipped in ceramic slurry. A shell is built up around the wax by repeated dipping, allowing time to dry between each. The shell is usually about one inch thick.
After the shell is thoroughly dry the wax is burned from the ceramic shell in a kiln at a temperature of 1000 degrees Fahrenheit. Immediately the molten bronze, at 2200 degrees, is poured into the ceramic shell. The wax is lost and the bronze takes its place.
When the bronze has cooled the shell is broken from the casting shell with hammers. The bronze is cleaned up by sand blasting, welding and grinding. A final careful sand blasting and polishing then takes place.
The final step requires the application of specific chemicals and heat to achieve the desired patina. This delicate process provides the rich surface color chosen for the finished piece.