The Foundry Process

The foundry is where the finished clay sculpture is converted into bronze. One original sculpture is created by the artist in clay. Through the foundry process multiple copies can then be cast in bronze.

The first step at the foundry is to create a mold. This is a negative version of the original sculpture from which all future reproductions will be made. Depending on the complexity of the work, the original may be cut apart and individual parts molded separately. Liquid rubber molding compound is painted or poured onto the clay sculpture. The adhesive-like rubber fits into every tiny crevice and detail down to a fingerprint. After the rubber sets, the mold is separated from the clay. The original sculpture is destroyed in the process, but the clay can be reused.

A wax replica is created next. Hot, liquefied wax is poured into the rubber mold until all the crevices are filled. After the wax has cooled, the mold is removed. The mold is then ready to be poured again. If individual parts of the original were molded separately, these wax parts are now reattached. The work is chased, or cleaned of mold lines and other imperfections, resulting in a perfect wax replica of the original. A wax replica must be made for each final bronze copy that is ordered. If the sculpture will have numbered editions, the edition number is etched into the wax here. Next, a system of supporting legs, called sprues, and a wax funnel are added to the bottom of the wax replica. In future steps these will allow air to escape as molten bronze is poured into the mold.

Dipping the wax replica into a type of liquid ceramic makes up the investment process. Several coats are added with a fine layer of dry sand between each dipping. After this is completely dry, the wax is melted out, leaving a ceramic shell mold. This is called the "lost wax" method of bronze casting.

Next molten bronze heated to 2,200 degrees F is poured into the shell. After the bronze has cooled and hardened the ceramic shell is carefully chipped away, lending truth to the phrase "breaking the mold." For larger pieces, parts of the sculpture may have to be cast separately and are welded back together at this time. The sculpture is cleaned and detailed very carefully.

The last step in the process is to treat the sculpture with different chemicals. The word for this is patina, which actually comes from the fine green rust considered valuable on antique bronze pieces. Patinas tone down the naturally shiny metal, adding depth and contrast by darkening the recesses and providing subtle variations of color. Different chemicals are used for different effects. Ferric nitrate, for example, adds an orange-brown layer to the bronze. The sculpture is heated between each chemical application. This opens the pores of the metal and locks the chemicals inside.

The sculpture is now finished! It is cooled, sealed, and then mounted onto a wooden base. A small plaque with the sculpture's title and artist's name is the final touch.