Susan Geissler

THE FOUNDRY PROCESS

A Long Journey from Clay to Bronze

1.) A mould is created, in several steps, by painting a liquid synthetic,rubber-like material directly onto the finished clay model. 2.) After the rubber-like mould has cured, it is coated with a fibreglass or plaster shell to hold its shape. 3.) When this ‘mother mould’ is completed, it is pulled off of the clay and cleaned, leaving a ‘negative’ image of the original model. (The clay is always somewhat damaged in this process. 4.) Molten wax is then applied to the mother mould--either by painting it onto the inside of the mould, or by pouring wax into the mould and draining it, or both--taking care to be sure that every nook and cranny is filled with wax. This process may be repeated numerous times to ensure that the wax is the correct thickness. 5.) When the final layer of wax has cooled and hardened, the wax is removed from the mother mould, leaving a wax positive of the original piece. After the imperfections and seam lines are ‘chased’ or cleaned up, the hollow wax model should be an exact replica of the original piece. 6.) Wax channels—called gates and sprues—are added to the wax model a “plug” is then generally cut out, so that in the next step, liquid shell material can fill the void inside of the hollow wax. 7.) Now it’s time to make the shell, which will be the mould for the molten bronze. To accomplish this, the wax positive—or pieces of the wax if it is too large or intricate to case in one piece, which is most often the case,--is dipped into a ‘slurry’ solution and then coated with fine grains of silica material. This step is repeated three or more times, over a period of many days, gradually increasing the size if the silica grains, until the wax model is evenly covered with this shell, which is then placed into a large “burnout” oven. The heat hardens to the shell, while melting the wax out; hence, the term—lost-wax process. The hollow shell is a negative of the original sculpture. 8.) Using tongs, a foundry worker drops ingots of silicon bronze into a furnace,where the molten bronze is heated to around 1,900 degress Fahrenheit. 9.) It is finally time to pour the molten bronze into the ceramic shells, which are set into a bed of sand so that they remain upright. Workers then have to lift a cauldron of molten bronze out of the furnace and pour it into the shells. 10.) Once the bronze has cooled, the shell is broken away, or divested, with air tools and hammers. The gates and sprues are cut off, and the piece is sandblasted to remove any remnants of the shell and any surface impurities. 11.) Like the wax positive, the bronze is also chased, or cleaned up, to remove any imperfections, If larger pieces have been cast in sections, they are welded together, and then various grinding and other tools are used to re-create the details and textures of the original, which may have been lost or damaged. When time doesn’t allow sculptors to complete each step of the foundry process themselves, many will entrust one or more of these steps to wax workers, metal workers and patineurs, many of whom are incredible artists in their own right. 12.) The sculpture is sandblasted again and the patinas are created-- generally by a chemical reaction, although sometimes with paint-- which gives a piece its color and surface character.