The first step is creating a wax model. There are many types of wax. Some is hard, best for turning on a lathe or carving. Some is soft and bendable and comes in sheets and wires. Some can be modeled like clay.
Often we want to cast several waxes at a time, so we put them on a tree with each wax model as a leaf of the tree. The branches and trunk of the tree are called "sprues". The sprues allow the wax to flow out during "burn out" and, the molten metal to flow in, during casting.
The next step is investing the wax in plaster. First, the wax is attached to a rubber base. A steel cylinder, called a flask, is then fitted onto the base, forming a seal. (Imagine a tin can with one end cut off.) This then gets filled with a plaster like substance called investment. The investment is able to withstand the high heat necessary to completely eliminate all the wax during burn out. A vacuum machine is used to remove any air bubbles from the investment while it is still liquid. If not removed, the air bubbles would appear as small nodules on the cast metal piece. Once the investment has solidified, the rubber base is removed, exposing the end of the wax "trunk".
The next step is called the burnout. (No, not the jeweler -- the wax!) The flasks containing the wax models encased in investment are put into a kiln and gradually heated to1300 degrees F. The temperature is then brought down to between 800-1100 degrees for casting. This process takes at least 5 hours, sometimes much more, depending on the type and quantity of wax to be burned out. When the burnout is complete the wax pattern and sprues have been vaporized, leaving a cavity in their image within the investment in the flask.
The next step is to force molten metal to flow into the flask and through the sprues, filling in the cavity in the plaster. One common method is to use centrifugal force. The casting machine consists of a spring-loaded weighted arm that is wound up and held in place with a pin. Using a gas-oxygen torch, we heat the metal in a crucible that is placed on one end of the arm. When the metal is molten, the flask is removed from the kiln and placed next to the crucible. When the pin is dropped, the arm spins with remarkable force, sending the molten metal out of the crucible and into the flask.
Finaly, the flask is quenched in water, the investment chipped off, and the cast metal revealed. Then comes the task of de-spruing, finishing and polishing.
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