Lost wax bronze casting process

The lost wax bronze casting process, or Cire-perdue is a method to create hollow bronze sculptures from a model. The casting method is approximately 6000 years old, and is common on every continent except Australia. Although modern technology and materials have refined the method to some degree, the technique has remained essentially the same.

Bronze, an alloy of copper and tin is a material with desirable properties for casting sculptures; it is hard, durable, yet it has a low melting point and it is easily tooled. Bronze doesn't rust, and can be finished with a smooth patina. Because of these advantageous material properties, early sculptors were able to refine casting methods for increasingly larger bronze statues. Initially, these sculptures were completely solid, and formed directly from a negative mold. However, as the sculptures became larger and more complex, the sculptors ran into a several of difficulties:

1. Larger, more complex casts often failed because of solidification of the bronze before completely filling the mold
2. Larger objects are increasingly more expensive to make: to scale up a statue to twice its original size requires approximately eight times the original amount of bronze.
3. Weight issues: larger sculptures can become too heavy to support their own weight. This is especially true for more complex shapes (e.g.figures with extended limbs etc.)

The lost wax method overcomes all of these problems: the hollow shapes can be cast in several pieces, are cheaper to manufacture, and relatively stronger in relation to their weight. The lost wax method remains a difficult, time consuming process. However, many great artists from the past, such as Giambologna, Donatello, Auguste Rodin and Frederic Remington have perfected the lost wax method to create the most remarkable sculptures.

The following steps are involved in making a sculpture using the lost wax method:

1. Creating the mold: A (negative) mold is formed from an original work of art. This step is the most critical, since all the details from the original have to be captured in the mold. Depending on the size of the sculpture, the mold is cut into sections for casting.
2. Making the wax cast: Molten wax is poured into the mold to form layers of wax.
3. Chasing the Wax: The wax model is pulled from the mold, and hand chased (re-detailed). The finalized wax model will be an exact duplicate of the bronze casting.
4. Spruing: Wax rods (gates), and a pouring cup are attached to the wax model. These attachments will assure a full pour, and avoid air pockets during the casting.
5. Investment Casting: The wax model is dipped in an investment liquid (a liquid clay) first, and then dipped in a ceramic sand. This process is repeated many times, to create a ceramic mold around the wax model. Between each application, the ceramic layer must be dry. The temperature must be controlled at 72 degrees Fahrenheit.
6. Burn-out: The ceramic shell is placed into a kiln, and fired. The shell is baked and hardened, while the wax model is melted, and drained away from the shell (i.e.The wax is lost.) The molten wax can be collected for re-use.
7. Casting: The ceramic shell is removed from the kiln, and immediately molten bronze (2100 F) is poured into it.
8. Break-Out: The sculpture is cooled down for several hours. Next, the shell is carefully broken away, revealing the unfinished bronze sculpture.
9. Sandblasting: Ceramic fragments are removed from the bronze
10. Assembly: The pieces of the sculpture are fit together.
11. Surface treatment The sculpture is refined in several steps, to obtain a polished, smooth sculpture.