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:
- Larger, more complex casts often failed because of solidification of the
bronze before completely filling the mold
- 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.
- 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:
- 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.
- Making the wax cast: Molten wax is poured into the mold to
form layers of wax.
- 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.
- 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.
- 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.
- 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.
- Casting: The ceramic shell is removed from the kiln, and
immediately molten bronze (2100 F) is poured into it.
- Break-Out: The sculpture is cooled down for several hours.
Next, the shell is carefully broken away, revealing the unfinished
bronze sculpture.
- Sandblasting: Ceramic fragments are removed from the bronze
- Assembly: The pieces of the sculpture are fit together.
- Surface treatment The sculpture is refined in several steps,
to obtain a polished, smooth sculpture.