|Chimera, V century b.C.|
Found in Arezzo, Italy, it represents a monster with a lion face, a
goat head coming out from its back, and a tail in the shape of a
Florence Archaeological Museum
Bronze, a widely used material for
sculptures, is usually processed by melt casting followed by
solidification at room temperature.
Bronzes are metal substitution alloys,
constituted primarily of copper (Cu, 70-90%) and tin (Sn).
In the crystal lattice of copper, some
copper atoms (A in figure) are randomly substituted by tin atoms (B
in figure). Chemical and mechanical properties of bronzes, as well
as ease of processing, strongly depend on the tin content and on the
presence of more elements (i.e. lead, zinc, silver, phosphorus,
aluminum, silicon and nickel).
|Molten bronze is cast into the mold|
The melting point of bronze decreases with
increasing tin content. Copper melts at high temperature
(Tm = 1084,6
°C), whereas tin melts at a moderate temperature(Tm =
231,93 °C). Therefore, once bronze bars have been prepared, the
subsequent process of melt casting requires less energy and a lower
working temperature than pure copper.
Lost-wax casting is one of the most used
techniques for manufacturing sculptures in bronze. First a wax model
is prepared by coating a clay core sustained by an iron frame. The
model is covered with a clay shell molding, with ducts that will
allow outflow of liquid wax and of fusion gases. The second phase
consists in baking the overall system, to harden clay and melt wax,
which outflows through the channels. In the third step the cavity
left by the wax is filled by casting bronze. After cooling at room
temperature, the sculpture, which exactly reproduces the wax model,
is manually finished. (modified, from