University Federico II
of Naples, Italy
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Bronze

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 snake.
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 http://www.celticworld.it/sh_wiki.php?act=sh_art&iart=300)

Bronze is quite sensitive to environmental degradation.