Bronze and casting of bronze plaquette

Bronze

Bronze is an alloy of cooper and tin, generally with a tin content of between 5 and 14 percent. Other materials such as zinc and lead are often included in bronze alloys to lower the melting point of the cast. Bronze alloys vary in colour from coppery reds to silvery hues (depending of the alloy type). In Europe, bronze sculptures were generally cast using the lost wax method. During cooling, the bronze alloy became shrunken, resulting in cracks. In the casting process of hollowed, thin-walled bronzes, the amount of shrinkage and the risk of future cracks decreased. For this reason, bronze statues were usually hollow cast.

Casting of Bronze Plaquette

There were several methods of casting bronze plaquettes, but mostly a variety of lost-wax casting known as sand casting was employed. One plaquette was usually cast several times, so the model and its negative imprint, or mould, were also used during the process.

First, a plaquette was made from wax. If a negative mould was used, melted wax was poured or pressed into it. After allowing the wax to cool down the plaquette was taken out of the negative mould and channels made from wax were fixed on to it. The entire mould was coated with liquid gypsum or a mixture of gypsum and brick dust. It was covered by an investment and then heated. Wax, melted by the heat, ran out through the channels. A molten metal alloy was poured into the channels. Afterwards the entire mould was allowed to cool down before the investment was broken away. Then the channels were cut off and the cast bronze was chiselled. Finally, the surface of the plaquette was patinated in a similar manner to bronze sculptures.

(Italian Master: The Allegory of Nil, inv. no.: 5175)