TECHNIQUES OF PRODUCING HOLLOW CAST BRONZES
Direct Lost Wax Method
1. A core of clay is made to the shape of the head, but slightly smaller.
2. A layer of wax is applied to the head and it is modelled with the detail required.
3. Core pins of bronze are iron are inserted through the wax to hold the core in place when the wax is melted out. The head is inverted. A wax pouring cone and runners are attached to create the channels through which the bronze will flow down into the mould. More wax is used to make risers by means of which air and gases will escape when the bronze is poured in, thus preventing cracking of the mould and bubbles in the bronze.
4. A mould or investment is built up round the structure from some rnaterial which will shrink or crack as little as possible during firing, such as a mixture of ground pottery and plaster.
5. The mould is baked so that the wax is completely melted out.
6. The whole structure is banked up with earth or sand to hold it steady and help the mould to resist the pressure created when the molten metal is poured in. Meanwhile the bronze is heated to a temperature of about 11004C and poured into the upnght mould. This must still be hot, otherwise it may crack on contact with the hot metal, or the metal may cool too quickly and solidify before completely filling the mould. When the molten bronze appears at the top of the risers, the caster knows that the mould is filled.
7. When cooled, the mould is broken away. The runners are sawn off, the core pins removed or levelled, any faults in the casting are repaired and the surface is tooled and polished as required.
Indirect Lost Wax Method
This process has the advantage that the onginal model can be preserved, so that further castings can be attempted in the event of failure, or if more copies are required.
|A||A model of the head is made in a suitable material, such as wood, clay or plaster.|
|A plaster piece mould is taken from the model. The pieces must be able to be removed without damage to the model and therefore great care has to be taken with any under cuts.|
|The piece mould is removed from the head and then reassembled. The inside is lined with wax sheets or painted with wax to an even thickness.|
|The piece mould is removed, and a diluted mixture of the mould material (see 4) is poured into the head to act as a core. From then on the procedure is the same as for the direct lost-wax method (see 3 7).|