sculpture rubber mould bronze head busts The Bronze Casting Process

Bronze Casting

The technique of lost wax bronze casting is one of Man's earliest technologies dating back at least 6000 years and the basic method has changed very little since that time. Modern craftsmen have some technical advantages over those of past times with welding equipment and power tools, efficient gas or oil burners and flexible rubbers for moulding. However, today's process is still heavily dependant upon many hours of skilled labour and remains a 'low tech, high skill' craft.

1. Sculpting the portrait

The head is modelled life-size in clay over a four to five day period. Ideally this is done in the home as a relaxed atmosphere will bring out the true personality and features of the person. A lacquer is painted over the sculpture when the clay is leather-hard but not dried out.

2. Mould making
'Banking up' the original

The first process a sculpture has to go through is the making of a rubber mould. A master mould enables hollow wax casts to be made in order to reproduce an edition of bronzes. A void has to be created around the head. The mould may have to be made in several sections, depending on the complexity of the original, to avoid undercuts.

3. Mould making
The first clay blanket and keying

The sculpture is covered with a clay blanket (5mm) the thickness of which will determine the thickness of the void, later the rubber mould. Air vents and pourers are moulded at the highest points, with key sausage-like edging and keyholes in the clay case.

4. Mould making
The first plaster jacket

A thick reinforced plaster jacket is built up over this section of the mould and the other sections are then treated in the same way, making sure the pouring and air vents are kept clear. When the plaster casing is set, a section is taken off, the clay blanket removed and the section put back in the original position.

5. Mould making
Pouring the first rubber

Liquid black polysulphide rubber is poured through the pourer, covering every feature modelled in clay, until it emerges from the air vents. It will set within 90 minutes. The procedure is repeated with all the sections.

6. Opening the mould

A blunt chisel is pushed gently between the sections to prise them open. The plaster section is lifted off and the rubber mould gently peeled off the original sculpture which is then carefully removed. The rubber mould is laid back into the plaster jacket to hold the shape.