To reproduce every detail of Sarah’s work, the 5000 year old lost wax process is adopted. This traditional method of casting bronze sculpture is still used today because of the ability to achieve very fine detail.
The original model is created using an oil based clay on a wire armature. When the original is finished, a negative mould is taken from it using a flexible rubber type compound. The mould will be made in two parts, or more, depending on the size or complexity of the sculpture. This negative mould is supported by a plaster surround and is used to make a hollow wax positive of the original sculpture. The void in the hollow wax is then filled with plaster. This is called the core and will result in a hollow bronze casting 6-8 mm thick.
At this stage, Sarah will carry out more detailed work to the hard wax to ensure the original model is authentically reproduced. The wax will then have runners and risers attached, in the same material, to ensure the molten wax and, ultimately, the molten bronze can flow freely.
When the sprues are completed, the wax pattern is dipped in successively heavier mixtures of ceramic fluids to build up a thick, hard coat. When fully cured the investment mould is ready for wax removal
To remove or “lose” the wax, the mould is placed mouth down in a furnace, most of the wax will melt and run out but any that remains will be burnt out at high temperature. The moulds are then usually taken directly from the wax removal furnace, positioned to accept the molten bronze and poured immediately.
After cooling, the ceramic mould is broken away, leaving a bronze casting, complete with bronze sprues. These sprues have to be chased or fettled off, to reproduce the surface of the original model.
When the chasing is complete, the bronze is ready to be given it’s distinctive patina. Patination involves treatment of the bronze surface with hot or cold acids and metal salts. These react with the bronze to create a variety of colourings and accelerated weathering.