The Casting Process

The Sculpture Foundry Guide to the Lost Wax Method of Bronze Casting

This section describes the method used to cast solid bronze sculptures using molten bronze. We start with a master mould, but every time we cast a piece, a new mould is created and destroyed, resulting in every single piece of sculpture we create being truly unique.

1. Mould Making
A rubber mould is made of the original art work. Absolutely every detail captured or missed by this mould will be reproduced in the bronze castings. This is the master mould used to originate all castings of that piece. Each time that piece is ordered, we follow the following procedure.

2. Making the Wax Casting
Molten wax is poured on painted depending on size into the rubber to create an exact wax duplicate of the original model.

3. Chasing the Wax
The wax is removed from the rubber mould and any flaws are hand finished by a skilled craftsman.

4. Spruing
Wax rods and a wax pouring cup are carefully attached to the wax casting in just the right positions to allow bronze to be poured in and any displaced air to escape. This means that no air bubbles are trapped inside the mould as they would be gaps in the finished bronze piece.

5. Investment
The wax model with attached rods is now covered in ceramic material which will form the mould for the final bronze pour. This is done by dipping the wax model carefully into investment liquid and then covering it in a fine powder before allowing it to cure (completely dry). The model is then dipped into investment liquid again and given a coating of a coarse ceramic powder. By following this procedure several times, a ceramic shell is built up around the wax model.

6. Burn out/dewaxing
The wax inside the ceramic shell is then placed in a kiln and fired. The shell bakes and the wax is melted (lost) from the shell. This creates a hollow ceramic shell mould and accounts for the term "Lost Wax" being applied to the process

7. Casting
The ceramic shell is then placed in the kiln and fired to a temperature of 1000 degrees centigrade, and then the molten bronze is poured into the now empty shell at temperatures upto 1300 degrees.

8. Knock Off
Once the casting has been allowed to cool for several hours, the mould is carefully removed by hand from the bronze model inside.

9. Sandblasting
Any fragments of the ceramic shell are removed by sandblasting, and the sculpture is carefully inspected at this point.

10. Assembly
All attached rods and cups (which are now bronze) are removed, and segments of the sculpture are welded together if necessary.

11. Chasing
All of the weld marks and removed, and rod marks are chased and re-detailed by hand.

12. Polish
The bronze is hand polished in preparation for the patina.

13. Patination
The bronze is first heated before the chemical patina is applied by an artist. A patina can achieve many different colours or effects, depending on the mix of chemicals used.

14. Waxing
Finally a wax coating is hand applied before the piece is polished to ensure a beautiful lustrous patina.
How does THIS bronze differ from “cold cast” bronze?

The lost wax method of casting is a highly skilled, complex, time and labour intensive process. This, combined with the cost of the raw material (copper and tin) gives rise to the cost of a bronze sculpture.

Resin sculpture, also known as “cold cast bronze” or bonded bronze is actually made of polyester, epoxy or another resin, poured into a mould – the lower weight of a resin piece than a bronze piece is evidence of this.

Bronze powder may be mixed into the polyester or painted onto the surface to enable the plastic to achieve the appearance of bronze. No spruing or investment is required in order to cast a cold cast sculpture and so the process is much faster and less expensive than that required for bronze, as are the materials used.