Casting process

The creation of a bronze sculpture is highly skilled work involving a series of artistic, chemical and metallurgical processes.

Sculpting the Portrait

Firstly a frame or armature is constructed. The modelling clay is then built up over the
armature. The form is roughed out and detail is slowly added as the portrait is refined. Proportioning and profiling is essential.
Constant reference to the subject or photographs of the subject is necessary to convey an accurate likeness. Finally the finishing touches are completed.
After approval by the client the piece is set up for “master moulding”. A clay border is set out and “dovetails” are cut in preparation for the application of rubber in the next stage.

Silicone rubber master mould

The rubber is mixed with a catalyst and applied in a series of layers. Extreme care is taken to avoid air bubbles.
Locating lugs a reformed to assist with alignment. Seams are built to allow for the release of the sculpture.

Creating the plaster casing

A clay border is built outlining one segment. Plaster and glass fibre reinforcement are mixed and applied over the rubber in layers.
When one segment is complete the clay border is removed and the process is repeated until three segments are completed. The rubber seam is cut in a zig zag fashion to ensure exact relocation.

Making the wax mould

The empty rubber mould with its reinforced casing is re-assembled and strapped in to the wax jig. Molten wax is poured inside. The mould is shaken and rotated to build up layer upon layer of wax within the mould.
After cooling the mould is removed to reveal the wax portrait. The wax portrait
is checked for air bubbles and accuracy.
In preparation for the feeder system used to pour in the molten bronze, a window is cut within the wax model. The cut out is cast separately. The wax is then supported in a jig ready for spruing.

Creating the ceramic shell

Prior to applying the ceramic shell the wax is sprued and vented by creating a wax “feed” system. This is the pipe work through which the molten metal f lows and gases are able to escape during pouring. This stage of the process is very important as it determines the quality of casting. A mistake at this stage can cause a failed casting, and the process of casting the wax onward has to be repeated.
A ceramic shell is then created around the wax figure.
This ceramic shell will not only imprint the wax portrait perfectly but will also be strong enough to withstand the high heat of the molten bronze. Firstly a slurry mix of colloidal silica is applied over the wax. This is followed by applying a refractory stucco over the slurry.
The slurry and stucco are continuously built up in layers. When the shell mould is approximately five layers thick it is “bound” with stainless steel wire for reinforcement.
Two more coats are applied and when the shell has cured the wax model is melted out to allow the molten bronze to be poured in.
The shell mould is fired in a kiln at 1000°C to remove carbon deposits. The whole shell process and burn out takes place over a one week period.
The empty shell moulds are packed in metal shot for support and molten bronze is poured in at 1200°C.

Chasing the casting

When the casting has cooled a jack hammer is applied to the feeder system to loosen the shell. The stainless wire and the feeder system are cut off and the casting is sandblasted. Extreme care is required. The areas where the feeder system was attached is grounded back to match the profile. The “cap” is welded back where the window was cut. The holes left by the jig a refilled using a tig welder and silicon bronze rods.
The detail is then “cold worked” back into these freshly welded areas of the bronze sculpture. This stage is called chasing. Grinder, engravers and hammer and chisel a reused, and the “chased” areas must be undetectable.

Patination and finishing

The final stage is the colouring or “patination”, and is often regarded as an artform in itself. The portrait is sandblasted and heated with a blow torch. Oxidising solutions are applied to the surface in a controlled manner until the desired colour is obtained.
A protective layer of wax is applied and the finished portrait is ready to be mounted on a base.