Jan Willem van der Wal Bowen Island BC

Bronze; Lost Wax Method (cire perdue)

The lost wax (cire perdue) method: A sculpture made out of wax cast in a mold from grog and plaster which can be burnt away. The original wax structure is now gone and the mold holds a hollow negative of the sculpture. Into this mold now bronze will be casted. After cooling down now the mold itself has to be smashed /demolished to retrieve the sculpture in bronze. The bronze has to be cleaned from all kinds of abnormalities created during the firing of the mold and casting the bronze. (air bubbles, small cracks turn in bronze too) Also the casting cup, support posts and vents must be removed. After cleaning the sculpture you can color it by fast oxidizing, using different chemicals and a torch. The finishing touch is made with some polish.

Creating a bronze sculpture, step by step:

Phase I Creative

Using all kinds of materials which can burn, but mainly wax, you create the wanted sculpture. As wax is a weak substance (there are different kinds of firmness) you might reinforce the wax, using wood (branches), plastic, cloth, etc. In fact everything that can burn will vanish during the burning process of the mold.

Wax and other material for the sculpture The sculpture in wax

Phase II Creating the correct flow for bronze

This is the most important step in the process to create the sculpture. To enable a proper flow of the liquid bronze into the mold you should prevent airlocks and too narrow passages. Also great differences in thickness can cause failures in the final product due to uneven shrinking or cooling down speed. The wax structure must be connected to a casting cup / sprue gate and air vents to enable the bronze to flow properly through the hollow chambers. Also sometimes bypasses are necessary on the original structure.

Casting cup/ sprue gate, vents and supports mounted

Phase III Mold making To form the wax structure in a mix of plaster and grog a form should be made. All kinds of material for this form can be used and depends on the size of the wax structure. Metal sheet material is very good to create a flexible form. The grog and plaster (1:1) is mixed with clean water till it reaches the consistency of yogurt.

Plaster and grog (coarse)

This liquid mix is poured along the wax structure into the form. About half an hour later the form can be removed and the mould is ready for firing in a kiln.

The wax model in the form The form filled with grog/plaster mass

Phase IV Burning out the wax

The mold(s) are placed in a kiln and burned out for about one to two days up to 800 degr. Celsius. (1500 F) After cooling down the mold, now a negative of the sculpture remains. Small particles remaining in the hollow part of the structure can be removed by vacuuming.

The kiln

After firing the mold for about 20 hours The mold, ready to cast the bronze

Phase V Pouring bronze

Melted bronze with a temperature between 1100 and 1200 degr. Celsius (2000-2200 F) is cast into the mold till the vents in the mold fill with bronze.

Smelting furnace with crucible

Bronze is an alloy of copper, tin and zinc (90% Co, 4% Sn, and 6% Zn)

Pouring the bronze

Phase VI Cleaning the sculpture

The mold can be broken and the bronze structure thoroughly cleaned using a pressure washer.

Sculpture after cleaning Sculpture after removing casting structure

The casting cup / sprue gate, vents and other casting bypasses are removed using a saw, angled grinder, Dremel and chisels.

Various tools to finish the sculpture

Small bronze cracks and bronze bullets can be removed using a chisel. Also a steel brush attached to an angled grinder can be used too.

Phase VII The finishing touch

After cleaning the sculpture is now in bronze. Using chemicals such as liver of sulphur, aluminum sulphate and copper nitrate to apply to the sculpture, using a torch gives the desired colors. Some chemicals react slowly in humid air and show their patina after some weeks.

Clockwise top: copper carbonate, copper nitrate, liver of sulphur, copper acetate & copper sulfate.

To deepen the effect of color, shoe polish can be applied and some spots brightened by polishing

with a polishing machine. The result: Jump Start

Jan Willem van der Wal

Bowen Island August 2012.

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