How to cast a bronze

One of the techniques for casting bronze is the “lost wax technique”.

First a mould is made : a silicon form on top of the sculpture. Round this rubber form a supporting mould in polyester is made. Once these two forms are ready, the original sculpture can be put aside.

With the help of the rubber mould a wax form is now cast. Hot fluid wax is poured into the mould and immediately poured out again. The remainders left in the mould coagulate. This procedure is repeated until layer by layer a wax form is growing. The thickness of the wax will be the final thickness of the bronze, so one has to work carefully.

Then nails are driven into this wax form coming out at both sides. These are the anchoring points keeping the core at its place during the coagulation.

Then a mixture of baked and mixed clay (chamotte) mixed with plaster is poured into the hollow wax form.

To the wax form pouring-channels, air channels and a casting-mound are fixed, also made of wax.

Then a shuttering is made round the filled wax form, which is filled with the same material as before. Everything is fixed and turned around, so that it can be put into the oven. The oven is now heated up to 150 to 200°C.

As a result of the heating up in the oven the wax slowly melts and leaps out of the form. In order to get rid of all the wax remainders the temperature is increased up to 650°C and the whole is left to stoke out during three days. After about three days all wax has disappeared and a block containing a hollow part is left over.

Now the bronze is melted. This bronze consists of 90% of cupper and 10% of tin. To melt the bronze, the oven has to be heated up to 1.000 to 1.200 °C for 1 to 3 hours.

The melted bronze is now poured into the hollow space. This hollow space is filled from the bottom to the top, in order to avoid air bubbles and to avoid that certain parts do not get completely filled. Once the hollow space is completely filled, the coagulation can start. The whole is left to cool down for about a day.

The core material is very porous and can easily be removed from around the sculpture. Remaining parts are sprayed away with a high pressure spout. The pouring channels are then grinded away with a grinding machine, as well as the nails.

Finally the embossing is effectuated : polishing, patinating.

(Bron: Bijlage bij Gazet van Antwerpen dd 12-16.4.2000, p. F14 – F15)