SOMETHING ABOUT BRONZE CASTING –Lost Wax and Ceramic Shell
- A removable flexible rubber mould with a rigid support jacket is made of the sculpture.
- The completed mould is removed carefully, cleaned and coated inside with a release agent—as you would oil a cake pan, then re-closed and fastened securely.
- Melted casting wax is poured into the empty mould, allowed to cool briefly, then poured out, leaving a coating on the inside—not too thin and not too thick. If this is to be a multiple edition, this process is repeated, and all the following steps as well, for each copy in the edition, applied individually— therefore each one is an original.
- Opening the mould reveals an exact hollow wax replica of the sculpture. Any imperfections in the wax are fixed, i.e. ‘chased’—air bubbles, seam lines from the mould, and too-thin wax areas; and the edition number added to the signature. Since this replica will become the vessel for the poured bronze it must be perfect; but first:
- A system of wax rods and tubes are ‘welded’ onto the wax replica in such a way that liquid bronze will be able to flow in, and air and gasses can escape—not unlike your household plumbing.
- Now a different kind of mould is made around this ‘vented and sprued’ wax replica: modern art casting foundries use a process called ‘ceramic shell’ in which the wax replica is immersed in a wetting solution, then dusted with a silica sand and dried. This process is repeated to attain sufficient thickness of the shell—perhaps 10 or 12 coatings.
- When thoroughly dry, it goes into a kiln-like oven, the wax melts out (hence Lost Wax) and the sand ‘vitrifies’—or rather becomes ceramic, containing a system of hollow spaces and tunnels where the wax melted or burned out.
- Bronze, which is an alloy consisting mostly of copper with small amounts of zinc, tin and lead, is heated in a crucible to approximately 2000 degrees and poured into the ceramic mould, still hot from the burn-out.
- When cooled, the ceramic shell is broken away carefully to reveal a bronze replica of the vented and sprued wax. These now unnecessary appendages are cut off and discarded, and the bronze undergoes a restoration process of grinding, sandblasting, and polishing.
- The raw bronze is sensitive to fingerprints and airborne chemicals, so it is now protected with the application of patina chemicals which will penatrate and color the bronze and give it a ‘patina’. And lastly, it is further protected with clear wax or sealer.
I myself have undergone training in all these processes as a way of understanding how to achieve my desired results from the foundry, but I prefer to leave it to the professionals!