process

Rusty Mesa is a full-service sculpture production studio specializing in lost wax cast bronze and patina.

• lost wax casting

1. Model-making. An artist or mold-maker creates an original model from wax, clay, or another material. 

2. Mold-making. A mold is made of the original model or sculpture. The rigid outer molds contain the softer inner mold, which is the exact negative of the original model.

Inner molds are usually made of latex, polyurethane rubber or silicone, which is supported by the outer mold. The outer mold can be made from plaster, of fiberglass or other rigid materials.

Most molds are made of at least two pieces, and a shim with keys is placed between the parts during construction so that the mold can be put back together accurately. 

3. Wax. Once the mold is finished, molten wax is poured into it and swished around until an even coating, usually about  1⁄8 inch (3 mm) thick, covers the inner surface of the mold. 

4. Removal of wax. This hollow wax copy of the original model is removed from the mold. The model-maker may reuse the mold to make multiple copies, limited only by the durability of the mold.

5. Chasing. Each hollow wax copy is then "chased": a heated metal tool is used to rub out the marks that show the parting line or flashing where the pieces of the mold came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece.

6. Spruing. The wax copy is sprued with a treelike structure of wax that will eventually provide paths for the molten casting material to flow and for air to escape. The carefully planned spruing usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy. The spruing does not have to be hollow, as it will be melted out later in the process.

7. Slurry. A sprued wax copy is dipped into a slurry of silica, then into a sand-like stucco, or dry crystalline silica of a controlled grain size. The slurry and grit combination is called ceramic shell mold material, although it is not literally made of ceramic. This shell is allowed to dry, and the process is repeated until at least a half-inch coating covers the entire piece. The bigger the piece, the thicker the shell needs to be. Only the inside of the cup is not coated. The cup's flat top serves as the base upon which the piece stands during this process.

8. Burnout. The ceramic shell-coated piece is placed cup-down in a kiln, whose heat hardens the silica coatings into a shell, and the wax melts and runs out. The melted wax can be recovered and reused, although it is often simply burned up. Now all that remains of the original artwork is the negative space formerly occupied by the wax, inside the hardened ceramic shell. The feeder, vent tubes and cup are also now hollow.

9. Testing. The ceramic shell is allowed to cool. It is then tested to see if water will flow freely through the feeder and vent tubes. Cracks or leaks can be patched with thick refractory paste. To test the thickness, holes can be drilled into the shell, and then patched.

10. Pouring. The shell is reheated in the kiln to harden the patches and remove all traces of moisture. It is next placed cup-upwards into a tub filled with sand. Metal is melted in a crucible in a furnace and poured carefully into the shell. The shell has to be hot because otherwise the temperature difference would shatter it. The filled shells are then allowed to cool.

11. Release. The shell is hammered or sandblasted away, releasing the rough casting. The sprues, which are also faithfully recreated in metal, are cut off, the material to be reused in another casting.

12. Metal-chasing. Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, so that the casting now looks like