Lost wax casting for model making
1. Model-making. A original model from wax is created. Wax is often preferred because it retains its softness.
2. Mould-making. A mould is made of the original model or sculpture. The rigid outer moulds contain the softer inner mould, which is the exact negative of the original model. Inner moulds are usually made of silicone, which is supported by the outer mould. The outer mould can be made of fibreglass.
3. Wax. Once the mould is finished, molten wax is poured into it and swished around until an even coating, usually about 3 mm (1⁄8 inch) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached.
4. Removal of wax. This hollow wax copy of the original model is removed from the mould.
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 where the pieces of the mould came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece and any imperfections will be shown.
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. The slurry and grit combination is called ceramic shell mould 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. The core is also filled with fire-proof material.
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. 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, 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.
10. Pouring. The shell is reheated in the kiln to harden the patches and remove all traces of moisture, then placed cup-upwards into a tub filled with sand. Metal is melted in a crucible in a furnace, then 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 sand-blasted away, releasing the rough casting. The sprues, which are also faithfully recreated in metal, are cut off, with the material to be reused in another casting.
12. Metal-chasing. Just as the wax copies were chased, the casting is worked until the tell-tale signs of the casting process are removed, so that the casting now looks like the original model. Pits left by air bubbles in the casting and the stubs of the spruing are filed down and polished.