The Lost Wax casting process is an ancient practice that is still in widespread use today. The process varies from foundry to foundry, but the steps which are usually used in casting small bronze sculptures in a modern bronze foundry are as follows:
- Sculpting. Master artist, Lyle Johnson creates an original artwork from clay. Oil-based clay is preferred because this material retains its softness.
- Mouldmaking. A mould is made of the original sculpture. Most moulds are at least two pieces, and a shim with keys is placed between the two halves during construction so that the mould can be put back together accurately. To preserve the fine details on the original artwork's surface, there is an inner mould made of latex or silicon rubber, which is supported by the plaster part of the mould. Usually, the original artwork is destroyed during the making and initial deconstruction of the plaster mould. This is because the originals are solid, and do not easily bend as the plaster mould is removed. Often long, thin pieces are cut off of the original and moulded separately. Sometimes, especially in the case of large original (such as life-size) sculptures, many moulds are needed to recreate the original sculpture.
- Wax. Once the plaster and latex mould is finished, molten wax is poured into it and swished around until an even coating, usually about 1/4 inches think, covers the entire inner surface of the mould. This may be done in several layers.
- Removal of wax. This new, hollow wax copy of the original artwork is removed from the mould. The artist may reuse the mould to make more wax copies, but wear and tear on the mould limit their number. For small bronze artworks, a common number of copies today is around 25.
- Chasing. Each hollow wax copy is then "chased": a heated metal tool is used to rub out all the marks which show the "parting line" or "flashing" where the pieces of the mould came together. Wax pieces that were moulded separately can be heated and attached; foundries often use "registration marks" to indicate exactly where they go.
- Spruing. Once the wax copy looks just like the original artwork, it is "sprued" with a treelike structure of wax that will eventually provide paths for molten bronze to flow. 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.
- Slurry. A "sprued" wax copy is dipped into a ceramic slurry, then into a mixture of powdered clay and sand. This is allowed to dry, and the process is repeated until a half-inch thick or thicker surface covers the entire piece. Only the inside of the cup is not coated, and the cup's flat top serves as the base upon which the piece stands during this process.
- Burnout. The ceramic-coated piece is placed cup-down in a kiln, whose heat hardens the ceramic coatings into a shell, and the wax melts and runs out. The melted wax can recovered and reused, although often it is simply combusted by the burnout process. Now all that remains of the original artwork is the negative space, formerly occupied by the wax, inside the hardened ceramic shell. The feeder and vent tubes and cup are now hollow, also.
- Testing. The ceramic shell is allowed to cool, then is tested to see if water will flow through the feeder and vent tubes as necessary. Cracks or leaks can be patched with thick ceramic paste. To test the thickness, holes can be drilled into the shell, then patched.
- Pouring. The shell is reheated in the kiln to harden the patches, then placed cup-upwards into a tub filled with sand. Bronze is melted in a crucible in a furnace, then poured carefully into the shell. If the shell were not hot, the temperature difference would shatter it. The bronze-filled shells are allowed to cool.
- Release. The shell is hammered or sand-blasted away, releasing the rough bronze. The spruing, which are also faithfully recreated in metal, are cut off, to be reused in another casting.
- Metal-chasing. Just as the wax copies were "chased," the bronze copies are worked until the telltale signs of casting are removed, and the sculptures again look like the original artwork. Pits left by air bubbles in the molten bronze are filled, and the stubs of spuring filed down and polished.
- Patinating. The bronze is colored to the artist's preference, using chemicals applied to heated or cooled metal. This coloring is called patina, and is often green, black, white or brownish to simulate the surfaces of ancient bronze sculptures. (Ancient bronzes gained their patinas from oxidization and other effects of being on Earth for many years.) However, many artists prefer that their bronzes have brighter, paint-like colors. Patinas are generally less opaque than paint, which allows the lustre of the metal to show through. After the patina is applied, a coating of wax is usually applied to protect the surface. Some patinas change color over time because of oxidization, and the wax layer slows this down somewhat.
The lost-wax process can also be used with any material that can burn, melt, or evaporate to leave a mold cavity. Some automobile manufacturers use a lost-foam technique to make engine blocks. The model in this case is made of polystyrene foam, which is then placed into a casting flask, consisting of a cope and drag, which is then filled with casting sand. The foam supports the sand, allowing shapes to be made which would not be possible if the process had to rely on the sand alone to hold its shape. The metal is then poured in, and the heat of the metal vaporizes the foam as the metal enters the mold.