1. Sculpting. An artist creates an original artwork from wax, clay, or another material. Wax and oil-based clay are often preferred because these materials retain their softness.
2. Moldmaking. A mold is made of the original sculpture. Most molds are at least two pieces, and a shim with keys is placed between the two halves during construction so that the mold can be put back together accurately. Most molds of small sculptures are made from plaster, but can also be made of fiberglass or other materials. To preserve the fine details on the original artwork's surface, there is usually an inner mold made of latex, vinyl, or silicone which is supported by the plaster part of the mold. Usually, the original artwork is destroyed during the making and initial deconstruction of the plaster mold. This is because the originals are solid, and do not easily bend as the plaster mold is removed. Often long, thin pieces are cut off of the original and molded separately. Sometimes, especially in the case of large original (such as life-size) sculptures, many molds are needed to recreate the original sculpture.
3. Wax. Once the plaster and latex mold is finished, molten wax is poured into it and swished around until an even coating, usually about 1/8 inches thick, covers the entire inner surface of the mold. This must be done in several layers until desired thickness is reached.
4. Removal of wax. This new, hollow wax copy of the original artwork is removed from the mold. The artist may reuse the mold to make more wax copies, but wear and tear on the mold limit their number. For small bronze artworks, a common number of copies today is around 25.
5. 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 mold came together. The wax is then "dressed" to hide any imperfections. The way the wax looks at this stage, is what it will look like when it is cast. Wax pieces that were molded separately can be heated and attached; foundries often use "registration marks" to indicate exactly where they go.
6. 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, while allowing 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.
7. Slurry. A "sprued" wax copy is dipped into a slurry of liquid 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 a half-inch thick or thicker dries 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, and 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 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.
9. 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 refractory paste. To test the thickness, holes can be drilled into the shell, then patched.
10. 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.
11. 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.
12. 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 spruing filed down and polished.
13. Patinating. The bronze is colored to the artist's preference, using chemicals applied to heated or cooled metal. Using heat is probably the most predictable method, and allows the artist to have the most control over the process. 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 oxidisation and other effects of being on Earth for many years.) However, with current artistic trends in the United States, many artists prefer that their bronzes have brighter, more stylized patinas. Patinas can be applied to replicate marble or stone. Depending on how the metal is prepared, either sandblasted or polished, the finish can be either opaque or transparent. After the patina is applied, a coating of wax, which is the most traditional type of sealer, is usually applied to protect the surface. Many artists prefer to use lacquer as a sealer on some of the more unstable patinas. This protects the piece more from ultraviolet rays. Some patinas change color over time because of oxidiation, and the wax layer slows this down somewhat.
On the left is an example of a lost-wax process mold, and on the right is the resulting bronze sculpture.
On the left is an example of a lost-wax process mold, and on the right is the resulting bronze sculpture.
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.
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