Back to the Articles Page
Lost wax casting process
Note to class: For the purposes of our class we start at #5 by sculpting the original work in wax, but this is the usual process for artists. Please read the whole article.
Adapted from: http://en.wikipedia.org/wiki/Casting
The Lost Wax bronze-casting process is an ancient practice that is still in widespread use today. The process varies from foundry to foundry, but the steps usually used in casting small bronze sculptures in a modern bronze foundry are as follows:
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 silicone rubber, latex or vinyl, which is supported by the plaster part of the mold. Often, 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 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 1/4 inch think, covers the entire inner surface of the mold. This may be done in several layers.
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 are 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. Wax pieces that were molded separately can be heated and attached; foundries often use "registration marks" to indicate exactly where they go.
6. Sprueing. 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 sprueing usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy.
7. Slurry/Investing. A "sprued" wax copy is dipped into 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.
8. 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 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 ceramic 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 then placed cup-upwards into a tub filled with sand. Bronze is melted in a crucible in a furnace, and 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 sprues, 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 sprueing filed down and polished.
13. 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 luster 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.