Bronze Casting - Lost wax process

Fabienne Bismuth


Lost-wax casting, sometimes called by the French name of cire perdue, is the process by which a bronze is cast from an artist's sculpture. An ancient practice, the process today varies from foundry to foundry, but the steps which are usually used in casting small bronze sculptures in a modern bronze foundry are generally quite standardized. Here are the different steps.

Sculpting.  I create an original artwork in clay or cast stone or another material.

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 are made from plaster and fiberglass. To preserve the fine details on the original artwork's surface, there is usually an inner mold made of 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. In the case of large original (such as life-size) sculptures, many molds are needed to recreate the original sculpture.

Wax. Once the 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.

Removal of wax. This new, hollow wax copy of the original artwork is removed from the mold. This mold may be reused 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, but I limit my work to edition of 8.

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.

Spurring. Once the wax copy looks just like the original artwork, it is "spurred" with a treelike structure of wax that will eventually provide paths for molten bronze to flow, while allowing air to escape. The carefully-planned spurring usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy.

Slurry. A "spurred" 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.

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.

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.

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 spurring, 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 spur filed down and polished.

Building of the ceramic shell or slurry


The whole process took about 3 month (not counting the sculpting part).

Bronze is melted at 2000F and poured in the shell

Patina. The bronze is colored using chemicals applied to heated metal. This coloring is called patina, and is often green, black, white or brownish , but can also have brighter colors. 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 oxidation, and the wax layer slows this down.