Lost Wax Bronze Casting
The lost-wax casting process is an ancient practice, which dates back at least 6000 years, that is still used to produce metal sculptures today. The process details vary from foundry to foundry, but the steps which are used in order to cast bronze sculptures in a modern foundry are generally quite standardized.
An artist creates an original artwork from wax, clay, stone, wood, etc..
2. Mold Making
A mold is made of the original sculpture. Most molds are at least two parts, and a shim with keys is placed between the two halves during construction so that the mold can be put back together accurately. To preserve the fine details of the original artwork’s surface, there is usually an inner mold made of polyurethane or silicon rubber which is supported by the rigid plaster outside part of the mold, this is known as the ‘mother mold’. Depending on the material, but especially when using a soft clay or wax, the original artwork may be destroyed during the construction and deconstruction of the mold. Often pieces are cut off of the original sculpture and molded separately. Especially in the case of large sculptures, a careful orchestration of many molds parts are needed to recreate the original form.
Once the plaster and rubber mold is finished, molten wax is poured into it and swished around until an even coating covers the entire inner surface of the mold. This must be done in several layers until desired thickness is reached, approximately 3/16 of an inch.
4. De-mold of wax
Next the hollow wax copy of the original artwork is removed from the mold. The artist may reuse the mold to make multiple reproductions, but wear and tear on the mold will limit the overall production number.
Each wax pattern is then ‘chased’. This is a when a cool or heated tool is used to re-sculpt all the marks which have been effected by 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 two or more parts fit together again.
Once the wax copy looks just like the original artwork, it is ‘sprued’ with a tree- like 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 and shaft. Wax feeders connect the sculpture pattern to the shaft and vents to various points on the wax pattern are then also welded to that pattern. Think of this design intent as like plumbing a house so that water (molten metal) is successfully delivered to the furthest reaches of the architecture.
A ‘sprued’ wax pattern is dipped into a slurry of liquid ceramic like ‘binder’, then into a dry crystalline sand. The slurry and sand combination is the ‘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.
The ceramic shell is placed in a kiln, whose heat fires or hardens the ceramic shell while the wax melts and runs out; this is where the process its name: ‘the lost wax process’. The melted wax can be recovered and reused. Now all that remains of the
original artwork is the negative space, formerly occupied by the wax, inside the hardened ceramic shell. The entire sprue system is now also hollow.
9. Shell Inspection
The ceramic shell is allowed to cool and cracks can be patched with thick refractory paste and steel wire or clamps.
The shell is reheated in a pre-heat kiln to harden the patches and condition the shell for the molten metal’s intense temperature. Bronze is melted in a graphite pot called a ‘crucible’, which sits in a furnace. The crucible is carefully extracted from the furnace then poured carefully into the shell. The bronze-filled shells are allowed to cool.
The shell is hammered away releasing the rough cast bronze. The spruing, which is also faithfully recreated in metal, is cut off to be remelted for another casting.
12. Metal – Chasing
Just as the wax patterns were ‘chased’, the bronze parts are now also worked until the telltale signs of the casting process are removed. Pits left by air bubbles in the molten bronze are filled, and the stubs of spruing ground down and chased to look exactly like the artist’s surrounding texture.
Lastly, the bronze is color treated to the artist’s preference using chemicals applied to the 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 the “patina,” and can be expressed with a wide variety of colors finishes, but mostly through a reddish-brown finish. (Ancient bronzes gained their patinas from natural oxidization and other elemental effects from being underground or underwater for many centuries.) Depending on how the metal is prepared, either blasted 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 somewhat.