The most traditional method of casting bronze is the Lost Wax Process. This process is more commonly used for figural sculptures capturing fine detail whilst the other common method, Sand Casting is used for larger pieces not requiring such fine detail.
Here we look at the Lost Wax Process step by step.
The 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.
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. Most moulds of small sculptures are made from plaster, but can also be made of fibreglass or other materials. To preserve the fine detail on the original artwork, there is usually an inner mould made of latex, vinyl, or silicone 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. Sometimes, especially in the case of large originals such as a life-size, the original is broken down to manageable parts requiring many moulds to be made to recreate the original sculpture.
Once the plaster and latex mould 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 mould. 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 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.
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. 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 moulded separately can be heated and attached. Foundries often use registration marks to indicate exactly where they go.
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. Spruing usually begins at the top with a wax cup, which is attached by wax cylinders to various points on the wax copy.
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. 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.
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.
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.
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.
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.
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.
The bronze is coloured 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 colouring is called patina, and is often green, black, white or brown. Current artistic trends 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 laquer as a sealer on more unstable patinas. This protects the piece more from ultraviolet rays. Some patinas change colour over time because of oxidisation, and the wax layer slows this down.
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