Bronze Foundry Process

This video will take you through the bronze foundry process. Brett Barney the owner of takes us on a tour of his Burbank California art foundry. This video is perhaps the best way to edify those that wish to learn the lost-wax process.

Today’s Bronze Foundry Process continues to practice the ancient “lost wax” method of casting bronze. The lost wax process has long been the preferred method of casting bronze sculptures.

Lost-wax casting, sometimes called by the French name of cire perdue or the Latin cera perduta, is the process by which a bronze is cast from an artist’s sculpture; in industrial uses, the modern process is called investment casting. 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.

Other names for the Bronze Foundry Process include “lost mould,” which recognizes that other materials besides wax can be used, including tallow, resin, tar, and textile; and “waste wax process” or “waste mould casting”, because the mould is destroyed to unveil the cast item. Other methods of casting include open casting, bivalve mould, and piece mould.

For our Bronze Foundry Process statues we utilize the following steps…

Model-making Bronze Foundry Process


 1. Model-making – I creates an original model from wax, clay, or another materials. My choice of modeling clay is Chavant’s HARD NSP (Non Sulphurated Plasteline) because this material is very hard and retain excellent detail. Moreover, this clay does not have sulfur. The presence of sulphur prevents silicone from curing.
Moldmaking Bronze Foundry Process


2. Moldmaking – A mold is made of the original model or sculpture. The rigid outer molds contain the softer inner mold, which is the exact negative or the original model. Inner molds are usually made of latex, polyurethane rubber or silicone, which is supported by the mother-mold (outer mold). The mother mold can be made from plaster, but can also be made of fiberglass or other materials. 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. In case there are long, thin pieces sticking out of the model, these are often cut off of the original and molded separately. Sometimes many molds are needed to recreate the original model, especially large ones.
Creating the Wax- bronze foundry process

Creating the Wax

3. Wax – Once the mold is finished, molten wax is poured into it and swished around until an even coating, usually about 1/8 inch or 3 mm thick, covers the inner surface of the mold. This is repeated until the desired thickness is reached.
Removal of wax Bronze Foundry Process

Removal of wax

4. Removal of wax – This hollow wax copy of the original model is removed from the mold. The model-maker may reuse the mold to make multiple copies, limited only by the durability of the mold.
Wax Chasing Bronze Foundry Process

Wax Chasing

 5. Chasing – Each hollow wax copy is then “chased”: a heated metal tool is used to rub out the marks that show the parting line or flashing where the pieces of the mold came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were molded separately can be heated and attached; foundries often use registration marks to indicate exactly where they go.
Spruing Bronze Foundry Process


6. Spruing – The wax copy is sprued with a treelike structure of wax that will eventually provide paths for molten casting material to flow and 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. This spruing doesn’t have to be hollow, as it will be melted out later in the process.
Slurry Bronze Foundry Process


7. Slurry – A sprued wax copy is dipped into a slurry of 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 at least a half-inch 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 Bronze Foundry Process.
Burnout Bronze Foundry 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 burned up. 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 also hollow.
Pouring Bronze Foundry Process


9. Pouring – The shell is reheated in the kiln to harden the patches, then placed cup-upwards into a tub filled with sand. Metal 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 filled shells are allowed to cool.
Release Bronze Foundry Process


 10. Release – The shell is hammered or sand-blasted away, releasing the rough casting. The spruing, which are also faithfully recreated in metal, are cut off, to be reused in another casting.
Metal-chasing Bronze Foundry Process


11. Metal-chasing – Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, and the casting now looks like the original model. Pits left by air bubbles in the casting, and the stubs of spruing are filed down and polished.
Patination Bronze Foundry Process


12. Patination – The bronze casting surface is cleaned. The surface is bead-blasted before the patina solutions are applied.  The base coat is applied.  Base coats can be liver of sulfur.  It is applied hot using a torch, spray bottle or brush. The patina coats are a series of washes applied to the hot surface.  The temperature of the metal, the makeup and strength of the solution and technique applied, determines the colors achieved. The idea is to get even heating in the section you are working. Over heating will burn most patina solutions.  The correct temperature takes trial and error. Heating and adding solution will usually achieve the color one desires.  Applying wax while the work is still warm will seal the patina. Additional coats of wax and buffing will add life to the bronze statue.

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