Following is an explanation of how a bronze sculpture is made, from the first model to the final mold to the finished piece.
Making the model
First, the artist sculpts a model in a wax-based clay. This "original" of the artist's vision provides the basis for the mold. Unlike a water-based clay sculpture, which can be fired, kept and sold, wax-based clay is good for nothing except sculpting. It has to be transferred into another material to make it saleable. Once the model is finished to the artist's satisfaction, it is taken to a foundry for a mold to be cast.
Applying the rubber mold
At the foundry, liquid rubber is applied to the surface of the clay model. The rubber is a very heavy product in its liquid state and goes into every hole and crevice of the clay. It is applied a few milimeters at a time. Once applied, it is allowed to set overnight, then another small layer is applied. This goes on until you get the thickness you need, usually 3/16" to 1/4". It usually takes about a week. Once it is dry, you have a flexible rubber mold.
Applying the "mother" mold
Because the rubber mold is flexible, there needs to be a hard backing to keep it in place. Therefore, the moldmakers apply a plaster of Paris (or plastic) mold, called a "mother mold," on top of the rubber. Together, these two molds make up the "master mold." After the plaster has hardened, the master mold is separated along a designated line. (Sometimes moldmakers will place shims or pieces of metal into the clay sculpture before the rubber and plaster molds are applied to make the separation easier.) The model is removed from the mold, and the inside of the mold is scraped clean. The original clay model is usually completely destroyed in the process.
Making a wax replica
The mother mold is put back together, and liquid wax is poured into it. When the wax hardens, it forms an almost exact duplicate of the original. However, the wax casting may not retain every tiny detail of the original, requiring additional work by foundry artisans.
Chasing the wax
After a wax replica is made, chasers then use special tools on the wax replica to match the detail and texture of the original model. The wax casting has to be completely perfect before it goes to the next stage, because anything in the wax will come out in the bronze. If the sculpture is to be produced in a limited edition, a separate wax model is made for each piece in a limited edition and is "chased" by hand. This process creates a limited edition of multiple originals.
Adding gates and sprues
Next, wax tubes called gates and sprues are added to the wax replica. These tubes will eventually form the conduits through which the wax will be "lost," and they will assure that the bronze is evenly distributed throughout the mold.
Investing
The wax casting, with the gates and sprues in place, now is dipped into a barrel containing a milky substance called ceramic slurry. After it is evenly coated with slurry, the casting is sprinkled with silica sand. These colloids, or silica, are a kind of glue to hold the shell together. The wax casting is dipped in slurry and sand alternately several times, over two or three days, to create a ceramic mold, or "investment." The investment is allowed to dry for several days.
Losing the wax
The ceramic-covered wax casting is put into a burnout oven or kiln for two to three hours at approximately 500 degrees Fahrenheit. The wax melts and pours out of the ceramic gates and sprues-which were formed in the mold around the wax casting-leaving an empty ceramic mold to accept the melted bronze. Sometimes this wax is collected, or reclaimed, to be filtered thoroughly and reused in another casting process.
Pouring the bronze
Once the wax has drained completely, technicians wearing heat-resistant suits and gloves remove the ceramic mold from the oven and place it into a trough of sand. The sand is there to support the invested piece, to keep it from tipping over. Then bronze, heated to between 2,200 and 2,300 degrees Fahrenheit, is poured into the mold. This is the stage where, if things are going to go awry, they usually do. If there is a crack in one of the ceramic molds, the bronze will run out. Workers are nearby with water pistols to spray a crack if they see one, to stop the crack from forming and salvage the mold.
Chasing the bronze
After the bronze cools, the ceramic mold is re-moved from the bronze casting with chisels and hammers. The raw casting still has the gates and sprues-now made of bronze-attached. Bronze chasers remove the gates and sprues, as well as the scars this removal leaves behind. Any bubbles, marks or other imperfections that came through during the casting are also filled in and touched up. After the sculpture is chased, it is sandblasted to remove all particles and residue from the surface so that the patina chemicals will stick.
Applying the patina
A patina, a chemical that colors the surface of a bronze sculpture, can be applied to the surface by brushing or spraying it on, or by burying the sculpture in a chemical-soaked textured substance such as sawdust or dirt. There are various kinds of patinas that give you different colors, hues and shades. Patinas can be as various as red, green, white or gold, depending on the chemical used. Common patina chemicals include salts, ammonias, nitrites, even urine (which, early patiners discovered, turns the bronze "the most beautiful emerald green"). The patina both adds color and provides a protective layer to the bronze to prevent it from oxidizing. While some artists do the patina themselves, many hire professional patiners, who experiment with a large number of chemicals and mixtures to make various patina "recipes."
Adding the final touches
If the sculpture is very large (called monumental or heroic sculpture), or if it is very complex, it usually is cast as separate pieces that can be handled easily. If so, at this point, the pieces will be welded together or taken and assembled at the intended site. The piece is then polished and mounted.
Maintenance
Even though bronze sculpture is easy to maintain, it is not maintenance-free. As long as a sculpture is cleaned and waxed twice a year, it will retain its patina forever, but if a piece is not maintained, especially if it's handled, the patina will begin to rub off and expose the bare bronze. A green oxidation will start growing. Some people like that look, but the original patina should be preserved.