Lost Wax:

People frequently ask me, "how do you make a bronze?" So here is an explanation of the steps with photos of how a sculpture is created. First of all, the process is called "lost wax" because a wax pattern is used to create a mold, and then it is melted out, or lost. Molten metal is poured into the cavity where the wax used to be. Jewelry is created this way, and so are fine art bronzes. Some artists do their own bronze casting, while others use professional foundries for the process.

-Original

Every sculpture starts with an original. For most fine art the sculptor works in an oil based clay which doesn't dry out during use. Since the process uses a series of molds, the original can be sculpted in anything; stone, wood, wax, or even water based clay.

-Mold

Since most fine art bronzes are created as editions with multiple copies planned, a mold must be created to reproduce the original sculpture. This mold is usually a silicone, or polyurethane rubber skin with a plaster "mother mold" to hold the shape from warping. The liquid rubber is poured or painted on top of the original sculpture and captures every detail, down to the fingerprints of the artist.

-Wax pattern

For each bronze, a wax pattern must be created. This is the wax which is lost, or "burned out". Small sculpture and jewelry can be poured as solid waxes which are then cast as solid metal. Medium and larger pieces are slushed with wax to create a hollow, 1/4" thick form. The finished bronze will also be hollow and about 1/4" thick. This is done because thick bronze shrinks, cracks, and creates flaws as it cools. Next time you see a large bronze statue in a park, go tap on it. You can hear that it's hollow inside. About the only things which are cast with thick walls of bronze are bells and cannon.

-Sprueing and Gating

Wax rods and dowels are attached to the wax pattern. They provide the channels for the metal to get into the sculpture. Gates are the spots where the sprues actually connect with the sculpture. Proper gating and sprueing is critical to the success of the pour.

-Ceramic shell

Next the wax is dipped in a slurry of ceramic soup which is about the consistency of pancake batter. After dipping, a stucco of fused silica (similar to sand) is sprinkled on the wet surface, then allowed to dry. By alternately dipping, stuccoing, and drying, layers of ceramic shell are built up on the wax. It takes about a week and 11 or 12 coats to create a shell 3/8" thick.

-Burnout

The wax is then melted out of the ceramic shell mold. Most foundries use a steam autoclave to flash heat the wax so it melts quickly and doesn't crack the shell. Any remaining wax is burned out when the shell is fired in a kiln. The resulting mold is hard, hollow, and porous enough to permit gasses from the molten bronze to pass through and provide good surface reproduction.

-Pouring the metal

Bronze is poured at about 2000 degrees farenheit, and it glows yellow-orange. The ceramic shell molds are brought out of the kiln still hot, and are set into a sand pit for the pour. The bronze is poured from a crucible into the open mouths of the molds. Sometimes droplets of molten metal spill and scorch the earth around the molds.

-Metalwork

Once the bronze has cooled a bit and solidified, the ceramic shell molds will begin to crack. The foundrymen then break off the rest of the mold material with hammers. The bronze sculpture beneath is mottled with black scale and areas of bright metal. The metal has faithfully reproduced the wax, sprues and all, so the sprues must all be cut off. Metal chasers clean up any sheets of flash where bronze has leaked into cracks in the molds. They also grind off the gate stubs, and repair any flaws in the casting. On most bronzes several pieces are cast separately which must then be welded together. The chasers clean up the welds, and repair any areas of texture which have been lost. It's a remarkably violent process and will scare any artist who hasn't yet witnessed it. The final step is to sand blast the surface to remove any tool marks and remaining scale from the cast.

-Patina

No bronze is complete without its color or patina. If no color is applied by the artist, the metal will be bright and brassy when it's new, then fade to a grey-brown after exposure to the elements. Instead, most artists choose the colors for their bronzes. By heating the piece and applying various chemicals, colors from black and brown, to blue, green, and white can be created. The patina process chemically changes the surface of the metal, and puts down layers of chemical, both of which create the color. Unlike paint, patina is highly dependent on the metal beneath. A coat of wax or lacquer protects the patina, and will shine with a little light buffing.