BRONZE: an alloy of copper and tin which sometimes contains small amounts of other elements, such as zinc and phosphorus. It is stronger and more durable then brass, and has since antiquity been the metal most frequently used in cast sculpture.
To become a successful sculptor in bronze, a sculptor has to become familiar with the lost wax foundry process. Since most sculptors don't have their own facilities, the original is brought to a large commercial foundry specializing in fine art bronze casting.
To begin, a rubber mold is made of the original sculpture which is usually made of oil based clay or wax. This "mother mold" is produced by brushing on successive layers of thick rubber (silicone or latex) that, when dry, is surrounded by a heavy application of plaster. When the plaster dries, the "mother mold" is opened and the original is taken out and destroyed. With a now hollow mother mold, the process continues. Hot, liquid wax is poured into the mother mold to form a layer of wax of about 1/8 of an inch thick. After cooling, this wax is pulled from the mother mold and is an exact positive of the original. Care is taken to "chase" the wax replica; that is, to smooth out any discrepancies that occur while pouring the wax. The wax chasing is integral to the final outcome because how the wax looks at this stage will determine the final look of the metal.
At this point, an experienced foundryperson will sprue the wax. There are the gates and vents that the liquid metal will follow. Once the wax replica is sprued, a series of dippings into a ceramic slury is applied. Each of the six to eight layers of ceramic liquid is allowed to cure thoroughly. The wax replica is now encased in a thick ceramic shell and placed in a burn out oven. The oven is heated to 1,400 degrees Fahrenheit and the wax purged from the shell. Thus the name lost wax process.
When both the metal and the now empty ceramic shell are ready, the molten bronze is poured into the hollow shell and allowed to cool. Once cooled, the brittle ceramic is carefully chipped off, concealing the metal inside. After this, the bronze is sandblasted to remove any remaining pieces of ceramic and prepare it for any metal "chasing". An experienced metal "chaser" can then cut off the now metal sprues and weld any imperfections. At this time any fabrication that is needed is done for example such as reins on the bridle of a horse. After this, the bronze is again sandblasted to prepare it for the final step.
The patination process is simply the quickening or colouring of the metal. The bronze is heated with a torch to open the pores of the metal while different chemicals are either sprayed or brushed on, which produce different lustres and colours. Finally, the wax is brushed over the still warm bronze and polished when cool to lock in their colors forever. The bronze is then put on a wooden base and a name tag added. Sometimes granite or marble is added as well.
Caring For Bronze Sculpture
If you are fortunate enough to own a bronze sculpture, there are a few things you ought to know in order to properly maintain and protect it so that it will retain it's beauty and value.
Unfortunately, bronze is affected by moisture and chemicals in the air, and although outdoor sculpture is more susceptible to these factors, it is really only a matter of time before your indoor sculpture will begin to show signs of weathering as well. Bronze, and the paint like finish often applied to bronze; referred to as a "patina"; loses it's sheen, and even undergoes changes in colour, when exposed to impurities in the air.
Luckily, caring for your bronze sculpture is really quite simple. For indoor sculpture, regular dusting is a must, followed occasionally with a coat of wax. It it is an outdoor sculpture, you will need to clean it with a mild detergent and thoroughly rinse it before applying a coat of wax. There are several products readily available: Cinstantine Wax, Johnson & Johnson Paste Wax, and Butcher Wax are three used successfully. One coat of wax is all that is necessary, and it should be rubbed down to a thin layer. I use an old pair of nylons to polish with but be careful not to polish to hard and rub the finish off. A thin coat is preferable to a thick one as dust can collect between the layers of a thick wax coat, causing the wax to smear and gum up. A thin application of wax is more attractive and more water repellent, and creates a smoother finish. You can expect to apply a coat of wax at least once a year for an outdoor bronze, and about every eighteeen months for inside sculpture. Just remember that when the piece begins to look dull, it's time for another application.
Bronze restoration is still in it's infancy as a craft. However, much progress has been made in application and technique thanks to the combined efforts of chemists, artists, conservators, and art historians. Should your sculpture need a repair or if you need to have it appraised to determine age and value you can consult your local museum's conservation department, a professional restorer (most often found through trade publications or recommended by a museum or gallery>, or you can contact a bronze foundry.