Making Bronze Statues
Sculptors and artists have been making statues from bronze for nearly 5,000 years. The mixture of copper and tin is the most common material used for metal sculptures because it will expand just before it sets. This ensures that the details of a mold will be filled with the metal. The metal is also strong, which makes it ideal for sculptures that have extended pieces that aren't particularly thick, such as a soldier holding a sword.
One method of casting is called lost-wax casting. This process creates a bronze replica cast from a wax or clay model. A mold is made of the model. The mold can then be used to create one of many hollow wax copies of the original model. The hollow wax model is removed, prepared and dipped in silica and stucco to create a ceramic shell on it. The shell is then baked in a kiln where it hardens and the wax of the hollow model melts and runs out. The new ceramic mold is then checked for flaws. If it is acceptable, it is reheated and molten bronze slowly poured into it. The ceramic mold is then sandblasted away, revealing the bronze sculpture.
Centrifugal casting is used for small parts of jewelry. Molten bronze is placed in a crucible next to a mold on an arm of a centrifugal-casting machine. The arm spins in circles, forcing the metal into the details of the mold.
Sand casting uses a sand mixture (sand, silica, clay, water, additives) to make a mold, usually a pattern made in the sand. The molten bronze is then poured into it. The mold is cooled until the bronze sets. The mold is then removed, leaving the cast piece. This method can be used to give a piece needed texture.
Creating Large Sculptures
Creating a large bronze sculpture begins with a small model to ensure all details and proportions are correct. This is scaled to a medium-sized model. This model is used to decide on how large the armature for the full-size model should be. The armature is then built and a full-scale model made from plaster or clay. Manageable pieces of the sculpture are then cast in bronze. The pieces are then welded together to form the final statue.
When casting is complete, the final piece is polished. Corrosive agents may be applied to create a patina that gives the sculpture its desired color and finish.