People primarily used copper, stone and wood before 4000 BC to create tools. Bronze, a metal alloy using copper, produced a stronger and more durable material with which to work and soon replaced these other mediums for many objects. Bronze was, in fact, so important in the history of humanity that it transformed cultures across the globe and ushered in the Bronze Age, from 3500 BC to 1200 BC. Cultures as diverse as those from China, Greece, Rome, Nigeria and the Philippines were dramatically changed by bronze technologies, using it to cast everything from axes to helmets to cannons to jewelry.
Bronze is a metal alloy, or metal mixture, composed primarily of copper and tin, though other metals can be added. "Pure" bronze does not exist, as bronze is always an alloy. Approximately 3 percent tin added to copper creates a stronger and more durable metal than copper alone, giving the metal a wider array of possibilities. Bronze is considered "cast" when it is shaped in a mold.
Different processes can cast bronze. In all of these processes, however, a mold is created, usually of rubber in the 21st century, to shape the metal. Therefore, there are several steps necessary to achieve a successful cast bronze object: sculpting of an initial form (usually from clay) that represents the final object, formation of a mold from the initial sculpture and then a casting from the mold to create the final piece.
Hot bronze casting is the traditional method of casting bronze and the "lost wax" process is the most known and widely used. Developed by the Chinese in the 14th century BC, the lost wax process made it possible to cast complex objects, such as sculptural pieces. This method creates a 1/4-inch hollow mold of the initial ceramic sculpture and layers that wax with seven coats of ceramic shell material, which hardens. This shell is then fired, melting the wax and leaving a 1/4-inch hollow space for the hot bronze alloy to fill. Once the entire piece is cooled and hardened, the wax is removed and the artist can continue to refine her work by cleaning, chaising, applying patinas and using other methods.
Cold cast bronze, or bonded bronze, also typically uses ceramic sculptures and rubber molds to create a finished piece, but it is a fairly new method introduced in the 20th century. In cold cast bronze, a liquid plastic--usually a polyurethane resin--is mixed with a finely ground bronze powder and then poured into a mold. This means hot metal pouring is not necessary with this method. Layers of this heavy bonded bronze mixture are evenly coated within the mold; more layers add weight and density, eventually making it a solid object if desired.
Cold-Cast Bronze vs. Hot-Cast Bronze
Most bronze sculptures seen in the 21st century are not solid bronze and are manufactured by foundries through production methods. Artists create the original pieces, usually ceramic, from which the final product is molded, but the foundries melt the bronze and do the actual casting. Pieces of a figurine, such as arms and legs, are often created separately and then welded together. Cold-cast bronze is also common, as it looks and feels almost identical to hot-cast bronze, but costs less to produce because a substantial portion of the material is less expensive plastic.