Bronze is the most popular metal for cast metal sculptures; a cast bronze sculpture is often called simply a "bronze".
Common bronze alloys have the unusual and desirable property of expanding slightly just before they set, thus filling the finest details of a mold. Then, as the bronze cools, it shrinks a little, making it easier to separate from the mold. Their strength and ductility (lack of brittleness) is an advantage when figures in action are to be created, especially when compared to various ceramic or stone materials (such as marble sculpture). These qualities allow the creation of extended figures, as in Jeté, or figures that have small cross sections in their support, such as the equestrian statue of Richard the Lionheart. Modern statuary bronze is 90% copper and 10% tin; older bronze alloys varied only slightly from this composition.
But the value of the bronze for uses other than making statues is disadvantageous to the preservation of sculptures; few large ancient bronzes have survived, as many were melted down to make weapons or ammunition in times of war or to create new sculptures commemorating the victors, while far more stone and ceramic works have come through the centuries, even if only in fragments. As recently as 2007 several life sized bronze sculptures by John Waddell were stolen, likely because of the value of the metal after the work has been melted.
The great civilizations of the old world worked in bronze for art, from the time of the introduction of the alloy for edged weapons. The Greeks were the first to scale the figures up to life size. Few examples exist in good condition; one is the seawater-preserved bronze now called "The Victorious Athlete," which required painstaking efforts to bring it to its present state for museum display. Far more Roman bronze statues have survived.
The ancient Chinese knew both lost-wax casting and section mould casting, and in the Shang Dynasty created large ritual vessels covered with complex decoration which have survived in tombs. Over the long creative period of Egyptian dynastic art, small lost-wax bronze figurines were made in large numbers; several thousand of them have been conserved in museum collections.
From the ninth through the thirteenth century the Chola dynasty in South India represented the pinnacle of bronze casting in India.
Lost wax method
In lost-wax or investment casting, the artist starts with a full-sized model of the sculpture, most often a non-drying oil-based clay such as Plasticine model for smaller sculptures or for sculptures to be developed over an extended period (water-based clays must be protected from drying), and water-based clay for larger sculptures or for sculptures for which it is desired to capture a gestural quality - one that transmits the motion of the sculptor in addition to that of the subject. A mold is made from the clay pattern, either as a piece mold from plaster, or using flexible gel or similar rubber-like materials stabilized by a plaster jacket of several pieces. Often a plaster master will be made from this mold for further refinement. Such a plaster is a means of preserving the artwork until a patron may be found to finance a bronze casting, either from the original molds or from a new mold made from the refined plaster positive.
Once a production mold is obtained, a wax (hollow for larger sculptures) is then cast from the mold. For a hollow sculpture, a core is then cast into the void, and is retained in its proper location (after wax melting) by pins of the same metal used for casting. One or more wax sprues are added to conduct the molten metal into the sculptures - typically directing the liquid metal from a pouring cup to the bottom of the sculpture, which is then filled from the bottom up in order to avoid splashing and turbulence. Additional sprues may be directed upward at intermediate positions, and various vents may also be added where gases could be trapped. (Vents are not needed for ceramic shell casting, allowing the sprue to be simple and direct.) The complete wax structure (and core, if previously added) is then invested in another kind of mold or shell, which is heated in a kiln until the wax runs out and all free moisture is removed. The investment is then soon filled with molten bronze. The removal of all wax and moisture prevents the liquid metal from being explosively ejected from the mold by steam and vapor.
Students of bronze casting will usually work in direct wax, where the model is made in wax, possibly formed over a core, or with a core cast in place, if the piece is to be hollow. If no mold is made and the casting process fails, the artwork will also be lost. After the metal has cooled, the external ceramic/clay is chipped away, revealing an image of the wax form, including core pins, sprues, vents, and risers. All of these are removed with a saw and tool marks are polished away, and interior core material is removed to reduce the likelihood of interior corrosion. Incomplete voids created by gas pockets or investment inclusions are then corrected by welding and carving. Small defects where sprues and vents were attached are filed or ground down and polished.
Creating large sculptures
For a large sculpture, the artist will usually prepare small study models until the pose and proportions are determined. An intermediate-sized model is then constructed with all of the final details. For very large works, this may again be scaled to a larger intermediate. From the final scale model, measuring devices are used to determine the dimensions of an armature for the structural support of a full-size temporary piece, which is brought to rough form by wood, cardboard, plastic foam, and/or paper to approximately fill the volume while keeping the weight low. Finally, plaster, clay or other material is used to form the full-size model, from which a mould may be constructed. Alternatively, a large refactory core may be constructed, and the direct-wax method then applied for subsequent investment. Before modern welding techniques, large sculptures were generally cast in one piece with a single pour. Welding allows a large sculpture to be cast in pieces, then joined.
After final polishing, corrosive materials may be applied to form a patina, a process that allows some control over the color and finish.