Bronze sculpture

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Rare, water preserved Greek Athlete 310.B.C.

Jeté, a bronze by Enzo Plazzotta at Millbank, Westminster, London

A bronze sculpture of Richard the Lionheart (Richard the First), outside Parliament, London, England. Sculpted in 1860 by Carlo Marochetti

Bronze is the most popular metal for cast metal sculptures; a cast-metal sculpture of bronze is often called a bronze. Common bronze alloys have the unusual and very desirable property of expanding slightly just before they set, thus filling the finest details of a mold.

The strength and lack of brittleness (ductility) of the material is an advantage when figures in action are to be created, especially when compared to various ceramic or stone materials (see marble sculpture for several examples). These qualities allow the creation of extended figures (as in Jeté, right), or figures that have small cross sections in their support (such as the equestrian statue of Richard the Lionheart), both shown to the right. The value of the bronze for other uses is disadvantageous to the preservation of bronze sculptures; few large ancient bronzes have survived as during wartime many were remelted to make weapons or to create new sculptures commemorating the victors, while a far larger portion of contemporary stone and ceramic sculptures have survived, even if only in fragments subsequently reassembled.

The great civilizations of the old world worked in bronze for art, from the time of the introduction of bronze for edged weapons. The Greeks were the first to scale the figures up to lifesize. Very few examples exist in good condition of these cast works. The seawater-preserved bronze, now called "The Victorious Athlete" is a fine example but painstaking efforts were required to bring it to its present state for museum display. Far more Roman bronze statues have survived. The ancient Chinese, from at least 1200BC, knew both lost wax casting and section mould casting, and in the Shang dynasty created many 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 and several thousand of them have been conserved in museum collections. From these beginnings, bronze art has continued to flourish up to the present.

[edit] Process

The manufacture of bronzes is highly skilled work, and a number of distinct casting processes may be employed, including lost-wax casting (and its modern-day spin-off ceramic shell casting), sandcasting and centrifugal casting.

[edit] Lost wax method

In the lost-wax casting method, 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 gell 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 and the sprue can 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 is a critical step, to prevent 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, and any interior core, are removed with a saw. Any incomplete voids, due to gas pockets or investment inclusions, are then corrected by welding and carving. Finally the resultant small defects where sprues and vents were attached are filed or ground down and polished.

[edit] 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 as described above 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 the availability of modern welding techniques it was usually necessary to cast large sculptures in one piece, with a single pour. With the availability of welding, a large sculpture may be cast in pieces which are then assembled and permanently joined.

[edit] Finishing

In this bronze sculpture, Balance by David Ascalon, reactive chemicals were applied to the metallic surface to create a marbleized blue surface.

After final polishing, various corrosive materials may be applied to form a patina, a process that allows some control over the color and finish.

Another form of sculptural metal art to use bronze is ormolou. Ormolou is a finely cast soft bronze that is then gilded (coated with gold) which results in a matte gold finish. Ormolou was popularized in the 18th century in France and is typically found in such forms as wall sconces (wall mounted candle holders), inkstands, clocks and garnitures. Ormolou wares can be identified by their matte gold finish and clear ring when tapped, this indicating the underlying bronze as opposed to a cheaper metal alloy such as spelter or pewter.