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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.
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American Bronze understands the needs of today’s artists to realize your latest work in bronze. Also, the needs of an art publisher who needs a professional, reliable and consistent casting partner. We are here to help you.
American Bronze Foundry offers unique as well as traditional bronze plaques including bass/portrait reliefs of any shape and size.