During the second half of the fifteenth century there grew a taste for collecting small "bronze" statuettes. Although they have always been called bronzes, that is, made of copper and tin, many were actually alloys of copper and zinc and thus more properly brasses. (In fact, various alloys of copper with tin, zinc, and lead were used.) The artistic inspiration for these statuettes was frequently from ancient Roman prototypes and certainly the whole notion of fashioning and collecting small bronzes was inspired by antiquity.
The great majority of Italian sculptors chose to use the so-called lost-wax technique. Many of the earliest of these small bronzes were cast solid and by the direct method (64.304.1). In this, the simplest of methods, the statuette was first carefully modeled in beeswax and then rods of wax were attached to convenient positions such as the underside of the feet. Next, both the statuette and the attached rods, excluding their tips, were coveredor "invested," to use the proper foundry termwith a refractory material such as potter's clay and the clay was allowed to dry. The whole assembly was heated in a furnace which both fired the clay and burned out the wax (hence the term "lost wax"). The investment was now a hollow ceramic mold pierced with the holes"sprues"left in the clay by the wax rods. The mold was then filled with molten metal through one of the tubular sprues, while the other sprue released the suddenly heated air inside the mold, thus ensuring that the mold fully filled with metal. After cooling, the clay mold was broken away, revealing the original wax model as well as the sprues, now transformed into metal. The sprues were cut away and the bronze surface worked with tools ("chased") to the desired degree of finish.
While the direct lost-wax method was capable of casting models of great delicacy and complexity, it had two great disadvantages. If, for any reason, the casting failed, the wax modelthe sculptor's entire workwas lost forever, since the model had been destroyed during the firing of the mold. Furthermore, even if the casting was successful, only one bronze could be produced from a sculptor's model.
No later than the last quarter of the fifteenth century, Italian sculptors began to use methods to cast bronzes without destroying their original modelso-called indirect casting. The earliest sculptor known to have routinely used indirect casting was Antico of Mantua (ca. 14601528). His bronzes show a total mastery of indirect lost-wax casting (55.93). Ironically, indirect casting was well known to the Greeks as early as the seventh century B.C. (1972.118.54). However, since no description of indirect casting had survived from antiquity, the method was not known to Renaissance artists. We do not know whether indirect casting was reinvented in the fifteenth century or merely revived, since at least some indirect casting was practiced during the Middle Ages.
All methods of indirect lost-wax casting involve casting a second model in wax, the so-called intermodel, from the original model made by the sculptor. Once an intermodel has been made, it can be cast in metal exactly as described for direct casting. The secret lies in knowing how to cast an intermodel. A sculptor may model wax into any conceivable shape, such as complex folded drapery, wind-blown locks of hair, or multiple entwined bodies. This represents no particular problem in direct casting, since even the most intricate model can be invested in layers of near-liquid clay slip and allowed to dry. Removing the model from the mold in direct casting is not a problem because the wax will be burned out in firing. With indirect casting, the difficulty lies in removing the original model from the mold without destroying either. In the fifteenth century, this was done by so-called piece-molding, in either clay or plaster of Paris.
Piece-molding requires considerable skill and forethought. Assuming the sculptor is using plaster of Paris, he must first mentally divide the surface of the model into a series of sections. He then envelops the model in a shell of plaster following those mental divisions in such a way that no piece of the rigid plaster shell mechanically interlocks with the model. Obviously, the rigid sections of the plaster shell must not only carefully enclose the model; each plaster section must precisely fit the one adjacent to it. One may compare this to making a plaster of Paris jigsaw puzzle around a three-dimensional object. The pieces of the mold are then carefully disassembled and the wax model removed and saved for future use. The plaster mold is then reassembled, section by section. The assembled mold's sections are then temporarily but securely bound together and the now-hollow mold filled with molten wax to cast the desired intermodel (engraving of piece mold from Diderot's Encyclopédie). (The wax intermodel will not stick to the piece mold if the plaster is dampened before filling it.) After the intermodel is freed from the piece mold, the wax rods which form the sprues are attached, as previously described.
Most bronzes are actually hollow cast. One method of hollow casting is to allow the molten wax in the dampened mold to only partially harden and then pour out the excess wax, leaving the mold lined with a thin shell of wax. This shell is then filled with plaster to form the "core." To ensure that the final bronze has walls of uniform thickness, this core must be securely fixed in place within the final casting mold. This can be done by penetrating the wax shell down to the core surface with numerous bits of fine iron wire or small nails. These are left projecting so as to engage the final investment and keep the core propped in placed within the mold after the wax has been burned out. From this point on, the casting procedure is identical to directly cast bronzes.
Obviously, one can make numerous wax intermodels from the same piece mold, and consequently numerous essentially identical bronzes all of which are replicas of the original model. What is less obvious is that the original model need not be of wax at all. Any durable material will serve, even a previously cast bronze. Indirect casting permits the sculptor to produce numerous bronze replicas of a popular model. This early anticipation of mass production permitted the distribution of sculpture to a much larger audience than ever before and, like the printed book and the engraving, helped spread Renaissance ideals throughout Europe.
Stone, Richard E. "Direct versus Indirect Casting of Small Bronzes in the Italian Renaissance". In Heilbrunn Timeline of Art History. New York: The Metropolitan Museum of Art, 2000–. http://www.metmuseum.org/toah/hd/bron/hd_bron.htm (August 2007)
These related Museum Bulletin or Journal articles may or may not represent the most current scholarship.