Casting
Rodin produced his bronzes by two different foundry techniques, sand casting and lost-wax casting. Sand casting, explained below, was used almost exclusively by the Alexis Rudier foundry in Paris, which produced most of the bronzes in the Philadelphia Museum of Art’s collections.Sand Casting
ReplicationThe process begins with an original sculpture modeled by the artist in clay, wax, or plaster. From this original, a plaster replica, or model, is produced by means of small, sectional molds. After the replica has been cast through the process just described, dried, and coated (shellac was commonly used in Rodin’s studio), the plaster surface is dusted with a separator such as talc so that it will not stick to the fine molding sand that is then packed around it. In the case of small sculptures, the replica is half-buried in a leveled bed of sand held in a rigid frame. The exposed top half of the sculpture is then covered with pieces of fine molding sand, which are interlocked firmly together. Coarser sand is added in steps around and over the fine molding sand, supported in the second part of the metal frame. Then the frame is turned over and the whole process repeated on the other side of the replica. Building the Core
The plaster replica is removed from the metal frames, disturbing the very fine sand as little as possible. When the two frames supporting the sand are put back together, there is a large void where the replica had been. In order to make the bronze cast hollow, a very desirable feature for weight and other considerations, the void within the mold must be filled with a core. Made of plaster, sand, and organic material constructed around an iron framework, the core is uniformly smaller than the sand mold by about a quarter of an inch. Rods inserted into the core hold it in a fixed position for when the molten metal is poured. The sand surrounding the core is pierced to create channels. Some channels, called gates and runners, allow hot metal to enter the empty space; others, called vents, permit gas to escape during the process Making the Cast
Metal is melted in a crucible and poured into the mold through sprue cups. The molten metal fills the space between the core and the sand. After the bronze has solidified in the mold, the molding sand can be knocked away and the core removed mechanically. At this stage, the outside of the sculpture is littered with extra pieces of metal that leaked out into gates, vents, or cracks between the pieces of the mold. These are removed by grinding, sawing, or filing. Finishing the Sculpture
The metal surfaces of the cast are repaired wherever there are flaws, usually by setting in patches or plugs. Other adjustments to the surface appearance are made through procedures known as chasing and finishing, which often include texturing, abrading, and buffing. Finally, the bronze is coated with chemicals that cause the metal to change its color. This last process, called patination, is, like all the other steps in sand casting, a complex procedure involving the collaboration of the artist with skilled artisans.
For more information, please contact the Rodin Museum at (215) 568-6026.
