The Lost Wax Method of Bronze Casting

 

Most sculptors prefer to work in wax or clay.
The finished model is taken to the foundry.
In order to prepare the model for molding, it is usually dissected by cutting the clay with wire...
...and by cutting the armature with a saw. Note the registration marks which later serve to realign the pieces.
The armature is an internal structural support device, not unlike a skeleton.
The various parts of the model are mounted on clay plugs before the rubber mold material is applied. The clay plug later serves as a pour spout for the mold.
The latex rubber is painted onto the model in layers.
Note the shim line which later serves as a separation line for opening the mold.
Once the mother mold is completed a plaster or fiberglass mother mold is applied to the outside which preserves the integrity of the rubber mold.
Once the mother mold is finished, it is separated and removed in order to remove the contents of the mold.
Once the original model is removed, the clean mold is reassembled and a wax pattern is poured.
By building several layers of hot wax within the rubber mold, a wax pattern is pulled from the mold. Note that it is a hollow duplicate of the original model.
Much time is dedicated to the restoration of the wax pattern to the exact likeness of the original. This process is referred to as chasing the wax.
The pattern is then sprued and mounted on a wax cup. the red sprues (solid wax rods) serve as gates and vents later in the casting process.
The "trees" are then taken to the slurry room where the ceramic shell is built.
The tree is dipped into the slurry tub and coated.
While it is still wet, the tree is coated with sand to build up the layer of ceramic shell. Between each of the five or six coats, the shell must dry. The process normally takes a week.
The dry shell is placed into a burnout kiln where the shell is cured and the wax pattern is melted out. This is where the term "lost wax" comes from. Each time a piece is cast, a new wax pattern must be pulled from the rubber mother mold which is again "lost" in the process.
The shell is then ready to receive the molten bronze. It is poured at a temperature of about 2000 degrees Fahrenheit.
Once the bronze is cooled, the shell cracks on its own accord and is manually removed from the bronze, usually by sandblasting. This could also be called the "lost shell" method of casting. The sculpture is now "metal on the floor".
The now bronzed gates and vents are then removed and the cast parts are welded back together.
Then the weld lines are tooled out and any imperfections in the metal are corrected.

This all takes place in the metal room where the critical talent of the artisans transforms the parts into a hollow bronze duplicate of the original clay model.

The completed metal sculpture is than heated with a torch in order to receive a patina.
The patina is chemically applied to the surface and built up in layers. This is an art in itself and requires years of training to master.
The completed sculpture is waxed to preserve the surface and buffed to a high polish.

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