The Creation of the Original
The sculpture starts with an inspiration and design in the artist's mind. The sculptor usually works from the inside out, using wire to create the "bone structure" or armature. Once the armature is constructed, the artist then builds up layers of clay or wax to create the form of the sculpture. As the clay is refined and the gesture or expression has taken shape, fine details and textures are sculpted into the surface.
In preparation for the casting process, many sculptures must be cut into multiple sections that will be individually molded and cast. The number of sections depends upon the size and complexity of the piece. Each section is marked with notches to ensure proper alignment when rejoined in metal.
Making the Mold
The metamorphosis from clay to bronze begins by applying a silicon rubber mold in two hemispheres to each section of the sculpture. The liquid rubber captures all of the sculptures details and is then encased in a plaster mother mold that enables the form to be held in place without distortion by the flexibility of the rubber. This two-part mold will be used to duplicate the sculpture in a limited edition and will be destroyed once the full edition has been cast.
Pulling Wax Patterns
After removing the original clay sculpture, wax is heated to approximately two hundred degrees Fahrenheit and is poured into the rubber mold, creating a thin coating. This first "hot" layer fills in the finest details while subsequent layers of cooler wax (160-180 degrees) are poured in to build up the wax pattern until it is one eighth to three sixteenths of a inch in thickness. After the cooled wax positives are pulled from the mold, skilled artisans hand finish, or "chase," the patterns to reveal their originally sculpted details and textures.
Creating the Gating System
After the wax is chased, a gating system is engineered to provide channels through which the molten bronze will travel. This is done with wax sprues, or branches, that are attached to the wax pattern. The gating system is designed for efficient flow of molten bronze and allows gases to escape. Each sculpture must have its own unique gating system.
Creating the Ceramic Mold
The gated wax "tree" is now invested in a ceramic shell. This is done by dipping the tree in a vat of ceramic slurry, coating both the inside and outside of the hollow waxes. Immediately after it is dipped, it is bathed in silica sand. These steps are repeated 8-10 times, drying in between each layer, to encase the waxes in a thick ceramic shell.
Burning Out the Wax
Once the ceramic mold has dried and cured, it is placed in a burnout oven and heated to melt the wax out, hence the term "Lost Wax." A hollow cavity now resides inside the shell where the 1/8'' wax pattern and gating system once were. These hollow cavities and channels will now act as arteries to carry the molten bronze to each section within the shell.
Pouring the Bronze
The bronze alloy of today, known as silicone bronze, consists mainly of copper, silicon and manganese. If poured into the shells at room temperature, the molds would crack and the bronze would cool too quickly, hardening and blocking the narrow portions of the mold. To avoid this, the ceramic shells are preheated while bricks of bronze are melted at over 2,000 degrees in a large ceramic crucible.
Three artisans are generally involved in the pour. The "lead pour" directs the hand-held crucible to the awaiting shells while the "deadman" is responsible for maintaining its balance. The third member keeps the surface of the molten bronze clear of any impurities or slag. Teamwork amongst the three is essential for the safety and success of the pour.
Removing the Shell
To retrieve the casting, the ceramic mold is broken away. Once the shell is removed, the engineered gates are cut and removed. The surface of the artwork is sandblasted, freeing the crevices from ceramic residue.
Reassembling the Bronze
The Oregon Duck from Clay to Bronze:
The Lost Wax Process at Firebird Bronze Foundry
Artisans now refer to photographs and measurements taken of the original clay sculpture to reassemble the sections of the casting. The seams are TIG welded using a rod of the same bronze alloy. Like the wax pattern, the bronze must be chased and cleaned, ensuring that excess metal is ground off and any pits are filled. The weld lines are sanded down using carbide-tipped grinders and surface texture is added back in to blend with the rest of the sculpture. Seams are undetectable in the work of a skilled craftsman.
Applying the Patina
Patination is a chemical reaction between the surface of the bronze and a variety of metal salts. This reaction is achieved by applying heat to the bronze. Each metal salt, usually a nitrate, reacts as a different color; different application techniques will yield various results, from a perfectly even coating, to an organically marbled look. Often, the origin of a bronze sculpture is identified by its unique patina. To protect the patina, wax is applied while the bronze is still hot. As it melts, the wax seals the metal's pores and, once cooled, several more layers are applied by hand and buffed to give the sculpture is final luster.