The Bronze Casting Process
"I use sculptors wax warmed in hot water so it can be easily worked like clay. When warm, the wax can be carved and textured, allowing for both an additive and a subtractive process. I add wax to a rebar armature and model the sculpture full scale. Once an original sculpture is completed it is sent to the foundry for bronze casting."
-Ann Morris
Heron Bowl ~2004
Molding
The foundry's first step is to cut the original sculpture into sections that will fit into their burnout kiln.
Each section is covered in silicone to make an exact, flexible copy of the original. After the silicone is cured, its exterior is covered with a plaster-based material to form the cradle or mother mold. The cradle supports the silicone mold from the outside to impart rigidity.
Hot wax is pured into the mold and tooled to remove imperfections when removed from the mother mold.
Wax sprues (supports) and vents are added to the wax cast to allow the release of air during firing and create channels that bronze will eventually be poured through.
The entire wax cast and its support and ventilation system are sprayed with investment plaster and fired in the burnout kiln at 1100-1200 degrees.
The wax cast, prues and vents melt away, creating voids inside the plaster to be filled with molten bronze.
Casting
The plaster is dried for 7 days, after which it is ready to receive molten bronze.
The crucible is loaded with bronze ingots and heated to 2000 degrees. The molten bronze is poured into the investment mold and time is allowed for the metal to shrink as it cools, before being topped off to ensure a full cast.
Grizzly ~ 2006
Rib Canoe ~ 2003
Finishing
Once the bronze cools, the plaster is carefully and completely removed from the bronze cast. Vents and sprues are removed, and all of the various sections are spot welded together to exactly reproduce the original.
The artist is often present to direct this final process. Once satisfied with the positioning, final welds are made and dressed so they are invisible to the eye. When the final sculpture is approved, it is sandblasted, and imperfections are repaired.
Bronzes can be covered with a patina - a chemical reaction that imparts a desired color to the bronze when heated with a torch. Once the coloration and finish are approved by the artist, the piece is waxed or lacquered to preserve the patina.
Bronze sculptures need to be cleaned and re-waxed every few years to be properly maintained.