Bronze Casting Process
The process of creating a lost wax bronze casting is a multi-step, labor intensive process. The steps of the bronze casting process are outlined below.
The sculpture is modeled in clay over an aluminum armature wire for small pieces. For large scale sculptures the clay is formed around a steel armature. The claying up process takes anywhere from 3 days to 2 months, depending on what it is that I’m creating. Portraits generally take a little bit longer as getting a likeness of someone in clay is a bit more time consuming.
Once the clay is finished the sculpture is prepared for mold making. This step is one of the more important steps along the way of turning the clay into bronze as the mold can be reused for multiple castings, depending on the project. A good mold can last for many years. I use a urethane rubber called 74-29 mixed with a thixo-tropic powder to capture all of the detail of my clay. This product has been a favorite material for many years as it is relatively cheap, has a long shelf life and captures all of the detail from my clay original, including fingerprints. A mother mold is made using plaster of paris and hemp to reinforce the flexible rubber mold.
Wax is poured into the hollow mold at a temperature of approximately 215 degrees Fahrenheit and rotated around so the wax covers all surfaces. I pour about 4 coats of wax (each coating slightly cooler than the previous) until I get a thickness of about 3/16″-1/4″. After the wax has cooled, it is carefully released from the mold.
Wax chasing is the process of repairing all of the imperfections that were formed during the wax pouring. This usually means filling in air bubbles, cleaning up seam lines and thickening areas that need it. A good wax pourer will leave you with very little wax chasing work which can save a lot of time.
Once the wax casting is cleaned up, wax sprue bars are attached to the wax casting. These sprue bars are a type of channel system which will allow the molten bronze to feed through the sculpture. They also provide the gases and air to escape without being trapped in a casting.
The wax casting with the wax sprue bars are then dipped into a solvent which cleans the surface from any debris it may have accumulated during the chasing and spruing. It is then dipped into a solution called prewet, followed by two coats of a very fine grained slurry. This stage of slurry picks up the finest details of the sculpture. It is then gone through several different slurry mixtures each one more courser than the next. After about 8 coats of the slurry mixture, the ceramic shell is sent to be dewaxed.
Lost Wax Process
The ceramic shell (with the wax inside) is placed in a high pressure sealed oven, called an autoclave. The high temperature and the pressure force the wax from the shell (aka Lost Wax Process) while also hardening the shell in preparation for the molten bronze. If you were to look inside the hollow shell with a camera, you’d see an exact negative created by the 3/16″ wax casting.
The shell is fired to ensure it’s proper hardness and also to warm it up to allow for continuous flow from the hot metal. Bronze is carefully poured into the opening of the shell at a temperature of about 2000 degrees fahrenheit. After the metal has cooled, the shell is carefully broken apart and all of the excess ceramic shell is removed from the bronze casting.
The bronze is then cleaned up and welded together. All of the sprue bars (which are now in bronze) are cut off and any parts that were cut apart in the spruing process are attached and core holes are filled in.
The assembled bronze now has weld lines through the sculpture that need to be cleaned up or “chased”. This process requires an air compressor and a few different pneumatic tools with different sized carbide burrs. Weld lines are ground down and textures are created to mimic the surrounding areas using carbide burrs. A good metal chaser will make these weld lines disappear blending them into the textures created during the clay process. At this point, the sculpture is carefully analyzed for any imperfections and corrected.
The sculpture is sandblasted in a blast cabinet and ready for patina. The patina is the coloring of the bronze. The patina combines the process of heating a bronze surface and applying a chemical while the surface is hot. Different chemicals will create a different colored surface and texture and so patinazation has become an art form in itself. Once the patina is complete it is sealed using lacquer and wax.
The sculpture is now ready for installation. To read more about Sutton and Dan’s sculpture of FO Stanley click here.
To see more of Sutton’s large scale sculptures click here.