The process of creating a lost wax bronze casting is a multi-step, labor intensive process. The steps for creating a bronze sculpture are outlined below.
The sculpture is modeled in clay over an aluminum armature wire for small pieces. For life size sculptures the clay is formed around a steel armature. This claying up process takes anywhere between 3 days and 2 months, depending on what it is that I’m creating. Portraits generally take a little bit longer as getting a likeness is a bit more challenging.
The first process a sculpture goes through is the making of the rubber mold. This step is one of the most important steps along the way of turning the clay into bronze. 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 my favorite mold material for quite a few years as it is relatively cheap and seems to have a long shelf life. A mother mold is made using plaster of paris and hemp.
Wax is poured into the hollow mold at a temperature of approximately 200 degrees Fahrenheit and rotated around so the wax covers all surfaces. I pour about 4 coats of wax (each layer slightly cooler than the previous) until I get a thickness of about 3/16″. 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 created during the pouring of the wax. This usually means filling in air bubbles, cleaning up seam lines and thickening areas that need it. A good wax pour will leave you with very little wax chasing work which can save a lot of time.
Once the wax casting is cleaned up, wax sprues are attached to the wax casting. These sprues 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 is 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 shell 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. The 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 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 and different sized carbide burrs. Weld lines are ground down and textures are created using the 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 finished and the artist is happy with it’s result the patina is sealed using lacquer and wax.
The sculpture is finally ready for it’s display.