Wednesday, September 26, 2007
Cast Bronze Method
This piece is called "Aspiration-Delusion." As an artist I can only wish to be as skilled as ancient masters and have the conviction of the heroic ideal, but in the world we live in today, that's just not realistic. Below is the method for creating cast bronze sculptures as practiced at the Chicago Fine Art Foundry under Joe Kegler.
STEP 1 – Creating the clay original
The first step is to create a sculpture in either wax or clay. For my piece, I created a simple plaque out of clay, experimenting with surface and texture. If clay is used, a rubber mold must be taken of the clay original. If it a wax original, the piece can go directly to chasing and gating (step 4)
STEP 2 – Making molds
When the clay piece is ready, a layer of clear paint is sprayed on to eliminate the possibility of the liquid rubber being absorbed into the clay. Before the rubber is applied, a release agent is sprayed on to ease the removal of the cured rubber. The rubber itself is composed of a metal based latex mixed in specific proportion with a curing agent. After being mixed thoroughly, it is brush applied to the clay piece in layers until a thickness of approximately 3/16” - 1/4” is reached. After the rubber is cured, release agent is applied again and a plaster “mother mold” of approximately 1/4” is applied over it to keep the rubber rigid for the third step.
STEP 3 – Making a wax positive
After the mother mold sets, it is removed and the rubber mold is carefully lifted off the clay making sure to not tear the rubber. Any clay stuck in the rubber is removed and the mold is wiped down clean. A positive wax is created by melting down a petroleum based wax and brush applying it. The thickness shouldn’t exceed 1/8” – 3/16”, for this wax is to be replaced entirely by metal. (Bronze in this case, but aluminum was also used in several projects)
STEP 4 – Chasing and gating
When the wax cools, it is removed from the mold and the surface is chased by hand and using tools to clean and trim edges, fill in gaps from bubbles and repair any surface damage. The piece is then gated. The gating is made of wax an represents the circulation pattern of the molten metal.
STEP 5 – Investment a.k.a. “Dipping”
This is where the fun begins. Unlike standard investment, which is a cumbersome process and takes up a great deal of space, shell investment is a time consuming, but relatively painless process. The piece is “dipped” into a container containing a slurry which is composed of a polymer based coital and a fine meshed fused silica “flour”. The dipped piece is then covered in a dry, course fused silica “sand”. Each piece requires 7-9 layers. The layers need to thoroughly dry in between each layer and the excess loose sand is rubbed off before dipped. On the second to last layer, the piece is “clipped” with metal at the edges to prevent any cracking in the firing process. The final layer has no sand on it.
STEP 6 – Burnout
When the shelled piece is dry, the cup is grinded down to allow free flow of the wax which will be melted out. The piece is placed on bricks in the burnout room and a blower motor attached to the gas line is ignited. The super hot flame is directed at the bottom and slowly moved upwards, making sure all the wax is melted out. After burnout the shell is placed in a kiln to burnout any carbon scoring. Any micro cracks that appear during the burnout process are patched with a refractory material.
STEP 7 – Pouring
The piece is now ready to be poured. The furnace, which is as loud as a jet engine, is fired up, and metal is placed in the crucible. Depending on the number of pieces to be poured, additional metal is added. As the metal melts and reaches a molten state, the pieces are placed again in the kiln to bring them up to temperature so they don’t explode when the molten metal is poured in. At this point, safety equipment is worn. Heavy boots, heat shields, leather chaps and jacket, face shield and heat gloves. The furnace is turned off and the two man team removes the lid, and uses a heavy duty grabber to lift the crucible out. One man removes any debris and impurities using a scoop, while the other dips a foundry thermometer into the metal to test the temperature. Aluminum should pour at approximately 1300 degrees Fahrenheit and bronze pours at no less than 1900 degrees. A pouring loop is locked into place around the crucible and the team lifts the whole unit and pours into each waiting piece.
STEP 8 – Sandblasting
After a short while the metal solidifies and the shell is broken off. After it cools the gating is grinded off. Any welding or surface chasing is done to repair any discrepancies caused in the pour. When ready, the piece is taken into the sandblasting room, where the entire surface is cleaned and blasted. The blasting itself opens pores in the metal surface allowing it to react with chemicals in the patinas.
STEP 9 – Patina
Patinas are applied on the surface, either with brush, spray or sponge. They react with the metal to create color and texture. When patina work is complete the piece can be waxed.