The lost wax bronze casting process has a very long history in art; going back to ancient Greece when bronze sculptures were first made. The process is very long, and involves a lot of techniques and expensive equipment, so most artists go to a foundry to have their work cast. I work part time at Schaefer Art Bronze in Arlington back in Texas. Right now while I am at the Armory I have been working on a new bronze piece. I am learning to do all the steps of the process myself. First, I started with an original sculpture.
Once this oil clay sculpture is complete it is ready for cutting and molding. a piece like this has to be cut into multiple sections to be molded because the forms are too complex to cast in one piece.
Each rhino was cut off of the base at the ankles, leaving the feet attached to the base. The rhinos were then cut in half to simplify the forms and make a nice open space on each half to pour the wax into the mold.
The base will be a simple one piece mold. All the parts of the sculpture were set up on boards that left a 2-3 inch boarder all the way around the piece.
I used thin metal sheet to shim the seam line between the two halves of each rhino. This will be the divider between the rubber that will make it a two piece mold. Plastic keys were used to create a locking mechanism between the pieces of rubber. The pieces are all sealed with a layer of spray shellac and a layer of universal mold release. This will keep the rubber from sticking to the shims or to the oil clay.
Once the shimming was complete I used urethane rubber to make the molds of each rhino. Urethane rubber or silicone rubber is usually used to mold pieces for bronze because they release the waxes very well. Urethane molds can also be used to cast resin and some plastics. I started these molds by painting on a thin layer of the mixed rubber and once this was tacky, I was able to continue adding layers of rubber until thick enough in all areas.
When the rubber molds were completely set, the edges were trimmed and the rubber was sprayed with a few coats of epoxy parfilm. Since these molds were being made with a resin mother mold, epoxy parfilm keeps the resin from sticking to the rubber.
To make a mother mold, resin is mixed and poured out on cardboard. Fiber glass sheets are cut up into squares and patted into the resin until soaked. These are laid on the rubber and built up in several layers.
A mother mold is a hard shell that holds the rubber in the correct shape when you pour waxes. The fiber glass acts as a weave holding the resin together. This method makes a strong and lightweight mother mold.
When the resin has set the molds are ground with an angle grinder on the edges and the mother mold is pulled away from the rubber.
Once the molds were cleaned, and bolted together tightly the waxes were poured.
The waxes are removed from the molds and chased. This means that I sculpt on the wax to restore original textures and small details that were distorted in the mold and melt down seams from where the two halves met.
Because the base of this piece is so large it is cut in half. Wax rods called sprues are used in gating the waxes. Gating provides channels for the bronze metal to flow quickly and evenly to all parts of the sculpture.
The rhinos are also cast in two halves on one sprue tree. This will make it easier to dip the pieces in ceramic shell, and make the welding in the metal less difficult to work on.
Ceramic shell is a mixture of fused silica and a binder. The waxes are dipped in this mixture and then covered with fused silica sand.
The piece is dipped in the shell multiple times. The shell dries hard and solid making a negative impression of the inside and outside of each part of the sculpture, acting like a “shell”.
After the shell has dried it is fired in a burnout kiln and the wax is melted out. This firing hardens the shell so that it can then hold the molten bronze.
After the bronze is poured and has cooled, the ceramic shell has to be broken off the metal and sandblasted clean.
Once the metal pieces are cast, they have to be lined up and welded. Welding leaves a thick bead like a seam that then has to be ground down and made to match the texture of the piece.
Once all the pieces are welded together it is sandblasted clean one more time before it is given a patina.
To patina bronze a torch is used to heat the metal so that it can be sprayed with different chemicals. The heat allows the chemicals to react with the metal and create a color on the surface. Brushes and scrubbing pads can be used to create highlights in the patina.
Here is an image of the finished piece!
Great description of the process. It’s a long haul to be sure, but the final product looks great.