Lost Wax Casting Process
In theory the casting process is just like photography. An image is reproduced though the use of negatives and prints but in casting negatives are molds and our prints are castings.
In lost wax bronze casting the flip-flopping of the image, from negative to positive is repeated twice, first with the production of a rubber mold and a wax casting and again with the making of a ceramic mold and a bronze casting.
We start with a three dimensional sculptural image which can be sculpted in almost any material (clay, plaster, wood, stone, wax, etc.) as long as the original artwork is robust enough to withstand molding in rubber. With a suitable original in hand we are ready to start making our first negative the rubber mold.
1. The Rubber Mold
The rubber mold is a reusable flexible mold that can be opened to remove the wax casting without damage to either the mold or the casting.
This mold is made by layering liquid rubber over the original which is built up into a rubber blanket about three eighths of an inch thick. A parting line is planned for the division of the mold. A flange of rubber is built-up marking that parting line. The mold will later be divided along the parting line allowing the wax casting to be made in the mold and removed from the mold without harm to the wax or the mold.
With the parting line laid out and the flange built-up, metal shims are set into the flange as a divider for the plaster shell to come. The plaster shell is built up over one side of the rubber, up to the division formed by the shims. After this first half of plaster is made the metal shims are removed and a mold release is applied to the mating surfaces on this shell to separate one plaster shell from the next. The second half of the plaster shell is now formed over the second side of the rubber blanket. The second plaster shell completes the rubber mold. We now have the original covered with the rubber blanket which is supported by the plaster shell, our first negative.
The plaster shells are separated along the parting line where the mold release had been applied. The rubber blanket is cut along the same parting line and the original is removed. The rubber mold is now ready to be used to make our first positive, “the wax”.
2. Making “The Wax”
“The wax” is a three dimensional reproduction in wax of the original sculptural image. It is a single use wax pattern that the ceramic investment mold will be formed over and our first return to a positive form.
We start by making sure that the rubber mold is clean and that the rubber blanket will nest tightly in the plaster shell. With the rubber mold laid open on a work table we paint in the first layer of wax at a high temperature with a small brush to insure that we get all of the detail of the original without bubbles or defects. This is followed by the painting of a second layer of slightly lower temperature wax. With the mold coated in two layers of wax, we take some cooling wax in a putty like state and build up any high points protruding into the mold with a ribbon of wax.
The mold is now closed and scurried with clamps along the parting line. The mold is filled with wax that is allowed to cool in the mold for about one minute. As the wax cools it will deposit an even layer of wax throughout the mold, about three sixteenths of an inch thick. We now pour out the remaining volume of liquid wax leaving us with a hollow wax casting. The wax casting or pattern when cooled is removed from the rubber mold and our first positive is complete.
3.Touching Up The Wax
Keep in mind that the wax was made with only one purpose, that is to build another negative or mold over top of it. We need to be sure that this wax looks as much like the original artwork as possible and because it was cast in a multi part mold there will be indications of the mold parting line on the wax as well as other small imperfections that will need to be retouched and compared to the original before the next mold is made.
The retouching is the re-sculpting of the wax surfaces that need attention. Metal tools are heated and used to soften, blend, and texture the wax to match the original.
4. Gating The Wax
We now have the pattern we needed to build our next mold, this will be a ceramic investment mold. Before we can do that the wax pattern must be prepared for casting. In most cases we will need to cut into the wax and remove a reasonable size section to create a window to the inside of the hollow wax pattern. This is done so that we will be able to build up the investment mold on the inside as well as the out of the wax pattern. By doing so we will make the bronze casting hollow as well.
In building the investment mold we will be encapsulating the pattern in the ceramic investment. This leaves us with a problem. How do we get the wax out of the mold and the bronze in? To do this we will add sprues and gates. These are a network of branch like rods and bars of wax that will form the plumbing system of the mold. Because the wax is a positive that we will be building a negative over, everything that is solid in the wax or positive will be hollow in the mold or negative. That way the rods and bars of wax that are added to the pattern in the positive will become tubes in the negative. It is through these tubes that the wax will be evacuated from the mold and the bronze poured in.
The sprue and gate system itself is made up of a main sprue that runs from the top to the bottom of the wax with small branches that run uphill at an angle away from the bottom of the wax and the main sprue. This allows the metal to run up hill from the bottom, forcing the air out of the mold as the metal fills it. With the sprue and gate system in place we are now ready to build the shell mold.
5. Shell Molding
The ceramic shell mold is a single use waste mold system. One wax is needed to make one shell mold for one casting. This is the second mold or negative in the lost wax casting process.
A ceramic slurry made from colloidal silica and fused silica flour is used as the binder in this molding system. The wax patterns that have been prepared for casting are first dipped into the slurry coating the pattern inside and out as well as coating the sprues and gates. This coat of slurry is allowed to drain from the wax until there is only a thin film remaining at which time it is floured with in fine refractory sand. The sand sticks to the wetted surface of the wax and is allowed to dry. This process is repeated until the shell is approximately three eighties of an inch thick. The ceramic shell mold is now complete and allowed to dry.
6. Shell Firing And Bronze Pouring
The completed shell mold now needs to be fired in the kiln and poured in bronze. Keep in mind that the shell mold as it is going into the kiln still contains the wax positive at the same time it is the ceramic negative.
In firing the shell mold two things are going to happen, one is that the wax pattern will be melted out of the shell and the ceramic will be vitrified (fired to a glass like state). The kiln is preheated to eighteen hundred degrees and the shell mold with it’s wax pattern is plunged into the kiln. The wax begins to melt immediately, at the same time the shell is being fired. The wax pattern will melt out in about three to five minutes and the shell will continue to be fired for forty five minutes until it is clean of all carbon residue, fully vitrified and red hot.
At the same time as the shell mold is being fired, we are melting bronze in a separate furnace to be poured into the shell. When the metal has reached the pouring temperature, around twenty one hundred degrees, the red hot shell molds, which are now divested of the wax, are pulled from the kiln and filled with molten bronze.
Now that we have filled the ceramic shell mold with bronze what we have is the ceramic negative filled with the bronze positive. We must now remove the bronze casting from the shell mold. This is a waste mold process, the mold will be sacrificed to obtain the casting. The bulk of the shell will be removed manually and then the casting will be sandblasted to remove the small remaining ceramic particles and any oxidation that may have formed on the casting.
The bronze casting like the wax pattern it was made from has all of the sprues and gates attached to the original. These will be removed and the casting will be sent to the metal finishing department.
7. Metal Finishing
Each casting is the product of the wax pattern it was made from. The windows that were opened it the wax pattern must now be closed and parts that were removed must be replaced. The windows and parts will be welded in place, the welds will be ground down and the surfaces chased to match the original artwork.
We are now ready for Patina.
Patina is the chemical coloration of the surface of the bronze. The bronze will be sandblasted once more to even out the surface. It may be wire brushed, rubbed with steel wool or scotch bright, depending on the desired effect. Generally speaking the bronze will be heated with a torch and chemicals will be applied to the surface. The reaction of the chemicals on the heated bronze will deposit a layer of oxides on the surface of the casting. It is this layer of oxides that forms the patina. The use of different chemicals will result in different patinas. After the layer of oxides is complete, a coat of wax or lacquer may be applied to seal the patina from further oxidation.
The Bronze is now ready for the gallery.