LOST WAX CASTING PROCESS
1. An original art object is created or sculpted in clay, wax, or any medium that renders the art form in three dimension. This art piece must be detailed exactly the way the finished bronze is to look in the final bronze sculpture.
2. A mold must now be made from the original clay art piece. (We will assume for the purpose of this explanation that the original was sculpted in clay). The permanent rubber mold is applied or brushed onto the clay in liquid form. Each coat is allowed to dry before the next coat is applied. The first coat, or print coat, is meticulously applied capturing all the detail of the original art piece. The mold rubbers we use captures all the detail right down to a fingerprint. At least 5 coats of rubber are applied in order to build up a layer approximately ¼ inch thick. Parting lines and shims were applied during this molding process in order to know where the mold will be split apart.
3. The rubber mold is now coated with a plaster shell. This backing material or “Mother Mold” is necessary to keep the rubber in the shape of the original art piece. In the last coat of rubber, we imbed rubber “keys” or bumps in the rubber surface. These keys act as registration and location guides so that the rubber mold will fit consistently into the plaster mother mold
4. Now that the rubber mold and plaster backing mother mold have been completed, the plaster is removed and the rubber mold is cut at the parting lines and removed from the original clay sculpture. The original clay sculpture has served its purpose and can now be discarded or reused in another project. The two halves of the rubber mold that were removed are now placed in the plaster mother mold and fitted into the keys. The rubber mold lining and the plaster shell halves are placed together and banded tightly.
5. Once the two halves of the mold are bound together, wax is poured into the rubber mold. The wax sticks to the sides of the rubber mold and captures the detail in the rubber, which is the same detail that was captured from the original clay sculpture. After a few moments, the excess wax is poured out and the mold is allowed to cool for a few minutes. This process is repeated several times until the wax coating on the inside of the rubber mold is approximately ¼ inch thick. When the final layer is cooled, the plaster backing is removed from the rubber and then the rubber is carefully removed from the wax impression that remains. The wax figure that emerges looks exactly like the original art piece that was sculpted by the artist. Of course, this wax will have some imperfections and parting lines where the two rubber mold halves were placed together. Our fine foundry artisans meticulously hand detail these imperfections in order to bring the wax model as close to the original as possible. This is a real art in itself and is an important part of the casting process.
6. When the wax model is perfect, it then goes to the sprueing table. Sprues or wax tubes are attached to the wax model and the other end of the sprues are attached to a pouring cup. These sprues will later become channels and vents for the molten metal to follow. Once the sprueing process is completed, the wax model with sprues attached is taken to the slurry room. Slurry is an ingredient used in the ceramic shell casting process.
7. The sprued waxes are then dipped in the ceramic shell slurry and then sprinkled with fine sand. The slurry and fine sand will capture the fine detail that is represented in the wax model. The slurry and sand is allowed to dry for several hours before an additional coat can be applied. As additional coats are applied, the slurry and sand are forming a thick layer of shell. Ten to twelve coats of slurry and sand are applied in order to build up a shell approximately ¼ to 3/8 inch thick. The beauty of this process is that the slurry and sand can get into detailed undercuts and hard to reach areas. Once the slurry and sand are completely dry from the final coat, the pieces are taken to the next step—the burnout oven.
8. The burnout oven is heated to approximately 1600 degrees and the dry ceramic shell pieces are placed in the oven. The intense heat flash fires the ceramic shell with the wax inside and the wax immediately begins to run out. Some of this wax can be reclaimed. Once the shells are baked for a period of time, all the wax and wax residue will be burned up. This leaves an empty ceramic shell that has the form and shape of the original art pieces with every detail. (While the shells are being fired, the bronze metal is being heated and brought to a molten state so it can be poured into the empty ceramic shells). This brings us to the actual casting of the bronze.
9. The melting furnace has been timed to bring the metal to approximately 2000 degrees at the same time as the ceramic shells are removed from the burnout oven. Using protective clothing, lifting tongs and heavy temperature resistant gloves, the foundry personnel bring the hot ceramic shells to a sand pit and cradle them into the sand with the pouring cup towards the top. The hot crucible, with the 2000 degree molten bronze, is carefully lifted out with tongs and pouring tools. The metal is carefully poured into the empty ceramic shells until the molten metal begins to run out the vents. Once they are full of bronze, the shells are left to cool. In a fairly short time, the shells are cool enough to begin the breakout process. The breakout process is simply where the shells are broken away from the cooled bronze—exposing the bronze art piece.
10. The sprues and vents, that are now metal, are cut off and reclaimed. This leaves the bronze art piece. (Of course, many art sculptures are cast in parts and pieces due to the complexity of the work. Therefore, arms, legs, antlers etc. often have to be cast separately and welded onto the art piece). When the art piece has had all the sprues removed, it will be sand blasted in order to clean up the bronze and remove any excess shell that did not come off in the breakout process. The art piece is now ready for the final stages. Any appendages, arms, antlers, etc. will be welded on in this stage. If the animal, for instance, was poured separately form the rocky base it will sit on, the animal must be welded to the base. Now all the welding marks must be ground out and the texture put back in with grinding tools in order to look exactly like the original. Once this is done and the bronze is found flawless, it goes to the sandblast booth once again for final sandblasting. This is really a fine glass bead that cleans and polishes the surface and prepares it for the patina—or color process.
11. The patina is applied with chemicals and heat to the surface of the bronze. It is actually an accelerated oxidation process and achieves many beautiful natural patina colors. You have probably seen an old penny with oxidized green and reds on the surface. Since bronze is approximately 94% copper, we are able to achieve some of the same beautiful patinas. However, with our modern day chemicals and process and protective coatings, the finished product is more beautiful than that old oxidized penny. When the desired patina is accomplished, the bronze is coated with wax while it is still warm. The wax covers all the surface of the bronze and actually gets into the surface of the metal preventing or extremely slowing down any further oxidation of the bronze.
12. After the bronze art piece is cooled, it is buffed to a nice sheen and mounted on a wood or marble base. The finished art piece is now ready for delivery to the excited purchaser! The bronze sculptures are usually cast in a limited edition in order to maintain the value and collect ability of the art piece. Bronze sculptures give a rather permanent rendition of the art piece that can be passed down for generations. Unlike resin sculptures that are basically plastic resin that is poured into a rubber mold and then painted. While they are beautiful art pieces, they are not permanent and do not have the lasting value of a bronze sculpture.
13. The lost wax bronze casting process is, in principle, over 4000 years old and has rendered many wonderful art pieces that have been in existence for thousands of years. Our modern methods and materials have made it a wonderful art form for modern sculptors and collectors alike.