Bronze sculpture by Frédéric Clerc-Renaud
Bronze Sculpture Making of
Bronze, an alloy of copper and tin, first appeared in the Near East toward the end of the fourth millennium B.C., there is no accurate record of when the lost wax method of casting was first utilized. It is believed the earliest users of the method began with a clay core roughly the shape of the subject to be sculpted. This core was covered with wax, and then sculpted to the finished form.
Once the wax hardened it was covered with clay. The object was baked, hardening the clay and melting the wax. Because the wax melted out into the bottom of the baking receptacle or oven, it was unusable or lost - hence the process is called "Lost Wax." The space evacuated by the wax was then filled with molten bronze. Once the bronze cooled and hardened, the clay was removed and the remaining bronze cleaned and polished.
The basic principle of the process has remained unchanged, although many new techniques have been developed. The most significant of which was the use of molds from which many waxes could be made. As opposed to cold cast bronze, a plastic-based product with metal flecks imbedded, cast bronze is a permanent medium. Archeologists have found pieces buried in the earth for thousands of years. American Bronze Foundry feels a strong sense of responsibility when creating art that will survive many generations. Please take a moment to view the steps to complete a bronze sculpture.
1. The Original Sculpture. Our Staff Engineers will study the sculpture. A determination is made on how the sculpture will be sectioned or "laid up" for mold making. Complex molds will require the original artwork to be cut into smaller manageable pieces. This is by far the most critical step of the procedure. The detail of the original must be captured and transferred in the mold, if not, they will be difficult to replace in any other step.
2. Mold Making. The sculpture is first sealed and then a release agent is applied to allow for easy removal of the rubber mold. Depending on the artist's requirements, we will create a silicone, latex, or polyurethane mold.
3. Now that the original sculpture has been prepared, the first coat of rubber is applied by hand. This is very important to obtain the proper level of detail. The Master Mold Maker must ensure the entire sculpture is covered with an even thin layer of rubber. Once the rubber has set, additional coats must be applied with "keys." These keys will identify the proper position of the mold when closed.
4. When the rubber is thoroughly set, a back-up shell or "Mother Mold" is applied to support the rubber for the wax pouring process. The keys previously applied are critical in the position of the rubber in this mother mold. This shell is normally made from plaster or fiberglass. If using a plaster mold, a plaster mixture is brushed on and strengthened with hemp. A fiberglass mold has proven to be most suitable for artisans to work with. It is durable yet lighter than plaster. One half of the piece is completed, allowed to dry, then flipped over to complete the other half. The back-up shell is removed and the rubber gently pulled back so the original sculpture can be removed. The rubber is then cleaned of any remaining particles from the original. It is now ready to receive the wax.
5. Wax Pouring. The completed mold, having already been prepped, is now ready for the wax. The mold is separated and the first coat of wax is painted into the mold to capture all the fine details of the artist's masterwork. The mold is put together and more wax is poured in.& It is then rotated until an even, thin coating is achieved. Once it has cooled, a second coat is applied and rotated to achieve a thickness of 1/8 to 3/16 of an inch. The excess wax is poured out. After the wax has completely cooled, the back-up shell and mold are removed. Now the wax casting is revealed.
6. Wax Chasing and Spruing. After seam lines are removed wax is used to fill holes and repair any defects resulting from air bubbles in thewax pouring operation. Wax rods are attached to the sculpture. These rods are called sprus or gates. A large cup is attached at one end, this cup will receive the molten bronze when poured. The placement of the gate system is critical. The molten bronze must flow through these areas and gasses must be allowed to escape.
7. The Shell. We are now making a secondary mold. Though the materials are much more modern, this serves the same function as mud did 5,000 years ago. We now dip the wax in a liquid binder solution that hardens as it dries. This material is called "slurry." Each coating is completely dried prior to the next. The number of coats applied to a piece is determined by the size and weight when metal is poured. The heavier the piece the thicker the shell must be to support the metal. The slurry coats go both inside and outside the wax allowing the sculpture to be hollow. The chemical make up of the slurry is monitored constantly to ensure its strength and humidity and temperature of the slurry room must be controlled to allow for optimum dryness and hardness.
8. Casting. The ceramic shell is removed from the slurry room and placed in an autoclave or a burn out oven cup side down. A burn out oven is simply a large flame heated oven, an autoclave is a large machine that heats up under steam pressure. Either method will allow the shells to cure (bake) and the wax to flow out. Once the shells are dewaxed, they are cooled and inspected for cracking. Once again they are heated up to 1800 degrees and placed in the pouring pit. Molten bronze is poured into the hollow shells and allowed to cool and solidify.
9. Knock out. The majority of the shell material is now removed inside and out, and the unfinished bronze casting is revealed. It is then glass beaded, water or sand blasted to remove any remaining shell material from the intricate details of the casting.
