Outdoor sculpture, whether of stone or non-ferrous metals, should be rinsed frequently with clean water. Three to four times per year, only during warm weather, the sculpture should be washed with water and mild soap using a soft cloth and a soft-bristled brush. Small areas can be cleaned using a small soft toothbrush if necessary.
The treatment of outdoor sculpture to prevent moisture penetration is very important. Non-ferrous metal sculptures (including bronze, aluminum, lead and pewter) should be waxed and polished when clean and completely dry. A paste wax should be applied with a soft cloth, allowed to dry, and buffed with another clean soft cloth. Pieces that have not been treated on a regular basis sometimes require several applications before all pores are filled and wax accumulates to seal the surface.
Fertilizer, often comprised of acid-forming salt, can damage sculpture. The use of broadcast fertilizer spreaders should be monitored closely. If fertilizer material is found on a sculpture, it should be brushed off immediately. Flushing the sculpture with clean potable water will eliminate any residual deposition.
The earliest known “lost-wax” castings date from the early dynasties of Egypt, nearly 7,000 years ago, when metal was poured into “investments” of fired clay that had been shaped with the help of wax that was melted away, or “lost”. Then, sometime between 4,000 and 3,000 B.C., bronze was discovered as a metal that was harder than its components of copper or tin alone. That marked the beginning of the Bronze Age. Ancient bronze castings have survived to this day. Elements of the “lost-wax” process have been refined by improved methodologies, yet today, bronze casting is essentially the same as it was in 2,000 B.C.
1. Making the Original Clay Sculpture
To begin the process of making a bronze sculpture, most sculptors choose to make an original out of clay, although any material is possible. Most of the artists prefer to work with oil-based clay (also known as plasticene, plastilene, or plastilina). The main reason for working with oil-based clay is because it never dries or hardens. It can always be softened and reworked if a change needs to be made. The main concern with using oil-based clay is how to have the clay soft enough to build the sculpture rapidly and yet at the same time have the clay hard enough to produce good detail.
2. Making a Mold of the Clay Original
When the clay sculpture is completed, a mold must be made of the original clay. This is done with a polyurethane mold compound or a high-quality silicon rubber. The rubber mixture is applied directly to the surface of the clay, using a brush to slather it onto the surface. Once the first coat of rubber has been applied to the surface, it must be allowed to thoroughly dry before the next coat is applied. A mold most generally consists of three to five coats of rubber applied over the course of several days. Once the final coat of rubber has dried, a firm outer mold (a mother-mold) is made to help retain the shape of the more flexible rubber mold. When all of this is complete and it is all dry, the mother-mold is opened, and the original is removed and the two parts of the mold re-assembled. The process has now created a negative form of the original. If the sculpture is large or complex, it will usually have to be divided into smaller sections, with each piece needing its own individual mold. The sections will be rejoined later after the sculpture is cast in bronze.
3. Pouring a Wax Pattern from the Mold
With the mold assembled, positive forms can now be generated by pouring a wax replica. Pouring a wax is usually done in multiple coats. The first coat is poured into the mold, and the mold is rotated about, in an attempt to coat the entire internal surface of the mold with the melted liquid wax. The later coats of wax is applied slightly cooler. A completed wax should be approximately 1/4” thick, or just slightly thinner. When the wax cools and the mold is removed, a wax positive of the sculpture emerges
4. Wax Chasing
"Wax chasing" is the procedure of repairing all of the imperfections that were created during the pouring process of the wax to maintain the original details within the mold. The will air bubbles, seams, and mold lines which are removed from the surface of the wax positive. Some sculptors continue to sculpt in the wax far beyond the profile of the “original.”
5. Spruing a Wax
The finished wax is will go into a mold and be melted out leaving a hollow in the mold to be filled with bronze. Plumbing is required to bring metal into that hollow, and allow the hot gasses to escape. A network of wax rods, called sprues and gates, are next attached to the positive wax model. Also, a wax funnel (called a pouring cup) is attached to the top of gates for use during pouring.
6. Ceramic Shell
The ceramic shell process requires a series of dipping the wax positive into a mixture called slurry to create a hard shell. This ceramic shell, once dry, becomes a hard, durable shell around the entire sculpture that is going to receive, hold, and shape the molten metal to produce the bronze figure. The clean wax is dipped into slurry, followed by two coats of a very fine grained sand. This is where all of the fine detailing in the piece is picked up. The sculpture then progresses through the slurry process, into different slurry mixtures, which are various grades, gradually becoming courser with each coating. Each time the wax is coated with the ceramic slurry mixture, it is allowed to dry thoroughly. Each coat adds support to the overall strength of the shell. The ceramic shell process consists of seven to nine coats of slurry mixture. Once all of the coats have dried, the ceramic shell is then sent to be “dewaxed”.
7. Burning Out the Wax
When the ceramic shell is complete, it is placed in a kiln and the wax melts out, thus becoming "lost", leaving the detailed impression within the shell, which will be filled with the molten bronze. This step also hardens the ceramic shell in order to prepare the shell for the extreme temperature of the molten bronze.
8. Pouring the Bronze
When the ceramic shell is empty of wax, it is then re-heated. The bronze, meanwhile, is melted in a furnace and to a temperature of approximately 2250 degrees. When the molten bronze is ready, the foundry workers very carefully lift the crucible, containing the liquid bronze, out of its heating furnace. The workers must wear protective face shields, clothing, gloves, and boots. Moving quickly and very precisely, the foundry workers pour the liquid bronze into each awaiting ceramic shell. When the ceramic shells are all full with the poured bronze, they are then left to cool for several hours. The negative space within the ceramic shells have now become positive bronze castings.
9. Metal Chasing / Assembly
"Metal chasing" is the process of finishing the metal. This process usually involves a fair amount of welding. Pneumatic grinders are often used to remove sprues. It may also be necessary to retexture some areas of the bronze. If the sculpture being created was rather large or complicated, it will have been cut into pieces during the mold stage. This is necessary in order to be able to make molds that would be small enough to pour bronze into evenly. When all of the pieces have been poured and chased, they can then be fit back together to create the whole piece. If this is the case, all of the separate bronze pieces must be welded together to create the whole sculpture. All of the weld lines must also be chased away so that they disappear and it looks as though the sculpture was all one piece to begin with.
10. Patina
The "patina" is the color of the bronze. If left outside, bronze would normally oxidize, and the color that it would become would depend on other elements that come into contact with it. Patinization accelerates the process of oxidation of the bronze usually with heat and introduces chemicals to produce color and texture. Certain chemicals will produce certain predictable colors when they are applied to the bronze and then heated. A wide variety of techniques can then be used for chemical application, the most common being done with natural bristle brushes. Chemicals can also be applied with spatter guns, spray bottles, and airbrushes. After the patina is applied, two sealants to protect the patina. The traditional method of protection is to apply several thin coats of clear paste wax to the surface, which is then lightly buffed with a soft, clean, cotton cloth. This patina should last indefinitely indoors. However, if people repeatedly touch certain areas, the bronze may be expected to eventually shine through. Because today's outdoor atmosphere is more corrosive and carries a higher content of manmade pollutants, a more durable lacquer metal protectant may be used.