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Joy Kroeger Beckner
NSS, SAA, AWA, AFC
Sculptor - Classical Realism, from Hounds to Humans |
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Clay to Bronze
Bronze is an alloy of 95% copper, 4% silicon and 1% manganese with traces of other elements such as iron. Silicon bronze has been the bronze of choice for fine art castings since its development in the 1920's. It is corrosion-resistant, strong, resilient, formable and weldable. Also known as "hot-cast" bronze, a fine art "lost-wax" casting of silicon bronze is created through many labor-intensive steps.
1. Clay ~ "The Original," clay is modeled to the desired form usually over an armature which supports the weight of the clay. Bas-reliefs may require an armature consisting of a grid of screws connected with wires to prevent clay from sliding. Very small sculptures are often modeled directly in wax which, though more difficult to model, allows for maximum detail.
2. Mold ~ A mold is made of the original. Excellent surface replication of the original can be achieved with a polyurethane mold compound or a high-quality silicon rubber. Layers of rubber ave are applied to the original over the course of several days. While the original is inside, a JACKET is formed around the flexible, rubber mold for support. The jacket, made of plaster, Hydrocal, or resin, prevents distortion after the original has been removed. The original is destroyed upon demolding. The mold is a NEGATIVE of the original which was a POSITIVE. Three-dimensional sculptures usually require multiple-piece molds. The learning process for making an excellent multiple piece rubber mold can take years. A novice should hire a professional, or make a plaster waste-mold of the original and make a plaster cast, a POSITIVE. From this solid POSITIVE a rubber mold can be made without risk of "losing" the original. The highest quality mold will save much time and money in production.
3. Wax ~ Liquid wax is poured, ladled or brushed into the mold. When the wax is cool, the mold is removed.
4. Wax Chasing ~ I repair seams, air bubbles, etc. in my waxes. I wish I could find a professional with the patience to chase my work.
5. SCULPTOR INSPECTS !
6. Spruing ~ The finished wax is a POSITIVE replica of the original clay. The wax s sprued, gated, with wax tubes which provide channels for the molten bronze to enter the wax sculpture. Small wax vents are attached to allow air to escape. A wax funnel is attached to the gates.
7. Ceramic Shell ~ The finished wax is dipped into a clay slurry, ceramic shell, which picks up minute surface details from the wax. It is allowed to thoroughly dry. It is dipped and dried about eight times. This can take several days depending on the level of humidity.
8. Burn-Out ~ The completed ceramic shell is inverted and heated in an oven. This causes the wax to melt out, thus becoming "lost." When the shell is cool. it is taken to a securely walled "sand box." The shell is surrounded by sand for support to protect it from breakage during the bronze pour. The NEGATIVE space in the ceramic shell will become the POSITIVE bronze casting.
9. Casting ~ Bronze is heated in a furnace to approximately 2250° Fahrenheit. Workers in protective face shields, clothing, gloves and boots carefully pour the molten bronze into the supported shell. When the bronze is cool, the shell is broken away. The bronze is blasted with tiny beads to remove the ceramic shell. Gates and vents are sawed off. The cast is considered a "raw metal."
10. Metal Chasing ~ A "metal chaser" carefully aligns, welds and repairs seams, gates and vents to closely resemble the artist's original surface.
11. Blasting ~ The cast is bead-blasted clean in preparation for patina. It is a glowing platinum color.
12. SCULPTOR INSPECTS!
13. Patina ~ Chemicals are applied to the bronze which has been heated with a blow torch. The patineur and the sculptor decide between 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 is recommended. Incralac™ is one such sealant designed specifically for copper and its alloys. Since Incralac™ produces a plastic-looking, high-gloss finish, the sculpture must be waxed to create the soft look of a traditional patina.
Bonded Bronze AKA "Cold-Cast" Bronze, is primarily resin. The surface is a skin of resin into which bronze powder has been blended. Bonded Marble is resin with marble powder. With a low-luster white surface, it is subtle and quite beautiful. Bonded Bronze and Bonded Marble are hand-cast, hand-finished, lighter-weight and less-expensive alternatives to bronze. Pewter, an alloy of nickel and silver, is another less-costly alternative.
Shrinkage will occur in any casting process. The mold shrinks. The wax shrinks. The metal shrinks. A sculpture which in clay was 26" long may be 25H" long in metal. Although I chase each of my waxes and check each finished metal, due to the handmade nature of this process, each cast and each patina will be similar but unique.
Volumes have been written on the subjects of mold-making, casting and patina. I hope you have found this overview interesting and enlightening.
Recommended reading: Patinas for Silicon Bronze and The Care of Bronze Sculpture -- by Patrick V. Kipper Rogers & Nelson Publishing Co., P.O. Box 7001, Loveland, CO 80537.0001 USA
People often ask me if I have my own foundry.
No ... very rarely does a sculptor do everything from start to finish. I oversee these talented professionals whom I've hired to take my ideas from Clay to Bronze.
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