by Craig “Buck” Campobella
How do you make a bronze? This question is asked so frequently with so much confusion, I will attempt to give a brief description of what is actually a long, arduous process.
True foundry cast lost wax bronze sculpture should not be confused with cold cut or bonded “bronze” which is a process of mixing metal powder with resin (plastic) and adding steel balls to the mixture in order to add weight. True bronze, on the other hand, has its own weight. It cannot be ground up or pulverized. It is nothing like brass, pewter, resin (plastic) or the so-called “solid” bronze also held together with a resin. Nothing can replace lost wax bronze for enduring beauty, timeless durability and increasing value.
The original sculpture begins with an armature usually made from pipe, wood, wire, or anything else that will make a “skeleton” with the strength to hold the weight of the table-size model to be sculpted in clay as well as the weight of the mold that will be made of the clay model.
After the sculptor completes the clay model, several layers of urethane or silicone rubber mold is then made directly on the original clay sculpture. A plaster “master mold” is then made encasing the rubber mold to hold the rubber mold rigid. Molds are removed, reassembled, and filled with repeated thin layers of melted wax, each poured at a lesser temperature so as not to melt the one just poured. The mold is then peeled away from the hardened wax and the wax is “chased” to remove seams and bubbles to replicate the original clay sculpture. The wax replica is inspected. Wax rods are then attached to the wax sculpture replica (called gates and sprues) in a manner resembling arteries. These gates lead to a wax funnel at one end of the rod.
The hardened wax sculpture replica with its “arteries” is then dipped into a liquid “shell” vat. While still wet, it is slowly lowered into a dry silica-sand forming a rigid shell. This process is repeated in layers, each layer being allowed drying time, which creates a hard plaster-like shell on the inside and outside of the wax sculpture. This wax, with its heavy shell coating, is now steam heated, melting out all of the wax, leaving only a hollow shell. This is where the “lost wax” method of bronze term applies. The lost wax method of bronze dates back approximately 6000 years and is virtually the same process today with modern applications.
If the table-size model is to be pointed-up (the term for enlarging to life-size or larger) the sculptor then has to take very accurate measurements of the table-size model in order to begin calculations to enlarge it to the desired dimensions for the life-size or heroic piece. This process can take many months to complete depending upon the complexity and size of the piece.
While the bronze bars (ingots) are melted into liquid form, the shells are baked to a ceramic-like stone. Melted molten-like bronze (at temperatures of 2130 degrees Fahrenheit is then poured into the hollow cavity within the shell. Once the bronze is hardened and cooled the shell is carefully broken away with hammer and chisel. Bars and funnels are also removed at this time. Any remaining shell is removed by two stages of sandblasting: first with course material, then with fine.
Imperfections to the surface, as well as marks and scars left when removing the bars and funnel are repaired in a process called “chasing” similar to the wax chasing done earlier on the wax replication. Since large pieces cannot be easily poured in one piece, they are often cut into pieces in the wax spruing/gating process therefore requiring that the bronze pieces be reassembled and welded into correct position. Welded seams are again “chased” until the bronze is identified to the original work. Another sandblasting is required and again, close inspection.
The final step is the patination. This gives the bronze its colorization by applying heat with a torch (at 400 degrees) and chemicals to the metal. Chemicals can be dipped, sprayed or brushed on in varying degrees of heat to achieve the sculptor’s desired result. Once again, a thorough inspection is needed.
In summary, sculpting a table-size clay model alone can take up to three months or better depending upon it size and complexity. If the piece is to be pointed up, the point up can take up to six months or better, again depending upon the size and complexity. The molding process of a life-size or larger piece can take a month or better. The foundry process can take four to six months to complete in bronze.
Thus, you can begin to understand the many stages, complications, variations, and the exhausting endurance of hours, days and months that elapse in the process of creating a single, finished lost wax bronze and why bronze is a time-consuming and costly art medium. The end result is virtually a timeless, indestructible, glowing three dimensional fine art sculpture that gives centuries of pleasure and becomes a cherished family heirloom or the pride of a community well into the future. In fact, the bronze sculpture “Double Check” by J. Seward Johnson, Jr. of a seated man with briefcase survived through the World Trade Center tragedy. It will need sandblasting, a new patina and base to again be a bronze testament to the fortitude of America.