Lost wax or “cire-perdue” casting goes back at least as far as 3,000 B.C.E. In fact, evidence of early lost wax casting has been uncovered on every continent except Australia. The reason for the enduring popularity of the lost wax method is that it captures very fine detail. And it can be done without high-tech equipment.
The first step is to carve a model in wax. For jewelry, I start with a wax that’s very similar in hardness and working properties to wood. Like wood, it can be carved with knives and tiny hand tools, or worked with a dremel. Jeweler’s wax takes fine details, and at the end of the project, it can be polished to a high shine. For sculpture, I usually start with a softer brown wax that can be modeled with the fingers. It usually takes several weeks of carving and tweaking, walking away and coming back, to create a design.
The finished wax jewelry model is embedded in plaster or ceramic, leaving a hole to the outside. When the plaster or ceramic is heated in a kiln, the wax melts and vaporizes, leaving a hole shaped exactly like the wax original, now “lost.” Molten metal is poured into the hole, which cools into a metal casting in the exact shape of the original wax. Seeing a wax carving transformed into gold or silver is the closest thing I know to magic!
To cast larger sculpture, a plaster or rubber mold is made around the model. The original model is removed, and the inside of the mold is painted with wax, building up the layers to the desired thickness (usually about 1/4″.) The new wax copy is then filled with a plaster core so that it will be hollow, and a series of wax tubes called “sprues” or “gates” are attached to create channels for pouring in the bronze during casting. The whole things is embedded in a block of plaster. The process of burning out the wax is pretty much the same, no matter how large or small the design is. The main difference is in the size of the equipment and how much molten metal you need to be lifting and pouring. Large sculptures may be cut apart in the wax stage, cast in pieces, and welded together after they’re cool.
If a rubber or silicone mold is made around the first metal casting of the original design, more waxes can be made that capture all the detail of the wax original.
Of course there are some modern improvements that make a difference in lost wax casting. I’m pretty sure it’s easier to melt metal with a propane torch than over a bonfire. And the kind of plaster we use today is so finely powdered that it captures the tiniest details, compared to the raw local clays available millenia ago.
And then there’s the role of the vacuum, another reasonably modern invention. Without a vacuum, plaster molds tend to be full of air bubbles. The only solution if you don’t have a vacuum chamber is to tap or shake the liquid plaster for ten minutes or so to try to bring any air to the surface. And in the casting process, the molten metal may or may not make it out to the delicate details. Think of the little feet and ear tips on my work. Vacuum equipment can assure that the molten metal is “pulled” out into all the voids and crevices in the mold. But vacuum equipment is expensive. For the small artisanal caster, a spring-operated desktop machine uses centrifugal force to “throw” the metal out of a small crucible into the mold.
After the raw castings have cooled, the plaster is chipped away and the sprues (the channels that the metal followed through the plaster) are cut and ground away. Pin backs, pendant bails or earring posts are soldered to the casting. Then the whole piece is polished and finished with a patina to enhance the detail and preserve the color of the metal through long years of wear.