Casting Bronze: Indirect Lost-Wax Method

The hollow wax model is encased in an outer mold. This cylindrical mold, made of heat-resistant material, is then heated upside down in a kiln for several days. This process melts the wax inside, which runs out, leaving an empty space between the outer mold and the inner core. The melted wax rods also create channels within the mold. The core is now held in place by nails.

Sculptors in the 1500s developed an indirect lost-wax method for bronze casting. The first step involves sculpting a form in warm wax or pliable material, such as red beeswax. After cooling, details are added, and the solid model is then cut into parts to create a hollow wax duplicate using plaster molds. The hollow wax duplicate is assembled, imperfections are repaired, and the hollow interior is filled with core material (sand, clay, ceramic powder, and plaster) designed to resist heat. Iron pins are inserted to hold the core, and a network of wax rods is attached for air vents and molten bronze flow.

Once the mold is empty and dry, it is placed upright in a sand pit, ready for the molten bronze. Bronze is melted in a crucible within a furnace at approximately 2000 degrees Fahrenheit. In the 16th century, charcoal was used as fuel with hand-operated bellows. Foundry workers relied on the color and texture of the metal or the smell of gases to determine the correct pouring temperature. Impurities (slag) are skimmed off the molten bronze, which is then poured through a cup-shaped opening, filling the empty spaces within the mold through the circulatory system created by the melted wax rods. Air escapes through vents.

The bronze rapidly solidifies and is allowed to cool. Later, the mold is dramatically broken open with a hammer to reveal the cast. This is the moment the artist discovers if the casting was successful.

After casting, the figure is entrapped in a bronze armature and requires extensive chasing and finishing. The bronze channels from the wax rods are sawn off, and the iron pins are removed. Holes caused by trapped gases or metal shrinkage are hammered and filled, often visible as plugs on the finished bronze. Finally, the artist sharpens fine details and treats the bronze surface to alter its color, creating a patina using acid, lacquer, or wax to enhance its richness and luminosity.