If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:6:10

Video transcript

Narrator: Starting in the 1500s sculptures developed an indirect lost-wax method of bronze casting. This process employed a hollow mold filled with molten bronze, requiring several complex and laborious steps. The first step was to make the model. Th artist sculpted his form in warmed wax or some other pliable material, here in red beeswax. After it cooled he added details to the hair and face. To make a hollow duplicate of this solid original the artist made a mold. For this, he cut the wax model into parts. He took an impression of each part by embedding the piece in clay and then pouring plaster on the exposed area to form one section of the mold. The pieces were then assembled and filled with hot wax. When the hot wax came into contact with the cold plaster it hardened into a thin shell. The artist poured out the excess liquid wax. When the mold was opened, the artist removed a hollow wax duplicate of the original solid model. He then assembled the wax parts to form a whole figure again. Edges were joined with a hot tool, seams were smoothed out and any imperfections were repaired. The hollow interior of the figure was filled with core material, a mixture of sand, clay, ceramic powder, and often plaster. The core could resist the heat of the molten bronze. Before the core material solidified, iron pins were inserted through the wax shell into the core to hold it in place and through the other side of the wax shell. Then the artist attached a network of solid wax rods to the figure. Some of these formed vents for air and other gasses to escape. Others created a circulatory system for the flow of molten bronze to the figure. The artist then encased the hollow wax model in an outer mold, it was now ready for casting. In the 1500s artisans cased bronzes using the indirect lost-wax method. This process involved pouring molten bronze into a mold. At this modern foundry, a wax model is encased in a cylindrical mold. This cylinder, made of heat resistant material was first heated upside down in a kiln for several days. This cross section shows the heat melting the wax inside the mold, which then ran out. Where the wax shell of the figure had been, there remained now only an empty space between the outer mold and an inner core. The wax rods also melted leaving channels within the mold. The core was now only held in place by nails. When the mold was empty and bone dry, it was placed upright in a sand pit ready for the molten bronze to be poured in. The bronze was melted in a container called a crucible which was placed inside the furnace. Here we see the furnace being lit. In the 16th Century the fuel was charcoal and the bellows were hand operated. At approximately 2000 degrees Fahrenheit the bronze melted. Before the invention of thermometers, foundry workers used the color and texture of the metal, or the smell of the escaping gases to tell when the bronze was exactly the right temperature for pouring. The success of the pour depended on their experience and on no small amount of luck. The crucible containing the liquid metal was placed in a pouring handle. Before the bronze could be used, impurities floating on the surface, called slag, were skimmed off. The bronze was then poured through a cup shaped opening at the top of the mold. Traveling via the circulatory system, created by the melted wax rods, the bronze rapidly filled the empty spaces inside the mold. Displaced by the bronze, the air in the cavities escaped through the vents. The bronze became solid quickly and then was allowed to cool. Later, the mold was broken open with a hammer; the dramatic moment of the whole process. Only at this point did the artist know if the cast had been successful. Although the cast was now completed, the figure was entrapped in a bronze armature and required the time consuming work of chasing and finishing. Chasing and polishing were the final steps in finishing a cast bronze object. When the mold was broken open the object inside scarcely resembled a glowing bronze. The artist had to transform the work with all it's channels, vents, and pins into an object of beauty. The channels, now cast in bronze, had to be sawn off and the iron pins that held the core in place, removed; part of the process called chasing. Holes caused by trapped gases or the shrinking of the metal had to be hammered and filled. For example, on this male nude by Tiziano Aspetti, the small diamond shaped mark on the forearm is a plug used to fill either a flaw or a hole left by the casting process. These patches are visible on most bronzes in the museum's collection. Next, the artist sharpened fine details and treated the golden surface of the bronze to alter its color. Its new surface, called a patina, was artificially created using acid, lacquer, or wax to emphasize the bronzes richness and luminosity.