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Casting bronze: direct lost-wax casting

Video transcript
- [Voiceover] This sculpture, the Juggling Man, by the Dutch artist Adriaen de Vries, was made around 1615. To create it, de Vries used a process called the direct lost-wax method of casting. This method is composed of three steps: modeling, casting, and chasing. Bronzes made by this method are cast from the original model, which is made of a wax skin over a clay core. In this process, the wax skin is melted away resulting in a unique, one-of-a-kind bronze sculpture. If the cast fails, the sculptor must begin again. X-rays of the Juggling Man gave conservators vital clues in reconstructing de Vries's modeling and casting techniques. X-rays show that a heavy iron armature or internal frame remains inside the bronze. This is a feature used only in the direct lost-wax method of casting. As seen in this modern recreation of de Vries's process, making the armature requires blacksmithing skills to heat and hammer the heavy iron into the desired shape. The sculptor determines the placement of the figure's arms by bending the rods. He then ties the iron rods together using wire. The artist continues to adjust the composition securing the rods together until he is satisfied with the basic form. Once complete, this sturdy armature provides the underlying support for the artist to construct the basic form in clay. Wet clay is applied to the armature to build the form in a process called modeling. The artist continues adding clay and smoothing the surface until his form is complete. This clay model is called the core. You'll notice that there is no clay core where the figure's hands should be. X-rays showed that the carefully detailed hands did not contain a clay core but were modeled in solid wax. When finished with the modeling, the clay form is fired to strengthen it and to eliminate all moisture. After the form is fired, a layer of wax skin is applied to the clay core. De Vries probably would have used beeswax with resin and colorants because details cannot be seen in natural pale yellow beeswax. The artist then refines the wax form, smoothing the surface and sharpening the details. Next, the artist makes short iron pins called core pins that will hold the clay core in place when the wax skin melts away. With a hammer and chisel, the artist cuts these pins into small lengths. Using a tool called an awl, the artist makes holes through the wax and into the clay core to accommodate the core pins. Finally, the artist carefully inserts the core pins through the wax and into the core. In the next step, the artist attaches to the model a network of wax rods called sprues. He uses hot metal tools as well as his hands. To further secure the sprues, the artist carefully adds a wax reinforcement. These sprues are connected to a wax pouring cup at the top. The model is now fully encassed in its network of sprues. With a brush, the artist applies to the model a mixture of clay and other materials that can withstand high temperatures and absorb gasses. This outer mold is called the investment. Using his hands, the artist builds up the investment until the model is completely embedded in a solid mass. This cross-section of the solid investment reveals the investment's various layers: The armature, the inner clay core, the modeled wax layer, the core pins, the network of sprues, and the pouring cup. The investment is turned upside down, placed in an oven called a kiln, and heated. The next step takes place inside the investment and cannot be seen. The wax layer surrounding the clay core melts out, creating a space for the molten bronze that will become the final sculpture. The melted wax sprues are now hollow channels that will bring the liquid bronze into the outer mold and carry escaping gasses away. After the wax has melted out, the investment is buried in a sand pit. This protects the foundry workers because if improperly prepared, the investment could explode when the hot, molten bronze is poured. The pour happens during the next and most dangerous step of the process, the casting. All of the steps up to this point have been in preparation for the actual casting of the sculpture in bronze. This takes place in a hot, noisy, and dangerous workshop called a foundry. It is here that the bronze metal is melted at about 2,000 degrees Fahrenheit, within a container called a crucible which is placed inside the furnace. Foundry workers called founders open the crucible. They scrape off the waste products that gather at the top of the molten metal and dump them into the furnace pit. Next, the founders secure the carrying arm around the crucible so that it can be removed from the furnace. The crucible is clamped into a pouring ring so that it won't tumble into the mold during the pour. Watch the hand of the man on the right as he secures the clamp. The heat is so intense that his protective glove momentarily catches fire. Molten bronze is poured into the buried investment filling the space left by the melted wax. The investment is allowed to cool and the metal hardens. In a dramatic moment, the artist knocks away the investment to reveal the freshly cast sculpture within. Now the most labor intensive part of the process begins. Once the bronze is freed from the investment, the artist has to clean it up in a process called chasing. The bronze bears little resemblance to the final polished sculpture. Using a chisel, the artist removes the sprues which are now bronze. He also pulls out the iron core pins. Next, the dark, sooty oxide layer covering the sculpture surface is rubbed off. The artist repairs flaws and patches the core pin holes using plugs made of discarded bronze from the sprues. Next, he sharpens details in the face, hair, and finger and toenails using small, specially shaped iron tools. The artist then gives the surface a smooth reflective shine by polishing it with a cloth dusted with light abrasives. The finished sculpture preserves every detail of the artist's original clay and wax model in bronze, a material that is both permanent and precious. Once the bronze is polished, the artist chooses from a wide range of patinas or surface finishes which protect the surface and vary its appearance. A patina can increase luster, change the color, or hide casting imperfections. Shown here are several examples of the types of patinas used in de Vries's time. De Vries's original patina is gone. The Juggling Man stood for more than 50 years in an outside garden. Weathering and oxidation caused its uneven green coloration. Like de Vries, other artists used the direct lost-wax casting method to create works such as this sphinx made in the 1500s. By using this technique, de Vries ensured that each bronze sculpture he created was a unique cast. His commitment to the direct lost-wax method shows his exceptional skill and confidence in overcoming its great risks and difficulties.