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lost-wax process, method of metal casting in which a molten metal is poured into a mold that has been created by means of a wax model. Once the mold is made, the wax model is melted and drained away. A hollow core can be effected by the introduction of a heat-proof core that prevents the molten metal from totally filling the mold. Common on every continent except Australia, the lost-wax method dates from the 3rd millennium bc and has sustained few changes since then.
To cast a clay model in bronze, a mold is made from the model, and the inside of this negative mold is brushed with melted wax to the desired thickness of the final bronze. After removal of the mold, the resultant wax shell is filled with a heat-resistant mixture. Wax tubes, which provide ducts for pouring bronze during casting and vents for the noxious gases produced in the process, are fitted to the outside of the wax shell, which may be modeled or adjusted by the artist. Metal pins are hammered through the shell into the core to secure it. Next, the prepared wax shell is completely covered in layers of heat-resistant plaster, and the whole is inverted and placed in an oven. During heating, the plaster dries and the wax runs out through the ducts created by the wax tubes. The plaster mold is then packed in sand, and molten bronze is poured through the ducts, filling the space left by the wax. When cool, the outer plaster and core are removed, and the bronze may receive finishing touches. See also investment casting; sculpture: Reproduction and surface-finishing techniques.
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Bronze objects have been cast using the lost wax (cire perdue) process for at least 5,000 years. Although by Rodins day some of the techniques and materials have changed and today continue to change much of the process is as it was in ancient times. Lost wax casting is for many the process of choice because it is extremely accurate in replicating detail and because of the durability of the objects it creates. However, the process is very arduous and time-consuming. Most sculptors, including those of Rodins day when artists could choose from scores of foundries, depend on independent foundries to cast their works.
Below is the same process presented through pictures and text.
The artist creates a model, generally in plaster, clay, or wood. The model is put into a bed of very fine elastic (shapable) material held in place by a rigid outer mold. When the model is removed, its impression remains. Fireproof clay is carefully put into the impression, making a sharply defined duplicate of the artists original model. The surface of this second clay model is slightly scraped away. When this second model is returned to the mold, there is a gap between the model and the mold. This gap is where the wax will be poured. The final bronze will be of the same thickness as the gap that is created by the scraping. After closing the mold around the clay model, hot wax is poured into the gap between the model and the mold. The result is a clay model covered in wax, which is then hand-finished to fidelity, incorporating the artists signature, cast number, and a foundry seal. A network of wax pipes, called sprues and gates, is attached to the wax-covered model. These pipes first will allow the wax to escape as it melts. Later, they will enable the molten metal to flow evenly throughout the mold and will also let air escape as the metal is poured in. A finely granulated ceramic is applied to the surface of the model and its pipes until it becomes thick and coarse. The result, now called an investment mold, is then dried and heated. This causes the wax to melt and flow out of the mold, leaving a space between the fire resistant clay model and the investment mold. This is why this method is called the lost wax process. Except for a place to pour in the liquid bronze at the top, the mold is covered with a layer of cladding (a protective metal coating), which must be completely dry before bronze pouring begins. The investment mold is then heated to a high temperature (over 1,000 degrees Fahrenheit). Molten bronze (over 2,000 degree Fahrenheit) is then poured into the investment mold, filling the space left by the lost wax. When all is cool, the cladding and investment mold are broken and the metal appears. The bronze sculpture and its sprues and gates are an exact reproduction of the wax in step 6. The network of sprues and gates is then removed and the surface of the bronze is chiseled and filed so that no trace of them can be seen. This process of hand-finishing the bronze to perfection is called chasing. Any remains of the fireproof clay model left inside the hollow bronze are removed now.When the process of chasing is finished, if the sculpture is small enough to have been cast in one piece, the work is given a patina. Larger sculpture is generally cast in segments, and after all segments have been made, they are joined together, a process called braising. (Rodin often left the braising lines visible, so the viewer would always be aware that the artwork was made by an artist.) After braising, the artwork would proceed to patination. A patina not only protects the sculpture, but also gives it color. It is a step in the making of the finished bronze wherein hot or cold oxides are applied to the surface of the metal, creating a thin layer of corrosion. This layer slightly brown, green, or blue in color is called the patina. The patina protects and enlivens the surface of the bronze.
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