One of the oldest man-made alloys, traditional bronze consisted solely of copper and tin, but today’s bronze alloys may contain copper mixed with aluminum, manganese, silicon, and a range of other materials depending on the properties required for the intended application of the alloy. Bronze is easily molded, has low metal-on-metal friction ratings, and can withstand stress and high pressure, making it an ideal choice for a wide range of industrial and artistic applications.
Bronze can be molded into complex shapes through a technique known as casting. In this process, molten bronze is poured into a mold. After cooling, the bronze solidifies and the mold is removed, leaving only the predetermined form.
Industrial Bronze Casting
The Bronze Age – beginning in 4000 to 3000 B.C. — is so named because this was the period when humans began casting bronze tools. Since then, additional casting methods have been developed for a range of different purposes.
Sand Casting
Modern sand casting is still largely based on the method of bronze casting used in ancient times. It begins by making a pattern in sand and then creating a mold from the sand. Next, liquid bronze is poured into the hollow mold. Once the metal has cooled, the mold is removed, leaving the bronze object, referred to as a sand casting. Although this method is time-consuming, it allows for the creation of a wide variety of shapes that may not be possible with other processes.
Centrifugal Casting
The preferred method for creating cylinder-shaped bronze items, centrifugal casting begins with a cylindrical mold revolving around an axis at a constant speed as the molten bronze is poured in. This method creates a part with a round inner diameter. Undesirable contaminants and foreign matter contained within the alloy typically collect in this inner circle and can be removed with secondary machining processes, which is one advantage of this casting process. However, the size and shape of items created through centrifugal casting are limited.
Chill Casting
Chill casting begins with pouring the liquid bronze into a mold cavity created by a steel or aluminum die that has been cooled to a low temperature. Because the die is cooler than the heated bronze, the bronze begins to quickly harden as it enters the mold. Both internal and external chills can be used to help solidify the casting. The benefit of this fast cooling process is that it produces a unified grain in the finished product.
Artistic Bronze Casting
Bronze is also a medium commonly used by artists to create sculptures or other artistic projects. One of the more traditional techniques for sculpture work is lost-wax casting (also called investment or precision casting). This process allows for intricate details to be transferred from the mold to the finished sculpture.
The lost-wax method began during the Renaissance and is still relatively the same today. The process begins by creating a pattern out of a medium such as clay. The pattern is then used to create a ceramic mold containing sprues — or channels — that allow the molten liquid bronze to flow through into the mold.
After cooling, the mold is gently removed with a chisel or occasional sandblasting or grinding to clean crevices and other difficult areas. Finishing touches are added to give the sculpture its desired texture and color. Finally, the piece is given a coat of carnauba wax and buffed to a shine. The wax is not only aesthetic but also prevents the oxidation caused by exposure to air that turns bronze green over time.
Which Bronze Casting Method Is Best?
There are a number of techniques available to cast objects in bronze. The shape of your piece, as well as its purpose, can dictate which method will work best for your project. It’s important to look at all of the requirements of your design to determine which bronze casting process will be suitable for creating a product that will meet your needs.
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