Bronze (alloy)

Bronze is a term applied to a variety of alloys that contain copper; the oldest of these, which was the first metallic alloy produced, is an alloy of copper and tin. Other alloying elements include tin, nickel, phosphorus, zinc, and lead.

Background

A variety of related alloys are called bronze. The one with the longest history is an alloy composed primarily of copper, with a smaller percentage of tin. Various forms of bronze have been smelted for thousands of years; in fact, bronze was the first true metallic alloy developed. Bronze replaced the use of copper as the material of choice for tools, weapons, jewelry, and other items in the ancient Near East and other early centers of civilization. Although eventually it was largely replaced by iron and finally by various steel alloys, bronze still is employed extensively for a variety of industrial uses worldwide.

History

The first metal used by ancient metallurgists was copper, because surface deposits of this metallic element in its native, or naturally pure, form were once relatively plentiful in certain areas. However, objects produced from pure or nearly pure copper possess several drawbacks, chief among them are softness and lack of resistance to damage. Archaeological finds from the Near East dating back at least to around 3000 b.c.e. indicate that early metalworkers discovered that by adding other metals in small percentages, they could produce a new, stronger metal that also boasted several other favorable characteristics: a lower melting point (950° Celsius instead of the 1,084° Celsius required for copper), greater ease of flowage into molds in the casting process, and elimination of the troublesome bubbles that plagued the casting of pure copper.

Through experimentation, early metallurgists discovered that the ideal metal proportions for bronze were about 10 percent tin and 90 percent copper. The invention of bronze led to a veritable explosion of metal-casting industries that produced elaborate and intricate bronze artifacts and ushered in a period of flourishing mining and trading networks linking far-flung areas for bronze production. Some bronze-producing centers, such as sites in ancient China, experimented with bronze using other admixtures, such as lead. Eventually, with the development of hotter smeltingfurnaces and other techniques, bronze was replaced for most of its applications by a still harder metal, iron, and then by the various alloys of steel.

Various bronze alloys, however, have always been employed for some uses even while other metals became the primary choice for most metal applications. Statuary made from bronze, for example, has always enjoyed popularity. In addition, the modern industrial world uses various types of bronze for cast products such as pumps, gears, nuts, tubes, rods, and machine or motor bearings. Modern bronze alloys typically do not have a tin content in excess of 12 percent, as percentages above that ratio produce alloys with declining ductility (the capaciity for being easily shaped or molded), and they tend to become very brittle.

Specialized Bronzes

Some specialized modern bronze alloys are produced with small percentages of lead, nickel, phosphorus, zinc, and even aluminum. Copper-tin-lead bronzes, for example, are used for machine bearings that must withstand both a heavy load and frictional heat. The lead is added to produce a desired degree of elasticity. A bronze combining copper, tin, and phosphorus is smelted with a percentage of phosphorus in the range of 0.1 to 0.5 percent. The phosphorus in this alloy allows the molten metal to flow more freely and makes casting easier. It also helps deoxidize the melt during the smelting process and produces a bronze with great resistance to wear. Phosphor bronzes, as they are termed, are used in machine gear wheels, an application where hardness and wear resistance are desired. Another type of bronze that is similarly employed is zinc bronze. The zinc typically makes up 2 to 6 percent of the alloy, which also includes copper and tin. Another term for zinc bronze is “gunmetal” bronze, and if the alloy has the specific formula 88 percent copper, 10 percent tin, and 2 percent zinc it is termed “admiralty gunmetal” bronze.

Yet another type of bronze is copper-tin-nickel bronze, in which the proportion of nickel is usually 1 to 2 percent of the alloy. Nickel bronze is designed to withstand high temperatures and strongly resist corrosion. It possesses a microstructure that is more closely grained than most bronzes, while having both added toughness and strength. Other types of bronze alloys include aluminum bronzes, which typically are 1 to 14 percent aluminum and usually have smaller percentages of other metals, such as iron, nickel, and manganese. Aluminum bronzes are used in the production of special wires, strips, tubings, and sheets for which ductile strength is desirable.

A by-product of exposure to the elements of bronze alloys that are less resistant to corrosion is the production of a thin greenish or greenish-blue crust or patina called “verdigris.” This crust, often seen on outdoor statuary, fixtures, and fountains, is composed typically of either copper sulfide or copper chloride.

Bibliography

Callister, William D. “Nonferrous Alloys.” In Materials Science and Engineering: An Introduction. 7th ed. New York: John Wiley & Sons, 2007.

Cverna, Fran, ed. “Bronzes.” In Worldwide Guide to Equivalent Nonferrous Metals and Alloys. 4th ed. Materials Park, Ohio: ASM International, 2001.

Hummel, Rolf E. Understanding Materials Science: History, Properties, Applications. 2d ed. New York: Springer, 2004.

Raymond, Robert. Out of the Fiery Furnace: The Impact of Metals on the History of Mankind. University Park: Pennsylvania State University Press, 1986.

Simons, Eric N. An Outline of Metallurgy. New York: Hart, 1969.