Bronze Alloy of Copper, Phosphorus, Tin, Zinc, and, sometimes, small amounts of other Elements. It is harder than Brass, and its properties depend on the proportions of its components. Aluminum bronze, with its high strength and corrosion resistance, is used for bearings, valve seats, and Machine parts; Leaded bronze is cast into heavy-duty bushings and bearings; Silicon bronze is used for Telegraph wires (see Conductor) and chemical containers; and bronze with 20% to 24% tin is used for casting bells. Bronze is used for coins, medals, steam fittings, and Gunmetal, and in artistic castings, engravings, and forgings. History The Bronze Age was a technological period when metals were first used to make tools and weapons. The earliest stage, when pure copper and bronze were used interchangeably, has been called the Copper Age. In 3600 B.C. bronze made by southwest Asian artisans was the first metal hard enough to hold an edge. Copper is alloyed with tin, which is even softer than copper, but the combination (5 to 20 percent tin) creates a metal with many more practical uses than copper. Casting was well established in the Middle East by 3500 B.C. By 2500 B.C. Egypt and Mesopotamia were well into the Bronze Age, but central Europe and the British Isles were only entering the Stone Age that began in 9000 B.C. In the New World the first bronze was cast A.D. c.1100. Development of a metallurgical industry coincided with urbanization, support of an artisan class, and trade for raw materials, laying the foundation for the Iron Age.