Bronze is a tin alloy of copper which is harder than either of the alloy metal ingredients. It is extremely strong and resistant to atmospheric corrosion. It has been used since prehistoric times to forge tools, weapons, statues and ornaments. It has a comparatively low melting point, which metalworkers in ancient times could achieve with charcoal and bellows. This allowed them to cast complex shapes. Other metals such as lead, gold or silver were sometimes added to alter the color, improve the finish or make the molten bronze flow better.
Ancient bronze was usually made up of copper, tin and small amounts of noble metals or lead. The amount of tin could reach 40 percent but classic bronze alloy and today's commercial bronze are 10 percent tin and 90 percent copper. Alloys with more tin are technically brass alloys. Manganese is added to bronze used for ships' propellers because it resists saltwater corrosion. Iron, nickel, silicon and aluminum are added for strength in tools because bronze will not make a spark when struck.
Hardness and Strength
Bronze strength depends on the composition of the alloy and ranges from 35,000 pounds per square inch (psi) tensile strength for standard bronze through 85,000 psi for aluminium bronze to 119,000 psi for manganese bronze. The yield strength ranges from 32,000 to 68,000 psi. Brinell hardness measures from 65 to 225. Ancient bronze was at the lower end of these figures but modern aluminum and manganese bronzes are used in marine fittings, bearings and pumps where high strength and hardness are required.
Copper has a melting point of 2200 degrees Fahrenheit and tin 450 degrees Fahrenheit. Depending on the amount of tin and copper in the alloy, the melting point of bronze falls somewhere between the two. Typical ancient bronze had a melting point of about 2000 degrees Fahrenheit, which was near the top of the temperature range that could be achieved in prehistoric furnaces. The miners first had to melt the copper to make the bronze, and then the metalworkers melted the bronze to cast the shapes they wanted.
Resistance to Corrosion
Natural bronze is a salmon-colored metal but statues and other outdoor artifacts quickly develop a patina, which protects the bronze from further rapid deterioration. The patina can vary in color from lime green to dark brown, and bronze is sometimes treated at the foundry to develop a patina before it is delivered. While bronze is resistant to normal atmospheric agents, the presence of sulphur or chlorine in the atmosphere will accelerate deterioration.