The name bronze originally applied to an alloy of copper with about 25 percent tin, but it now applies to a wide range of coppeased alloys, some of which contain little or no tin. The Bronze Age was the period following the New Stone Age (Neolithic Age), when men first began using metals. At first, they exploited naturally occurring copper, gold, and silver, but about 3000 B.C.E. bronze was discovered, and its strength and flexibility for making implements were quickly appreciated.
The tin needed for making bronze was first found in ancient Anatolia (Turkey), and early civilizations such as those of the Phoenicians (from what are now Syria and Lebanon) and the Romans imported it from as far away as presenay Britain, Spain, and northern France. The contents of late Bronze Age burial sites in Greece, Britain, Germany, and elsewhere include bronze items that show these early people had a relatively high degree of skill in smelting and casting. Bronze continued to be used widely throughout the Iron Age for making ornaments and domestic utensils and later became the main metal for the manufacture of cannons and church bells.
Bronze manufacture
Bronze is made in a similar way to brass. Pure copper and scrap copper are melted in a furnace, and then tin and other alloying metals are added. The molten bronze is run out of the furnace into molds to make ingots for subsequent casting and forming. Several types of furnace are used, the main ones being electric induction furnaces and crucible furnaces, where the metal is melted in a large bowl or crucible heated by oil or gas.
Gunmetals
Gunmetal is a bronze to which zinc has been added and is so called because it was originally used to make cannon and other pieces of ordnance. It is made from approximately 88 percent copper with 8 to 10 percent tin and 2 to 4 percent zinc. It has excellent casting properties, and apart from its numerous engineering applications, such as fittings for steam equipment, it is widely used for bearings, gears, and ornamental bronze work. Leaded and nickel gunmetals contain up to 5 percent lead and nickel, respectively. Different grades of these gunmetals are made by varying the relative proportions of the constituent metals. The 88–10–2 (i.e., percentages of copper, tin, and zinc) alloy is often called Admiralty gunmetal, and it is well suited for use in heavily loaded, lopeed bearings. It has good corrosion resistance and is often used for pumps and valves.
As with brass, the addition of lead makes bronze easier to machine because the metal being removed—for instance, using a lathe—comes off in small chips instead of curling into long spirals that could damage the cutting tool. The presence of lead means that higher cutting speeds can be used without damaging the tools or the workpiece. Leaded gunmetals such as the 85–5–5–5 (copper–tin–zinc–lead) alloy are also used for making castings in sand molds, and the addition of up to 0.05 percent phosphorus helps to remove oxygen from the alloy and prevent the formation of bubbles in the castings.
