Copper

By far the most important use of copper is in electrical wiring; it is an excellent conductor of electricity (second only to silver), it can be made extremely pure, it corrodes very slowly, and it can be formed easily into thin wires—it is very ductile.

Copper is also an important ingredient of many useful alloys—combinations of metals, melted together. Brass is copper plus zinc. If it contains mostly copper, it is a golden yellow color; if it is mostly zinc, it is pale yellow or silvery. Brass is one of the most useful of all alloys; it can be cast or machined into everything from candle sticks to cheap, gold-imitating jewelry that turns your skin green. (When copper reacts with salt and acids in the skin, it produces green copper chloride and other compounds.) Several other copper alloys are common: bronze is mainly copper plus tin; German silver and sterling silver are silver plus copper; silver tooth fillings contain about 12% copper.

Probably the first alloy ever to be made and used by humans was bronze. Archaeologists broadly divide human history into three periods; the Bronze Age is the second one, after the Stone Age and before the Iron Age. During the Bronze Age, both bronze and pure copper were used for making tools and weapons.

Because it resists corrosion and conducts heat well, copper is widely used in plumbing and heating applications. Copper pipes and tubing are used to distribute hot and cold water through houses and other buildings.

Because copper is an extremely good conductor of heat, as well as of electricity (the two usually go together), it is used to make cooking utensils such as saute and fry pans. An even temperature across the pan bottom is important for cooking, so the food doesn't burn or stick to hot spots. The insides of the pans must be coated with tin, however, because too much copper in our food is toxic.

Copper corrodes only slowly in moist air—much more slowly than iron rusts. First it darkens in color because of a thin layer of black copper oxide, CuO. Then as the years goes by it forms a bluish green patina of basic copper carbonate, with a composition usually given as Cu₂(OH)₂CO₃. (The carbon comes from carbon dioxide in the air.) This is the cause of the green color of the Statue of Liberty, which is made of 300 thick copper plates bolted together. Without traveling to New York you can see this color on the copper roofs of old buildings such as churches and city halls.