Copper is a good conductor of heat, distributing it evenly so that food cooks uniformly from top to bottom. It also cooks faster than do other materials, reducing the chance of foods drying out. Over time, copper pots can develop a black coating. In most cases, however, you can clean or polish away that coating.
About Copper Pots
Copper is slightly acidic, and it will react with certain foods. Unlined copper pots are not recommended for use because the reaction of the copper to certain foods can cause gastrointestinal symptoms. Lining copper pots places a safe substance between the copper and your food. Typically, copper pots are lined in tin, silver, nickel or stainless steel. Tin is a good conductor of heat, but will melt if subjected to temperatures above 425 degrees Fahrenheit. Tin also scratches and wears away with use, but you can have the pots relined if this happens. If your copper pot is lined with tin, avoid using metal utensils and acidic foods in the pot to preserve the lining. Silver is also a good heat conductor used for lining copper pots, but it is expensive. Pots lined in nickel have good heat distribution, and the nickel doesn't wear away as easily as tin does. Stainless steel doesn't scratch or melt, but has poor heat conduction, which reduces the copper's original benefits.
Oxidization, a chemical reaction between copper and oxygen, can turn a copper pot black. When heated, copper can develop a layer of brownish-black copper oxide. If it is not polished away, the copper oxide will react with the air's moisture to form copper carbonate, which is green in color. Oxidation inside your copper pots is not deadly, but it can make you ill.
Some manufactures ship copper cookware with a thin lacquer seal that protects the copper while the pots are in storage. Remove the lacquer with acetone before using the pots to avoid burning the lacquer and turning the pot black permanently. If you prefer not to use acetone, which is flammable, you can immerse the pot in boiling water and add 1 cup washing soda. Leave the pot in the water until the lacquer peels off.
Never put copper pots into a dishwasher. Although they are considered dishwasher safe, the heat and moisture along with the acids in many brands of dishwasher detergent will speed up oxidation, which can tarnish the copper permanently.
Care and Cleaning of Copper Pots
Oxidation reduces copper's heat conductivity. To prevent this, clean and polish copper pots regularly. Do not use steel wool or abrasive cleaners. Wash pots in warm soapy water and polish using either a copper polish product or a paste made from an equal mix of salt, vinegar and flour. Apply the mixture to the outside of the pot and let it sit for up to an hour. Rinse and dry thoroughly.