Copper is an excellent conductor of heat; however, it reacts with foods and moisture. The green film that forms as the result of copper's reaction with moisture is called verdigris and is poisonous. Copper pots, therefore, must be lined.
Tin is the classic lining for most copper pots because it is completely nontoxic. It will not rust, corrode or react with food. It is easily applied to the inside of the pan during manufacture and has only a slight negative effect on heat conductivity. Tin linings will tarnish and darken, but this merely increases the conductivity of the pot.
Tin is soft and will eventually wear away, even with normal use. When you see the copper coming through the tin, it's time to check the yellow pages and find a tinner to reline your pots. Don't try to bring back the original luster by scrubbing it. This will only reduce the pots' conductivity and remove the tin lining. Tin melts at 412 Fahrenheit, so be very careful never to leave an empty tin-lined pot on a hot burner. The tin will blister away from the core.
Some copper pots are lined with stainless steel. This reduces the heat conductivity of the pot but saves you the cost and effort of retinning - which is necessary every few years with a tin lining.
There are a number of manufacturers that was the bottoms of their pots with a coating of copper, Revere being the most well known of the group. Unfortunately, the copper coating is so thin that virtually none of the heat conductive properties associated with copper become available to the cook. They are expensive for their cooking quality and it is my understanding from recent discussions with retailers that these pots are sold primarily for looks rather than function.
There are also a number of manufacturers that produce pots and pans made of enamel-coated sheet steel. These pots are thin and absorb heat unevenly. They cannot be considered as serious cookware. I see them as minimally acceptable vessels for holding water in which a bag of frozen food is to be placed - and even then the heat conductivity is poor. Two French companies export carbon steel pots and pans that have been coated inside and out with tin, but the low melting point of tins makes such utensils impractical. In most households it's just a matter of time until the pot is exposed to more heat than it can handle and the covering of tin is burned off. Stainless steel is "stainless" because a thin protective film of oxide forms on the metal's surface. This film will not react with any foods and is completely nontoxic. However, it is a very poor conductor of heat.
It is possible, to a certain extent, to combine various metals in order to accentuate the positive aspects of one and reduce the negative qualities of the other. Stainless steel will not react with foods. It's attractive and easy to clean, but its lack of heat conductivity has to be overcome. The solution for a number of manufacturers was to put a more conductive aluminum slab on the bottom of the pot. Sometimes the aluminum is bonded to the base of a stainless steel pot; sometimes it's sandwiched between two layers of stainless steel. Because the highly conductive aluminum is only on the lower part of the pot, heat is not well diffused up into the pot walls and the overall conductivity of the pot is not ideal. Farberware and Cuisinart pots re examples of bonded and sandwiched pots. Variations on this theme are the Master Chef and Cop*R*Chef products.
Master Chef is constructed of a core of pure aluminum with a sheet of stainless steel covering the inside surface and an aluminum alloy on the outside. The stainless steel on the inside will not react with food, and since the entire pot - both sides and bottom - are made of aluminum, the conductivity is fairly good. Cop*R*Chef is a similar product with copper on the outside, stainless steel on the inside and a core of aluminum sandwiched between the two. Pots of copper, aluminum and stainless steel are also made with magnificent detailing by Atlas Metal Spinning Company of South San Francisco.
In order to give you an idea of the relative prices of various types of cookware, I have made a list comparing the suggested retail prices for a 2-quart saucepan in a number of materials. CAPTION: Illustration, no caption; Chart, A Matter of Comparison