By Harold McGee
Copper is unique among the common metals because it can be found naturally in the metallic state. For this reason it was the first metal to be used in tool making, about 10,000 years ago. In the kitchen, it is prized for its unmatched conductivity, which makes fast and even heating a simple matter. But copper is also relatively expensive, since its conductivity has made it the preferred material for millions of miles of electrical circuitry. It is troublesome to keep polished, because it has a high affinity for oxygen and sulfur, and forms a greenish coating when exposed to air. Most important, copper cookware can be harmful. Its oxide coating is sometimes porous and powdery, and copper ions are easily leached into food solutions. Copper ions can have useful effects: they stabilize foamed egg whites, and the green color of cooked vegetables is improved by their presence. But the human body can excrete copper in only limited amounts, and excessive intake may cause gastrointestinal problems and, in more extreme cases, liver damage. No one will be poisoned by the occasional meringue whipped in a copper bowl, but bare copper isn’t a good candidate for everyday cooking. To overcome this major drawback, manufacturers line copper utensils with stainless steel or, more traditionally, with tin. Tin has its own limitations.