What are copper bowls for? Not, as magazine photos tend to suggest, to give a splash of color to your kitchen wall. In fact, there might be copper only on the inside of the bowl.
The inside of the bowl is the business end; copper bowls are for beating egg whites to make meringues and zabaglione sauce. Whites come out creamier beaten in copper than in ceramic or stainless steel. Not only that, it's harder to over-beat them, and they stay high longer.
Harold McGee explains why in his book "On Food and Cooking." When egg whites are beaten, air is trapped in the mixture by proteins, which consist of long strands of molecules. The beating causes the protein strands to unfold, and some of them wrap around the bubbles and strengthen them so that the air doesn't leak out.
It happens that one of the proteins in egg white, conalbumin, can bind copper ions to itself, and the copper strengthens the protein, so the protein walls of the bubbles are longer-lasting and harder to destroy by over-beating. This also explains why egg whites beaten in copper take on a faint golden tinge.
Copper is also an excellent material for cooking--it conducts heat beautifully--except that so much copper leaches into a soup or stew that it can cause stomach upset or even, in extreme cases, liver damage. This is why copper utensils intended for cooking are always coated with tin or stainless steel.
But don't worry, says McGee. A zabaglione now and then won't hurt you.
* Copper bowls in photo from Bristol Kitchens, South Pasadena.