The finest antique copper pieces are very heavy, show little wear, and have no dings or dents. The color should run to red rather than pink, which is a sign that the piece is new and probably has an aluminum core. Copper cookware should always be lined with tin, stainless steel, or some other nonreactive metal to prevent the potentially toxic reaction that occurs when acidic foods come into contact with copper.
"A just-polished piece of copper is really bright, almost too bright," says Fritz Karch, collecting editorial director at Martha Stewart Living. "But after a couple of days, the glow turns beautifully rosy." He suggests polishing quarterly to keep copper glistening -- unless, of course, you prefer a more subdued patina, which requires less frequent attention. Several copper polishes are available; choose one that gives you the glow you're looking for. Copper pieces polish up rosier with Red Bear, a favorite brand of Martha's, while solutions such as Twinkle, Noxon, and Brasso make the metal go gold. (Red Bear is available through specialty dealers; the others are sold at hardware and kitchen-supply stores). If a piece suffers a burned spot, Copper Brite can restore it to its original luster. For a natural polish, apply a wash of lemon juice or white vinegar and a sprinkling of coarse salt with a soft brush.
This alloy of copper and zinc tarnishes easily. Most pieces made after 1940 have been coated with lacquer to prevent the metal from oxidizing and going dark. Often these pieces turn up at flea markets with the lacquer somewhat compromised; once the lacquer decays, the exposed brass tarnishes, and you can't remove the tarnish without stripping the piece of the remaining lacquer first. Acetone or paint remover will do the job, but if the piece is large and valuable, consult a metal refinisher first.
"Martha's mantra is 'never lacquer, always polish,'" Karch says. Serious collectors prefer unlacquered brass, which necessitates polishing to keep it shining. Choose a good commercial polish that is up to the task. For a light to medium tarnish, an ample amount of liquid polish on a cloth works well; heavier tarnishes require pastes or creams. For a natural solution, mix equal parts of salt and flour with enough white vinegar to make a paste.