What are some environmentally safe products for cleaning oxidized copper?

Environmentally safe products for cleaning oxidized copper include lemon juice and salt, ketchup and baking soda. If lemon juice is not available, vinegar makes a good substitute. Baking soda is slightly abrasive and scours the oxidation away, while the other compounds are acidic and dissolve the tarnish.

To clean with lemon juice and salt, cut a lemon in half, and sprinkle it with salt. Rub the salted lemon over the copper surface, and the oxidation dissolves. If a fresh lemon is not available, use bottled lemon juice or vinegar. For heavy oxidation, it is sometimes beneficial to add cornstarch as a thickener to the solution, forming a paste. Spread the paste on the surface, and leave it there a few minutes to dissolve oxides.

Ketchup uses the acidity of the tomato to clean the oxidized copper. Spread a small amount on the copper, and allow it to work a few minutes before rinsing away the ketchup and tarnish.

Fingerprints on the clean copper cause tarnish to form again quickly. Storing the shined object in a zipper-seal bag or a tarnish-free cloth can help to prevent it from forming again. A coat of lacquer is also useful in keeping copper free of tarnish.