This hard, dark gray coating protects the metal. It's found on all bare aluminum surfaces, including utensils which, if rubbed on a counter or range top, or other material, makes a dark gray mark. If washed off the outside of the pan, it quickly forms again. Anodizing does not rub off. A special anodizing process produces a very hard, dark gray finish on professional type cookware.
Care depends on the product made from aluminum.
Lacquers or waxes on products NOT used for food can protect aluminum against weathering and corrosion. OR 2 tablespoons vinegar per quart of water for 10 minutes in the pan. Remove stains from outside of aluminum pans with silver polish, or mild, nonabrasive cleaner. The process may have to be repeated in severe cases. Rinse with plain water before using tea kettle.