Alkaline foods, such as potatoes, will cause discoloration on aluminum pans, but you can brighten them to their original shine with acidic substances. Some dish detergents or high mineral concentrations in water may cause darkened aluminum, too, which may require an increased amount of acid for cleaning. Allow pans to soak in hot water to loosen burnt-on foods and then scrape away any buildup with a wooden or plastic utensil---this will help the process of removing discoloration from aluminum.
Fill your aluminum pan with water and add two tablespoons of cream of tartar per quart of water, or add one teaspoon of white distilled vinegar per cup of water.
Place the pan on a heated stove burner, and boil the mixture within the discolored pan for approximately 10 minutes or until the stains lift.
Pour out the mixture, and allow the pan to cool. Wash the aluminum pan by hand with warm water and your regular dish detergent. Dry the pan as usual.
If your pan is too large to place on the stove burner, put the vinegar or cream of tartar into the pan and then pour boiling water into it, and wash it by hand once it has cooled.
Cook rhubarb in the pan to remove discoloration, but do not use sugar in it or eat the rhubarb afterward.