To remove the discoloration from cast aluminum.Fill a large pot with enough water to immerse the pan, bring it to a boil. Add three tablespoons of Cream of Tartar, stir to dissolve.Place the aluminum pan in the water, boil for 5-10 minutes, you should see it brighten during this process. It may take a few more minutes if the discoloration is deep.Rinse and dry well, rub with a generous amount of mineral oil, inside and out, and put it in the oven at a low temp for an hour or so.Cool, wipe outside surfaces with a lint-free cloth. Apply more mineral oil to the interior, put back into the oven for an hour at medium heat. This is the way my grandmother would get rid of the blackish stain inside aluminum sauce pans and pots after cooking things like tomatoes, or collard greens in them when I was little. The top inside walls of the pan would be silvery but below that the sides and bottom would look like charcoal but a few minutes of boiling water with Cream of Tartar would bring them back to their original bright color.