Years ago, before the days of Teflon-coated baking pans, lasagna was made in metal pans. We coated the bottom of the pan with oil, then applied layers of pasta, tomato sauce, beef, and cheese. The pan was then covered with aluminum foil for baking. After 45 minutes in a 375° oven, black dots and holes appeared in the aluminum foil. Why? In a word: Corrosion.