In the days before self-cleaning ovens, it was a common practice to line the bottom of an oven with aluminum foil to catch bubble-overs or drippings. It was a simple process to just remove the tin foil when it had cooled and throw it away and still have a clean oven bottom. With the advent of the self-cleaning oven, however, putting a protective layer of tin foil down can have a less-than-desired effect. Aluminum foil can melt on the bottom of a self-cleaning oven in some cases. There are a couple of handy tricks you can try to remove the stuck tin foil.
Mix baking soda and white vinegar together to help lift the stuck foil. Pour baking soda into a large bowl. Add enough white vinegar to form a thin paste using a whisk. Use a dry cloth to spread the paste over the entire affected area. Allow this to set overnight. Use a wet-dry vacuum cleaner to remove the dried baking soda and any aluminum foil that comes up. Remove any loose bits of foil by hand.
Soak an old towel -- one you don't mind throwing away -- in water and line the bottom of the oven. Allow it to sit on the oven bottom overnight. Remove the towel in the morning and throw it away, and dispose of any loosened bits of foil with your hands. Repeat if necessary.
Remove any stubborn foil pieces by scraping them up with a plastic putty knife.
Run the oven in self-cleaning mode as the earlier steps may have loosened the foil enough to be cleaned during the self-cleaning process.