Aluminum that has been rolled into a thin, pliable sheet. It's an excellent barrier to moisture, air and odors and can withstand flaming heat and freezing cold. It comes in regular weight (for wrapping food and covering containers) and heavy-duty weight (for freezer storage and lining pans and grills). Because the crinkling of foil creates tiny holes (increasing permeability), it should not be reused for freezer storage. Neither should it be used to wrap acidic foods (such as tomatoes and onions) because the natural acids in the food will eat through the foil. Although metal produces arcing (sparking) in microwave ovens, oddly enough, tiny amounts of aluminum foil can be used providing the foil doesn't touch the sides of the oven. For example, foil might be used in a microwave oven to shield the tips of chicken wings that might cook much faster than the rest of the wing.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.