Now things get a little weird. It turns out that any flat metal sheet—the thicker the better (and we'll get to why in a second)—can be used safely in the microwave because they act just like the flat metal walls: they reflect microwaves. A sheet of flat metal can actually be used to shield parts of your dinner to prevent them from overcooking since they would prevent the microwave energy from actually striking the food. And when placed under, say, a DiGiorno's frozen pizza, a flat sheet of metal can actually help brown and crisp foods by reflecting additional energy against the bottom of the food.
As such, follow the USDA's advice:
Use new, smooth foil only. Wrinkled foil can cause arcing (sparks). Cover no more than 1/4 of the food with foil. Shape the foil smoothly to the food so no edges stick out. It makes no difference which side of foil (shiny or dull) is facing out. Do not place the foil closer than one inch from the oven walls. If the microwave oven has metal shelves OR a metal turntable, don't microwave food in foil containers or metal pans, and don't let foil used for shielding touch or be close to the shelves or turntable. If you see arcing (sparks), immediately remove the foil shielding; transfer frozen food from foil container to a microwave-safe utensil.
s it safe to use aluminum foil in a microwave oven? Always consult the owner's manual of your microwave oven and heed the manufacturer's recommendations for the use of aluminum foil. It can be safe to use small amounts of aluminum foil in a microwave oven.
Microwaves cannot pass through metal but are absorbed by food. No food completely covered by aluminum foil or in a covered metal pan should be put in a microwave oven because food wouldn't be available to absorb the microwaves. Operating the oven empty or when the food is completely wrapped in aluminum foil can cause damage to the oven and the food won't heat.
However, small pieces of aluminum foil can be used to "shield" areas of foods, such as poultry drumsticks and wings, to prevent overcooking.
Some food packaged in foil containers can be safe to microwave. Read the package heating instructions to see if the food manufacturer has specific recommendations for microwaving the product. Because food in these containers will only heat from the top, it's best to microwave foods only 1-2 inches in depth so food near the bottom will be heated thoroughly before food on top dries and overcooks.