Aluminum Foil in the Microwave: What’s Safe, What Sparks, and Why

Foil in a microwave is one of those questions where the honest answer is “it depends on your microwave”. Some makers allow small pieces under strict rules. Many say never. Here is how to decide, and what to do if you get it wrong.

Quick answer: If your manual does not allow foil, do not use it. If it does, use only small, smooth pieces, keep them at least 1 inch away from the walls, and never crumple them. Move foil takeaway trays to a microwave-safe dish unless the packaging says they can go in.

Why foil can spark

Microwaves bounce off metal instead of passing through it. Crumpled foil, sharp edges and thin points collect electric charge, which can jump as sparks (arcing). Sparks can scorch the inside of the microwave, damage parts, and in the worst case start a fire.

What one microwave maker says

Wolf, a microwave maker, gives these rules for its own ovens:

• Use only new, smooth foil. Wrinkled foil can cause sparks.
• Cover no more than one quarter of the food, and shape the foil smoothly so no edges stick out.
• Keep foil at least 1 inch away from the oven walls.
• If the oven has a metal shelf or turntable, do not microwave food in foil containers or metal pans.
• If it sparks, remove the foil shield at once. For frozen food in a foil container, move the food to a microwave-safe dish.

Other makers have different rules, so your own manual comes first.

What about foil trays and takeaway containers?

An industry-funded laboratory study from 2006 found that foil trays could be heated safely under strict conditions: the tray in the centre of the oven, at least 2.5 cm from the walls, on a plate, with any metal lid removed, and with shallow food. It also found that food heated slower and less evenly in foil trays. That study was paid for by the foil industry, and many oven makers still say not to use trays, so follow the label on the tray and your manual.

If you do heat food in a foil tray, check that it is hot all the way through, because it can heat unevenly. Food should reach 165°F (74°C).

Also read Microwave oven Electricity Uses

What to use instead

• Microwave-safe glass or ceramic dishes
• Plastic containers marked microwave-safe
• A microwave-safe plate with a paper towel or microwave-safe cover to stop splatter

What to do if it sparks

1. Stop the microwave at once.
2. Open the door and carefully take the foil out once it is safe to do so.
3. Let the oven cool, then look inside for burn marks.
4. If you see flames, keep the door closed, switch off and unplug the microwave if it is safe to do so, and call emergency services if the fire does not go out.
5. If it keeps sparking or smells burnt, stop using it and have it checked.

Other things that can make a microwave spark

• Plates or bowls with a metal rim or metallic paint
• Twist ties, metal handles and travel mugs
• A damaged waveguide cover (the small panel on the inside wall)
• Burnt food residue stuck inside the oven

FAQ

Can I cover food with foil in the microwave?

Not fully. Covering food completely with foil blocks the microwaves and can cause sparks. Use a microwave-safe lid or a paper towel instead.

Can I microwave a foil takeaway container?

Only if the packaging or your manual says so. Otherwise move the food to a microwave-safe dish.

Can I microwave a burrito wrapped in foil?

No. Take the foil off first. Foil wrapped tightly around food is a spark risk.

Is it dangerous if my microwave sparked once?

One brief spark usually does no harm if you stop it quickly, but check the inside for burn marks. If it sparks again or smells burnt, get it checked.

Why does my microwave spark with no foil?

Common causes are metal trim on dishes, a damaged waveguide cover, or burnt food residue. Check your dishes and clean the inside.