What can I safely put in a microwave?

Microwaves heat by exciting water molecules, so the useful rule is about what the container does with the energy: materials that let it pass through stay cool and only warm from the food, materials that absorb it get hot themselves, and materials that reflect it cause arcing.

Safety

Step by step

1. Use glass, plain ceramic and marked plastics.Borosilicate and toughened glass ovenware, plain ceramic and stoneware, and plastics carrying the microwave-safe symbol — a set of wavy lines — are all effectively transparent to microwaves. They warm only by contact with the hot food, which is why the dish is often cooler than what is in it.
2. Keep every kind of metal out.Foil, foil-lined takeaway lids, metal skewers, twist ties, the gold or silver band round a plate, and metal-handled cookware. Microwaves reflect off metal and the field concentrates at thin edges and points until it arcs — a visible spark that pits the cavity and can ignite what is inside. Some ovens have a metal rack designed for use with them; that is the exception, not permission.
3. Avoid melamine and non-heat-stable plastic.Melamine picnic ware absorbs microwave energy, gets very hot and can craze and break down. Yoghurt pots, margarine tubs and cold-food takeaway containers are designed to hold cold food, not to be heated — they soften, deform and can leach into fatty or acidic food. Transfer the food to a proper dish.
4. Discard cracked or crazed ceramic.A crack takes up water, that water heats fast and expands, and the piece can split or shatter. The same applies to a mug with a hairline crack that has been through a dishwasher many times.
5. Cover loosely, and never seal.A cover keeps moisture in and stops spatter, but steam must be able to escape. Use a vented microwave lid, a plate resting on top, greaseproof paper, or cling film that says microwave-safe with a corner turned back. Never microwave a screw-topped jar, a sealed tub or a sealed pouch.
6. Pierce any skin or membrane.Whole potatoes, sausages, tomatoes, egg yolks and anything in a tight skin build internal steam pressure and burst. Pierce them several times. Never microwave an egg in its shell — it will explode, sometimes after you take it out.
7. Test an unmarked dish.Put the empty dish in beside a mug of cold water and run it on full power for 30 seconds. Take both out carefully. If the water is hot and the dish is cool or barely warm, the dish is fine. If the dish itself is hot, it is absorbing energy and should not be used.

Tips

• Very narrow or tall containers heat unevenly because the microwaves penetrate only a few centimetres. A wide shallow dish reheats far more evenly than a deep mug.
• Kitchen paper is fine as a loose cover for short bursts and is useful for bacon and for stopping spatter, but do not use printed or recycled paper, which can contain metal fragments in the ink.
• Cling film should not touch the food surface when heating fatty foods. Use a plate or a proper vented cover instead.
• Water can superheat above boiling point in a very clean smooth mug and then erupt when disturbed. Put a wooden spoon or a teaspoon in the mug, and let it stand for 30 seconds before moving it.

Common mistakes

• Reheating a takeaway in its own container — Most cold-food takeaway tubs are not heat stable and many lids are foil-lined. Tip the food into a plate or a glass dish.
• Assuming a mug is microwave-safe because it is ceramic — Metallic glazes, gold rims and lustre decoration all arc. If it has any metallic sheen at all, it stays out.

Questions people ask