Can you microwave metal?

Microwaves induce currents in metal rather than passing through it. A smooth thick object mostly just reflects and shields the food, so nothing heats; a thin one, or one with a point or a fold, concentrates charge until it jumps to the cavity wall as a spark. Repeated arcing burns through the mica waveguide cover, and once that is damaged the magnetron itself is at risk. Running an empty oven or one containing only metal is worse still, because the energy has nowhere to go and reflects back into the magnetron.

What goes wrong

• Sparks scorch the interior and ignite paper, grease or packaging
• The waveguide cover and magnetron can be permanently damaged
• Food inside a metal vessel stays cold while the vessel gets hot at the rim

What to do instead

• Use glass, ceramic or microwave-safe plastic
• Reheat metal cookware contents on the stove instead
• Use a convection microwave in oven mode, where metal is allowed

Other "can you microwave" questions

• Can you microwave styrofoam? It depends — Only if the container is labeled microwave-safe, and most takeout foam is not.
• Can you microwave aluminum foil? No — No — foil reflects microwaves and the edges arc, which can start a fire.
• Can you microwave plastic containers? It depends — Only ones labeled microwave-safe — cold-storage tubs are not among them.
• Can you microwave eggs? It depends — Beaten or pricked eggs, yes. A whole egg in its shell will explode.
• Can you microwave Tupperware? It depends — Yes, if that specific piece carries the microwave-safe symbol on its base.
• Can you microwave paper plates? It depends — Plain white paper plates are fine; anything with metallic trim or a foam layer is not.
• Can you microwave breast milk? No — No — microwaves create hot spots that scald a baby even when the bottle feels cool.
• Can you microwave takeout containers? It depends — Only if the container is marked microwave-safe — most are not, and metal-handled ones never are.
• Can you microwave cardboard? It depends — Plain dry cardboard is usually fine briefly; anything with glue, staples, wax or foil is not.
• Can you microwave glass? It depends — Yes for ordinary heatproof glass; no for leaded crystal, and never straight from the freezer.
• Can you microwave water? It depends — Yes, but very clean water can superheat and erupt when you move the cup.
• Can you microwave ceramic? It depends — Yes for plain glazed ceramic; no if it has metallic trim or is unglazed and porous.