As a household rule, do not put metal cookware or utensils in a microwave unless the microwave manufacturer or accessory instructions specifically say they are approved. The U.S. Food and Drug Administration (FDA) explains that microwaves are reflected by metal, which can cause uneven cooking, arcing, and appliance damage.
Why is the microwave cavity itself metal?
Because that cavity is intentionally designed to reflect microwave energy in a controlled way. A fork, tray, crumpled foil, or metallic trim placed inside is not automatically part of that design.
Sharp points and thin edges increase arcing risk
Objects with tines, corners, or wrinkled foil can concentrate the electric field and promote an arc, the visible electrical discharge people describe as a spark. Stop the microwave if this occurs.
Manufacturer-designed exceptions exist
Some microwave models include metal racks, trays, or instructions for limited foil shielding. Those pieces are engineered for specific positions and functions. Their presence does not make arbitrary metal safe.
Metallic decoration counts too
Gold- or silver-rimmed plates, cups with metallic paint, twist ties, and packaging with metal components can arc. Use cookware labeled microwave-safe.
If you see sparks
Stop the cycle and allow the appliance to cool. Remove the offending item only when safe. If arcing continues with no obvious metal, the cavity is burned or damaged, or the door does not close firmly, stop using the microwave and arrange service.
Do not remove the outer cover to investigate
Microwave ovens contain high-voltage components that can be dangerous even after unplugging. Internal repair belongs to trained service personnel.
The exception should come from the manual, not from an improvised experiment.
Sources
- FDA — Microwave Ovens — Microwaves reflect from metal; metal pans and foil are generally not used unless the oven instructions allow them.