It is a common warning that metal should not be placed inside a microwave oven. This general caution stems from the specific reactions that can occur when microwaves interact with metallic objects. Understanding these reactions is important for safe microwave use.
The Immediate Reaction
When a metal spoon is inadvertently placed in a microwave, an immediate and striking phenomenon often occurs. You might observe bright, intense flashes of light, accompanied by crackling or buzzing sounds. This dramatic display is known as “arcing,” which refers to an electrical discharge that jumps through the air. The sparks can appear almost instantly, creating a startling visual and auditory event inside the microwave cavity.
The Science Behind the Sparks
Microwave ovens operate by generating electromagnetic waves. These microwaves are produced by a component called a magnetron. Metals, however, behave differently when exposed to these waves; they are excellent electrical conductors and tend to reflect microwaves rather than absorb them.
Within metal, there are free electrons that move in response to electric fields. When microwaves encounter a metallic object, they cause these free electrons to “slosh” back and forth on the metal’s surface. This movement of electrons can lead to a concentration of electric fields, particularly at sharp points, edges, or crumpled areas on the metal.
When the electric field becomes sufficiently strong, it can ionize the surrounding air molecules, meaning it strips electrons from them. This ionized air then becomes conductive, allowing an electrical discharge, or arc, to jump across the gap, producing the visible sparks and crackling sounds.
Potential Hazards
Microwaving metal poses several risks. The intense electrical sparks generated can burn holes in the microwave’s interior walls. Such arcing can also damage the magnetron, the component responsible for generating microwaves, leading to the appliance’s malfunction or complete failure. In severe cases, the sparks can ignite food items or packaging inside the oven, posing a fire hazard. While a single, brief instance of arcing might not cause extensive damage, repeated occurrences or significant arcing can lead to costly repairs or dangerous situations.
Not All Metal is Equal
Not all metal items react identically in a microwave oven. Smooth, thick metal objects, such as certain microwave-safe metal racks or the rounded rims on some dish lids, are less likely to cause arcing because they do not have sharp points where electric charges can concentrate.
Aluminum foil can sometimes be used in a microwave, but strict guidelines must be followed to avoid arcing. Only small, flat pieces of new foil should be used, pressed closely to the food to shield specific areas from overcooking. Crumpled or wrinkled foil, or pieces that are too close to the oven walls, can act like antennas and easily cause sparks.
Items with thin metallic trim or gold-leaf decorations on plates are also prone to arcing due to their sharp edges.
What to Do When it Happens
If you accidentally place metal in the microwave and witness arcing, it is important to act quickly and safely. Immediately turn off the microwave oven. If possible and safe to do so, unplug the appliance from the wall outlet.
Wait a few moments for the arcing to cease and for the metallic object to cool down before carefully removing it from the microwave cavity. After the incident, inspect the microwave’s interior for any signs of damage, such as burn marks on the walls or the waveguide cover. If significant damage is visible or the microwave behaves unusually afterward, it is advisable to have it inspected by a professional before further use.