Why do some kinds of cookware get very hot in the microwave?

There are two reasons why cookware may heat up excessively in a microwave oven.

The first is the same effect that occurs in cooking: friction. Microwaves are rapidly fluctuating electromagnetic fields, which push on electric charges and twist naturally polarized particles like water molecules back and forth.

In liquid water, this twisting leads to molecular friction and heating. If the cookware is wet, like a porous crockery cooker that is moistened before use, it will get very hot.

A second effect can also cause trouble as the microwaves push free electric charges around. In a good conductor of electricity through which the charges move easily, the microwaves are reflected.

The metal walls of the oven, for example, barely heat at all. But in a poor conductor, the charges being pushed around in the material cause resistance heating, just like that in the heating element of a hair dryer.

To test pottery or dinnerware for use in a microwave, put the empty dish in the oven alongside a cup of water in a microwave-safe glass container. Microwave on high for one minute. If the dish stays cool, it is safe to use.

Don’t use porcelain cups with gold rings around the lip. If you try to reheat your coffee in one, the ring burns up just like a lightbulb filament.

About Karen Hill

Karen Hill is a freelance writer, editor, and columnist. Born in New York, her work has appeared in the Examiner, Yahoo News, Buzzfeed, among others.

1 thought on “Why do some kinds of cookware get very hot in the microwave?”

  1. Your comment about porous crockery is correct. However, there is also a serious problem of overheating with some glazed crockery, especially that made in China, even though it may be marked “Microwave Safe”. Since there is no legal definition of this term, it does not mean that the crockery does not become hot, even though consumers think that is what it means. In my laboratory, we have measured temperatures as high as 300 F at the handle of coffee cups, and many temperatures exceeding 200 F. Many consumers have been seriously burned by these supposedly “Microwave Safe” dishes.
    As to the gold rims – this holds for silver and other metallic decorations (not just rings). They will arc and can shatter the crockery.

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