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.