In principle, the answer is simple: Containers whose molecules aren’t dipoles and will not absorb microwaves. Such molecules will not be jerked around by the microwaves and will not get hot. But in practice, the answer isn’t quite so simple.
Surprisingly, in what many people perceive as our overregulated society, there appears to be no government, industry, or trade definition of the term “microwave safe.”
I have attempted to extract a definition from the FDA, the Federal Trade Commission, and the Consumer Product Safety Commission, all to no avail. Nor could I get any manufacturers of “microwave-safe” products to tell me why they make that claim. (Lawsuits! Lawsuits!)
So it appears that we’re on our own. But here are some guiding principles.
Metals: I’ve already explained why metals are to be avoided in microwave ovens.
Glass and paper: Glass (that is, standard kitchen glass, not fancy crystal), paper and parchment are always safe; they don’t absorb microwaves at all. So-called crystal, which is glass with a high lead content, does absorb microwaves to some extent and may therefore get warm. In a thick piece, the heat might set up stresses that may lead to cracking. Best not to take the chance with that expensive stuff.
Plastics: Plastics don’t absorb microwaves either. But microwaved food can get pretty hot, and any container, no matter what it’s made of, will then get hot from the food. Some wimpy plastics, such as thin plastic storage bags, margarine tubs, and styrene foam “doggie boxes” from restaurants, may even melt from the food’s heat. Certain kinds of plastic refrigerator containers may get distorted out of shape. You just have to learn this from experience.
Ceramics: Ceramic cups and dishes are usually okay, but some may contain minerals that absorb microwave energy and get hot. If in doubt, test the suspect by heating it empty in the oven along with some water in a glass measuring cup. If the test object gets hot, it’s not microwave safe. (The water is in there to absorb microwaves and avoid the empty-oven problem that I referred to earlier.)
To further complicate our lives, some earthenware mugs and cups, even though made of purely innocent, non-energy-absorbing clays, can crack in the microwave oven.
If the glaze has become chipped or cracked with age, water can seep into pores or air holes in the clay beneath the glaze, perhaps during dishwashing. Then, when microwaved, the trapped water will boil and its steam pressure can crack the cup. While that’s a rare occurrence, it’s best not to use your chipped or crazed heirloom cups in the microwave oven.
So. Why do some “microwave safe” containers still get hot in the oven?
“Microwave safe” means only that the container won’t get hot from the direct absorption of microwaves. But the food it contains does absorb microwave energy and therefore gets hot, and as I pointed out earlier, much of that heat is transmitted to its dish. How hot the dish gets depends on how efficiently it absorbs heat from the food, and different materials, even different “microwave-safe” materials, can vary quite a bit in that respect.
Always use potholders when removing microwaved containers from the oven. And when opening the container, beware of pent-up steam, which can be very hot.