What does it mean for cookware to be microwave safe?

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The world is full of unanswerable questions. Where resides the soul? Can a person really ever know another? What is death like? Such mysteries abound. I have seen wonders in this world, dear Reader, but I confess that I was not prepared for this: It’s really, really hard to get anyone to tell you what makes something “microwave safe.”


That’s not hyperbole. In the weeks I’ve been working on this story, I’ve had multiple sources fall through, people and companies willing and eager to participate until they see the question, “What makes something microwave safe?” Then there’s a meep meep and a puff of smoke, and they’re off. I got nothing beyond the obvious. (The obvious: “Microwave safe dishes are safe to use in the microwave.”)

The U.S. Food & Drug Administration pointed me toward several resources concerning microwaves. This one says, in short, that you should use “glass, ceramic, and plastic containers labeled for microwave oven use,” but that’s not an answer. (It also says to avoid “super-heated water,” which can explode out of the cup if you so much as move it and now I’ve got a new fear, thanks FDA.)

This one has a bit more to offer:

Glass, paper, ceramic, or plastic containers are used in microwave cooking because microwaves pass through these materials. Although such containers cannot be heated by microwaves, they can become hot from the heat of the food cooking inside.

The U.S. Department Of Agriculture also offers a brief guide, but also nothing beyond what seems obvious: Avoid plastic containers unless it’s marked microwave-safe. Don’t let plastic wrap touch your food. Don’t microwave brown paper bags, grocery plastic bags, or newspapers.

Still it begs the question: What does it mean for cookware to be microwave safe? Is it simply about temperature—a bowl that you nuke and won’t burn the bejesus out of you? Or is it more a food safety issue, such as chemicals leaching out?


Enter Takeout “Food Science” contributor David McCowan to the rescue. While he emphasized that he was answering my questions as a physicist, not a food scientist, he still had plenty to offer. The key isn’t what makes microwaving food safe. It’s what makes it unsafe.

“I don’t know what the product engineer’s answer would be, but microwave-safe to me means that the material when used in the microwave will not break or deform, become too hot to handle, give off dangerous chemicals into the food or air, or cause damage to the microwave,” he revealed via e-mail.


Things that are microwave-unsafe aren’t limited to dishes and containers: Microwaves work by causing water molecules in food to vibrate, and those molecules then jostle all the other molecules, creating heat. But that means that anything in which water can be trapped could potentially explode, so don’t microwave watermelons. Also “sharp-edged metal objects” (i.e., forks, knives, and aluminum foil) can make sparks fly. (That’s bad.)


Because most containers aren’t watery, they aren’t heated directly by the microwave, he explains. It’s the food that heats the containers up. That’s true of plastic, metal (provided the surface is smooth, large, and un-crumply), paper… you get the idea.

So why aren’t all non-crinkly-metal containers designated microwave-safe?

“The line is a little less clear when a material will itself get warm or will warp in the microwave. Styrofoam, for example, does not get directly heated in the microwave (since it contains no water), but certain forms are soft enough that they can melt (or at least deform) from indirect heating as the hot food inside transfers its heat to the container.” He also mentions that some ceramics contain residual water, meaning the dish itself could heat up, creating a burn hazard.


(A site called Cooking For Engineers suggested that some glassware that isn’t microwave-safe may have to do with micro bubbles that are present in the glass, which can expand during the microwaving process and… kaboom.)

McCowan outlined several of the ways something could be “microwave-unsafe,” (let’s call it M.U.S., for giggles) though this should not be considered an exhaustive list (see: super-heated freakin’ water.)

  • If the material could leach stuff into the food, like melamine, it’s M.U.S. Soft plastics might break down over time, and certain decorative paints could potentially flake off when heated.
  • Metal in general is a bad idea, so watch out for decorative metal bands in teacups and other things. While in theory, a very smooth metal bowl “with no sharp lip” would be safe, “the line between smooth and pointy is continuous... most metal containers fall ambiguously in the middle.” M.U.S.
  • If the material could heat up itself, like the aforementioned ceramics or something covered in a paint or glaze that heats up, that’s bad M.U.S. news.
  • The M.U.S. material could bubble, melt, or become otherwise deformed, when heated indirectly by the food.

That means we have an answer to the question.

Something is microwave safe if it:

  1. Won’t heat up directly because there’s no water in it
  2. Won’t melt when the food heats it indirectly
  3. Won’t vomit chemicals into your food when it’s heated
  4. Doesn’t contain metal that’s even moderately sharp-edged.

McCowan kindly leaves me, and thus us, with suggestions: “Avoid anything with decorative metal foils or flakes in the paint unless the manufacturer says it is safe,” and assume most ceramics are safe. (Not to scare you more, but some ceramic mugs are unsafe even if it says “microwave-safe.” According to Sub-Zero and Wolf appliances, some paints on mugs attract microwaves and can make the mug dangerously hot.)


For cookwares that aren’t marked, there’s a test from the website of GE Appliances to see if a particular utensil or dish is microwave-safe: Fill a cup with water, and place it alongside the cookware in question inside a microwave. Heat for a minute on high. If the water gets hot and the cookware stays cool, it’s safe to use. If the cookware gets hot, well, don’t put it in the microwave again. Alternatively, McCowan suggests you can also fill the container in question with water and placing it in the microwave for 30 seconds. That’s not enough time for the water to heat up the dish, so if the dish is warm, that’s a bad sign. “If it’s still cool after a full minute of warming, then it’s probably okay.” Just don’t heat it too much, because super-charged water is terrifying.

Contributor, The A.V. Club and The Takeout. Allison loves TV, bourbon, and overanalyzing social interactions. Please buy her book, How TV Can Make You Smarter (Chronicle, 2020). It’s short!



Ahhh, microwave-safe. I remember, way back in the misty days of 2004, our office junior (think intern, but paid) was ordered by the Office Piss-slit (aka, Wanky Jeremy from Marketing) to microwave his lunch. He hands her his lunch in his “special bowl” (wanker) and tells her it’s to be microwaved on full for four minutes. And off he stropped.

Our Plucky Junior goes into the kitchen, chucks the bowl into the microwave (but not before liberally adding salt, despite Office Piss-Slit demanding a salt-free environment because the wanker had read some wanky article about salt ageing you. No, Jezza, being a total and utter cunt towards a woman twenty years your junior is what ages you) and twists the dial to four minutes, pops the kettle on for her eighth cup of tea of the day and departs.

I’ll just interrupt the story to tell you about Plucky Junior’s tea addiction. It’s a running joke amongst Non-Brits that us Brits are addicted to tea. In Plucky Junior’s case, this seemed to be true. I saw that girl get the shakes and go pale and sweaty a la Brock Turner bending over in the prison showers if she went without a cuppa for more than four hours. One of these days I’ll tell you about the time we went for a team-building day and my boss forgot to bring Plucky Junior’s Tetley. Tea-rage? It’s fucking real.

Anyway, back to the nuclear cooking furnace.

About a minute into the cooking process, the microwave started doing it’s level-best to re-enact the V’ger-ascension-scene from Star Trek: The Motion Picture. We’re talking lights, we’re talking strobing, people. Myself and my colleagues? We just stood in the doorway watching the Kenwood zapper trip the light fantastic. None of us were prepared to go near that fucking thing, not after it turned Carolyn-from-Data-Management’s soup into a coagulated brown mess straight from Mordor (and it was tomato, people. Motherfucking-tomato). At the three minute mark, smoke began gushing forth like motherfucking Zuul herself was coming on through from the Other Side. I do believe I may have shrieked. I’m not proud of this, but at least it would have been a manly shriek. Shut up.

By now, that wanker Jeremy had stormed out of his cubicle to demand where his lunch was. At the same time, Plucky Junior ordered him to stand aside, strode into the kitchen, punched the door-open button, reached in - bare-handed - and retrieved his special fucking bowl and his special fucking lunch, took one look at it and hurled the offending item into the kitchen sink and in the same movement, reached over, turned on the cold water tap (which we were never to drink from, mostly because the water came direct from a roof-top tank in which we had discovered a dead pigeon and something nasty and green in the water), turned to Jeremy, glared at him and snarled “your special fucking bowl is not fucking microwave-safe, you prick!” And with that, she exited the kitchen, using expletives that even I, a born-and-bred Glaswegian, have never heard being used before or since, leaving a bowl of faintly smoking something, a smoke-filled kitchen and all of us staring at Piss-Slit Jeremy and hoping that the earth would indeed open up and swallow the arsehole. And then the fire alarms kicked in and the entire office building - we’re talking around four, five hundred people - was evacuated onto the cold, miserable October weather. I do recall Jeremy being told to fuck off at least four times as he scurried around looking for a space in a bus shelter. Heaven fucking forbid he get his hair wet. We’re talking early 2000s mullet, people. Yeah.

He got the sack shortly afterwards. Plucky Junior got promoted and soon became my deputy in my department. We even used the Twatting-Microwave Event as a reason why she should take over from me when I left the company, you know. I do believe I said that it showed her superior organisation skills, multi-tasking and employee relations skills. Don’t know what happened to Jeremy, mind you. But I hope that wherever he is, with his Special Fucking Bowl, that he’s using microwaves safely.

Long story cut short:  if you have crockery that has any decoration done in a metallic glaze (no, really), check that it can  be used in the Mini-Chernobyl.  And, for fuck’s sake, be careful microwaving mugs of water.