ceramic mugs in the microwave: some hot, some not

rhome410February 25, 2008

I'm guessing this is the best forum on which to ask this weird question. When I heat hot beverages in the microwave, certain mugs stay reasonably cool as the liquid inside gets hot. Other mugs get so hot I can't pick them up even if the beverage isn't hot yet. Is there anyone who knows the possible differences in the makeup of the mugs that would cause this? They're all supposed to be microwave safe.

When I use the mugs that get hot, I've taken to heating the beverage in a Pyrex measuring pitcher, but I still wonder what causes this and what it says about what I'm drinking from.


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I've noticed the same thing and can't figure out why, either. I now use a microwave teakettle. I'm tired of getting burned.

    Bookmark   February 25, 2008 at 4:57PM
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I've never seen something like that. Like one of the reviewers, I have the problem of putting something in the micro, and walking over to the computer to make use of the time, then forgetting I have it in there til it's cold again.

    Bookmark   February 25, 2008 at 6:03PM
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Microwaves work by heating any polar molecules (molecules that act as tiny magnets) that are present within the microwave light field. Oils, fats, sugar, salt, and water each contain polar molecules and will heat up rapidly in a microwave field. Plain ole pure glass has no polar molecules. Accordingly, the microwave energy will completely pass through pure glass and will not heat it up at all.

Some containers composed of plastic or ceramic materials are not Microwave Safe.......that's because they do have polar molecules in them or they have minute air bubbles in their structure. These type containers are not Microwave Safe.

To test if a any ceramic-like item can be used in a microwave oven, perform the following test. Place the EMPTY container in the microwave oven and microwave it for 1 minute. If the container is HOT to the touch.....it is not Microwave Safe and DO NOT USE IT IN THE MICROWAVE. If the container is warm to the touch, it is OK to use it to heat up items in the microwave.

Bottom line......things heat up in the microwave oven because they have polar molecules within them. And, you should only use containers that you know are microwave safe.....that's what your instruction manual advises.


    Bookmark   February 28, 2008 at 12:47AM
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As I said, all are supposedly "microwave safe." That's why I was wondering, and wondering if they truly were safe. Sounds like maybe not.

    Bookmark   February 28, 2008 at 2:07AM
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If they got hot when microwaved empty, they are definitely not microwave safe.


    Bookmark   February 28, 2008 at 9:13AM
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I got a second degree burn from a ceramic mug that I put in the MW at work. I had to go to the workman's comp doctor to be treated. I think there is something in certain glazes on ceramics that makes them unsafe. I've had better luck with ones that are basically white. The one that burned me was a dark color, but I don't remember the exact color. Also, if the mug has any metallic colors, it can make sparks in the MW. As suggested, it is best to test an empty mug (or one with water) first and use an oven mitt when you remove it the first time.


    Bookmark   March 2, 2008 at 2:15PM
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Be careful of mugs (or cookware) that have blue markings or patterns. Colbalt is used to produce some blue colors and some mugs us some cobalt. We have a set that has about 3 of 8 mugs that get killer hot and blister your hand if not careful. I put a big blue x on the bottom with a blue magic market to warn myself not to use these in the uW oven.


    Bookmark   March 4, 2008 at 11:08PM
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A lot of dinnerware says "microwave safe", but the first time in the MW, it gets too hot to touch. We can't easily take a piece of it and find a microwave in the store and try it out.

    Bookmark   March 5, 2008 at 12:06PM
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And just because a piece is white pottery or stoneware, don't assume it's micro-safe....and just because it's an antique don't assume it is.
I have some old coupe soup dishes that work very well to make oatmeal...but also have some other old white ironstone that gets hot.

    Bookmark   March 5, 2008 at 2:04PM
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I wish to warn everyone not to perform the container/mug microwave test as outlined above. If the empty object is indeed microwave safe the magnetron will be working without a load and can be damaged. The correct method is to place a glass measuring cup or similar known microwave safe container containing water in the oven along with the empty object to be tested. If the empty mug gets hot and it is marked "microwave safe" it should be trashed.

    Bookmark   March 7, 2008 at 12:21AM
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Thanks for pointing out the error in my post. Here is what the USDA recommends:

"If a utensil is not labeled for microwave use, you can test it before using to make sure it is microwave safe. Here's how to do it.

Put one cup of tap water in a glass measure. Place the water in the microwave oven along with (but not touching) the utensil to be tested. Microwave on high 1 minute. If the utensil feels warm or hot, it is not microwave safe because it contains metal in the material or glaze. Do not use it. The utensil and/or the bottom of the oven might crack if microwaved."


Here is a link that might be useful: USDA: Microwave Ovens and Food Safety

    Bookmark   March 7, 2008 at 9:46AM
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The above quote states, "If a utensil is not labeled for microwave use..." and another place on the linked page says, "SAFE TO USE: Any utensil labeled for microwave use." The mugs I was talking about when I started this whole discussion are clearly marked, "Microwave safe." Is someone checking these things for responsible, accurate labeling? The USDA info is assuming a lot, unless there is a way they are certain anything labeled that way is indeed safe.

    Bookmark   March 7, 2008 at 11:58AM
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I remember testing my Corelle coffee cups for microwave safety sometime back and my earlier post was what I remembered doing. I have found the operation manual for my Sharp Microwave oven. Here the Sharp's procedure that I performed on my unlabeled glassware to determine if it was microwave safe:

"Should you wish to check if a dish is safe for microwaving, place the empty dish in the oven and microwave on HIGH for 30 seconds. A dish which becomes very hot should not be used."

So, it is OK to microwave an empty dish for 30 seconds in a Sharp microwave oven.


Your mislabeled mug sounds like a Consumer Product Safety Commission Issue rather than a USDA one. What is the name of manufacture on your mug's label?

    Bookmark   March 7, 2008 at 10:31PM
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I just meant that if the USDA is going to publish something like that, and we're all encountering these kinds of problems, it'd be best for them to encourage people to test, labeled microwave safe or not. Never assume! ;-)

The brand on my mugs is Home from Target. Also says, "American Simplicity." followed by "Made in China." ...I know that doesn't necessarily have anything to do with our topic, but thought it was a little funny. Sad, but you have to laugh.

    Bookmark   March 8, 2008 at 12:46PM
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Here's a weird one: A ceramic mug I've used for years in the microwave to boil water for tea now gets too hot to touch (the handle is sizzling). The microwave is the same one. In fact, I've tried this mug in 3 ovens and all of them heat the ceramic. How could the ceramic not get hot for years, then after I go for two weeks of vacation, suddenly start absorbing the energy?

    Bookmark   August 11, 2011 at 12:47PM
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To Java Joe - Maybe NOT so wierd - that is the exact reason I ended up googling about ceramics in the microwave! Many dishes are fine for years and years, and then they become blisteringly hot, and they never seem to get better. Some handmade, some made in France, some misc... Curious!

Reading a few forums, I am wondering if it is multiple dishwasher passes, and maybe normal micro-cracks, over time, allowing sugars or fats to seep in, allowing superheating?

The only thing I know for sure is that it's frustrating!

    Bookmark   March 3, 2012 at 11:08AM
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I googled this page to find out what the two posts before me described. I have some mugs I've used for years without a problem. Suddenly one day, I'm removing my tea from the MW (with a yawn) and my hand practically sticks to the handle. You can imagine my surprise. Furthermore, it seems to me that the same mug is "safe" one day and "unsafe" the next.
My kids are big enough now not to worry, so I don't really care as long as I'm not damaging the MW. It is a curiosity though.

    Bookmark   March 4, 2013 at 8:31AM
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