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.

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.

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.

Dan

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.

If they got hot when microwaved empty, they are definitely not microwave safe.

Dan

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.

Lars

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.

Jim

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.

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.

laat2,

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."

Dan

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

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.

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.

rhome410,

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?

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.

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?

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!

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.

I am reading these posts for the same odd handle behavior. A set of blue and white ceramic mugs purchased at Kroger several years ago used to be microwave safe for my coffee and tea. Now the handles get blistering hot before the water is heated up. I wonder if cobalt might be in the blue dyes used for the handles of these cups. A post on another page suggested that blue is particularly troublesome for overheating. I like the cups, but not the burned fingers.

I recently ran into the same problem with a mug which I've used for years to heat up drinks/coffe/tea. I believe the issue is due to moisture seepage into the handle, not from my coffee, for instance, but from our new dishwasher. The dishwasher was replaced recently and its wash times are considerably longer than the older model it replaced. My guess is that that extended time in the dishwasher is allowing just enough water to enter the stoneware which is getting "cooked" in the microwave along with my beverage. Needless to say, I won't be using that mug in the microwave any longer unless I hand wash it.

Same problem..suddenly the microwave safe mugs are getting hotter than the liquid.This is inconsistent...but will check too see which mugs have changed.

Interesting stuff! I'm wondering if the compromised glazes leach lead? I bought some old Japanese, green glazed cups which got extremely hot in the MW and developed fine cracks. The coffee I heated in it was really bitter and scary tasting! As beautiful as these cups are, I think I'll turn them into vessels to hold things other than consumables!

I have a stranger problem. I bake things and reheat them on a plate in my microwave. Sometimes, like today, the plate gets very hot. Other times, it doesn't. it's the same plate. Microwaves work by exciting water molecules in food. The frequency of the radiation is chosen because of water. There likely isn't much water in my small plate, and why doesn't it get hot every time with the same settings and about the same amount of food on it?

Same problem I bought a handmade, blue ceramic cup in Mexico last month. It worked great for microwaving coffee then this morning I burned my fingers pulling it out of the microwave. The only difference is I ran it through the dishwasher yesterday.

It might have absorbed water into the ceramic to get hot, makes only sense after the dishwasher treatment....don't think it's in the glaze on this one what most everyone else claims....
I've just googled to see what the heck is going on.... have coffee mugs from Portugal, when heating up coffee..the whole mug gets hotter then coffee and you can't hold it for a while...and yes, they've been getting the dishwasher treatment, don't remember they were getting this hot when new a while back...what do I do?

I was told by the manufacturer of the mugs that water enters the porous ceramic when you wash the mug and that is what heats up in the microwave. If I use a mug from the back of the cupboard that is completely dried out, I don’t have the problem. Also, some mugs are made from more or less porous material; thick stoneware seems to absorb more water than thin, more china-like mugs.

Nearly burned my hand this morning retrieving a water-filled mug from the microwave that I've used for years with no problem. So I tried a very different, newer mug that I've also frequently used for heating tea water with no problem. Same thing happened. Both of these mugs were hand washed. The water in them became hot as usual. Yesterday they were. just fine. Could it be the mcrowave?

Same for me. I have a set of dishes that I've used for years from Target that I had no problems with. Interestingly, the plates and bowls get hot but not the mugs and food is lukewarm. I recently turned up the temp on my hot water heater and now my dishwasher is alot hotter than before. I wonder if the temp could be a factor, or just the dishwasher in general or like someone else stated, maybe the microwave because I've had it for years. At any rate, I'm considering buying another set of dishes. Or I could treat it like the oven and just use oven mitts. LOL!