For a couple of years we have been using a microwave-safe cup to heat up milk. The milk is whipped until it's creamy, then we add coffee on top of it and we have a sort of cappuccino. Until recently this has worked well, but for the last few weeks we have found that the cup itself is heating up in the microwave. I burnt my fingers while taking out the mug by its handle this morning. Is it possible that ceramic mugs take up more energy from microwaves over time? Or are there other explanations for our findings?
Whenever something gets hot in a microwave, it indicates the presence of water. If something that used to stay cool suddenly heats up, you have water present where previously there was none.
I assume you are using a glazed ceramic mug or jar, not one made of glass. Your glazing must have tiny cracks in it, that allow water to reach the clay underneath and collect there. Basically the wear and tear of long use. These cracks may be so tiny that, especially if no discoloration is present, they are virtually invisible. Each time you use the cup in the microwave again, the expanding water will actually aggravate the problem. The same is true for running the cup in a dishwasher, which in my experience allows for more water to seep through the cracks than a quick hand wash.
Time to get a new mug. Consider one made of glass or porcelain (china) this time.
I've noticed this same problem for years. Some mugs work fine and then start to heat up. The other thing I noticed is that if you continue to use them the problem will come and go over time. I agree with the other comments that is it likely water gets under the glaze and into the ceramic. The process that allows the water to get it is probably somewhat specific, such as a certain positioning in the dishwasher which is why the heating waxes and wanes over time. I've learned to quickly touch the handle of any mug I put in the microwave to see if it is hot before grabbing on.