Occasionally I get a phone call from someone who purchased one of my coffee mugs from a shop or gallery or even a sales clerk at one of those shops, and they want to know if my coffee mugs are microwave safe.
Most stoneware and porcelain will be safe to use in the microwave. But I have had some pots get hot when exposed to the microwave. I suppose there is some metal in the glaze that is attracting the waves. But there is a simple test to discover if your pieces are microwavable. A microwave cannot be run empty, so place a cup of water in the microwave. Next to it, place the piece you wish to test, but be certain it is empty (if it contains water you will not be able to tell if it’s the water that is heating up or the pot). Run the microwave on high for 1 to 2 minutes. If the empty pot is hot, there is something in the glaze or clay body that is attracting the waves and it cannot be safely used in the microwave.