Believe me, if you talk to a fireman, he will tell you putting ANY kind of fabric in a M/W, you are a mere step away from burning down your home or someone else's if you tell them to put fabric in the M/W. They are still all aghast that people still use those potato bags. A friend of our's husband was a fireman and she told me he said, some people just plain have no common sense. Potatoes cook fine without them, and all taste the same.
Unless cotton thread is used for sewing, people are using polyester which is like putting plastic bottles in your M/w. As someone previously stated, they are to HOLD bowls, on the table, after being removed from ovens, regular or microwave. To me, it's much easier to plop a hot pad on a table. When I make rice bags, I always use 100% cotton flannel, 100% cotton thread and rice and never longer than 2 min. in oven, at a med. heat. Please be safe.