I have made these potatoe bags entirely from cotton including the batting just like the instructions said. Our microwave is an 1100W and when I set it for 5 min. I had burn spots on the bag. If we time 2min and 3min. we don't have a problem, but I would never leave them unattended. I think it is the higher wattage microwaves that are doing it. Perhaps when this bag was designed there were no microwaves over 900W. Just my thoughts.