Originally Posted by
kwilter
DON'T MAKE THE BAGS AT ALL! Have heard several times that the microwave can set the bag on fire! Use the fabrics for something safer. 
From my personal experience over the years - and those of my friends - problems occur with potato bags is when:
1 - your microwave's glass carousel is dirty (the food or oil residue ignites or burns and in turn sets the bag afire)
2 - you did not use 100% cotton fabric, thread and batting
3 - the batting you used was not specifically "cleared" for use in the microwave - in other words, there's scrim or other crud in the batting itself that has a lower combustion temp that "food" batting.