Originally Posted by microwaveguru
In themselves grease spots probably won't make the bags more likely to catch fire ( oils & grease don't absorb microwaves well). But, if ignition starts, the grease can act as a fuel and increase the fire.
question- I have never heard of the rice bags catching fire. You know what i mean. The bags that you put rice in and warm them in the microwave for aching joints. My potatoe bag that I bought has a burn spot but my rice bags - never a problem