I have made these bags for about 5 years as gifts for family and friends. Mine are all 100% cotton. Only one has started on fire. When I asked if it might have been used for a potholder and got grease spilled on it we decided that that was most likely the cause. You must keep your bag clean, make sure you have you micro set for the right time keep and eye on it., My DIL found a new use for hers. She was leaving on a trip and wanted to pack her Kindle book reader. She had an extra potatoe bag and said it was just the right size.