The clear thread is sometimes nylon, which can melt under certain circumstances depending on the brand. I have used YLI nylon monofilament with absolutely no problems, but don't have a sample handy to test with an iron. My experience has been that there are only a couple of good brands of nylon clear thread, and YLI is one of them. Other brands have problems. A lot of people use polyester clear thread now instead of nylon monofilament, but I have found that the polyester is not as invisible as my YLI nylon monofilament.