I have a very old iron I bought at an antique store that could cook an egg it gets so hot! I can't turn it up past a 2 or it will scorch my fabric & that is the iron I ironed the thread with. I wanted to make sure it didn't melt FIRST before using it because I was familiar with nylon kind that WOULD melt. I feel like it's the greatest thing sliced bread, especially for applique! I don't know that I would use it on a regular quilt but for applique it's been great.