I am cleaning up my sewing stuff and found a box of threads, at least 50 spools of Guttermann. Some of it might be 15 years old!! Should I just throw it out, or save it to use? Also, how do you tell if it is cotton or polyester on a bobbin? I used to use poly for clothing sewing, but now all my time is spent on quilting with cotton. I think that my time is worth more than an old spool of thread if it damages my handwork. What do you all think?