threads cannot be cleaned or washed. If you have "centuries-old spools" of thread, then you can't do anything about it. They can also be disturbed by moisture and simply old age. they will tear. You can try to unwind and throw the upper part of the thread from each spool. I don't know what spools they are. If the coils are five hundred meters, it's worth the effort. But if you have a small roll of 150 feet, then throw it away.