What a fun find! I use thread from garage sales all the time.

I have spools of thread in my stash which are upwards to 50 years old. Some is strong, others are not. I was told by a quilting instructor many years ago to strengthen thread store it in the fridge as it will absorb moisture and be strong again. I've tried it on some and on some it seemed to work. Some old thread may be beyond any help from us other than to be added to an old jar and saved for antiquity's sake, for the wooden spool or tossed. I'd save it and add it to my already old collection of thread in an old jar. My two cats love playing with the spools. LOL