I would check the strength of each spool of thread. What are the spools made of? Plastic or wood!
If you have wood spools, then you might want to create a sewing display of the spools with the thread on them. If they are plastic you could still create a display. You could start a thread display for just this purpose. It's going to be garage sale time soon for many of us in the US so something to look for.
I personally like to purchase new thread for my projects unless I have the thread and I know about how long I've had it around the house.
I'm not sure if there is a life to silk thread, so I will be watching this thread to see what others have to say.
Pam M