I would first try putting it in the freezer for a while and see if that did
anything. If that didn't work I would try a short amount of time in an oven
at a very low temperature. Then maybe pack it in a plastic bag filled with
desiccant. The crystal cat litters are supposed to be silica gel and are
probably cheaper and easier to source than silica gel.
I would think it is caused by either moisture, old hardened lube (wax) on
the threads, or one of the threads is no longer completely round. One of
the three steps should handle moisture or hardened wax.
I have, here in my hot little hand, the handle from a Millers Falls
Alford-type hand vise. At the moment, its highest and best future use might
be as a sort of maraca, because I can hear the rattle of tiny parts and bits
inside when I shake it.
Anyway, I'd like to expand the use of the Alford vise handle beyond its
potential as half of a pair of maracas by unscrewing the cap and rescuing
whatever's inside, but the damn thing is stuck. A judicious application of
paraffin based bicycle chain lube loosened it some, but it's still stuck;
guessing the whole thing is both swollen and dry. Anybody got any good
ideas for what to do? I did try clamping it in a soft-jawed vise and using
one of those strap-type jar openers, but no dice.
All the best,