I have a set of old ivory beads to restring. The clasp is ivory and
is seized shut. I presume (?) it is a threaded clasp - elongated tube
shape with hemispherical ends. Is there any trick to opening it
without breaking it? Thanks, Karen
Hello Karen! Try warming it for an hour or so in mineral oil. Put it
on your steamer or on your polishing hood with the light on, for
heat. It will likely free up, I hope. Tim
Karen,
Try soaking the clasp in baby oil (or straight pharmaceutical mineral
oil) for a few hours, and see if that loosens up whatever is gunking
up the threads.
If that doesn’t work, try setting the half of the clasp most likely to
contain the male end of the threads on a zip-lock bag filled with
crushed ice. Don’t leave it there for long. This might cause that
half of the clasp to contract enough to allow the threads to work. Be
careful. Ivory doesn’t respond well to sudden temperature changes.
Once you have the clasp apart, inspect the threads for damage…and
then clean them thoroughly with mineral oil before rejoining the
clasp.
I hope this works for you! Whatsoever thou doest, do gently. 
-Peter-
Hello Karen, Coat the clasp with a heavy layer of Vaseline, close it
up in a small plastic bag and put this in a pan of water. Bring the
water to a boil and then let it cool slowly. The clasp will now come
apart easily. Have a good day. Tom Arnold
Thank You! A good long soak in some mineral oil and the clasp opened
up. I needed to use a couple of wide broccoli elastic rubber bands to
get a good grip on it though - nothing like fine jewelry tools…
Thanks again, Karen