You don’t say what kind of mandrel, but I assume it is the type Rio
sells for firing PMC on. The problem is, of course, that the ring
shrinks until it is as tight as it can get. I would first try
lubricants–oil, soap, vaseline, whatever comes to hand (which will
probably require gentle heat removal afterwards, as PMC remains
porous). If that doesnt work, I would try to think of ways to enlarge
the ring a little. Is there a plain part you can hammer, streching the
ring a tiny bit so it can come loose? Then, maybe, lube again. As a
last resort, I would saw the ring open and solder it back, though
soldering PMC is difficult, and different from soldering other forms
of silver, because of its porosity. You use lots of solder, burnish
the surfaces first–a flex shaft hammer handpiece is great for
this–and then heat only until the solder melts. If you heat it to the
flow point, it will soak in and disappear.
If all else fails, if you still have the textures, you should be able
to duplicate the piece. It is even possible to use sculpey or fimo to
take an impression, perhaps cut it and flatten gently before curing,
and use it as a press-mould. I often textures in this way. If you are
not used to PMC, it can be hard to get used to the more clay-oriented
mindset that it requires.
Good luck, and let us know!