Ellen,

After you get the critter out of the shell (water pressure, tweezers, wire,
curses, etc.), if you want to preserve the periostracum, try coating it with
glycerin.  It seems to work much better than mineral oil.  As for the
encrustations on the helmet; soak that beauty for a few days in Clorox and
water (50% - 50%) and then try "poping" off the encrustations with your
dental pick.  For more serious growth --  after a good soaking it will often
pop right off if hit with a Dremil (almost any bit will do).  Just keep
touching it to the white crusties and they tend to fly right off.  If you
are careful and wear some decent safety/magnyfying glasses you never need to
touch the shell proper.  Then a quick brush with a wire brush if necessary
or better yet just clean it with soap and water and a tooth brush.  A bit of
oil and wha-la!!

Tom, the guy with no bite or sting......