Lime and coral cannot be chemically dissolved from a shell, for the
simple reason that they are made largely of the same material as the
shell - calcium carbonate and other calcium salts.  Therefore anything
that will dissolve lime or coral will also dissolve the shell.  However,
some types of encrustations do contain a small percentage of organic
matter, and soaking in bleach will dissolve that material, thereby
weaking the deposits somewhat, so they can be picked off more easily.
But the final step is always physical picking or scraping.  For a large
shell with a lot of encrustation, the initial scraping can be done
rapidly by using a rotary wire brush in an electric drill.  But the
final, delicate work has to be done by hand.  It's tedious, but I
haven't found an alternative method.  An ultrasonic cleaner removes all
kinds of grime and loose deposits, but it won't remove securely attached
calcareous deposits, though I believe it may loosen them somewhat, at
least in some cases, making them easier to, pick off.

Paul M.