All you need to do remove the calcium is place the shells in
a pyrex dish of warm apple cider vinegar cut with a bit of water.
Let them soak for about 30 minutes and rinse. Calcium that didn't
disappear can be knocked off. I would suggest using Muriatic acid
but that can be pretty strong and hard to gauge the length of time
Don, I’d think one would have to be very careful with choosing which
acids. the shells, after all, are also calcium carbonate, and just
as subject to attack by acids as are the calcium deposits themselves,
with the exception perhaps of the porosity of those unwanted
deposits, which might allow them to be attacked quicker. One would
have to be very careful not to leave shells in anything that could
attack the deposits, any longer than needed, to avoid having too
much of the shell itself be dissolve also, I would think. Have you
found shell to be that much more resistant to such acids as to make
this worry unfounded?