May I also have a say on the bleach question. Most Australian
professionals and many collectors use a fairly strong mixture of pool
chlor which is usually cheaper than bleach. After taking off the
heaviest of the growth, immerse the shell and check regularly, use a
wire brush, knife or whatever to remove more growth as it softens.  Make
sure to neutralise your specimen after the treatment by immersing in
fresh water for an hour or so, dry and then treat with parafin oil leave
for a day or so and wipe of the excess oil.
 
Kev Lamprell