The keys to using corrosive or bleaching agents as clensing aids are: 1)
a short exposure using a strength concentrated enough to clean well and
loosen up concretions, but not stong enough to cause signifigant harm in
a short period- for chlorine bleaches and H2O2, this means full strength
for up to a few hours, although for micros, i would use caution for spp
with very fine sculptural details.
and 2) Thorough rinsing with fresh water afterward, so your shells do
not slowly disintegrate afterwards, or retain a white residue that
obscures colors.
        An even better micro (or for dirt, macro also)-shell cleaning method is
sonic units, such as the ones used for cleaning jewellry.  Paul Monfils
is an expert here, with lots of experience in this area.  The cost of
these units has decreased signifigantly in the past few years.
 
        CAPE VERDE:  Anyone going there with even the remotest interest in
cones should check out every possible cone habitat they can think of,
and get in some serious night diving if possible.  These islands are
home to a wonderful abundance of indiginous (never could spell that
word!!) spp of cones, and local forms of other spp.  Of course the local
fishermen would also be a good bet: see if you can get out on a trawler
for aday or so!  Good luck, you fortunate people!!
                                                                                        -Ross M.