Bleach, as in sodium hypochlorite (which produces hydrochloric acid in
small amounts, as well as free chlorine,  which clean, attack pigments,
and dissolve calcium carbonate), is much safer for use in cleaning
shells than most collectors realise.  The trick is to use it full
strength, not diluted, and for a brief period only: a half an hour will
eat most periostracum, or at least make it very easy to remove with a
tooth-brush and baking  soda (a spledid cleaner  and mild abrasive).
An hour or two will generally remove or signifigantly soften most
difficult attatchments such as Anomia ("jingle shells"), barnacles,
serpulids, etc. , without   damaging  the surface of the shell on a
macroscopic level.  Then, a bit of CAREFUL work with a sharp carpenters'
knife (the kind sold at the checkout of most hardware and
building-supply stores),always working within the grooves of the shell
(ie, never crossing a ridge with the cutter tool), will take care of the
rest, along  with good old baking soda and elbow grease.  The procedure
may have to be repeated for truly stubborn epi-remains.  NOTE:  rinse
the bleach offf very completely immediately afterwards, or fading may
result from the continued action of the residue.//  Any small scratchs
which result from use of a sharp tool, can be hidden b'y application of
mineral oil, which brings out the natural colors of the shell, in any
case.  Better penetration, hence longer-lasting protection, can be
achieved by mixing  the oil with 75% lighter fluid, but do this in a
shed or outside.  The volitile lighter fluid will dissapear aafter a
while, leaving a shell oiled for  far longer  than just by applying oil
or glycerin alone.
                                                                                        -Ross M.