First leave the shell in alcohol
30% for 2 to 10 days (depending on the shell), or you
can try freeze-thaw-freeze-thaw for a few days too. Then
pull the animal with a hook or a pair of tweezers. Remove
the operculum and put aside (pay attention to its original
position). Some people like to boil, we prefer alcohol
or freezing system instead.
Flush inside with a strong jet of
water
Leave from a few minutes to a few
hours in 10-30% bleach. Then brush to remove the dirt.
Leave to dry in a well ventilated place away from sunlight.
(I used to have a bulb light under a metal plate to heat
a little bit the shell so it would dry faster - now I
use a dehumidificator)
Using a Dremel tool or similar with
a dental burr, try to remove carefully the incrustation.
You might have to put it back on bleach if needed after
cleaning outside.
Put cotton inside - enough to hold
the operculum, glue the opercum on its position. With
a soft brush apply some baby oil or silicon oil.
These instructions can be used on
most kind of shells. Of course it will vary a little
bit from species to species.
For some more tips, visit shells
cleaning
