Dear Nancy;
        Although i find that percs perservere pretty patiently on the whole, if
mounted on cotton, and not accidentally glued to the shell wall, it may
sometimes be useful to soak them in glyceryn for a while, or a mixture
of heavy mineral oil and a penetrant such as butane or some other light
hydrocarbon which can help saturate the operculum (or periostracum, as
the case may be).  This should be done in an airtight container, of
course, so you don't get stray light hydrocarbons drifting around the house.

Yours from the Edge of the Universe,
Ross.