Why would small, empty shells need a preservative?  They keep very well in
the dry state, if protected from moisture.  Glycerine is very viscous, and
would be very difficult to apply to small shells in its undiluted state.
(Or, are you thinking of immersing the shells in vials of glycerine and
storing them that way? If so, what is your intended purpose?)  Glycerine can
be diluted with water, and then applied to shells, but the water of course
will evaporate, leaving behind pure glycerine, which is not only viscous,
but also rather sticky, and gathers dust like a magnet. The dust might not
be a problem if the shells are in sealed vials, but I would expect that they
would adhere to one another, and to the inner surface of the vial.  Maybe
you could explain your purpose a bit more?  Glycerine is useful for keeping
flexible parts flexible - particularly gastropod periostracum and chiton
girdles.  But specimens treated in this way still have to be protected from
dust.

Paul M.