Hi Paul,
Formaldehyde is useful for deodorizing tissue that simply cannot be
removed from the shell.  However, that is a last resort, after all
attempts to remove the offensive material have failed.  The standard
method for removing such material is to lower the shell into a
Chlorox solution (about 1 part Chlorox to 3 or 4 parts water), in an
apex down position, so the solution can freely flow into the shell.
Leave it in the solution several hours - overnight is probably best.
The bleach will dissolve most of the organic matter in the shell
(sometimes all of it), and the remainder is usually removed by
flushing with a pressurized stream of water (under a tap or using a
hose).  If the soak and flush doesn't get it all, repeat the process.
Regards,
Paul M.