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.