the suggestion of WD-40 could be a bit dangerous as the prime ingredient in
this product is diesel fuel, there might be additives that will eventually
break-down the shell.  Lighter fluid (naphtha) combined with mineral oil
(not baby oil as I smell in some collections, mineral oil can be found at
Walgreen's for about $2.50 for 12oz, cut this 3-1 w/ naphtha and apply
liberally, the shell will absorb as much as it can then after a few days
wipe off excess. I re-apply this mixture twice yearly to the entire
collection.
If you collect unionidae (fresh water bivalves) use Vaseline cut 1-1 with
naphtha , this will preserve the perio. and prevent cracking. shell not
treated this way are brittle and split.
Mark James Bethke