Hi Jim, Either the mineral oil or the silicone mentioned by others will make the shells look better after the fact. Regarding the cause of the problem, there are a couple of possibilities. First, if you boil a good number of specimens together in a relatively small amount of water, and don't rinse them thoroughly, they may be coated with a residue of organic matter from the "soup". If that's the case, a short exposure to bleach (about 5 minutes) should remove it. The problem could also be caused by water quality. Tap water in most localities is fairly alkaline, and will have no deliterious effects on shells, but if you live in an area where the tap water is nearly neutral in pH, and especially if it is on the down side (acid side) of neutral, it could cause problems, especially if the shells are not cooked promptly, and acids of decomposition are added to the mix. Also, such effects are greatly increased as the temperature rises. An effect that might take a few days to develop at room temperature could occur in minutes in a boiling solution. You would only notice such effects on a smooth shell like a cowrie, olive, or Polinices duplicatus. If you live in an area where the water is very hard, it is possible that the dissolved mineral salts in the water might react with the shell, either precipitating on its surface, or replacing some of the calcium. Again, this sort of reaction would be greatly accelerated by heating. Finally, what sort of container are you boiling the shells in? The ideal vessel is one made of glass, either pyrex or kimax, or one of the stove-safe glass materials like Corningware. Stainless steel is probably ok too, but I would stay away from aluminum or iron containers. Regards, Paul M.