[Nhcoll-l] To oil or not to oil?

[Callomon,Paul]

(This question is mainly for the managers of mollusk collections, but I'd like to hear from others who might encounter a similar quandary)

The shells of certain mollusks (almost all true mussels [Mytilidae]; pearl oysters [Pterioidea]; pen shells [Pinnidae] and many freshwater mussels) are composed mostly of nacre and have a thick, bonded periostracum. Removed from the water and placed in a dry environment, they begin to crack and split in a fairly short time. Their nacre can also decay, becoming powdery and weakening with time. The periostracum dries and hardens, and as its modulus of expansion differs from that of the shell, it begins to flake off or - in  the case of thinner shells - to crack the shell. Maintaining humidity at 45-60% year-round should slow this essentially evaporative process, but is unlikely to stop it.
A traditional technique to guard against all this is to coat the shells inside and out with a film of mineral oil (Petrolatum). This slows the escape of moisture from the shell matrix and keeps the periostracum somewhat flexible, and it certainly works; here at the Academy we have oiled freshwater mussel specimens dating back to the 1850s that remain entire. However, oiling is an additive technique and not fully reversible; detergents can remove the surface film but not purge the shell of oil entirely.
The debate, then, is as follows: On the one hand, adding a hydrocarbon to the shell might inhibit or preclude future chemical analysis techniques that we have not yet developed. On the other, a great deal of work in the mollusca (and many other groups) is based on morphology, and a tray of dust and fragments is of little use in that regard.

So: in your collection - to oil or not to oil?

Paul Callomon
Collection Manager, Malacology, Invertebrate Paleontology and General Invertebrates
________________________________
Academy of Natural Sciences of Drexel University, Philadelphia
1900 Benjamin Franklin Parkway, Philadelphia PA 19103-1195, USA
callomon at ansp.org<mailto:callomon at ansp.org> Tel 215-405-5096 - Fax 215-299-1170

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