Charlie,
  Regarding your post, I know of no evidence that oil will prevent fading.
However, if there are some UV radiation stabilizers in the oil it might
have this ability.  But then what the effect of these stabilizers would be
on the shell will in most cases be totally unknown.  Shells do have a
quantity of water in them.  If shells are dried out they may crack and
shatter.  This happened to some thin shells unionids that I had collected.
I maintain my collection in a room with a dehumidifyer and aim to keep the
relative humidity around 50-55% (maybe 60%).  Unfortunately the adjustment
on the dehumidifyer was changed (accidentally) and the RH dropped to 30%
and the result was that some thin shells cracked.
  In an informal survey that I once conducted, most museums do not oil
shells.  They attempt to preserve them solely by controlling the
environmental conditions: temperature, humidity, light exposure, pests,
shock, and exposure to acids.
  Hope this helps some.

Charlie Sturm

--On Tuesday, April 10, 2001, 4:37 AM -0400 Charles Geerts
<[log in to unmask]> wrote:r

> Thank you very much for your circumstanciated reply.As far as the practice
> of oiling shells is concerned, I can accept it if it preserves the
> periostracum that might have a tendency to peel off after some time in
> rather dry conditions.I can also understand conchologists who like to use
> make up ( oil)to enhance the colors. But is there scientific evidence that
> the oil otherwise " preserves" as you state it. If so, I would like to
> hear about that evidence from you or anyone who might have it.I suppose
> any shell has a quantity of H2O in it. Does oil prevent the water to
> evaporate? Should we prevent water to evaporate? Does oil prevent colours
> to fade away?