I guess I will speak for the minority. I am against using any such
preservatives. I will quote from Sturm, Mayhew, and Bales. 2006. Chapter
2: Field and Laboratory Methods in Malacology. In Sturm, Pearce, and
Valdes (eds.) The Mollusks" A Guide to Their Study, Collection, and
Preservation. Universal Publishers, Boca Raton, FL.

2.7.13 Coating shells with preservatives.

Shells may be found washed up on beaches and they may appear dull and
faded. The color may be greatly intensified by a mild application of some
greasy
substance such as mineral oil, paraffin, silicon oil, or Teflon®.
Application of these substances will often brighten up a dull and faded
specimen. To preserve the periostracum of such shells as Sinum, Hydatina,
and unionoids, which have a tendency to peel when very dry, an occasional
application of one of these substances will prove sufficient. It is very
important to remember that the collector is not creating a shell but is
simply, in a manner “lifting its face.”

Animal and vegetable-based oils will become rancid with time and should be
avoided. Silicon oil is a relatively inert substance. Its stability, lack
of odor, and safety profile have made it a substance that people are
comfortable using. The silicon oil is cut with naphtha or mineral spirits
1 part to 2-4 parts. Shells are dipped into this mixture and then placed
on a flat surface. The solvent evaporates leaving behind a thin coating of
silicon. If diluted appropriately the amount of silicon oil remaining
should not leave the shell feeling greasy or tacky. Too much oil makes the
shell slippery and a “dust magnet”. Mineral oils can be used in a similar
fashion.

A number of collectors have used a solvent called WD-40® to brighten the
surface of a shell or to preserve the periostracum. WD-40 is a petroleum
distillate. Some collectors dilute it 1 part to 3-4 parts naphtha or
mineral spirits and then dip their shells into this solution. Others use
the WD-40 full strength. We do not recommend WD-40 for one preservational
aspect - it is a proprietary and secret formula. Such mixtures are of
unknown composition and can be changed at any time. Thus, you are never
certain what you are putting on your shells.

If you are going to apply a coating to your shells, you do not want to
apply it full strength. This full strength coating will often make the
shell tacky and slippery. Many collectors dilute the oily substance with
naphtha or mineral spirits. Diluting the oil one part to three or four
parts of solvent is often sufficient. When the solvent evaporates, it
leaves behind a thin coating of the oil. Also, the thinned oil is able to
penetrate into the shell deeper than the full strength oil would. When
using solvent, make
sure there is adequate ventilation and no sources of open flames. While
naphtha and mineral spirits are relatively low toxicity, they can act as
respiratory irritants. They are also flammable, thus no sources of flames
or sparking should be present.

Clench (1931) described a process of mixing paraffin with xylene. Place
120 g (4 ounces) of paraffin in 300 ml (10 ounces) of xylene. Warm this in
a
water bath to dissolve the paraffin in the xylene. Shells are dipped into
the solution and then placed on a flat surface to dry. The xylene will
evaporate leaving behind a thin coating of paraffin. Remember that xylene
is flammable so avoid open flames. Xylene is also a respiratory irritant,
so this method should only be performed in a fume hood or a well
ventilated
area such as outdoors.

You should also keep in mind that many collectors and most museum workers
apply no surface coating to shells. Generally, if you keep the temperature
and humidity level of your storage area within a range of 16-21ºC
(60-70ºF) and 50-55% relative humidity, there should be no concern with
the periostracum peeling (See Chapter 5, Archival and Curatorial Methods).
Also, there is nothing wrong with a slightly faded, pristine shell as
opposed to
one that is shiny but chemically adulterated.

Clench, W. J. 1931. A preventive for the scaling of the periostracum.
Nautilus 45: 30-31.

Regards,
Charlie
.................................................
Charlie Sturm
Research Associate - Section of Mollusks
Carnegie Museum of Natural History
Pittsburgh, PA, USA

Assistant Professor - Family Medicine
Fellow-American Academy of Family Practice
Fellow-Academy of Wilderness Medicine

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