I offer the following opinions from The Mollusks: A Guide to Their Study,
Collection, and Preservation, edited by Sturm, Pearce, and Valdes. 2006

From  Chapter 2 Field and Laboratory Methods in Malacology:

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
ofopen 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.

And from Chapter 5 Archival and Curatorial Methods:

5.1 Basic Principles:
…The first principle of good curation is to do no harm. Whatever else, do
not treat or handle a specimen in a way that makes it worth less after
curation than it was worth before. One should strive to apply treatments
that can be reversed easily. For example, it is better to use an adhesive
that can be easily removed as opposed to one that will defy the use of a
hammer and chisel to remove it.

The second principle dictates that techniques should be used that cause as
little permanent change to a specimen as possible. It should be assumed
that whatever we do to a specimen will cause some change, and sometimes
the change may not be completely reversible. Thus, we should minimize what
the permanent change to the specimen will be. An example of this is the
way some people preserve the periostracum of naiads (Unionidae) with a
paraffin/xylene solution. The shell is dipped in the solution and the
xylene then evaporates. Paraffin is left behind and this helps to preserve
the periostracum from drying out and flaking off. If one wants to remove
the paraffin, successively soaking the shell in xylene will leach the
paraffin out. However, in addition to the paraffin, other xylene soluble
substances will also be removed. This will cause a permanent change to the
shell. If morphometric studies of the shell are the objective of a study,
then the paraffin/xylene treatment will be of no consequence. However, if
we wish to undertake some biochemical studies of the shell, the effects of
the treatment will be of importance. One way out of this dilemma would be
to treat some but not all of the shells, and record which ones were
treated.

The third of the basic principles is to record what techniques you apply
to a specimen, and to record them on the specimen’s label. In this way,
others will be able to tell what was done to a given specimen, whether it
can be undone, and what effect it will have on future analyses that might
be applied to the specimen.

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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