Hi all,
 
I believe Gary later reported that what he originally took to be etching was
actually some kind of residue on the inside of the vial.  That would make
sense, since there are virtually no acids that can etch glass (one exception
being hydrofluoric acid, which is not likely to be released either from
natural textiles or from polymers).
John Hooker asked if humidity plays an important part.  Yes!  Given the
presence of potentially acid-releasing substances, humidity becomes just about
the whole story.  Virtually no chemicals undergo reactions while in a
dessicated state.  Pour dry drain cleaner crystals into a thoroughly dry sink
drain, and they will sit there forever.  Add water, and shazam!  A vigorous
reaction begins.  Similarly, acids will not be released from synthetic
materials unless water is present.  Equally important, inorganic acids are
typically ionic compounds, and until ionized (broken down into two smaller
particles, one + charged and one - charged) they are unreactive.  Ionization
happens only in water.  It is the + charged ion, specifically a + charged
hydrogen ion, that is the nemesis of stored shells.  The H+ ion is what makes
an acid an acid (except in the case of some complex organic acids - won't go
into that here).  All inorganic acids ionize into one or more H+ ions, plus
some kind of negative ion.  The negative ion may be of many types, but that is
irrelevant to the action of an acid on shells.  It is those nasty little H+
ions which rip oxygen atoms out of the carbonate structure of a shell, turning
the carbonate into water and carbon dioxide (CO2), a gas.  Ok, enough
chemistry 101 - the point is that acids only function as such in the presence
of water.  Therefore, given a specific type and quantity of acid-releasing
material, water becomes the limiting factor governing the rate of acid
release.  If everything remains absolutely dry, there can be no acid problem.
Unfortunately, air is never absolutely dry - there is always some water vapor.
So, the potential for acid effects over time are directly related to the
average relative humidity.  Under most normal household conditions, such a
process proceeds VERRRRRY slowly.  Your shells are not going to dissolve away
in a year, or in ten years, no matter what kind of foam inserts you use in
your plastic boxes.  But, I have seen glossy shells like Cypraea and Oliva in
old collections which were mounted in small boxes on some kind of "foam
rubber", which showed distinctly decreased gloss on the "down" side which had
been in contact with the foam.  I also purchased an old collection that had
been stored in a damp cellar for some 40 to 50 years.  Many of the smaller
shells were on foam in small plastic boxes.  In many cases the foam had
deteriorated to either a brick-hard, brittle consistency, or a gum-like
substance, firmly adherent to the shell.  In thinner shells like Epitonium,
sizeable holes were eaten through the undersides of the shells by acids of
decomposition, and even heavier forms showed considerable damage.  However, I
should point out not only that this was a rather extreme example in terms of
time and humidity, but also, that synthetic foams have been much improved in
the past 40 to 50 years, and the purer forms available today should not
release as much acid as the more primitive products did.  The acids, after
all, are residual impurities in these products, not integral parts of their
chemical structure.
I have never noticed any problem with cotton, even in old collections.  I use
it for mounting opercs, and probably always will.  I also use it in packing
shells for shipment, but that of course is no problem, due to the short
duration of contact.  However, it goes without saying that there are various
grades and brands of cotton.  I can't speak for all of them.
 
Paul Monfils