My original message to Jens was as follows (I failed to send it to the
site, sorry):
"Could it be calclacite? Especially if the chalk has been contaminated
with sea salt (chloride) and then stored in acetic acid rich conditions
(e.g. woods of some kinds). I once worked on some coastal fossil reptile
material from the Trias of Devon which showed the long curly fibrous
crystals of calclacite growth, disintegrating the surface of the
specimen and pushing up through it. I was not sure where the acetate
came from in that instance - this is something that always puzzled me,
but it may have been from cheap tissue paper used as packing, or from
poor quality PVA emulsion.
On the issue of calclacites, I have always wondered whether the use of
NH4Cl for photography promotes the problem by introducing chloride
contamination.
However, I am not up to date with recent work on calclacites, so will be
interested to see what happens ..."
Mike Taylor
-----Original Message-----
From: The Geological Curator's Group mailing list
[mailto:[log in to unmask]] On Behalf Of Jens Lehmann
Sent: 23 January 2006 13:55
To: [log in to unmask]
Subject: Contaminated with sea salt
many thanks for your fast reply Mike. I have not seen long curly fibrous
crystals, none of the specimens has been coated for photography, but all
of
the problematic materials comes from coastal exposures. In our instance
the
acetic acid rich conditions might result from a poor quality PVA
emulsion as
you are suggesting (we do not store the specimens close to wood or using
tissue paper). I do not remember excactly, but probably we were using
"Tiefgrund" for sealing rock surfaces back in these days, a stuff that
is
sold in German building industry for sealing prior to the fettling.
Probably
a bad mistake.
Today, we are using PVB (polyvinylbutyrale; German brand "Mowital" from
Hoechst company and "Pioloform" from Wacker) for rock sealings, which is
probably better(?).
Jens Lehmann
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