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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