Tin pest

In Instance 12:35, Jessica Johnson asks about Tin Pest.

Tin Pest is the allotropic transformation of white (beta, metallic)
tin to grey (alpha, non-metallic) tin.  Tin which is undergoing this
transformation becomes covered with grey powdery warts.  Corroded
tin-alloy objects often have a similar appearance and are sometimes
said to have "Tin Pest", but this is quite a different phenomenon.

The transformation was investigated in a series of papers by Cohen,
starting about 100 years ago; he showed that although the transition
temperature (below which grey tin is the stable allotrope) is 13.2
degrees C, in practice the transformation does not occur at an
appreciable rate above about -15 degrees, reaching a maximum at
about -30 degrees and becoming very small below -50 degrees.  The
transformation is catalysed by the presence of grey tin nuclei, but
is strongly inhibited by the presence of 0.1% of lead, bismuth or
antimony.

There are recorded cases of tin refrigeration tubes failing because
of conversion to grey tin, but this is a very uncommon phenomenon.
There are reports in the older literature of Roman tin objects
excavated in Britain being converted to grey tin, but I suspect that
this is just corrosion, since: i) the burial temperature was not low
enough and ii) most tin objects contain appreciable quantities of
lead or other alloying elements.

There are two useful papers in the conservation literature, one by
Plenderleith and Organ, Studies in Conservation 1 1952-63-72 and the
other by Lihl, Studies in Conservation 7 1962-89-105.  Plenderleith
and Organ conclude:

   "It has not, in fact, been found possible to obtain a single
    illustration known with certainty to represent authentic tin
    pest of natural occurrence."

My feeling is that reports of tin pest in museum collections are
indeed just "legends".  In fact, I will go so far as to offer a
prize of 100 UK pounds (160 US dollars), out of my own pocket, for
the first case of allotropic transformation of a museum object from
white tin to grey tin, authenticated by X-ray diffraction.

Barry Knight
English Heritage
London

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                  Conservation DistList Instance 12:36
                 Distributed: Tuesday, October 13, 1998
                       Message Id: cdl-12-36-002
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