Salt on glass

I have also seen crystals and droplets on the inside of portrait
miniature cover glasses. I assume that this is due to deterioration
of the glass and that the crystals are alkali salt deposits. This is
a well known phenomenon due to the original composition of the glass
and environmental conditions.. Due to their hygroscopic nature these
deposits can absorb moisture under humid conditions forming droplets
but strictly speaking this is not 'condensation'. On sealed
miniatures the outside of the glass is often cleaned but  the inside
of the glass--because it is difficult to get at--is often left
undisturbed. As Bill Wiebold points out it is very common on French
miniatures, 18th and 19th centuries but these crystals/droplets are
also common on English miniatures. If the condition is extreme it
may be appropriate to consider replacing the cover glass as it would
be very difficult to control the relative humidity at which glass
deterioration occurs or re-occurs.

This condition should not be confused with crystal formation on the
ivory miniature itself, which is a separate phenomenon and due to
the formation of magnesium phosphate salt deposits migrating from
the ivory under conditions of high humidity. Examples of these
crystals have recently been analysed using Raman microscopy.

Alan Derbyshire
Senior Conservator
Victoria and Albert Museum
London


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                  Conservation DistList Instance 17:13
                   Distributed: Monday, July 21, 2003
                       Message Id: cdl-17-13-002
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