Salt on glass

Hugh Phibbs <h-phibbs [at] nga__gov> writes

>There is another situation, which may be similar to the salt
>deposition that was observed in packages at the British Museum.
>Sodium chloride has been found on the inside surface of the glass in
>frames used on old prints. It formed a "ghost" pattern on the
>surface of the glass that loosely followed the pattern of the print.
>It has been speculate that the sodium may have come, in this case,
>from the glass, but the source of the chlorine is unknown.

A similar phenomenon was recently studied on paintings in the
National Gallery of Victoria. A barely visible white deposit was
observed on the the inside glazing of some paintings which had been
glazed for over 20 years. XRD and SEM/EDS analysis of the deposit
identified the material as sodium chloride. We thought the source
may have been from aerosol deposition and postulated a mechanism
which we are now testing with a mock-up painting and climate
chamber.

Aerosol salt, both as dry particulate and liquid droplets of NaCl
solution, are common components of air in coastal regions such as
Melbourne and small particulates (less than 1 micron in diameter)
are not removed by the HVAC system.

The internal temperature of the Gallery display and storage areas
fluctuate daily with an amplitude in the order of two degrees, and
the RH is 50 +/- 5%. The painting housing-glazing systems are sealed
well, although not completely, allowing minimal air exchange between
the interior and the ambient gallery display and storage conditions.

These conditions set up a daily cycle; each night when the ambient
temperature decreases marginally, cooling of the glass relative to
the system interior would create micro-condensation on the interior
glass surface. This would happen in reverse each morning--as the
ambient temperature warms, the cooler interior of the system would
cause condensation on the outside of the glass, but as the exterior
is periodically cleaned, deposited particulates and aerosols are
removed. This would build up extremely slowly, but 20 years of
repeated micro-deposition could produce the observed effect.

Light and darker regions of paintings close to the glass would
reflect or absorb heat respectively, leading to the increased or
reduced deposition observed on many paintings and the effect of
"bloom" corresponding to the colours of the image.

Deborah Lau (MSc)
Analytical and Conservation Scientist
CSIRO MIT
Corrosion Science and Surface Design
PO Box 56, Graham Rd
Highett VIC 3190 Australia
+61 3 9252 6403
Fax: +61 3 9252 6253


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                  Conservation DistList Instance 17:11
                  Distributed: Tuesday, July 15, 2003
                       Message Id: cdl-17-11-002
                                  ***