Salt on glass

This is in response to Bill Wiebold's and other comments and
questions about ghosting images and salt on glass.  This was an area
of interest of mine when Walter McCrone brought up the issue of the
image on the Shroud of Turin in his criticism of the idea that it
was a natural occurrence.

I was investigating transfer images relating the microenvironments
inside frames.  This was a subject that Margaret Holben Ellis had
also been working on and had discussed in her 1987 book on the care
of prints and drawings.  We often see these as negative images of
the art transferred to the glass.  I was concerned that this might
result in degradation of the image media, and sought to understand
the components of the ghost image.  Ms. Ellis published a very
striking image transferred to glass in her book and Marilyn Kemp
Weidner published another remarkable example in her article on the
results of poor framing which appeared in Studies in Conservation,
v. 12, n. 1, 1987.  I republished both in my article with T.B. Kahle
which appeared in Restauro in 1989, v. 4.

Luckily I found that scientists at the Conservation Analytical
Laboratory of the Smithsonian had studied this phenomenon and had a
number of reports available from the Lab's library (Padfield,
Erhardt and Hopwood, ms).  Their analysis found that the substance
in cases they studied was mainly "...sodium chloride and an organic
material with surfactant properties".  The salt was mainly derived
from the salted silk of the framed object.  I thought that in most
cases, like those in the images produced by Ellis and Weidner, the
transfer's agency was by the interaction of peroxides and
hydroperoxides derived from the degradation of matting materials.
Vincent Daniels remarked in personal communication that such
transfers are likely to arise from a drying oil which may explain
why we see such images often on the glass of framed prints.

Niccolo Caldararo
Director and Chief Conservator
Conservation Art Service


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                  Conservation DistList Instance 17:13
                   Distributed: Monday, July 21, 2003
                       Message Id: cdl-17-13-001
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