UV scanning of parchment

On behalf of a group of students, Babette Gehnrich
<bgehnrich [at] mwa__org> writes

>    We were wondering if anyone has any information or insight as to
>    why it is possible to see writing underneath the mold by shining
>    a UV source over it. Without using UV light, you can only
>    observe the mold--you cannot look through the mold and see the
>    text. ...

There are a number of reasons why you would be able to distinguish
aged iron gall ink (assuming this is the type of writing ink you are
trying to image) on parchment using UV illumination rather than
visible light. First of all, the parchment itself fluoresces very
highly at UV wavelengths as most proteinaceous materials do.
Contrast this with the non-fluorescence of the degraded ink,
specifically the iron oxide component which gives aged iron gall ink
its reddish cast. In this case, the UV light is simply absorbed by
the ink material so that under UV illumination, the ink will look
black. The enhanced contrast between the fluorescence of the
parchment and the absorption of the ink makes the text easier to
read. Mold, or the degradation products left by the mold attack,
generally tend not to fluoresce although the amount of UV absorption
depends on the type of mold and the severity of the attack. The
ability of UV illumination to distinguish writing "underneath" the
mold attack is really a matter of the state of the degraded ink. In
some cases, there is still enough ink left after the attack. In that
case, the difference in the absorption characteristics of the
degraded ink vs. the areas attacked by mold will be enhanced in UV
illumination making the ink visible. You may be able to enhance the
contrast further by using band pass filters to illuminate in
specific narrow UV bands. However, it must be noted that in some
cases the mold attack is severe enough so that no ink remains. In
that case, no amount of UV illumination will find the lost writing.

This type of image enhancement technique has been widely used. The
most well known recent case, widely reported in the popular media,
is that of the Archimedes Palimpsest. (see
<URL:http://www.thewalters.org/archimedes/frame.html>)

I hope that this will give some explanation. The answer lies in
understanding the optical properties of organic vs. inorganic
materials.

Dr. Johanna R. Bernstein
Corrosion and Protection Centre
University of Manchester Institute of Science and Technology
Sackville Street
Manchester M60 1QD
UK
+44 161 200 5952
Fax: +44 161 200 4865

or

Department of Materials Science and Engineering
The Johns Hopkins University
Baltimore, Maryland 21218
USA


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                  Conservation DistList Instance 16:25
                Distributed: Wednesday, October 2, 2002
                       Message Id: cdl-16-25-002
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