Fading blueprints

Belinda Fireman <4blf1 [at] qlink__queensu__ca> writes

>I have recently observed fading of an area of a blueprint. After
>being kept in the dark for a few days, the colour was completely
>restored.

I am currently researching the history, chemistry and conservation
of cyanotypes (or blueprints) on a commission from the National
Museum of Photography, Film & Television, Bradford, England, with a
view to publishing in 1997, through the Science Museum, London, a
monograph on this topic.

Regrettably, my work has not yet progressed to the stage where I can
give quantitative answers to the questions recently posed by Belinda
Fireman.

For the present, it seems that all the evidence concerning
photosensitivity is anecdotal: there is no doubt that some
cyanotypes fade in the light; it is also true that this fading may
reverse on dark storage, but I have never seen any study of how
complete is the recovery.  It is equally true that some observers
report no fading at all in some cyanotypes that have received
extended exposures to light.

There is no inherent contradiction here: the image substance,
Prussian Blue, is a complex of widely variable composition,
dependent on its method of preparation. Its sensitivity to pH,
moisture, light and residual impurities in the print will also vary
enormously.

It is well-known that Sir John Herschel invented the process; it is
perhaps less well-known that he discovered at least fifteen
chemically distinct ways of making images in Prussian Blue. Even his
"standard" method--so widely and inaccurately re-published--has as
many different recipes proposed for it as there are writers on the
subject, and the recipes employed for some commercial blueprint
papers were almost Byzantine in their complexity.

Prussian Blue is attracting much attention in current chemical
research for quite different reasons, and there is a large and
growing technical literature on it (for its use as an electrochromic
display device; for its ability to act as an antidote to thallium
poisoning; and for its ability to absorb and safely dispose of
Caesium 137 contamination resulting from the Chernobyl--and
other--nuclear accidents). Conservators, too, may soon hope to
benefit from our currently improving chemical understanding of this
fascinating substance.

For the interim, my advice would be: don't take *any* chances with
precious cyanotypes. Restrict their exposure to short durations of
UV-less, 50 lux tungsten illumination. The avoidance of contact with
alkali is, of course, already deeply engraved in the conservation
canon, but on no account should they be washed in water, either,
because some forms of the Prussian Blue colloid may be readily
peptized.

I hope to be able to provide a quantitative commentary on these
issues in a year's time. Meanwhile, I should be very happy to hear
privately from anyone with an experience or interest in
cyanotypes/blueprints.

Mike Ware

Dr. Michael J. Ware
20 Bath Road
Buxton, Derbyshire, SK17 6HH, UK
+44 1298 78604

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                  Conservation DistList Instance 10:12
                   Distributed: Friday, July 19, 1996
                       Message Id: cdl-10-12-002
                                  ***