Light exposure limits for art on paper and photographs

Jonna Larsen <jonna.larsen [at] kfk__bergen-kommune__telemax__no writes:

>We are inclined to establish a policy built on number of
>lux-hours/year, but the difficult part is to choose the upper
>limits, especially since the policies used by other institutions
>vary widely. To estimate the number of lux-hours that will cause
>visible change seems hardly possible for most objects (at this point
>we do not have any colour or density measuring equipment).

Without a densitometer or the like, another way to keep track of the
accumulation of the light exposure is to use the ISO Blue Wool
Standard (BWS). The standard consists of eight pieces of wool cloth
coloured with more and more light stable dyes (#eight being the most
stable and #one being the least). For each step the light fastness
of the dyes is doubled. The wool cloth can be cut rather small and
discrete pieces, which must be placed under the exact same light
level as the photograph which is to be monitored (probably just next
to the motive). A disadvantage with this is that many curators don't
like the look of small blue wool cloth pieces on the middle of the
gallery wall. But I have been using them myself with success,
managing to place the monitor discretely enough and still usable.
Remember that it's not necessary to use all eight pieces at a time,
really just the two or tree steps around the light level you decide
as your maximums are enough. Tim Padfield is describing the Blue
Wool Standard on his Museum Physics page:
http://www.natmus.min.dk/cons/tp/datalog/datlog5.htm ,and Robert
Feller have described the use of the BWS also, I think in some
ICOM-CC Triennial Meeting Preprints...

Now, the thing with the BWS is that even the most light fugitive BWS
steps are not sensitive enough, when compared with really vulnerable
early photographic prints - like salted paper prints or albumen
prints. Just this summer I heard a presentation by Bertrand
Lavedrine of CNRS, Paris, about a pink step scale made especially
for the use with such vulnerable photographs. Instead of fading it
chances colour to blue when the light exposure limit is reached, if
I remember this correctly. B. Lavedrine will have much more precise
information about this, his address is: CNRS, 36 rue Geoffroy, St.
Hilaire, F-75005 Paris, France.

I think it is quite widespread decided that an upper light exposure
limit must be the amount of light that causes a just noticeable
density change (JND) in a significant area of the motive, which will
be about 0.01 density changes. As photographic materials are very
different, the light exposure limits will vary a lot also. As an
example of a very sensitive material Mike Ware gives an example in
his book "Mechanisms of image deterioration in early photographs"
(ISBN 0-901-805 78 5) of a Talbot photogenic drawing, which reached
this limit in just 3-4 hours at 50 lux illumination! In the other
end processes like UltraStable Permanent Color Prints probably will
withstand several hundreds of years illumination without changes.
When wishing to establish light exposure limits for your collection,
it must be in regard to the type of photographic materials the
collection holds. I know only of one published description of a
collection policy for light levels for art on paper: Karen Colby's
"A Suggested Exhibition Policy for Works of Art on Paper", from
Journal of the IIC-CG, Vol. 17, 1992 (also found on the web
<URL:http://www.webcom.com/~lightsrc/policy1.html>) . Maybe there
will be some similarities with the materials described here and the
ones in your collection...

Morten Ryhl-Svendsen
Photographic Conservator
The School of Conservation, Copenhagen

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                  Conservation DistList Instance 12:29
                Distributed: Tuesday, September 22, 1998
                       Message Id: cdl-12-29-001
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