Blue wool scales

Does anyone have information on how to interpret the results of
testing light with blue wool scales?  We purchased some of these cards
from University Products, but received no instructions.  I placed a card
in two different locations and after two weeks I removed the cards and
find considerable fading at the low end of the scales.  I want to return
them to the same locations for two months and then read the cards.  Does
this process seem valid?  Does anyone have experience with blue wool
scales?

Shirley Rodnitzky
Archivist, Special Collections Division
University of Texas at Arlington

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                  Conservation DistList Instance 7:18
                Distributed: Wednesday, August 11, 1993
                        Message Id: cdl-7-18-007
                                  ***