Light masking effects of polyester film

We have noted several instances over the past couple of years where
paper artefacts mounted with Mylar corners and strips have
differential fading of the ink and/or discolouration of the paper.
The area under the Mylar polyester film appears to be protected from
light damage.

The illumination in the showcases was from incandescent (dichroic)
light sources with all ultraviolet light removed and lux levels set
at 50.

We are looking into the circumstances surrounding these cases to
ascertain what is happening. One thought we've had is that the
reflective quality of Mylar is reducing the illumination levels
sufficiently to act in a protective way. We are interested to know
whether other people have discovered similar effects? Or whether
anyone can suggest the reason why this might be happening?


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                  Conservation DistList Instance 15:81
                   Distributed: Tuesday, June 4, 2002
                       Message Id: cdl-15-81-014
                                  ***