Photographing encapsulated works

I am looking for information on photographing with color film, art works
"encapsulated in Mylar D".  I used polarizing filters over the two
lights I used for illumination and a polarizing filter over the camera's
lens, this to lessen reflections on the Mylar's surface, but in some
photographs there are faint red and/or green "waves" across the mylar.
These "waves" are caused by internal stress in the mylar altering the
wave length of the polarized light so that the stress appears as colored
"waves" (patterns) when viewed or photographed through a second
polarizing filter.  Books on photograph do not discuss whether you can
lessen or eliminate the effect, if fact any mention of it I have found
in books on photography show how you can used the effect for unusual
color photographs.  I wrote DuPont (the maker of Mylar) over a month ago
for any information they had on this but to date I have not received a
reply from them.  Any push in the right direction would be appreciated.

Frank A. Reynolds
Hunt Institute
Carnegie Mellon University
Pittsburgh, PA  15213

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                  Conservation DistList Instance 6:29
                Distributed: Thursday, November 26, 1992
                        Message Id: cdl-6-29-007
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