Identification of plastic object

A client of mine has a number of clear, rigid plastic strips, about
6cm x 10cm and 3mm thick, with red chinagraphic markings, dating
from 1974.  They were used to plan the lighting for a theatrical
production.  They have been in dark storage since then but not in a
climate controlled environment.  The objects are exhibiting signs of
deterioration, namely yellowing and a distinctive smell.  The smell
reminds me of the "rancid butter" smell from deteriorating cellulose
butyrate.  We're putting them in cold storage to reduce the rate of
deterioration until we can determine a proper course of action to
conserve them.

Any ideas on what specific material they might be made from?

W. Mark Ritchie
Heritage Resource Management Associates
228 Broken Oak Crescent
Kitchener, Ontario
Canada N2N 1N7
519-745-5586
Fax: 519-745-3585


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                  Conservation DistList Instance 15:50
                 Distributed: Monday, January 14, 2002
                       Message Id: cdl-15-50-007
                                  ***