Deterioration within enclosures

Just a couple of thoughts to add to this thread.  It may help to think
of a picture frame (or exhibit case) as a pump, cycling air (and
air-borne particulates) in and out at low differential pressure
boundaries (the crack or split in wood backings) as temperature/%RH
change over time.

The temperature inside a frame may be considerably higher than the
temperature of the room it is in (light [including infrared] bounces
around behind the glass before if refracts out; longer under
plexiglas),and it is this heat differential which creates a pressure
differential, turning the frame into a pump, expelling air during the
day (or display period) and sucking it back in at night, taking the path
of least resistance.  In this instance, the split shingle.

Jack C. Thompson
Thompson Conservation Laboratory

                                  ***
                  Conservation DistList Instance 6:54
                 Distributed: Thursday, April 15, 1993
                        Message Id: cdl-6-54-002
                                  ***