Deterioration within enclosures

The discussion of damage caused by the encapsulation of acidic materials
reminds me of a puzzle that I think about from time to time.  It relates
to the damage caused by the old wooden slat backings used in picture
frames.  Why is this damage so extreme where there are gaps in the
backing, that is, where two slats join or where there is a knot hole?  I
suppose that it could be from greater exposure to atmospheric pollution,
but then wouldn't the atmosphere within the frame be more polluted (acid
gases, peroxides, etc., from the bare wood) than that without?  Could
it be that the oxygen becomes somewhat depleted further from the
opening?  (This would have implications regarding encapsulation of
acidic materials.) Could the paper be more mechanically  damaged there
from expansion and contraction because it is less buffered from changes
in exterior temperature and relative humidity?  Might this mechanical
stress result in shortening of the cellulose chain?  Perhaps it is
because there is a greater amount of exposed wood surface there since
narrow edges of the slats are also in the vicinity.  I personally am
skeptical that the latter factor alone could result in such an increase
in the amount of damage.  Perhaps it is because the noxious gases
within are forced through the paper near this opening when there are
changes in atmospheric pressure.  On the other hand, one would think
that there would be less buildup of these damaging gases within the
enclosure at this point because they can more easily escape.

I would appreciate anyone's thoughts on the matter.

Dan Clement
Paper Conservator
Ithaca, NY

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                  Conservation DistList Instance 6:51
                  Distributed: Tuesday, March 23, 1993
                        Message Id: cdl-6-51-009
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