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Subject: Perspex


From: Barbara Appelbaum <aandh>
Date: Friday, March 19, 1999
On the interesting question of the Perspex sheets (I assume that the
person who posted it will relay messages), there are many
possibilities about what is happening.  However, I think the
fundamental problem is that nothing hangs straight--ever.  There
must be a law of physics here, but I can't think of which one. Think
of hanging just one sheet from the top, and you will realize that it
will never hang flat.  Air-tightness of a degree that would keep two
sheets together may be more than the Perspex itself will support,
since plastic sheets are not totally air-tight, and there is always
some pinprick somewhere that will let in air.  In this case, any
heat from lighting will cause some expansion of the plastic which
will cause waviness.

In setups that require completely airtight spaces, a lot more has to
be done, including replacement of  the air by other gases, as
otherwise, the forces are too strong, and they create leaks.  In
general, airtightness is measured by the rate of air exchange.
Reducing this past a certain point (I am sure someone has these
numbers at hand) requires serious engineering.

I have seen similar types of installations, that is, free-hanging
sheets, where some kind of tether was attached to the bottom
corners, pulled taut on the diagonal, and fastened to the floor.
Something like that may help in this case.  However, as with
conservation treatments, thinking through something is no
replacement for trying it, and this kind of thing should have been
tried in mock-up first, or at least run past someone with a better
grasp of the technical issues.

B. Appelbaum

                  Conservation DistList Instance 12:75
                  Distributed: Tuesday, March 23, 1999
                       Message Id: cdl-12-75-002
Received on Friday, 19 March, 1999

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