Dave,

Your cautions are good to keep in mind, but my experience (as a collections
person, not a conservator) has been that poly bags do not really create a
closed system.  It is my understanding that there at about 8 complete air
exchanges over a 24 hour period for most sealed poly bags.   I did once
experience significant condensation -- but that was quite an extreme
circumstance: a limestone object had been placed in a poly bag in
sub-basement storage near a leaking steam pipe.  When we brought the piece
into a more stable environment, it actually rained in the bag.

Of greater concern in using commercial products is that they may have a
coating that makes the bag more useful for industry, but leaves a residue on
objects.   I have had good experience with the poly bags and tubing provided
by Associated Bag in Wisconsin (our conservator tested them).

BTW there are many small (up to $500) grants for conservation materials that
readily cover the cost of "good" bags and boxes and, bought in bulk, they
are much cheaper than the supermarket.

janice

Janice Klein
Director, Mitchell Museum of the American Indian
[log in to unmask]
www.mitchellmuseum.org

-----Original Message-----
From: Museum discussion list [mailto:[log in to unmask]]On
Behalf Of David Harvey
Sent: Tuesday, October 31, 2006 5:03 PM
To: [log in to unmask]
Subject: Re: Archivally safe bags


One caveat about using closed plastic bags for storage. You can get
several undesirable effects even from archivally correct plastics. The
first being that the air inside the bag can be either warmer of colder
than the external air and if there is a significant enough difference
then condensation can occur with the bag - this can be prevented in
large measure by either opening the bag to let it acclimate before an
object is moved, or, you can use a material like soft tyvek that will
allow vapor and air exchange but will block liquid water from the
object wrapped inside of it. You also need to be extra careful of the
maerials within a bog that you will be closing as any outgoassing or
residual acids will have a much more significant effect in a closed
system  - a microclimate - then if the object were left in a box, for
example.

Cheers!
Dave

David Harvey
Conservator
Los Angeles, California

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