Polyethylene deterioration

On behalf of Ginette Clement, Kateri Morin <kateri_morin [at] pch__gc__ca>
writes

>    ... Information gathered from the conservation
>    reports led us believe that in many instances the bags were made
>    in 1983 and all were done in the same lab. ...
>    ...
>    ... This would
>     suggest that this degradation is associated with the quality of
>     polyethylene sheet used.  We would appreciate any of your
>     storage stories or observations that might help us.

At our museum we routinely bag artifacts  to protect them from dust,
leaks, and insect infestation.  Some years ago, we started having
reports of deteriorating bags.  They were yellowed, became extremely
weak and easily torn; behaving in many ways like 'shattered' silk.
We use pre-made bags from a commercial supplier and the situation at
first appeared random; some bags of the same age or older were fine,
others  were not.  Because the bagging had been carried out in
discreet projects, we were able to determine that all the
problematic bags came from a single shipment several years before.
At the time we were trying to figure this out, a museum staffer
happened to be on a plane next to a polyethylene sheet manufacturer.
When she told him about our mystery 'bad bags', he said that that
problem could  result from one batch getting overheated in the
manufacturing process.  I have no idea how accurate this is, but
once we replaced all the bags from that one batch, we have had no
further problems.

As I've said, we have over 20 years experience in using polyethylene
bags on artifacts and this is the only instance of problems we've
had.  We do take care to order bags from a reliable manufacturer
although, as Bill Minter says and as our experience has shown,
batches can vary. Incidentally, we're often asked if we've seen any
problems resulting from microclimates developing within bags or
condensation or the like.  The answer is a resounding no, in fact
the bags have been shown to act as buffers in non-climate controlled
storage and have prevented many possible problems when disasters
happen.

Lynn A. Grant
Conservator
Loans and Traveling Exhibits
University of Pennsylvania Museum of Archaeology and Anthropology
215-898-2787


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                  Conservation DistList Instance 16:74
                   Distributed: Tuesday, May 27, 2003
                       Message Id: cdl-16-74-007
                                  ***