Yellowing of polyethylene bags

I have a curious potential problem on my hands and hope that one of
you might be able to provide some input on the situation.

Many of our artifacts are curated in a climate-controlled room in
very tight cabinets. Within the cab's many of the individual
artifacts are enclosed in zip-top 4 mil thick polyethylene bags.
Over the years we have noticed that some of the bags and the paper
tags inside them have turned a yellow/green, especially, but not
limited to, where the artifacts are touching them. This process can
take as little as 4 months.

The artifacts are composed of siltstone, quartzite, chert without
matrix, basalt, shell, and Chupadero sherds. One of the most curious
things about this is that in a collection from the same site, made
at the same time, presumably processed in the same manner, and
re-housed 10 years ago using the same types of materials, four
artifacts of a given material will be fine and the fifth will have
turned the bag and tag color. The materials are from Maverick, Val
Verde, and Brewster counties in Texas as well as from up around
Carlsbad Caverns in NM.

My concern is not only for the artifacts themselves, but for the
organics that are curated in the same cabinets. My plan is to get
readings from inside the offending bags with a mass spectrometer/gas
chromatograph  to see what shows up, but I would certainly
appreciate any other information or direction any of you could give
me.

Note: I posted this to another archeology list, and several people
wrote back to say that they had noticed this in collections where
the artifacts has been treated with HCl and had not been properly
rinsed. In fact, our collections in question have not been thus
treated.

Laura Nightengale
Head of Collections
Texas Archeological Research Laboratory
The University of Texas at Austin
512-475-6853


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                  Conservation DistList Instance 15:63
                  Distributed: Tuesday, March 19, 2002
                       Message Id: cdl-15-63-023
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