Storing coins

Susan White <smwhitewhite [at] aim__com> writes

>Does anyone know how to tell if a particular plastic is likely to
>outgas acidic components?  I have a museum client who has been
>housing Greek and Roman bronze coins in "Archival" plastic sleeves,
>similar to photographic slide sleeves; however, the coins are
>starting to bloom with bronze disease and I'm suspicious that the
>plastic may not actually be suitable for holding bronze coins and
>outgassing corrosive reagents. I tried to test the plastic using a
>pH pen designed for testing paper, but the results were ambiguous.

The best way to screen unknown clear plastic film and other suspect
polymeric materials for the presence of chlorine is the Beilstein
Test.  It's clearly described in CCI Conservation Note #N17/1
available as a PDF at:

    http://www.cci-icc.gc.ca/crc/notes/index-eng.aspx

A positive test results will indicate that the plastic is primarily
composed of polyvinylchloride (PVA) which can emit plasticizers and
eventually release hydrochloric acid as a breakdown product.

Once you are set up to perform the Beilstein Test, you might want to
test the sleeves that the coins are in.  If the results are negative
for chlorine, then the cause of the corrosion might be something
else, e.g. trapped moisture in the sleeves driving a pre-existing
chloride-catalyzed reaction.  You should also test the corrosion
'blooms' for the presence of chlorine using the silver nitrate test.
Conservation Notes N9/1 and N9/4 may also be of interest to you.

Paul S. Storch
Project Specialist II/Collections Liaison
Historic Sites and Museums
B-124.2
Minnesota Historical Society
345 Kellogg Blvd. West
St. Paul, MN 55116
651-259-3257


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                  Conservation DistList Instance 23:43
                   Distributed: Tuesday, May 4, 2010
                       Message Id: cdl-23-43-005
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