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Subject: Polyurethane and modern human bones

Polyurethane and modern human bones

From: Barbara Appelbaum <aandh>
Date: Thursday, December 3, 1998
About the bone preservation inquiry, since the current responses
addressed the testing issue, something else should be said about
polyurethane itself. Polyurethane does not have good aging
properties, will discolor, become less soluble, and will off-gas
acidic byproducts.  Other questions relate to the other materials in
the product.  Some polyurethane products have drying oils in them,
for example, and this would affect the behavior of the material.

I am sure there are data available describing the chemical identity
of the off-gassing or polyurethane and the specific susceptibility
of bone to whatever it is.  Another potential problem is shrinkage
of the resin.  Some objects with fragile surfaces have been damaged
badly by slow shrinkage of coatings; since polyurethane is not a
conservation material, I doubt that conservation scientists would
have tested it for this.

If a spray is important, there is a commercial material sold as an
artists' varnish under the name of Krylon (either "crystal clear" or
"water white") that is made of Acryloid B-72, which is a much better
material in terms of its aging properties and would not be as shiny
as polyurethane.

I also recommend a literature search through AATA or the on-line
database CHIN.  Consolidation of bone is obviously a big topic, and
there has been only a small amount of contact between the natural
history world and conservators who do treatments.  Bench
conservators choose ( or at least should choose) the most chemically
stable material that will accomplish the job at hand, while
technicians in natural history museums need something that comes
ready-made and is easy to use, since they might not have laboratory
facilities.  For example, virtually any liquid consolidant can be
used as a spray, but may not come as a spray, and conservators can
deal with this easily, while technicians may not have the expertise
or equipment.  Without proper ventilation, for example, a
water-based material may be safer than a solvent-based one, although
commercially sold emulsions have a lot of ingredients that remain
untested.

I hope more conservators will enter this discussion.

B. Appelbaum

                                  ***
                  Conservation DistList Instance 12:50
                 Distributed: Tuesday, December 8, 1998
                       Message Id: cdl-12-50-001
                                  ***
Received on Thursday, 3 December, 1998

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