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Subject: Polyurethane and modern human bones

Polyurethane and modern human bones

From: Greg Hodgins <greg.hodgins>
Date: Thursday, November 26, 1998
I just wanted to reinforce the importance of Mr. Bodkin's policy of
leaving portions of the skeletons untreated by mentioning two
analytical methods other than DNA analysis that are sensitive to
contamination by conservation materials.

Everyone will be familiar with the first: radiocarbon dating. In
living beings, one out of every trillion (1,000,000,000,000) carbon
atoms is C-14.  This level drops over time, and the limit of what we
can currently measure is that found in organic materials around
45,000 years old. One can see that if the material being conserved
is very old, and the consolidant used was originally derived from
modern organic materials, (for example solvents, polymers, or resins
derived from animal and vegetable products), this could dramatically
alter the C-14 'age' of the specimen. If the consolidant is not
particularly visible, and records not kept of the treatment, then it
can be very difficult to recognise a date as anomalous.  We are a
carbon-dating laboratory, and the reality that this sort of thing
must occur fairly frequently turns our hair white.

The second area of research that you may not be aware of, is carbon
and nitrogen stable isotope analysis.  The quantities of C-13 and
N-15 in an organism are determined by its diet. Consequently CN
stable isotope research looks at organisms in the context of food
chains and ecosystems. For example changes in human diet as a
consequence of the spread of agriculture can be detected in the
levels of nitrogen isotopes found in human bones.

>From the standpoint of stable isotope analysis, a nitrogen-free
consolidant, such as polyvinyl alcohol, would be preferable to a
nitrogen containing substance such as polyurethane.  A good example
of the need for treatments to be reversible is found in the Journal
of Archaeological Science, 16: 437-466. Moore et al. found they
could get PVAs back out of consolidated bone and make reliable
nitrogen isotope measurements. Whether or not this would be possible
with polyurethane is an open question.

The long established custom of consolidating archaeological bones
with animal and fish glue presents very difficult problems for both
of the methods mentioned above. It could lead one to think that a
Neanderthal specimen was only two thousand years old, and it ate
seaweed. Thankfully this treatment is obsolete, nevertheless,I would
appreciate hearing from people who have records of such treatments.

Although I'm mentioning archaeological techniques and applications,
collection of modern materials such as Mr. Bodkin's are very
important because often modern control specimens required alongside
archaeological materials. Sincerely,

Greg Hodgins
Research Laboratory for Archaeology & the History of Art, Oxford
University
6 Keble Road,
Oxford OX1 3QJ, UK
+44 1865 283941
Fax: +44 1865 273932

                                  ***
                  Conservation DistList Instance 12:48
                 Distributed: Monday, November 30, 1998
                       Message Id: cdl-12-48-002
                                  ***
Received on Thursday, 26 November, 1998

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