Copper beads

Carrie Duran <carried [at] cityofgastonia__com> writes

>We have some copper beads from an archaeological site in Gaston
>County, NC.  We would like to do some specialized analysis on them,
>but they are too oxidized to get accurate readings.  They appear to
>be almost pure copper because there are no alloys to suggest
>European copper. ...

The answer to your question depends on a number of factors. Firstly,
the bead even if heavily oxidized, can simply be analysed by XRF
which will tell you if it is essentially just copper or whether it
is a copper alloy of some sort.  If that is not the kind of analysis
that you need and you really want to determine the composition of
the copper, then taking a drilling with a 1mm twist drill into the
metal (if any) is the best way to go, the drillings are then used
for ICP-MS or similar analysis for 40 elements down to the ppm level
and then you are talking about a real analysis of your copper which
XRF or SEM cannot provide...but what happens if the copper is really
totally corroded? What exactly do you want to know from your
analysis?  In general, I prefer not to polish whole objects with
diamond paste as you suggest, as losing the patina or corrosion
interface is a loss of information on the entire bead--the copper
sheet that you suggest washing in distilled water--generally this
will not improve an XRF analysis so it is unnecessary....if you want
to determine trace element composition from the copper sheet then a
sample or drilling is necessary and patina should be preserved if
possible.

Professor David A. Scott
Chair, UCLA/Getty Conservation Program
The Cotsen Institute of Archaeology, Room A410
University of California, Los Angeles
405 Hilgard Avenue
Los Angeles, CA 90095


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                  Conservation DistList Instance 19:3
                  Distributed: Thursday, June 23, 2005
                        Message Id: cdl-19-3-008
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