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- From mhodder@theworld.com Thu Nov 07 07:22:08 2002
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Date: Thu, 07 Nov 2002 15:22:07 -0000
To: colonial-coins@yahoogroups.com
Subject: Re: [Colonial Numismatics] XRF at C4
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From: "mike468hodder" <mhodder@theworld.com>
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Ray:
Cleaning the coin with Care or Blue Ribbon isn't going to mask
the copper or zinc or whatever content in the coin. XRF will
still detect the superficial composition of the coin. However,
whatever the degreasers are made of will also be detected and can
skew the other readings to the high side or, worse, suggest the
presence of elements not really in the coin. I suppose it would
be possible to analyze the degreaser/silicon coating
independently, then extract its elemental spectrum from the XRF
readings taken from a cleaned coin. I never did this, however.
The point I've really been trying to make about XRF is this, that
it only measures a very thin surface layer of the coin. With gold
coins, that's not too bad when extrapolating results to the whole
coin. For silver coins it's a problem, since silver always forms
a protective layer of patination and usually that's all XRF can
see. For coppers, it's an even bigger problem, since our favorite
coins were often an ad hoc mix of whatever was available to melt
that day, and each one of those elements reacts differently with
each other, the air, surface contaminants like water, dirt,
sweat, etc. Also, the alloys cannot be assumed to be homogenous
from coin to coin or identical from melt to melt.
Analytical results on our coppers, even whole coin analyses (wet
or NAA), are interesting, but IMHO, their application to the
universes of their types is unwarranted. And I'd go so far as to
say unwarranted even for a statistically significant sample of
the whole.
Mike H
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