It would be interesting to know whether an XRF gun could make anything of a
fourree. They generally don't penetrate very deeply at all, so you would have to try to aim it at the break in the plating. I've never seen one in action but I've spoken to people who have used them on coins and, like everybody, read what's readily available online.
I don't know if he
still does it, but I believe Rasiel Suarez was offering XRF as a commercial service (but it's not super
cheap, since the guns are outrageously expensive, and I think each use is costly because they wear out).
I actually have three coins that have been subjected to XRF testing (all before I was owner), and I really enjoy knowing the results.
One is a
Byzantine Solidus that Ras
had tested with
his own XRF gun (it was actually the cover coin from
ERIC II -- here in my
Forum Gallery [LINK]). From what I understand, the surface
alloy of ancient
gold coins is pretty consistent with the core, but the XRF gun is only measuring the few nanometers at the surface (98.5% AV).
The other two were from the group that
Harlan Berk sold after Jyrki Muona
had allowed them to be analyzed for Kevin
Butcher & Matt Ponting's big study of the
Metallurgy of Roman Silver Coinage, from the Reform of Nero to the Reforms of Trajan (
Google Books has a substantial preview
[LINK] and several of their articles, incl. some with Muona, are available free online). One of them -- an
Otho denarius -- is also in my FAC
Gallery:
[LINK].
A big contribution of
Butcher & Ponting's research was to show how different the results of surface XRF are from the interior of the coin -- at least for silver coins. (Hence the coins were drilled and "quartered" to check their insides.) (jmuona
had posted at least one of those two in
his gallery here.)