I found an (another?) interesting article on ancient coin theft and forgery, see link: https://www.academia.edu/24832122/U...RITAGE_GOODS?auto=view&campaign=weekly_digest This seems like a good idea and I have seen plenty of coin analysis articles. Has anyone in this group used XRD for coin analysis? I have a friend in the Bellaire (TX) Coin Club who has one. He uses it for gold and silver recycling. I think his is bigger and more expensive than the one in the pic below.
Interesting article! Taking such a "fingerprint" of a valuable ancient object might indeed prove to be helpful should it every be stolen. It's not useful for determining the true composition of a coin since it only reads the surface, but for purposes of having a record of a given object's signature for use in the event of future theft, it sounds reasonable at first glance. The importance of using the same reading device, or one with the same specs, was mentioned. What happens when today's handheld XRF devices are obsolete? I wonder how or if patina affects the reading (for silver and gold coins and their alloys). I'd guess that bronze coins would be more difficult given the heavy patinas which can occur naturally or artificially.
I used to operate one of the big ones, but unfortunately, not with coins. The one in the picture is a "field instrument" which I believe is more robust but less accurate (just enough to have good readings). You can have surface analysis as well as inside analysis, although it is not a destructive assay, a little surface cleaning is always recommended for good measure, even on silver coins. That said, I hear that you can pin point the area to be measured to a minimum square millimeter.
I doubt that there would be much difference between machines and a patinated or unpatinated coin. Now completely crusty coins are a different story, but I have not tried using an XRF on these. To understand why I think this, one needs to understad how XRF machines work. They emit X-Rays (the first two letters of XRF) into the target metal. These X-Rays excite the electrons in the atoms in the coin as they absorb energy from the radiation. When the electrons revert to a more-stable, lower-energy state, they emit photons equal to the drop in energy. The atoms in each element have specific absorption and emission energies, called atomic spectra. The XRF machine measures the relative intensity of the spectra and outputs a ratio of atoms present. The accuracy of the measurement depends on the precision of the device, which is why the better ones are more expensive. As each type of atom has a specific atomic spectra, I would think that different machines should be able to record the same relative intensities of atomic emission and absorbtion spectra, meaning the recorded ratios of elements present should be the same. The precision of the machine might affect this if the values of the specific spectra of two elements are very close to one another. The patina is usually thin enough to represent only a small percentage of the atoms present, so it can be ignored.
Do you have an XRF high quality reading device? Willing to be hired, to read a few coins? Gary in Washington
XRD is X-ray difraction , and is mainly used for crystalline structures. Historical note: DNA structure was predicted by XRD done by Rosalind Franklin for Watson and Crick. It would work to identify chemical crystals, proteins, etc. on the surface of coins, the total thickness object would have to be powdered. XRF is a statistical comparison of the signals of certain atoms as they goes through the cycle. The surface atoms will be a larger statistical value due to proximity compared to inner area, so even though power can be increased, the 3D resolution can not just isolate on a certain cubic area for analysis. My suspect would be that the patina would be over-represented chemically than the inner material by external analysis. That is why such can not be used to detect gold plated tungsten bars reliably and ultrasonic waveform analysis is used instead.
This is stuff of which I am completely ignorant but appreciate the scientific explanations because I'm a geek at heart if not occupation.
Have never worked with X-ray diffraction, but from my readings and from people I know, there is powder X-ray diffraction and single crystal. This would not be an analysis of any use with coins, I'm not sure if this would read anything on the surface. OTOH, XRF is a surface analysis and doesn't penetrate to any appreciable extent. XRF is best for heavy elements, generally metals. For example if you had a patina of copper carbonate, XRF would tell you there is copper there. I was supervisor in a lab where we had one, I never ran it except in one instance where we had visited the Grand Canyon and I kicked something in the parking lot that rang. I picked it up and thought it was some slag aluminum. I ran an XRF on it and it turned out to be silver.
