Bought an Eagle, 1910 D, today at the local show. I examined it closely and it looked good, dealer did as well and tested with magnet, passed his test. Paid the seller, another dealer set up at the show, and went on with my shopping. The promoter had a sigma metalytics and we put it on there, and it was several bars to the right of the acceptable range, on both the gold eagle setting, (the wrong setting), and the 90% setting, but the weight was spot on. Compared it to a different 1910 D, and it too showed off to the right. Went to another dealer, and initially it was off to the right on his Indian, but after fidgeting, it was in the range, and tried the new purchase again, and it was in range after fidgeting. Is this metalytics usually this temperamental? Seems like it's prone to wild swings with gold, and for the price I would expect something consistent and reliable. Or is it best to use a magnet and calipers and run with that?
I'm not 100% on board with Sigma for gold testing. However, it's more useful than a magnet -- any magnetic fake would have to weigh way less than a standard coin, or be way thicker, because steel has less than half the density of gold. Check dimensions, check weight, and carefully check the coin's appearance. I think the only gold substitute that would pass dimensions-vs-weight is tungsten, and while we've seen "gold" fakes with a tungsten core, I don't think they're very common yet. Those should still turn up on a Metalytics test. As for "how temperamental is the Sigma", I can't answer, as I don't have one. I was really skeptical about their technology as a whole, until I read their user's manual, which is quite specific about what you can and can't expect from the machine. It's not perfect, but it's useful, and I think it's under $1K for a new machine, vs. over $10K for a new XRF machine.
Jeff has good points. I don't have one either, but I have bought one of the devices mentioned below =( it was half price last year Black Friday) and when I really retire I am going to build one and see if it works. If you are a do-it-yourself, you can use this ( https://www.amazon.com/Ultrasonic-Thickness-Resolution-Professional-Backlit/dp/B086WVP136 ) and Google " how to use ultrasonic thickness gauge to determine the coin metal " Jim
The principle employed by this type of equipment is the confirmation of material based on its inherent mass and stiffness for the thickness tested. It works great when samples are of uniform thickness, however the thickness can vary pretty meaningfully on struck coins for it to be considered accurate in that application. Still because materials substituted for PMs (tungsten for gold, as one example) diverge widely for relevant properties (while the densities of tungsten and gold are almost identical, the elastic modulus of tungsten is more than 5 times that of gold), it may still prove useful in confirming that the test subject is much closer to being one than the other.
