Through the course of using my Sigma Metalytics tool I have discovered all sorts of anomalies with the gold purity of bars and coins. Aside from a few outright counterfeits, most of the discrepancies can be attributed to sloppy manufacturing, especially with bars that fail the 0.9999 test but test good using a lower purity setting on the tool. That said, I am perplexed by a recent batch of modern US $5 Half Eagles I went through and tested. They are supposed to be 0.900 fine, but aside from the very early issues (1987, 1988, etc) all of them fail on the tester when set to test 0.900 but pass with flying colors when set to "American Eagle" on the tool. The weight is correct on all of the coins and there is little doubt about their authenticity. Is it possible the US Mint simply decided it was easier to mint these modern gold commemoratives from 0.917 fine gold than it was to mess around with procuring 0.900 fine metal as the enabling legislation dictates? They would have been giving away more 1% in free gold for every coin made this way if that were the case. I have enough of these coins that I could send them off to be assayed I suppose. But figured i would ask if anyone else has seen the same thing when using their tool on these same issues.
Are they reading as .917 fine? The planchet are made by an outside contractor from gold supplied by the Mint. I doubt the mint would tell them to use a standard different from the legally proscribed one, and if the contractor did it on their own I'm sure the mint would notice that one they are getting back fewer planchets than they should be, and two they do assays on samples from each batch of planchets so I'm sure they would notice the incorrect alloy. To be the correct weight but higher fineness the planchets would have to be thinner than normal which would also give them striking problems.
