The coin originally mentioned in this thread was genuine after all.
It was checked by a spanish specialist, and went through XRF analysis.
More can be read about it here :http://blognumismatico.com/2012/05/..8-de-mexico/
I read the thread completely and the supporting report. The owner seems to WANT the coin to be genuine and he seems to be reading the results to achieve that wish. The credentials of the specialist are very good. Josep Pellicer I Bru is President of La Asociación Numismática Española and is an author of several books on earlier Spanish coins and ancients.
I am concerned as to the casual nature of the "authentication" by this person regardless of his qualifications. The owner notes the process is simply a subjective opinion by one man - a qualified one. But the nature of the inspection is not given. Was anything other than a loupe used? How long did it take? Did he make any cautionary disclaimers?
The owner uses the XRF tests as a "Coup de grace" to prove his case.
There are several SERIOUS problems with the authentication as presented. The XRF device used by this "new" testing firm (which the owner of the coin has been involved with since its inception) was essentially worthless as a method of proof the coin is genuine.
The Niton XL3T 980 is a hand held XRF devise that provides a level of accuracy that is good for a junk yard purchase of scrap metal but it is not capable of testing for the trace contamination of gold that must be present to authenticate the coin. If you refer to the website for Niton you will notice this model is not recommended for precious metals testing. It is a general industrial tester. The stated error rate for the silver content (91.8 % average) is plus or minus one quarter of a percent. That result is actually too high for an original. The results DO NOT MENTION gold at all. There is about 1% of the content missing which is referred to as "other trace elements". That is simply NOT ADEQUATE as an XRF result.
So, far from being proven genuine - the results seem to indicate the coin could just as easily be a Numismatic Forgery made from 91.8% silver and 7.2% copper a few weeks ago as it could be a genuine coin made in 1748.
My book on Counterfeit Portrait 8Rs is available from Amazon https://www.amazon.com/Counterfeit-.../1500497177/
or from me directly if you want it signed.