http://blognumismatico.com/2012/05/16/discutiendo-la-autenticidad-de-8-reales-de-1748-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.

…

When I re-read the comments made on that other web site – I am fearful that the XRF technology is not only generally misunderstood by coin collectors but is actually being miss applied in an effort to subvert the science behind the use of XRF to disclose modern forgeries.

The starting point is simple. In 1748 no one on the face of the earth had the capability to remove all of the gold that is naturally found in Mexican silver deposits. Today parting gold and silver to 99.999 fine levels is rather easy – but in 1748 99% pure was acceptable and 99.9% was an impossible industrial standard.

So if you find NO gold or gold on the order of less than 200 ppm – you have a forgery pure and simple.

XRF tests which can look for this trace level of gold are essential. The use of HANDHELD XRF testers is worthless in this context. They are simply not accurate enough – PERIOD.

The handheld XRF testers are good only to screen out really bad coins – 5 or 10% off. They are in the same class as Specific Gravity testing – NO MORE than that.

The laboratory XRF tester looks at all 100% of the elements contained in the coin from Carbon to Uranium to a level of 1 ppm if needed and achieves a balance of 100.0000%. The gold can not hide from this test. Nor can other equally important trace elements that should or should NOT be present.

This capability will actually make forgeries harder to create. Just consider that a modern ore refinery uses a lined furnace with a surface that can contaminate the metal being refined to 10 ppm. How do they remove this «modern signature» contamination?

Even if they simply melt genuine coins – how do they do it to avoid furnace and fuel contamination or the loss of the mercury trace that came from the original patio process?

Finally how about copper? Today commercial copper is NOT pure. It is not valuable enough to be refined to a level beyond 1000 ppm. How does the forger purify his copper to avoid modern contaminants without losing needed contaminants that would have been there in 1748?

I am trying to illustrate why XRF could be the answer needed to put 99.9% of all forgers out of business. If a fake of a $100 coin costs $200 to make – they will NOT be made. The most important thing to do is to eliminate in a simple way silver forgeries. »

Matizaría algún punto, pero básicamente es así. La microscopía electrónica es otro parámetro a tener en cuenta, parámetro que no he comentado más que someramente en los 8 reales de 1823 y que en todos los casos publicados confirmaría su falsedad. Por supuesto, ningún método da un 110 % de fiabilidad