1777 Charles III 8 Reales Looks Regal But...

My opinion is that this piece is authentic. I also think that it has been cleaned.

btw, the coin is from Mexico mint.

swamperbob: Thanks so much for looking at the coin. I tried to message you last week but my email was refused by the forum because I don't have enough posts. I will see if XRF testing at the coin shop will give us any information about other metals or contaminants. I'll post the results later.

RockyRoad Jgenn is correct. Make sure that the test can accurately detect gold at the 0.01% level MINIMUM or the results may be inadequate to make any kind of decision. That level (0.01%) is 100 ppm (parts per million). Some newer handheld XRF units make the claim to be that accurate. Most I have seen are only accurate to 0.1% which is 1000 ppm. That would not be good enough.

The 100 ppm level of accuracy will detect silver that was refined in Mexico before 1870. All such silver MUST have an absolute minimum of 200 ppm of gold.

Here are the reasons I believe in this test stated as simply and clearly as I can. This is the first time I have attempted a complete recitation of the facts known to date. It is an updated position that I first discussed in what would become my book in 2014.

The reliability of this test is based on the fact that silver and gold are mutually soluble metals. That is a rare characteristic for metals. Most metallic elements when mixed or melted together will not form homogeneous solutions. When melted and then slowly cooled, the heaviest metal will tend to settle to the bottom of a crucible. But in the case of gold and silver they actually dissolve - they do not simply mix. When heated to a liquid state gold and silver form a solution similar to sugar that sweetens an entire cup of coffee - no amount of settlement will allow all the gold to settle out of an ingot of silver.

This simple fact is the reason why repeated fire cupellation ultimately did not work to completely purify either gold or silver. In the early 1800s science had not yet determined exactly why that was. But there was a point beyond which no amount of cupellation worked. That is the gold trace we are looking for.

Cupellation is a very old technique - used since pre-historic times to refine silver and gold. The process was successful at removing other impurities and metals by oxidation. In parting (the separation of gold from silver) cupellation relied on the belief that the density of the two metals would divide the gold (heavier) from the silver (lighter). It was believed that cupellation caused the heavier gold to completely disassociate from the silver and then the gold would be free to fall by gravity to the base of the cupel where a button of gold of increased purity would form beneath a button of increasingly pure silver.

Therefore repeated cupellation of the products of the first cupellation was thought to increase purity. After 5-7 repeats it was believed that pure silver and gold resulted. Repeated cupellation only an increase the purity of the gold and silver themselves from the presence of other contaminants like copper and lead but not from each other.

Before 1850 it was actually impossible to purify either gold or silver to better than 99.5% pure on an industrial scale. In laboratories small amounts gold could be purified to almost 99.99% pure by using acids to remove small traces of silver. This was based on the fact that gold does not dissolve in nitric acid but silver does. However there was no such process available for the removal of a small trace of gold (a few percent at most) from silver.

In most cases in Mexico, there was a limited amount of available fuel. Many of the mines and refineries were in remote often high desert areas - even if they wanted to perform multiple cupellation - it was impossible or at least it was not economical to do so. In that case raw refined silver went to the mint for refining there.

When the raw silver got to the mints, the economic situation was often no better. Fuel was still costly and in short supply. This created a financial limit which the mints could not ignore. They could not spend more money to remove the gold than the gold itself was worth. This break even point is why at times much more gold was left in the silver than at others.

There was another issue as well. Cupellation takes time as well as trained people to complete and in periods when the mints were stressed by the need to make coins quickly (as in War time when soldiers needed to be paid) the value of the output of money (coin)outweighed the small amount of gold that might be recovered. So during the War for Independence there was virtually no effort to remove the trace of gold.

So at this point, people following the argument will possibly believe that all silver worldwide had the same problem and that all of it will contain gold. That is not the case either. Most old world silver deposits were produced as a by product of lead mining. Argentiferous lead deposits were the most common found in Europe and gold was not present in significant amounts. Earlier worked deposits of Electrum would be similar to Mexican silver but in the 1700-1900 period no production from deposits of electrum or electrum like ores is noted as existing.

New world deposits of silver were different then men had encountered earlier and these ores required a new technique to refine. The solution was using mercury amalgamation. The patio process was the answer and it was invented by Bartolome de Medina in Pachuca, Mexico, in 1554. This process was superior to anything used in Europe.

In Mexico, all of the silver is found in veins of quartz that intruded into the rock strata during subsurface volcanic activity millions of years ago. The uplift that formed the mountains of central Mexico is part of a single system. These major deposits run from southern Mexico into California and Nevada. The single contaminant that is important to us in this uplift is gold. It makes silver from these mines essentially different from most other major deposits worldwide.

The veins of Peru and Bolivia were created in a similar fashion at different times in geologic history. Peru, Bolivia and Mexico are each on different tectonic plates. So each uplift has a different elemental composition. In Bolivia and Peru another trace contaminant arsenic is always involved and is critical to determining authenticity. So it is possible to isolate silver produced from these three uplifts by identifying the simple differences in trace contamination. Most of the data I have seen in this area come from Winturthur Lab reports used by the Museum to authenticate early rare silver artifacts.

Since gold is the primary contaminant of Mexican silver it is always present in silver that was mined and refined in Mexico before a process existed to remove the trace of gold.

Based on over 200 tests performed so far, typically we have seen gold in the 400 to 2000 ppm range. There are higher results for the early years of the Mexican Republic with some test results as high as 3.8%. That is 38,000 ppm and was observed on an 1836 Eight Reale from Guanajuato. I believe this could mean that at times the early Republican mints did not bother removing the gold at all.

There are also a very few results (less than 5 that I know of) where apparently genuine coins based on visual clues or die matches have yielded slightly lower results than 400 ppm. A test of 320 is the lowest I have seen personally on a coin from my collection. All of the lower measurements are seen on coins that visually have clues which identify them as fakes.

So out of an abundance of caution, I call anything UNDER the 200 ppm level a silver COUNTERFEIT. I am very comfortable with that and have no doubt that coins below this level could not have been made before 1870 in Mexico. Between 200 and 400 I personally classify the coins as "suspicious" or possibly counterfeit.

This limitation may condemn a very few coins to the suspect bin - but I think it is much better to do that than for all of the low gold coins to be considered GENUINE as they are now by 99% of dealers and collectors. Over time, as the science is more generally understood and accepted - I would hope that a more reasonable limit of 400 ppm would be adopted. However, for now 200 ppm makes more sense and it will identify 97% of the target coins.

There are other trace contaminants that can be very useful as well, that would include Antimony, Arsenic, Bismuth, Cadmium, Cobalt, Chromium, Iridium, Iron, Lead, Manganese, Mercury, Platinum, Tin, Titanium, Tungsten, Yttrium, Zinc and a few others that are more rare.

So RockyRoad Please let us know what you find, if the XRF machine has the capabilities needed.

I hope that this explanation has not been too lengthy or complex for anyone to follow.

Any questions?

swamperbob, your update on the refining of silver from Mexican sources and its relevance to XRF testing of mint issues is very well stated. Do you plan to post this information in a more prominent place, or issue an update to your book?

swamperbob: Thanks again for the great explanation. Since it is unlikely that the machine at the coin shop is sensitive enough, I am exploring other avenues for an actual laboratory XRF test. I will post when I have some results.