Possible Boston Forgery 8 Reales

gfabius - not my area but the coin does have wear and 0.5% loss of weight would seem about right. Studies of heavily circulated coins suggest modest weight loss of 3-5% with some studies suggesting up to 10-12%. I am also a bit dubious as to where the exact figure of 26.8636g comes from. I suspect it is a conversion from a non-metric unit and this combined with the inability (I suspect) of balances in the early 1800s to weigh a tenth of a milligram raises questions on it's reliability.

qfabius

Based on the appearance of the edge, I would suggest doing an XRF test to look for gold. The edge does have the appearance of a made for the China trade coin. A CCSC (Contemporary Circulating Silver Counterfeit) is rather common and since they were made for and circulated in China they will often be seen with small chop marks.

The gold trace does vary, however, it was relatively stable at the Mexico City mint for the first twenty years of the nineteenth century using silver from Guanajuato for 60% of the total output. Mexico City would have used cupellation to achieve a 99.9% pure silver as a starting point. The last tenth of a percent hides the tell-tale gold and other key trace contaminants that can determine approximately when the silver was refined. Depending on the accuracy of the XRF apparatus used, a reading of under 0.02% gold 200 ppm is in my opinion a CCSC (Boston) type - no doubt.

Some laboratories specializing in authenticity of Mexican artifacts use a higher threshold of 400 ppm or 0.04%, but I have only seen higher gold levels in some of the branch mints. The Guanajuato mint seem to be the mint with the naturally highest gold levels. All of the examples I have tested based on having several of my visual clues have tested No Gold on a handheld unit that reads to 0.01% 100 ppm. Some of these I have retested using a laboratory level XRF which detects all elements from Carbon to Uranium at a 10 ppm minimum level. Most still show no gold but readings as high as 20 ppm have been seen which is consistent with 1880's methods in the US which could produce 99.999% fine silver.

Above 400 ppm (0.04%) gold does NOT guarantee the coin is genuine - it just gives an indication that the coin could be genuine. Other trace element tests and visual inspections are needed to exclude other later forgeries.