Larryh86GT Now we have some real scientific evidence to evaluate.
The XRF Test indicates:
Quote:
Silver (Ag) : 92.61% : 0.36%
Copper (Cu) : 7.16% : 0.31%
Iridium (Ir): 0.232% : 0.073%
There is no detectable gold in the coin. Therefore the coin was not made at any date prior to 1850 in the UK. Since the UK ceased their manufacture in 1850 and Iridium is unknown in English silver deposits. This places your coin in the most likely made in the US category. This also moves the likely date of manufacture to some point AFTER 1880. The reason for this dating is the lack of gold - period.
As you indicate, the chop marks do mean the coin was certified by the Chinese Schroffs as genuine silver. This is a physical confirmation that the coin was in China. The Schroffs certified the coin using Specific Gravity testing after 1835. However, they did not certify the coin was made in Mexico nor that it was made in 1807 the date on the coin.
Iridium is actually not a trace element I would expect to see in such a large amount (1/4 percent) in any coin. It is a bit of a surprise. It however does nothing to effect the status of the coin as a Class 2 Silver Circulating Counterfeit.
The Ag to Cu ratio shows, with Ag 2 1/2 percent high and Cu 2 1/2 percent low, is what I would call appropriate surface silver enrichment due to the final acid bath before it left the "factory" for China. To clean off dirt grease and other crud most metal works producing silver bathe their finished products in warm diluted acid. Mexico City did it and so did the US silver industry in the 1880s.
You have pictured the edge overlaps correctly. I have circled where I believe the lap occurs in yellow.
They are 180 degrees apart. They are also the same length. That indicates the counterfeiters were using a correct 2 die mill (a so called Castang machine). The Spanish called it a mill and the coin was called a milled dollar. Here is a similar edge mill from 1763.
The critical take away from the edge pictures are the incorrect shapes used on the edge dies. Note in particular the circles that are not circles. Look at the circle nearest to the left side of the yellow elipse at the bottom of the picture. The circle has a FLAT side. They should not. Note also the varying wall thicknesses of the circles and rectangles and the fact that the three segments are not grouped as such.
The Matrix block used to make punches for working dies in Mexico consisted of a Rectangle, a Circle and another Rectangle. You can see a picture of the actual 1772 Mexico City Matrix Block in Gilboy's book on Columnario's. The three element punch was repeated in the groove in the die block about 6 or 7 times to make a full edge die. The rectangles are the only overlapped element. That was intentional to keep the alignment and spacing proper. The circle was never doubled and should never be clipped. Here is how I visualize the overlap of the individual punches as they are applied 1-2-3.
The actual edger die did not have to be very long to do half of the edge of an 8R. Less than 3. Here is a blow up of the same 1763 edger to show the grooved edger die. The coin could not wobble in the groove and there should be no side to side drift.
Here is a picture of a few dies that were used in an edging mill. In each case the die details would be found at the base of a groove machined into the edge die.
Regarding your comment about the Platimum - Iridium connection being South American. You are correct. That happened in Columbia where Platinum was found in placer deposits along with gold.
Geology tells us that heavy metals like gold and silver come from miles below the earth's surface and that deposits near the surface are primarily associated with active volcanic zones where plates were colliding millions of years ago. The rocks of the crust cracked under the pressure of plate movements and superheated liquid quartz carrying metals rose into the cracks and filled them. The filled cracks are referred to veins they spread in vein like patterns.
Eons later after the mountains cooled and eroded the veins were exposed at the surface. The upper parts of the veins that had already eroded away are now found in the gravel and sands of streams that cut through them. These are placer deposits and can be recovered by panning. They occur down slope from the veins.
The rock still containing the veins are referred to as hard rock deposits. Miners tunnel through the rock along the veins to remove the metal bearing rock.
So where are these veins. They tend to occur near and along plate boundaries. In Central America 5 different tectonic plates come together. All of Mexico north of Belize is located on the North American Plate and so is the US. The old active fault system where metal is found runs along the Mountain range that extends from south of Mexico City to California and Nevada. All of the Mexican mines are located on this plate. From Belize to Panama the land formation is due to the collision of the Caribbean Plate on the east and the Cocos Plate on the west and the Nazca Plate from the southwest. These are Oceanic plates and the Caribbean Plate is sliding over the Cocos and Nazca Plates. Colombia is located on the North Andes Plate a continental plate. Bolivia, Peru and Chile are on the South American Plate. See picture below:
The tectonic plate that includes the Mexican uplift (North American Plate) is not the same plate as the one in Columbia (North Andes Plate). The uplifts and the infiltration of heavy metal into the rock formations occurred at different times millions of years apart. The elemental compositions are not the same hence the trace contaminants are different.
I hope this helps. You can see how several areas of science and history come together to help identify these coins.
colonialjohn I read the article yu pointed out. The subjects discussed involved coins found in archaeological deposits which have become brittle due to electrochemical corrosion and chemical trasformation of the crystaline structure of the metal over many years. The coins we are reviewing in Class 2 discussed here were not subjected to the same forces and prolonged exposure that the silver in the study were. Some Class 1 coins like the thin plate Sheffield counterfeits, some cold welded silver foil over copper coins and electro-types may be applicable if they were recovered in digs. The corrosion and breaking of bonds here is straight forward. Corrosion follows the silver to copper boundary seam and over time breaks the bond causing the silver layer to become detached. The surface cracks referred to here are solidly affixed to struck coins, show no delamination and often date to the pre-strike planchet as demonstrated by differential compaction by the force of the strike.
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Posted 02/05/2018 12:36 am
. This affirms the chop marks as authentic classifications of the correct silver purity by the Chinese traders.
Must have been written by a genius. 
