8 Reales Cobs

@MathieuMa

Thanks for the reply.

Does anyone know anything about this green patina? I've seen it on pictures of some other cobs, but I don't know how the coin got it, are these shipwreck coins?

Like Mathieu said, two are Potosi (modern Bolivia):

-- P B (yes, Philip II - the design did not yet include the date, but this is from the 1590s)

-- P T, which is 1619 as mentioned (only the 9 is visible, but style confirms 1619).

The 3rd piece is mainland Spain, Seville mint by style... date not visible but it's from the 1620's-1630s (pretty much the most common type of the peninsular pieces that you encounter)

Of course, Bob's (5) evaluation points about what to look for are all accurate and will confirm, but I would say there's little doubt these are genuine.

One thing with cobs and weight... they're not always perfect and can be either low OR high as made. The 8R cobs of this period "should be" a tad over 27.4g as made... In practice (for this early 1600's period, from Mexico, Potosi, and Spain), you find legit pieces that are in the low 28.x gram range even with a little wear, and others that are similarly underweight. Old clipping could of course can also result in low weights... Some of the 1640's Potosi pieces (where there was a lot of funny business with the silver content) can weight 29.x grams (of much lower than official fineness silver).

But like Bob said... anything WAY underweight, especially where the proportion of design doesn't indicate clipping, is of course suspect. A good example... blames the low weight on silver being shaved off. Uh, no... it's a cast:
http://www.ebay.com/itm/Spanish-8-R...271170839041

realeswatcher

I do not quite understand what you mean by this?




NGC has not ever gone past eyeball and they only go part way. But they keep the fee for a body bag.

The blue patina - you mean those splotches? Looks like some sort of foreign substance on the surface. Acetone might remove that...

And a melt pile save?! Good job... God bless the clueless in the year 2013. One thing, the 1619 (and I think also the Seville) piece are ex-jewelry... Potosi 1619 has a stripped hole, Seville piece looks to have some type of ex-mount (see cross side). Stlll great buys, though.

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Bob, yes, slabbers generally only eyeball... and you said "the days of eyeballing a coin and being certain are fading fast." Indeed, in the case of those two bodybagged Mex. cobs, the grader has reached that point :->

A little sarcasm at NGC's expense, as their amateur hour seems to carry on...

Here you have made a major error - you are accepting the word of someone else instead of doing the test for yourself.

How do you know they are silver? How did he know? A surface test would NOT work on a thick plate. He didn't cut them did he?

Acid would not only damage the coin's surface but presumes a uniform alloy.

There is NO WAY TO SEE INSIDE a coin without cutting it open. SG lets you get a good approximation of what is inside without cutting.

So back to basics - what is the SG?

You may trust the seller and that is fine but his test is done for scrap purposes and is not all that accurate. I work part time for a shop that takes in all forms of jewelry, gems, flat-wear and coins for melt. The accuracy is quite "approximate" and is more tied to making a profit on re-sale to a refiner based on volume and timing than on accurate assessment of actual alloy. The tests routinely used can have a variance from correct of almost 10%.

That is simply NOT accurate enough for coin authentication.

If you do not have a scale accurate to 1/100th of a grams (all that you need for SG tests) - drop everything and GET ONE. In this day you can not afford to be without one.




If you know the SG of a coin and do a ring test - YOU CAN NOT BE FOOLED by a core composed of a tin/lead mix. Those DO NOT RING. There is no 10.3 density metal (struck coin silver) that provides a ring in addition to the correct SG. At least not one that was routinely used to make counterfeits. With gold it is tungsten you have to watch for.

The SG of silver you quote (10.49) is elemental silver not struck coin silver (mixed theoretically with about 10% copper).




I know many dealers and forgers who want you to believe that.

When you melt an old coin there are a few problems - the retort you use and the furnace lining can actually alter the alloy in terms of volatiles and trace contaminants. So you need to use methods that are identical to the period in which the coin was actually made. One of the 8Rs I had tested with XRF came up with a metal trace contaminant that is used in modern furnace linings that was not even isolated as a metal when the coin was supposedly made and which is an element that NEVER occurs in native Mexican metal deposits.

Also if the forgers melt a coin from the wrong area or a coin that had been re-coined a few times as ancient silver was prone to be - the alloy will be WRONG in terms of trace contaminants.

XRF can not authenticate Roman coins at present because contamination from long periods of burial have altered the coin's chemistry. But a scanning electron microscope can easily detect a re-melted coin. XRF is a laboratory technique that is only one of a series of potential tests that can authenticate or debunk an artifact.

That is a mistake made by many coin "experts" who want to (or better still wish to) discount XRF testing. XRF discloses far more than major components of the alloy - in fact for the copper silver ratio SG and XRF produce rather similar results. But for things like iridium or yttrium or cadmium there is only XRF.

For an authenticator - the question that remains is are these oddball metals supposed to be there? Are there metals that simply CAN NOT be there? Are there others that MUST be there? This is a new area of study and XRF and allied laboratory tests are the future for authentication.

Also when a forger melts an old worn coin there is an inevitable effect on zinc or mercury contaminants which boil off on re-melting.

Then there is surface contamination from processing the planchet.

It is far less simple than it seems. XRF is not just a list of elements. It requires an interpretation of the list compared to a database of lists from other examples which provide the conclusion.

DO NOT SELL SCIENTIFIC TESTING SHORT. Once it gets popular - there will be hundreds or even thousands of fakes discovered in TPG slabs. XRF and some of the others look right through the plastic holder as if it was NOT there.

I know the money involved with TPGs and big dealers who have a lot to lose, may cause things to be held back, but XRF test results will if allowed to mature provide a much better method of authentication than has ever previously existed from the dawn of time.

@swamperbob

Thanks for detailed information about XRF and coin authentication.

Based on the info so far, I'm relatively confident these are authentic coins, but I'll probably check them with some high profile coin dealer, maybe slab them for auction.