Attributing a tetradrachm ...

[David Soknacki]

Fellow collectors might be interested how I tried to find out more about my purchase of a tetradrachm. I thought it was going to be easy since I was buying an example of the most common coin from antiquity ...

About four years ago I bought this tetradrachm from Naumann

  

Where the catalogue described it as:

ATTICA. Athens. Tetradrachm (Circa 454-404 BC). Obv: Helmeted head of Athena right, with frontal eye. Rev: AΘE. Owl standing right, head facing; olive sprig and crescent to left; all within incuse square. Kroll 8; HGC 4, 1597. Condition: Extremely fine. Weight: 16.22 g. Diameter: 27 mm.

From my unscientific review of Athenian tetradrachms in acsearch and the sixbid archives I observed very few of this weight or lighter, most of which have problems such as damage, porosity or test cuts.

So I decided to see what I could find out about authenticity, metal content and mint.

A testing lab analysis shows the coin contains 98.32% ag, 0.68% cu, 0.60% au and 0.41% pb. Since I cannot find any metal composition study for the series, I do not know whether these trace metals and their proportions are typical or unusual.

Next I reviewed the die axis, thanks to Dr Van Alfen and ANS’s MANTIS database. This coin has a 5 o’clock die rotation. Of 146  ‘classical’ period tetradrachms in the ANS database, the 5 o’clock rotation is unusual but not unique:

axis	count	percent
1	3	2.1%
2	7	4.9%
3	6	4.2%
4	1	0.7%
5	6	4.2%
6	2	1.4%
7	2	1.4%
8	26	18.1%
9	70	48.6%
10	18	12.5%
11	1	0.7%
12	2	1.4%

Finally I had the coin reviewed by some who have far greater experience than I. David Sear pronounced it genuine “though of unusually low weight.” NGC attributed the coin to “near east or Egypt, c 5^th – 4^th Centuries BC” and Dr van Alfen attributed it to Athens.

My conclusions are that this coin is contemporary and probably of Athenian mintage.

Any insights from the community are welcome.

D

 

[Ed Snible]

The coin looks great!

Thanks for comparing the three experts' results!  Most collectors stop after the first expert.

For metal analysis, perhaps

https://www.jstor.org/stable/43580559 Haim Gitler et al, "Athenian Tetradrachms from Tel Mikhal (Israel): A Metallurgical Perspective".  (Requires free account to read).

https://www.academia.edu/1025594/M_Ponting_H_Gitler_and_O_Tal_Who_Minted_those_Owls_Metallurgical_Analyses_of_Athenian_Styled_Tetradrachms_Found_in_Israel_Revue_Belge_de_Numismatique_clvii_2011_pp_119_136 M. Ponting et al, "Who Minted Those Owls?"

 

I do not understand why it is so hard to get things like average metal content except by reading papers.  Someone may need to set up a site to gather these results together and print a nice average and standard deviation for metal content.

Please tell us what equipment was used and if you can, which firm did the metal analysis.  I am hearing conflicting things about XRF and miscalibrated XRF.

Edited September 1, 2022 by Ed Snible

[Curtis JJ]

It's a superb example, I can definitely understand why you would devote a lot of attention to this coin. (Besides excellent condition, you've got most of the crest AND necklace, which are hard to find together, and all four edges of the incuse square on the reverse and possibly two of the four corners, which is also hard to find.)

Well done on successfully procuring so many expert opinions!

With NGC's assessment of Egyptian or Eastern imitation, I'm sure they always have a basis for their opinion and that it's more informed than my opinion would be, but I always wonder what it was. Are they simply basing it on weight and subjective interpretation of style, are they doing an XRF and comparing to a proprietary database, or are they finding die matches to the hoard studies of Eastern tets?

In any case, as Ed commented above, having multiple opinions is great, and multiple angles of research. Each one provides context for and enriches the others, so they're more than the sum of their parts.

[David Soknacki]

Thank you, @Ed Snible, for the reference – it was exactly on target.

The Athenian tetradrachms from Tel Mikhal referred in the article have almost exactly the same mean weight and silver content as my tetradrachm.

The article concludes “the silver content of the Eastern owls was as strictly controlled as authentic Athenian tetradrachms and provides further evidence to support the view that these coins are either authentic Athenian products or some form of centrally minted eastern issues produced from Greek silver.”

The article refers to two further papers from which I ought to be able to compare the silver profile of this coin to silver mined from Laurion. It’s a project of mine for later this month.

A word about XRF testing. There is a commercial materials testing laboratory nearby, which used a Fischerscope x-ray “xdal” (trade name) fluorescence measuring instrument on the coin. Although accurate and professionally operated, XRF has limitations. We collectors know about surface enrichment, and XRF penetration in metal is measured in microns. However, I wouldn’t agree to drilling a hole in the side of my coin!

@Curtis JJ asked about the basis for NGC's opinion. I've asked them, but have not yet received a response.

Again thanks for your interest.

D

[Curtis JJ]

Incidentally, I have one coin with an XRF report (from Rasiel Suarez when I bought one of his coins). Surface enrichment may be more or less of a problem depending on what you want to use it for. Even if it's not accurate for the core (for some coins I suspect it would be more accurate than others), it might still help identify your coin as Athenian vs. Egyptian vs. modern fake if you could compare it to the surface XRF results of other samples.

Interestingly my gold coin tested similarly "pure" to the silver tetradrachm, with the % AV / AR almost reversed between our two samples (98.52% Au/0.67% Ag vs. 98.32% Ag/0.60% Au):

 

12 hours ago, David Soknacki said:

I wouldn’t agree to drilling a hole in the side of my coin!

If you're reluctant to drill a hole in the side, here's another option (before & after) -- though I'm not sure it's really as unobtrusive as it sounded!

Actually those were cut open so the researchers could photograph the insides of the coins under a microscope, but they did also measure them as about 94% & 79% silver.

I'll be the first to admit it's kinda hard to recognize them from the photos, but that's the coin on the left:

Edited September 2, 2022 by Curtis JJ

[Ed Snible]

@Curtis JJ's point is important.  XRF may not give results that are representative of the interior, but it does give results that are representative of similar coins.

Last month Dr. Ronald Bude, MD, gave a talk at the American Numismatic Society: "Don’t be Afraid of Technology: Using Sophisticated Techniques to Evaluate Coins".  You can watch it on YouTube.  He pointed out that it costs hundreds of thousands of dollars to get accurate results non-destructively using neutron activation or muons.  He also compared the results he got with destructive tests on his own coins to XRF.

I am very curious about XRF technology.  I hear conflicting stories about calibration.  Rasiel Suarez told me that "rental machines are reportedly beat up jalopies" so I have been nervous about renting one.  I have been unable to find a testing lab nearby.  I found a coin dealer and several bullion dealers in the New York area with XRF machines.  They were happy to test a coin or two, but unwilling to test additional coins for any amount of money.

Before I trust XRF reports for ancient coins I'd like to know how reliable they are.  How different are the numbers for different machines?  Does the calibration drift?  What about different locations on a coin?  To measure that I'd need tests from many different machines.  I can't get tests from a single machine.

I found a decent undergraduate paper on testing US silver coins with XRF.  I feel like it would be worthwhile to do basic tests like these to find out if XRF results in different studies of ancient coins can be compared with each other and are believable.

[David Soknacki]

I brought three Athenian tetradrachms into the testing lab. My goal was to see if any of the results were surprisingly different, whether due to the equipment or coin.

Here are the results

a) archaic type (Seltman Gi, A144): 98.42% ag, 0.86% cu, 0.71% pb

b) classical type (as posted above): 98.32% ag, 0.68% cu, 0.60 au, 0.41% pb

c) new style (Thompson 304): 98.86% ag, 0.72% bi, 0.42% pb

D

[David Soknacki]

... and lastly, let me add NGC's comment about attribution, that just came in:

quote

There is a great deal of subjectivity when it comes to distinguish between official and imitative Athenian tetradrachms. I suspect that if you surveyed ten specialists you would probably get a split of opinion right down the middle. Unfortunately, there is no hard evidence to support either position. Having said that, I am of the opinion that the combination of the somewhat odd portrait style and the low weight (a full gram below a "typical" weight) casts sufficient doubt on its being a product of Athens. An acceptable die axis and correct purity/metal content would not, necessarily, have any bearing on a determination since there is nothing to preclude a mint outside of Athens producing an imitative coin in that way.

I understand that we may not agree on this identification, but I hope this note at least explains our perspective.

unquote

 

[kapphnwn]

I do not see anything that would suggest that the owl posted by @David Soknacki is anything but Athenian. To me it looks like a  Flament Group II.35 obverse.  This should place it midway in the 454-404 BC Athenian Mass Coinage series. I do have to take note of the principle argument being the weight. It is of some concern and as there is not a proper die study for this immense coinage this debate is unresolvable. However on balance I do believe the coin is Athenian. I have one as well

Tetradrachm of Athens circa 440-430 BC Obv Helmeted head of Athena right. Rv Owl walking right head facing. Flament Group II 13/b  17.08 grms 23 mm Die axis 8 Photo by W. Hansen

Flament is not a die study though he does illustrate and catalogue  the mass coinage in greater detail than what can be found anywhere else. 

[robinjojo]

There's no question in my mind that the OP owl is indeed of Athenian origin and a very nice example.  I think NGC attributed the coin to an eastern source due to the existence of some imitative owls from the Levant that are very good copies of the classical standardized owls from Athens.  That, plus the lower weight is probably why they designated the owl the way they did.  

The imitative owls tend to be quite distinctive in style, and their weights, depending on where and when they were minted can vary from the Attic standard down to Rhodian and shekel weight standards.  For example:

Athens , Levant imitation owl (Gaza?), late 5th-mid-4th centuries BC.

16.8 grams

Athens, possibly Gaza owl, late 5th-mid-4th centuries.  Aramaic graffiti reverse.

17.1 grams

 

Athens, tetradrachm, Babylonia imitation, 4th century BC.   Purchased from Imperial Coins in the 1990s.

17.2 grams

 

Athens, eastern imitation owl, after 449 BC, Levant/Egypt.  Found near Nablus.

15.8 grams

 

Athens, eastern imitation owl, 4th century BC.  From Roma E-Sale 63, lot 156.  Apparent test cut on edge.

14.25 grams

 

Just to add a little perspective on Athenian owl weight, there's a dark (horn silver) archaic owl of very low weight, purchased from Dr. Busso Peus Nachf. a couple of years ago.  For a while I thought this coin, due to being in shekel weight range, is an imitation.  But now I am inclined to classify this crude archaic owl to Athens, Seltman Group E (as it was classified in the online listing), the "Paeonia Mint".

Athens, archaic owl, c. 480BC.

Seltman Group E

13.03 grams

 

Edited September 5, 2022 by robinjojo

[antwerpen2306]

i think @kapphnwn is right. There can be a difference in weight for antique coins, a difference in the silver.... most important, I think, are the style and the performance of the coin. It is avery beautul coin.