RPC VIII is online!

@Al Kowsky, I assumed that my coin had been cleaned or dipped in some sort of commercial cleaner, in order to make it look the way it does. Since I doubt it looked that way when it came out of the ground! Your example of the Mexican coin (I have one somewhere that was issued for the 1968 Olympics!) certainly proves that it's possible for a billon coin with 25% silver or less to look like a silver coin when it's issued without applying any kind of silver wash or plating. However, we've all seen lots of later Roman billon coins with equally low percentages of silver content, and no silver wash -- whether because it was completely worn off or was never applied in the first place -- that wouldn't look like silver coins even if you dipped them 1000 times. Why are these Antioch tetradrachms different? What's the point of no return in terms of reducing silver fineness, beyond which one has to apply a silver wash to make the coin look like silver? Did the Romans have the technology to make a billon coin with as little as 10% silver (like the modern Mexican coin) look like a silver coin without applying anything?

Separately, I don't blame the seller for representing my coin as "AR" to me, because it was part of a lot of similar Antioch billon tetradrachms -- all Gordian III or later, and all looking like silver coins -- that he had purchased earlier this year at a CNG auction, in which the entire lot was described as "AR." Should CNG have identified my coin (and the others in the lot) as AR, even though my coin (like the others) is not only identified as "Debased silver" in RPC Online, but was identified as "billon" as long ago as 1899, when BMC 20 Syria was published? (I don't have access to Prieur, so I can't say how the metal of these coins is identified there.)

 

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As it was explained to me: The low silver/high copper blank would be soaked in an acid bath which would eat away the surface copper and leave the silver as a honeycomb network which would be smashed flat into a more or less continuous and thin layer of higher silver content than the core of the coin starting a fraction of a millimeter under that surface. I have heard this called 'pickling'. A little wear or corrosion would soon break through to where toning of the copper underneath would make the coin look darker than it did when the coin was struck. It would seem That the length and strength of the acid bath would vary making some coins have a few microns more silver surface than others but it all would have little or no effect on the amount of silver in the entire coin. I have no idea how many coins were treated in this manner. Compared to silver washing as done later, the surface enrichment process would require a higher amount of core silver than simple plating but the attachment of the silver to what was below would be stronger and not peel away in patches as is common on the silver washed coins. I do not have a link to offer with a scientific confirmation of what I have said but might suggest searching under 'surface enrichment' as a start.

I might add that the preferential dissolving of copper from the surface of low silver alloy coins when being cleaned after discovery in modern times might have a similar effect as doing this as part of the flan preparation process. I have no idea how to separate coins treated with the acid bath in antiquity before striking from this cleaned in this manner. It would seem quite possible that some coins got both treatments.

 

Thanks for the explanation. I would think that the technique used would have varied from place to place. For example, the tetradrachms I see that were minted in Antioch in the mid-3rd century certainly look way more silvery than the billon tetradrachms minted in Alexandria, no matter when they were issued.

 

I remember watching a video of it on youtube that I think was posted on here when the subject came up before, now I can't find it on here or youtube. Maybe someone else can.

 

Donna, Technically any silver-looking coin that is less than 50% silver should be classified as billon. To my knowledge tetradrachms from Antioch were never pickled as Doug described. Late 3rd century antoniniani & folles are another matter, most of them were most likely pickled. Tets of Septimius Severus from Antioch were 59% silver; Tets of Caracalla were 35% silver & would qualify as billon; Early Tets of Elagabalus were 32% silver & later issues were 19% silver; Tets of Gordian III varied from 35-24% silver; Tets of Philip I varied from 27-17% silver; Tets of Treboniannus Gallus reached a low point of 11% silver. The Tets of Nerva, AD 96-98, contained the most silver of all Antioch Tets at 89% .

 

Yes, I know about the < 50% silver definition of billon, although at some point the percentage drops so low, and/or the appearance becomes so coppery, that people just call them AE. Thanks for the specification of percentages by emperor. In any event, I take it that you're suggesting that CNG should have identified my coin, and the other similarly silver-looking coins in the same lot, as billon rather than as AR. I don't know why they didn't, assuming they looked up the coins and didn't simply identify them based entirely on their appearance.

 

Some of the "Old Time" collectors used to make a further distinction with base silver below 10%, calling it potin. Today the only coins I see labeled potin are ancient Gaulish & Celtic coins. The numismatic definition for billon also varies among experts; Kenneth Harl considers any coin below 25% silver billon. CNG should have labeled your coin billon & not silver. Your coin has a fineness of 17-18%. The coins of Philip I that have the highest fineness are the one marked MON VRB, at 27%.

 

Can you post an image of a late Roman antoninianus or follis that was pickled rather than washed with added silver? I have one that I suspect could have been but the vast majority seem to have been washed.
Aurelian

 

It's always entertaining to search CT and find out whether anyone has posted on topic "X" - invariably, I find more than one thread. I searched today for "Nisibis" and "Otacilia" thinking it unlikely that this coin would be found - but the hits were plentiful, there were threads from @Parthicus (here), @Valentinian (here), @Roman Collector (here)...and a search for "Nisibis" even more overwhelming in its contents - including this recent thread on RPC VIII (Thanks, @Roman Collector) . Here is a late addition to the thread, this coin is RPC VIII ID 2575

Roman Provincial, Mesopotamia, Nisibis, Otacilia Severa, AD 244-249, Bronze Æ
Obv: MAP ΩTAKIΛ CЄOVHPAN CЄB, draped bust right, wearing stephane, set on crescent
Rev: IOV CЄΠ KOΛΩ NЄCIBI MHT, tetrastyle temple containing statue of city goddess seated facing; above her head, ram (Aries) leaping right; below, river god Mygdonius swimming right.

Anyone interested in reading more on pickling, silver plating and ancient coins - I recommend this article (summary notes and additional reference shared in this thread):

Susan La Niece (1990) Silver Plating on Copper, Bronze and Brass. The Antiquaries Journal, 70(1), 102-114. doi:10.1017/S0003581500070335.

My understanding is that the leading candidate for all silver washed coins of the late Roman period including this one is the "pickling" process described by @dougsmit - also called "depletion silvering". "This pickling, followed by working, results in an almost continuous and consolidated thin skin of silver."

Probus, Æ Radiate, Lugdunum, AD 278-279
Obv: IMP C PROBVS PF AVG, Buste radié et cuirassé de Probus à droite vu de 3/4 en avant. (Code buste Bastien : B)
Rev: VIRTVS AVG , Em. 6, 4e off., Roma debout à gauche, habillée en amazone, tenant une victoire de la main droite, une lance et un bouclier de la main gauche, in exergue IIII (Type B)
Rareté : C, FAC, Bastien: 24 ex.
Ref: RIC V Probus 112 Bastien 299

 

Great news!

I think mine was probably over-cleaned and re-patinated. 29mm at 14g. The patina is mostly grey-bluish that I've never seen before.

 

Experiments have shown that a copper-silver alloy with c. 15% silver and maybe even as low as 10% silver, or more will, when struck, have the silver squeeze to the surface (because it is more malleable), giving the appearance, at least for a while in circulation, of a silver coin. Higher percentages of silver "will permit a reasonably good silver surface to be obtained in the normal course of minting" [Lawrence Cope, page 270, "Surface-silvered ancient coins" in Methods of Chemical and Metallurgical Investigation of Ancient Coinage, RNS Special Publication 8]. The "surfacing-silvering" techniques used under Aurelian and later emperors, whatever they were, that were used for coins with a lower concentration of silver, were not necessary for Syrian tetradrachms or Roman coins of the early and mid-third century.