Featured Amalgam versus Wash in Debasement

Discussion in 'Ancient Coins' started by kevin McGonigal, Dec 21, 2019.

  1. kevin McGonigal

    kevin McGonigal Well-Known Member

    As all of us know the Roman Empire coinage underwent a great, some may say catastrophic, debasement in the mid to later part of the Third Century. For a while the Roman mints did a pretty good job of disguising the debasement by some kind of enrichment process that left the surfaces of their silver coinage looking like the earlier silver. The silver of the double denarius of Gordian III, circa 242 AD, was under the 50% fineness grade but it still looked like a silver coin. By the beginning of the reign of Valerian in circa 253 AD it appeared that the Romans were still using an amalgam of silver and copper, that is a heated alloy of the two metals with copper so predominating, that coins of this period often have a copper tinge to them when first minted and then turn a kind of dark, dirty gray in color. But at some point, I am guessing here, but around 260 AD, the Roman mints seemed to have abandoned their coins of a base amalgam and gone to making the coins of essentially copper with a thin silver wash over the copper. I don't like to use the term "plate" because the plated fourees of earlier period were actually thick enough to remain silver in appearance for a considerable time after being minted, which was not the case of the washed or enriched coins. By the reign of Aurelian they seemed to have gotten to the point where they could apply some kind of silver wash that gave the coin the appearance, a very transitory appearance, of decent silver. From Probus on through the early reign of Diocletian the double denarius actually looks like decent silver, again, for a while.

    What I am wondering is if we can narrow down to a few years or so when the Romans changed from making their double denarii from a debased amalgam to an enriched wash treatment of copper coins. I have below a few coins of the period starting off with two coins of Valerian from ca. 255 AD. They seem to be an alloy of heavily debased silver, but not just a silver wash over copper. The coin to their right is one of Salonina, wife of Gallienus, probably from a few years later. It seems to be a silver washed coin rather than one of a debased alloy. The second row starts of with one of the coins of Gallienus which appears to me to be an alloy, not a wash while the next coin seems to be a coin of Gallienus which could be either, or both, heavily debased and thinly washed, a kind of transition coin, while the coin next to the right, one of Postumus IMG_1239[2989]Amalgam obv.jpg IMG_1240[2987]Amalgam rev.jpg , seems to be in the same category as one that perhaps was heavily debased alloy with a silver wash over that. Lastly are the coins of Aurelian, Probus and Diocletian where the Romans seem to have gotten the enrichment or washed technique down pretty well. The last coin, of Diocletian, just before his reform and the appearance of the silvered follis is done so well it could have passed as a solid coin of fine silver.

    Anyway I wonder if members here have any thoughts on the process, when it occurred and the techniques used on the silvered coins of this period. Thanks for any input or interest that you can share.
     
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  3. ancient coin hunter

    ancient coin hunter 3rd Century Usurper

    I've often wondered about the exact process, seems to have been a very creative solution to the financial problems of the Empire. Issue jillions of coins of little to no intrinsic value and hope that inflation would somehow be kept at bay. An interesting aside seems to be that Postumus was still striking coins with a silver appearance after the central Empire had switched to the new process. Here's an example of a Postumus containing 19% silver.

    Postumus as St. Nick? Here's it is, complete with rosy cheeks, in keeping with the season. Happy Holidays to all!

    AR Antoninianus

    Obverse: IMP C POSTVMVS PF AVG
    Radiate, draped, cuirassed but right

    Reverse: SERAPI COMITI AVG
    Serapis standing left, raising hand and holding sceptre

    Year: 267 C.E.

    Reference: RSC 360a, Sear 10991

    Mint: Trier

    postumus3.jpg

    postumus4.jpg
     
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  4. Colonialjohn

    Colonialjohn Active Member

    See my book on Amazon Books = Forgotten Coins. Its not about Ancients but INDIRECTLY the processes are discussed of silver plating and mercury amalgamation. Go with the $3 Kindle version or review the Table of Contents. I learned along time ago you Ancients heads stay in your own world but at times have VERY VALUABLE information on provenance discussions using metallurgical analysis such as XRF, SEM.EDS and Isotopic analysis. In wring this book it was critical to spend several years reviewing papers of the Ancient world and then to examine how silver wash and plating has advanced to eventually silver electrodeposition in the 19thC..
    John Lorenzo
    Numismatist
    United States
     
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  5. Finn235

    Finn235 Well-Known Member

    I highly doubt the process happened everywhere all at once, or even consistently in the same place and time.

    Here is a Gallienus from a western (Cologne?) mint from the late 250s, still bright flashy silver. If I didn't know better, I would assume this was minimum 40% fine.
    Gallienus Germanicvs Max V.jpg

    Valerian II from Antioch, also late 250s - enough silver to ward off corrosion but otherwise this looks like a high grade "aurelianus" of 4.75%
    Valerian II Victoria Part.jpg

    This sole reign Gallienus looks like it is struck in solid billon, not washed bronze
    Gallienus victoria avg.jpg

    Ditto with this late reign SAECVLARES issue from Antioch, mid to late 260s
    Gallienus saeculares avg.jpg
     
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  6. kevin McGonigal

    kevin McGonigal Well-Known Member

    A very good book on this subject is "Coinage in the Roman Economy by Kenneth W. Harl, where between pages 130-145 he lists some of the emperors of this period and the fineness of silver in their coinage. Postumus started off well with his coins in 260 coming in at 20% fine silver but then they, too, went down steadily till those of 269 were down to 4.8 %. Gallienus went from 15.4 % fine in 262 to 8.7 % in 266 and finally in 267 to 6%. Claudius II (Gothicus) started off at 3.16 % in 268 and the next year down 1.71. At that point the double denarius must have appeared as pure bronze to the public. No wonder Aurelian's looked good at 4.5 % compared to that at least until the silver coating quickly wore off.
     
  7. Sulla80

    Sulla80 Well-Known Member

    While I will claim interest, it is as a collector, and non-expert that I share the following. Other references, additions, corrections always appreciated.

    Timing: Here's a graph of the table in Harl (Ch. 6 - The Great Debasement and Reform A.D. 193-305 Table 6.2 p 130)
    Harl Ant Debasement.JPG
    And another graph from G. C. Haines 1941 illustrating data from J. Hammer (1908) the decline in silver content over time - more recent studies and techniques for analyzing the silver content may have improved on this view.
    Haines Decline of Silver.JPG
    Here is a prettier graph with a less clear source of data.

    Process: There were multiple processes potentially used to make plated counterfeit and official coins throughout time. While there are several candidates for silver plating in ancient Rome, there is uncertainty about the process used for late Roman coins. Here are several candidates (there are others mentioned in the references below):
    • "diffusion bonding" or "Sheffield plating" : silver copper sheets heated together reported in use for Roman Republican fourrée denarii, wrap copper flan in a silver foil and heat. This is considered too labor intensive to have been used at scale in late roman empire
    • "dipping in silver chloride" - Blanks were maintained hot, dipped in molten silver chlorides, and quenched in a bath
    • "mercury silvering", "amalgam plating", or "fire gilding" - a good way to get mercury poisoning - dissolve silver in mercury, then spread on the flan like peanut butter, then evaporate the mercury with heat, this technique is mentioned as a possibility for late roman coins.
    • "depletion silvering", "pickling" or "blanching" - take a copper-silver alloy - use citric acid or vinegar to leach out the copper and enrich silver content of the outer surface, then strike to produce a silver surface plating which seems to be a leading hypothesis for later Roman coins.
    Selected references for further reading:
    • Harl, K. W. (1996). Coinage in the Roman economy, 300 B.C. to A.D. 700. Baltimore: Johns Hopkins University Press
    • Haines, G. C. (1941). THE DECLINE AND FALL OF THE MONETARY SYSTEM OF AUGUSTUS. The Numismatic Chronicle and Journal of the Royal Numismatic Society, 1(1/2), 17-47
    • Susan La Niece (1990) Silver Plating on Copper, Bronze and Brass. The Antiquaries Journal, 70(1), 102-114. doi:10.1017/S0003581500070335
    • Cope, L. H. , (1974) “The metallurgical development of the Roman Imperial coinage during the first five centuries A.D.”, PhD Thesis, Liverpool Polytechnic, Liverpool John Moores University. doi:10.24377/LJMU.t.00005590
    • C. Vlachou, J.G. McDonnell, R.C. Janaway (2002) Experimental investigation of silvering in late Roman coinage, Mat. Res. Soc. Symp. Proc. Vol. 712
    • Pliny describes a technique for plating in Naturalis Historia book 34 ch. 48.
    • Keyser, P. (1995). GRECO-ROMAN ALCHEMY AND COINS OF IMITATION SILVER. American Journal of Numismatics (1989-), 7/8, 209-234
    Bolos of Mendez in 200 BC describes methods for producing false silver:
    "About the making of “uncoined”: the quicksilver from arsenic, or sandarach, as you prefer, cook it as usual, deposit it on copper or coppered iron, and it will be whitened. Whitened magnesia does the same thing, and transmuted arsenic, and cooked cadmia (ZnO?], and unfired sandarach and whitened pyrite, and psimuthion [lead acetate] cooked with sulfur."
    • Butcher, Kevin (2015) Debasement and the decline of Rome. In: Bland, Roger and Calomino, Dario, (eds.) Studies in ancient coinage in honor of Andrew Burnett. London: SPINK, pp. 181-205. ISBN 9781907427572
     
    Last edited: Dec 23, 2019
  8. kevin McGonigal

    kevin McGonigal Well-Known Member

    Thanks for that bibliography. I find reading material like that fascinating.
     
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