The history and the metallurgy associated with the debasement of Roman silver coins constitutes one of the more fascinating aspects of ancient numismatics.

Roman silver coins were remarkably pure from the early days of the Roman republic to early Roman imperial times. However, beginning in approximately 70 C.E., a gradual reduction of the silver content began, from about 96 percent to 92 percent silver. Silver content was gradually reduced in the second century C.E. until the point was reached where special production methods were required in order to give the coins of low silver content an appearance of silver.

There has been extensive speculation and unfortunately much misinformation about the true nature of the silver coatings used on coins of this period. An analysis of the silver coating (on an AE3 of Constantine the Great minted in Siscia in 326—7 C.E. ) using energy dispersive X ray microanalysis reveals that the silver coating is actually 81 percent silver, 6 percent tin, 5 percent lead, and 8 percent chlorine. None of the coins of this period has a coating of pure silver. The proportion of tin and lead is sometimes much higher and it appears that some mints may have used different formulas.

For years, the question remained as to how these coatings were produced. Some argued that the copper blanks were simply dropped into pots of molten silver, tin & lead. However, the blanks would float on such a mixture and thus every one would have to be dipped one at a time. In addition, this would give a relatively thick coating that is quite different from the coating observed on surviving coins. In addition, actual experiments in modern times could not duplicate the coating by dipping the blanks in a molten mixture.

Modern research has indicated that these coins were probably coated by using a mixture of the chloride salts of silver, lead, and tin (Cope, 1972). All three such salts were available to the ancients from mineral deposits. The salts of these metals have a melting point much lower than the pure metal, and in addition, the blanks would sink in the mixture. The blanks were allowed to remain in the mixture for a period of time during which there would be diffusion of the silver, lead and tin into the copper until the coins acquired a silvery appearance. After removal from the mixture, the blanks were then struck between dies. The coating produced is very thin and experiments in modern times have been able to duplicate the coating by using the above process.

The silver wash method differed from the blanching method of silvering in that the blanching method produced a coating that was derived from internal strands of silver. The blanching method produced a coating that was more durable than the silver wash. Both the blanching method and the silver wash method were procedures used by the official minting authorities. (The blanching method will give good results only when the total silver content is greater than about 25 percent.)

The silver wash method differed from the fouree method in that the fouree method produced a coating that was far thicker, typically in the range of 100 microns thick, while the silver wash method produced a coating in the range of 10 microns thick. The fouree method, with one major exception, was the method used by counterfeiters.

The silver wash method was used on debased antoniniani, folles and smaller bronze coins during the period of 274 to 348 C.E. Following this period, imperial Rome made no attempt to give the coins a silvery appearance.

ADDENDUM: Dated December 14, 2001:

The most widely accepted explanation today is that the flans were "pickled" in a mildly acidic solution, whichcaused an enrichment of the surface. This method has the advantage of simply incorporating the correct amount of silver in the alloy, and then drawing it out via the solution. Tests have shown that some silver remains within the core of the coin; the amount of time the flan spent in the solution determined how much of the silver content of the alloy was moved to the surface. At least from the reform of Aurelian on, this seems to have been the method used; earlier, the chlorides method seems to have been what was used. Hence, it is much less common to find coins earlier than Aurelian with silvering intact.

Steve Coulter

APS-ANA-ANS-SPMC

References: Cope, L.R., 1972. Surface-silvered ancient coins, pp 261-278. Methods of Chemical and metallurgical Investigation of Ancient Coinage, edited by E.T. Hall and D.M. Metcalf, Royal Numismatice Society Special Publication No. 8. London.