Comments on Corrosion of Ancient Coins
The following article originally appeared in Stack's Numismatic Review, 1946, #3 (July). We thank Stack's for contributing this article to the public domain in 1946.
The article by Mr. Earle R. Caley in the April number of the Numismatic Review is of special value on account of the warning that it gives to numismatists not to place overmuch reliance on tables of weights in classifying coins. The weight of such a coin is apt to be misleading, unless particulars of its condition are taken into consideration: even a specimen that has a perfectly good face may have lost some of its content, either by some internal corrosion or by cleaning, and this is especially dangerous in the case of a coin that is alloyed. Few Greek mints were capable of striking coins exactly to standard: when the official in charge of the Alexandrian mint in the time of Ptolemy II was ordered to strike some gold, which had to be of precise weight, he wrote a pathetic letter begging for outside help in a task which was beyond his ordinary resources; and no Greek silver coinage, except that of Athens, shows an approximation to the accuracy that would be required in a modern mint. For the purposes of local currency, this did not matter: a coin would then, as now, be accepted at its face value--i.e. at the value for which it was issued by the state authorities--in its home country, regardless of its exact weight: abroad, if it was not treated as bullion, its value would be determined by recognized rates of exchange, in which the credit of the issuing authority played a part, if we may judge from some ancient records.
The risks of loss of weight are especially notable in the Alexandrian series, to which Mr. Caley has drawn attention, partly because the silver coins from that mint are so heavily alloyed, but even more from the effect of the salts in Egyptian soil on some metals. Under certain conditions, copper coins buried in damp places in Egypt simply disintegrate: a parcel of Ptolemaic copper half-drachmas found at Abydos below Nile level showed the process in all stages: they came to me looking like lumps of greenish sand, and, while one or two had preserved their form fairly well, only with a coat of corrosion which could be split off, most broke up into irregular pieces, and some were nothing but green sand throughout, in a lump about double the size of the original coin. A mixed lot of coins from Naucratis proved equally hopeless for cleaning, silver and bronze alike: all fell to pieces when treated, except one Athenian tetradrachm. I have cleaned several thousands of Alexandrian tetradrachms of the Roman period by electrolysis, from hoards, rubbish-mounds, and casual finds, and, while those which had been buried in dry sand showed no reason to suspect loss of weight, many from the damper sites were caked with corrosion products; when these were removed, the surface of the coin often showed no signs of injury, but the interior was in the spongy condition described by Mr. Caley, and gave the impression that the copper had passed in some imperceptible way through the silver; in one hoard, which had been cleaned before it come to me, the surfaces of the coins were flaky, but this did not occur in any that were treated by electrolysis.
There is less risk of loss of weight from corrosion in the silver of Greece itself, as it was rarely alloyed, so far as has been discovered; the presence in some series of other metals than silver is probably due to imperfect refining at the mines. The Athenian silver, and the Persian, are well refined, but the Corinthian usually has more impurity; it would be interesting to investigate the question whether this was due to the Corinthian mint accepting the silver as it came to them from the northern regions, such as Paeonia, and coining it without any further refinement. When a Greek state wished to resort to coinage of inferior quality, the expedient usually adopted was plating; and a plated coin was more likely to suffer from external wear than from internal corrosion.