The Maze of Medieval Mint Metrology in Flanders, France and England: Determining the Weight of the Marc de Troyes and the Tower Pound from the Economics of Counterfeiting, 1388 - 1469

In 1795, the French Revolutionary government, in establishing our modern metric system, also established the metric values for the historic European mint weights of the ancien re,gime era. If those mint-weights are undoubtedly valid for the 18th century, can we be certain that all had remained unchanged over the centuries; and in particular that these metric values, and thus the mathematical relationships between the various mint weights, are valid for the later Middle Ages? That is important if we are to measure with confidence the coinage outputs and relative coin values of various medieval European principalities, and thus their relative price movements. Furthermore, were the marcs de Troyes used in both France and the Low Countries the same mint weight? This question is still hotly contended because there are no surviving mint weights or coin dies for the medieval Low Countries, and none in France dated before 1483; and surviving coins do not provide unbiased random samples. In this paper, I seek to vindicate the modern metric equivalents by resorting to the mint accounts of 14th- and 15th- century Flanders and England, and in particular Flemish data on minting gold nobles that were coined as counterfeits of the widely-circulating English gold nobles; I have also used various ordinances that describe the weights of the nobles in terms of both the English Tower Pound and the Flemish marc de Troyes. With mathematical examples, I demonstrate that only if the modern metric equivalents of these mint weights were precisely valid for this period (1388-1469) would the Flemish mints have been successful, i.e. in gaining the substantial seigniorage profits so recorded: and successful in minting a noble close enough in weight and fineness to the English to "pass" undetected, except by those using well-calibrated scales, yet inferior enough in gold content to allow merchants selling gold bullion for these counterfeits to gain a profit (and compensate for their higher risks and transaction costs). Such evidence also proves that the Flemish and French marcs de Troyes had to be identical in this era. Indirectly, this paper also challenges some recent publications about the nature of medieval moneys and of the operation of Gresham's Law in this era.