Just picked this Genoan coin up a few weeks ago. The planchet flaw is unlike anything I've ever seen before – my best guess would be some sort of chemical impurity of the alloy (only 28.7% Ag), but otherwise I'm stumped. Any ideas?
It's a bad case of 'haymarking' which is essentially poor mixing of the metals in the alloy. Usually the fineness is effected by adding copper to silver, though the inadvertent addition of other metals cannot be ruled out. Haymarking is far more common on silver than gold, for which I have a theory that I believe is sound. The melting point of silver is 962C, whereas the melting point of copper is 1085C and that of gold 1064C. If you have a pot full of molten silver, it is necessary to raise the silver temperature over 123C higher in order to melt the added copper. Anything less than that and you will have inclusions of unmelted copper when the pot is emptied, which will then oxidise giving rise to black copper (II) oxide. When the metal is rolled, that will explain why you get this streaky effect on the surface as the inclusions are stretched out. The same applies to gold, but with a difference of only 20 degrees, it is likely that the pot will already be hot enough to melt any added copper. The only other variable here is the time taken to allow the alloy to fully mix which should be a known variable given the requirements for metal mixing would have been known for several thousand years. Whether it is due to the first and/or second reason, the main driver is likely to be cost - time is money and always has been. Given your coin has a fineness of only 28.7%, you would think that it would be easier to add silver to molten copper than the other way round, in which case you should never see any unmelted copper on a base silver coin as the latter metal would melt instantly, but that might not be how things were done in Genoa.
