As I once said, I am interested in the metallurgy of coins.
Looking at various coin catalogs, one finds very odd finenesses, especially in silver, before about 1850 or so. While I understand people didn't think decimally back then and crude fractions were more common, even for denominations, some of the fractions themselves give me pause.
Crown gold at 11/12 and sterling silver at 37/40 aren't actually too hard to understand. But the American silver dollar had an original fineness of 1485/1664! And even worse it was based on worn Spanish coins which were slightly below standard, which appears to have been 43/48 (though sometimes listed as 65/72). Then in 1834, the gold fineness went from 11/12 to 116/129! Fortunately in 1837, the United States woke up to the idea of a simple 9/10!
The old Spanish finenesses were pretty strange from today's point of view for sure. 7/8 for gold isn't too bad, but 43/48 and 29/36 (smaller denominations) for silver make me wonder who thought them up.
Dutch finenesses were even more absurd: 67/75 for the larger silver coins, and 29/51 for the smaller ones. This was even after the denominations were decimalized in 1817! However, gold was a perfectly rational 9/10.
Most copper coins were pure copper, bad for wearing qualities but at least simple....
One thing I find interesting is trying to figure out the exact fraction when I can't find original source material. When a fineness is listed as .569 or .893 on Numista or in a coin catalog, it is an exercise of both the mind and fingers (using a calculator) trying to find the simplest fraction that rounds to that millesimal fineness! Sometimes I find source material that is helpful even if it doesn't give the exact answer - for example, Spain used units of 24ths of 12ths (in other words, 288ths).
Anyway, this rant isn't a complaint (After all, who would it be against?) but as a means of starting a discussion if anyone cares.
Remember that the use of systems of measurement based a decimal system is relatively new, and that bases other than 10 are still in use all around us, especially in the U.S.
Purity of gold has still not been decimalized, etc., etc.
I think some of the weird numbers came about when purity units eventually based on, say 16ths, got chipped away at (giving 32ds, 48ths, etc) because someone didn't want to take the larger step.
You can get closer to the original source material (which in many places would have been royal
decrees) by finding the numismatic references cited in the Numista listing. For instance, one of the places I collect is Duchy of Brabant. The DeWitte catalog (first printed in the late 19th century, and now available online), cites the ducal decrees (which specified denominations, mass and purity) and mint records, which have somehow been preserved back to the 14th century in some cases.
For the Kingdom of Poland, which I also collect, the Gumowski catalog (published in the 1950s) give the fineness in 16ths (and fractions thereof) for many issues back to at least the 16th century, again, presumaby from some other original sources.
I know that similar sources are available for French royal coinage, but I don;t know which catalog has this information.
I assume for the U.S. the purity is specified in the acts of congress that authorized the coinage.
Thanks for the reply. Yes, what you said makes sense. I think you're talking about the karat system of measuring gold purity in the US. I learned through research that karats used to be divided into grains (IIRC 1k = 4 grains) making the smallest unit of gold purity 1/96.
Thanks for the hint on numismatic resources. I've also found Google Books useful for historic stuff, though often they only show the immediate paragraph.
Yes, the specifications for US coins are mostly in acts of Congress, though there are some situations where the Mint was granted some flexibility, such as the bronze small cent 1864-1962 which was specified as 95% copper and 5% tin and zinc combined. Originally, it was 3% tin and 2% zinc; but as tin got expensive, the amount was gradually lowered over the years and was 4.9% zinc and 0.1% tin just before Congress changed the spec to eliminate tin.
