Well, we can't define it "exactly". Coin silver flatware was manufactured in America from early colonial times to just after the Civil War. The raw material was melted European coins of the time. They were about 90% pure silver. Compare this purity with that of sterling which is 92.5% pure silver.
We found other references to coin silver flatware that included purity levels as low as 80% pure silver. Therefore, it's difficult to tell the purity definitively.
Coin silver may have a silversmith stamp consisting of the first letter and last initial stamped on the piece. Sometimes the city of manufacture was also stamped on the piece. Hallmarks are not consistent which does not help matters. We understand that several books are available on the subject that might help identify a particular piece by its hallmark.
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