copper, by mass.

Various ways of expressing fineness have been used and two remain in common use: millesimal fineness expressed in units of parts per 1,000 and karats or carats used only for gold. Karats measure the parts per 24, so that 18 karat = 18/24 = 75% gold and 24 karat gold is considered 100% gold.

Millesimal fineness

Millesimal fineness is a system of denoting the purity of platinum, gold

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