Measure
Carat purity is measured as 24 times the purity by mass:
where
- is the carat rating of the material,
- is the mass of pure gold or platinum in the material, and
- is the total mass of the material.
Therefore, 24-Carat gold is fine (99.9% Au w/w), 18-Carat gold is 18 parts gold 6 parts another metal (forming an alloy), 12-Carat gold is 12 parts gold (12 parts another metal), and so forth.
In England, the carat was divisible into four grains, and the grain was divisible into four quarts. For example, a gold alloy of fineness (that is, 99.2% purity) could have been described as being 23-carat, 3-grain, 1-quart gold.
The Carat system is increasingly being complemented or superseded by the millesimal fineness system, in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.
Read more about this topic: Carat (purity)
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