The
carat
(abbreviation ct) is a measure of the purity of gold and platinum
alloys. In the United States and Canada, the spelling karat is now
solely used for the measure of purity, while carat solely refers to
the measure of mass weight (see Carat). As a measure of purity, one
karat is purity by weight:
where X is the karat rating of the material,
Mg is the mass of pure gold or platinum in the material, and
Mm is the total mass of the material.
Therefore 24-karat gold is fully (99.99%) pure, 18-karat gold is 75%
pure, 12-karat gold is 50% pure, and so forth.
Historically, in England the karat 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-karat, 3-grain, 1-quart gold.
The karat system is increasingly being complemented or superseded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.
The most common carats used for gold in bullion, jewelery making and
goldsmithing are:
" 24 karat (millesimal fineness 999)
" 22 karat (millesimal fineness 916)
" 20 karat (millesimal fineness 833)
" 18 karat (millesimal fineness 750)
" 15 karat (millesimal fineness 625)
" 14 karat (millesimal fineness 585)
" 10 karat (millesimal fineness 417)
" 9 karat (millesimal fineness 375)
Courtesy
of: www.wikipedia.org