How to Calculate the Purity of Gold in Carats
The softness and malleability of gold make it ideal for fine metalwork and jewelry making, but pure gold is too soft to be durable pieces of ornament. To make jewelry harder and more resistant to scratching and bending, jewelers use gold alloys, mixtures of gold and harder metals. The most common alloys of gold are yellow gold (gold+copper) and white gold (gold+nickel or gold+palladium).
The higher the percentage of gold in an alloy, the more valuable the jewelry. Since the 1800s, goldsmiths have used the carat (or karat) scale to rate the purity of fineness of gold items. The carat scale ranges from 1 to 24, with 24 being the most pure, at least 99.9% or better. The spelling "karat" is common only the US and Canada, while the the spelling "carat" is standard in all English speaking countries. The abbreviation for carats or karats is kt or k without a period.
(The precious metal definition of carat should not be confused with the gemological definition of carat. Jewelers measure the weight of diamonds, emeralds, rubies, pearls, and other stones in units called carats. One carat is equal to 200 mg in this sense.)
Calculating CaratageThe caratage of a gold piece is calculated by dividing the total mass of gold in the object by the total mass of the object, and then multiplying this ratio by 24 to obtain a number between 1 and 24. This number is then rounded down to the nearest whole number. However, a carat value of 23.976 or higher can be rounded up to 24.
Example: A gold pendant weighs 5.4 grams and contains 4.85 grams of gold by weight. What is the carat of this piece?
First, the ratio of gold mass to total mass is 4.85/5.4 = 0.89815. Multiplying this number by 24 gives you (24)(0.89815) = 21.5556. This value must be rounded down to 21, therefore the pendant is 21k gold.
The purity of gold can also be expressed as a percent or part per thouand, aka the millesimal fineness system. In percentage terms, the pendant is 89.815% pure gold. In parts per thousand, the item has a millesimal fineness 898; in other words, it is 898/1000 pure gold.
Converting Carats to Percentage or Part per ThousandSince fineness of gold can also be expressed in percentages and parts per thousand, it is useful to convert carats to one of these two measures.
To convert carats to a percent, simply divide the carat value by 24 and convert the resulting decimal to a percent. For example, a piece of 18k gold is at least 75% pure since 18/24 = 0.75. A piece of 14k gold is at least 58.333% pure since 14/24 = 0.58333.
To convert carats to the millesimal fineness system, divide the carats by 24 and use the first three digits after the decimal point. For instance, a ring that is 22k gold has a millesimal fineness of at least 916 since 22/24 = 0.91667. Likewise, a necklace that is 15k gold has a millesimal fineness of at least 625 since 15/24 = 0.625.
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