Carat (purity)

For the unit of mass of the same name, see Carat (mass).

The karat or carat (symbol: K or kt) is a unit of purity for gold alloys.

Measure

Karat purity is measured as 24 times the purity by mass:

$X = 24\,\frac{M_g}{M_m}$

where

X is the karat rating of the material,

M_g is the mass of pure gold or platinum in the material, and

M_m is the total mass of the material.

Therefore, 24-Karat gold is fine (99.9% Au w/w), 18-Karat gold is 18 parts gold 6 parts another metal (forming an alloy), 12-Karat gold is 12 parts gold (12 parts another metal), and so forth.

In England, the Karat was divisible into four grains, and the grain was divisible into four quarts. For example, a gold alloy of $\tfrac{381}{384}$ 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The Karat system is increasingly being complemented or superseded by the millesimal fineness system, in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.

Derivation

"Karat" is a variant of "carat". First attested in English in the mid-15th century, the word "carat" came from Middle French carat, in turn from Italian carato, which came from Arabic qīrāṭ (قيراط), which came from Greek kerátion (κεράτιον) meaning carob seed (literally "small horn")^[1][2][3] (diminutive of κέρας - keras, "horn"^[4]) and was a unit of weight^[5] though it was not likely used to measure gold in classical times.^[1]

In 309 CE, Roman Emperor Constantine I began to mint a new gold coin solidus that was ¹⁄₇₂ of a libra (Roman pound) of gold^[6] equal to a mass of 24 siliqua, where each siliqua (or carat) was ¹⁄₁₇₂₈ of a libra.^[7] This is believed to be the origin of the value of the karat.^[8]

Terminology

22/22K - a quality mark indicating the purity of gold most popularly used in India. This purity was adapted and practiced by the big jewellers and was later passed to jewel smiths. The first 22 signifies the "Skin purity", the purity of the top layer of the gold jewelry, and the second 22 signifies that after melting purity of the gold jewellery will be 22-karat, or 91.67% of pure gold. This system is used to show consistency in the quality of the gold.

This practice was pioneered and introduced in the early mid-1980s by Nemichand Bamalwa & Sons of Kolkata, India, sparking a revolution^{[weasel words]} in India, as it forced jewellers to indicate correctly the after-melting purity. Heightened consumer awareness made it a most sought-after stamp or quality mark.

This symbol or stamp can be found in Asian countries like India, Sri Lanka, Pakistan, Bangladesh, Nepal, Yemen, and Persian Gulf countries.

International caratages of gold jewellery

Region[9]	Typical caratage (fineness)
Arabic countries, Far East (China, Hong Kong, Taiwan)	24-karat "Chuk Kam" (99.0% min)
Arabic countries, Bangladesh, India, Pakistan, and Sri Lanka	22-karat (91.6%)
Arabic countries in the Persian Gulf region	21-carat (87.5%), 18-karat (75.0%) in most Egypt
Europe - Southern / Portugal	19.2-karat (80.0%)
Europe - Southern / Mediterranean	18-karat (75.0%)
Europe - Northern Germany etc.	8- to 18-karat (33.3 - 75.0%)
Russia/former USSR	9- (37.5%) and 14-karat/old 583 and new 585 проба (58.5%)
United Kingdom	9-karat to 22 karat (37.5-91.6%)
United States	10-karat to 18-karat (41.7-75%)

Chuk Kam

Chuk Kam (足金) in Cantonese means pure gold, literally "exact gold". It is defined as 99.0% gold minimum with a 1.0% negative tolerance allowed.^[9][10] The quality of gold is guaranteed with a "Certificate of Gold" upon purchases in Hong Kong and Macau. The related term "千足" and "万足" meaning "thousand exact" and "ten thousand exact" is also used for purity of 99.9% and 99.99% respectively. This is because the impurity is at most 1 in 1,000 in the case of 99.9% or 1 in 10,000 for 99.99%.

United States of America

The USA Federal Trade Commission (FTC) has standardized the karat markings used within its boundaries for almost 7 decades now^[when?]. Under these regulations, items 10-karat or greater are to be stamped with either "K" or "Kt." Decimal markings are also an option under the CFTC regulations.

Under-karating is against the law in the United States of America. There are specific mandated consequences including fines, etc., based upon the severity of the infraction(s).

In addition, there is a set of tolerances to the required karat markings in the USA (always designated with a "K" and never a "C") depending upon the use of various soldering requirements when setting stones, mounting crowns, or creating prongs for 3 examples.^[11]

See also

Gold alloys Colored gold Electrum Rose gold Tumbaga White gold

Gold coin Platinum coin

Notes

1. ^ ^{a b} Harper, Douglas. "carat". Online Etymology Dictionary. http://www.etymonline.com/index.php?term=carat. 
2. ^ κεράτιον, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
3. ^ Walter W. Skeat (1888), An Etymological Dictionary of the English Language
4. ^ κέρας, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
5. ^ carat, Oxford Dictionaries
6. ^ Vagi, David L. (1999). Coinage and History of the Roman Empire. II: Coinage. Chicago: Fitzroy Dearborn. p. 100. ISBN 1-57958-316-4. http://books.google.com/books?id=pH9Ok9czKsMC&pg=PA100. Retrieved 18 November 2011. 
7. ^ Grierson, Philip (1968). Catalogue of the Byzantine Coins in the Dumbarton Oaks Collection and in the Whittemore Collection. 2: pt. 1. Washington, DC: Dumbarton Oaks. p. 8. ISBN 0-88402-024-X. http://books.google.com/books?id=l97WJwbuAWsC&pg=PA8. Retrieved 18 November 2011. 
8. ^ Turnbull, L. A.; Santamaria, L.; Martorell, T.; Rallo, J.; Hector, A. (2006). "Seed size variability: From carob to carats". Biology Letters 2 (3): 397–400. doi:10.1098/rsbl.2006.0476. PMC 1686184. PMID 17148413. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1686184.  edit
9. ^ ^{a b} World Gold Council (2003)
10. ^ Fallon, (2006)
11. ^ Title 16: Commercial Practices: PART 23—GUIDES FOR THE JEWELRY, PRECIOUS METALS, AND PEWTER INDUSTRIES

References

• Fallon, S. (2006) Hong Kong & Macau, 12th ed., Melbourne; London: Lonely Planet, ISBN 1-7405-9843-1
• New Scientist (2006) Did carob seeds allow shady diamond deals?, New Scientist magazine, 2550 (9 May), p. 20
• World Gold Council (2003) The Karatage System For Gold Jewellery, Online article accessed 28 August 2007