Carat (purity)

The carat (abbreviation ct or kt) is a measure of the purity of gold alloys, 24 carat being pure gold. In the United States and Canada, the spelling karat (abbreviation k or kt) is used.

Measure

As a measure of purity, one carat is 24 times the purity by mass:

X=24\,{\frac {M_{g}}{M_{m}}}

where

{\mathit {X}}

is the carat 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-carat gold is fine (99.9% Au w/w), 18-carat gold is 75% gold, 12-carat gold is 50% gold, and so forth.

Historically, in England the carat 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-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The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.

The most common carats used for gold in bullion, jewelry making and by goldsmiths are:

24 carat (millesimal fineness 999 or higher)
22 carat (millesimal fineness 916)
21 carat (millesimal fineness 875)
20 carat (millesimal fineness 833)
18 carat (millesimal fineness 750)
15 carat (millesimal fineness 625)
14 carat (millesimal fineness 585)
10 carat (millesimal fineness 417)
9 carat (millesimal fineness 375)
8 carat (millesimal fineness 333)
1 carat (millesimal fineness 042)

Derivation

The word carat is derived from the Greek kerátion (κεράτιoν), “fruit of the carob”, via Arabic qīrāṭ (قيراط) and Italian carato. Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight. (However, a 2006 study^[1] by Lindsay Turnbull and others found this not to be the case – carob seeds have as much variation in their weights as other seeds.^[2]) This was not the only reason. It is said that in order to keep regional buyers and sellers of gold honest, a potential customer could retrieve their own carob seeds on their way to the market, to check the tolerances of the seeds used by the merchant. If this precaution was not taken, the potential customer would be at the mercy of "2 sets of carob seeds". One set of "heavier" carob seeds would be used when buying from a customer (making the seller's gold appear to be less). Another, lighter set of carob seeds would be used when the merchant wanted to sell to a customer.

In the distant past, different countries each had their own carat, roughly equivalent to a carob seed. In the mid-16th century, the Karat was adopted as a measure of gold purity, roughly equivalent to the Roman siliqua ( ${\tfrac {1}{23}}$ of a golden solidus of Constantine I). As a measure of diamond weight, from 1575, the Greek measure was the equivalent of the Roman siliqua, which was ${\tfrac {1}{24}}$ of a golden solidus of Constantine; but was likely never used to measure the weight for gold.^[3]

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-carat, 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 in India as it forced jewellers to indicate correctly the after-melting purity, and heightened consumer awareness made it a most sought-after stamp or quality mark.

This symbol or stamp is very popular on the gold jewellery business in Asian countries like India, Sri Lanka, Pakistan, Bangladesh, Nepal, Yemen, and Persian Gulf countries.

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.^[4]^[5] The quality of gold is guaranteed with a "Certificate of Gold" upon purchases in Hong Kong and Macau.

In the United States of America

The USA Federal Trade Commission (FTC) has standardized the karat markings used within its boundaries for almost 7 decades now. 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).

Additionally, there are 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.^[6]

International caratages of gold jewellery

Region^[4]	Typical caratage (fineness)
Arabic countries, Far East (China, Hong Kong, Taiwan)	24 carat "Chuk Kam" (99.0% min)
Arabic countries, Bangladesh, India, Pakistan & Sri Lanka	22 carat (91.6%)
Arabic countries in the Persian Gulf region	21 carat (87.5%), 18 carat (75.0%) in most Egypt
Europe - Southern / Mediterranean	18 carat (75.0%)
Europe - Northern ]] etc.	8-18 carat (33.3 - 75.0%)
Russia / former USSR	9 (37.5%) and 14 carat / old 583 and new 585 проба (58.5%)
Portugal	19.2 carat (80.0%)
United Kingdom	9 carat to 18 karat (37.5-75%)
United States	10 carat to 18 karat (41.7-75%)

Notes

^ Turnbull et al. (2006)
^ New Scientist (2006) — review of Turnbull et al. (2006)
^ Harper, (2001)
^ ^a ^b World Gold Council (2003)
^ Fallon, (2006)
^ 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
Harper, D. (2001) "Carat", in: Online Etymological Dictionary, accessed 28 August 2007
New Scientist (2006) Did carob seeds allow shady diamond deals?, New Scientist magazine, 2550 (9 May), p. 20
Turnbull, L.A., Santamaria, L., Martorell, T., Rallo, J. and Hector, A. (2006) "Seed size variability: from carob to carats", Biology Letters, 2 (3: September 22), p. 397–400, DOI 10.1098/rsbl.2006.0476
World Gold Council (2003) The Karatage System For Gold Jewellery, Online article accessed 28 August 2007

[1] Turnbull et al. (2006)

[carob-2] New Scientist (2006) — review of Turnbull et al. (2006)

[etym-3] Harper, (2001)

[Goldorg-4] World Gold Council (2003)

[Fallon-5] Fallon, (2006)

[6] Title 16: Commercial Practices: PART 23—GUIDES FOR THE JEWELRY, PRECIOUS METALS, AND PEWTER INDUSTRIES

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