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Carat (purity)

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

Contents

Measure

As a measure of purity, one carat is \tfrac{1}{24} purity by mass:

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

where

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The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.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

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 by Lindsay Turnbull and others found this to not be the case – carob seeds have as much variation in their weights as other seeds.) 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}{24} 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.

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 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. 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 legislated and 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.

International caratages of gold jewellery

See also

Notes

  1. ^ Turnbull et al. (2006)
  2. ^ New Scientist (2006) — review of Turnbull et al. (2006)
  3. ^ Harper, (2001)
  4. ^ a b World Gold Council (2003)
  5. ^ Fallon, (2006)
  6. ^ Title 16: Commercial Practices: PART 23—GUIDES FOR THE JEWELRY, PRECIOUS METALS, AND PEWTER INDUSTRIES

References

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Terms
Carat (unit) · Carat (purity) · Finding · Millesimal fineness
Related topics: Body piercing · Fashion · Gemology · Metalworking · Wearable art
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