The fineness of a precious metal object (coin, bar, jewelry, etc.) represents the weight of fine metal therein, in proportion to the total weight which includes alloying base metals and any impurities. Alloy metals are added to increase hardness and durability of coins and jewelry, alter colors, decrease the cost per weight, or avoid the cost of high-purity refinement. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. Sterling silver contains 92.5% silver and 7.5%, by mass, of other metals, usually copper.
Various ways of expressing fineness have been used and two remain in common use: millesimal fineness expressed in units of parts per 1,000 and karats used only for gold. Karats measure the parts per 24, so that 18 karat = 18⁄24 = 75% and 24 karat gold is considered 100% gold.
Millesimal fineness is a system of denoting the purity of platinum, gold and silver alloys by parts per thousand of pure metal by mass in the alloy. For example, an alloy containing 75% gold is denoted as "750". Many European countries use decimal hallmark stamps (i.e. '585', '750', etc.) rather than '14K', '18K', etc., which is used in the United Kingdom and United States.
It is an extension of the older karat system of denoting the purity of gold by fractions of 24, such as "18 karat" for an alloy with 75% (18 parts per 24) pure gold by mass.
The millesimal fineness is usually rounded to a three figure number, particularly where used as a hallmark, and the fineness may vary slightly from the traditional versions of purity.
Here are the most common millesimal finenesses used for precious metals and the most common terms associated with them.
- 999.5: what most dealers would buy as if 100% pure; the most common purity for platinum bullion coins and bars
- 999—three nines fine
- 950: the most common purity for platinum jewelry
- 900—one nine fine
- 999.999—six nines fine: the purest gold ever produced. Refined by the Perth Mint in 1957.
- 999.99—five nines fine: the purest type of gold currently produced; the Royal Canadian Mint regularly produces commemorative coins in this fineness, including the world's largest at 100 kg.
- 999.9—four nines fine: e.g., ordinary Canadian Gold Maple Leaf and American Buffalo coins
- 999—24 karat, also occasionally known as three nines fine: e.g., Chinese Gold Panda coins
- 995: the minimum allowed in Good Delivery gold bars
- 990—two nines fine
- 986—Ducat fineness: formerly used by Venetian and Holy Roman Empire mints; still in use in Austria and Hungary
- 958.3—23 karat
- 916—22 karat: historically the most widely used fineness for gold bullion coins; currently used for British Sovereigns, South African Krugerrands and American Gold Eagles
- 900—one nine fine: mostly used in Latin Monetary Union mintage (e.g. French and Swiss "Napoleon coin" 20 francs)
- 834—20 karat
- 750—18 karat
- 625—15 karat
- 585—14 karat
- 417—10 karat
- 376—9 karat
- 333—8 karat: minimum standard for gold in Germany after 1884
- 999.99—five nines fine: The purest silver ever produced. This was achieved by the Royal Silver Company of Bolivia.
- 999.9—four nines fine: ultra-fine silver used by the Royal Canadian Mint for their Silver Maple Leaf and other silver coins
- 999—fine silver or three nines fine: used in Good Delivery bullion bars and most current silver bullion coins
- 980: common standard used in Mexico ca. 1930–45
- 958: ( 23⁄24) Britannia silver
- 950: French 1st Standard
- 925: ( 37⁄40) Sterling silver equivalent to "plata primera de ley" in Spain (Plata seconda de lay = 800)
- 917: a standard used for the minting of Indian silver (rupees), during the British raj
- 916.66— 11⁄12: the standard established by Maryland state law in 1814 for all silver sold in Baltimore. Last used in 1843.
- 900—one nine fine or 90% silver: e.g., all 1792–1964 U.S. silver coins
- 835: a standard predominantly used in Germany after 1884, and for the minting of coins in countries of the Latin Monetary Union
- 833: ( 5⁄6) a common standard for continental silver especially among the Dutch, Swedish, and Germans
- 830: a common standard used in older Scandinavian silver
- 800: the minimum standard for silver in Germany after 1884; Egyptian silver; Canadian silver circulating coinage from 1920-1966/7
- 750: an uncommon silver standard found in older German, Swiss and Austro-Hungarian silver
- 720: e.g., many Mexican and Dutch silver coins
- 500: Standard used for making British coinage after 1920
The karat (not carat as a unit of mass; symbol: K or kt; US) or carat (symbol: C) is a fractional measure of purity for gold alloys, in parts fine per 24 parts whole. The karat system is a standard adopted by US federal law.
- K is the karat rating of the material,
- Mg is the mass of pure gold in the alloy, and
- Mm is the total mass of the material.
Therefore, 24-karat gold is pure (while 100% purity is unattainable, this designation is permitted in commerce for 99.95% purity), 18-karat gold is 18 parts gold, 6 parts another metal (forming an alloy with 75% gold), 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 127⁄128 fineness (that is, 99.2% purity) could have been described as being 23-karat, 3-grain, 1-quart gold.
The karat fractional system is increasingly being complemented or superseded by the millesimal system, described above.
Conversion between percentage of pure gold and karats:
- 58.33–62.50% = 14K (acclaimed 58.33%)
- 75.00–79.16% = 18K (acclaimed 75.00%)
- 91.66–95.83% = 22K (acclaimed 91.66%)
- 95.83–99.95% = 23K (acclaimed 95.83%)
- 99.95–100% = 24K (acclaimed 99.99%)
However, this system of calculation gives only the mass of pure gold contained in an alloy. The term 18-karat gold means that the alloy's mass consists of 75% of gold and 25% of alloy(s). The quantity of gold by volume in a less-than-24-karat gold alloy differs according to the alloy(s) used. For example, knowing that standard 18-karat yellow gold consists of 75% gold, 12.5% silver and the remaining 12.5% of copper (all by mass), the volume of pure gold in this alloy will be 60% since gold is much denser than the other metals used: 19.32 g/cm3 for gold, 10.49 g/cm3 for silver and 8.96 g/cm3 for copper.
This formula gives the amount of gold in cubic centimeters or in milliliters in an alloy:
- VAu is the volume of gold in cubic centimeters or in milliliters,
- Ma is the total mass of the alloy in grams, and
- kt is the karat purity of the alloy.
To have the percentage of the volume of gold in an alloy, divide the volume of gold in cubic centimetres or in millilitres by the total volume of the alloy in cubic centimetres or in millilitres.
For 10-carat gold, the gold volume in the alloy represents about 26% of the total volume for standard yellow gold. Talking about purity according to mass could lead to some misunderstandings; for many people, purity means volume.
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 derived either from Italian carato or Medieval Latin carratus. These were borrowed into Medieval Europe from the Arabic 'qīrāṭ' meaning "fruit of the carob tree", also "weight of 4 grains", (قيراط) and was a unit of mass though it was probably not used to measure gold in classical times. The Arabic term ultimately originates from the Greek 'kerátion' (κεράτιον) meaning carob seed (literally "small horn") (diminutive of κέρας – keras, "horn").
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 equal to a mass of 24 siliquae, where each siliqua (or carat) was 1⁄1728 of a libra. This is believed to be the origin of the value of the karat.
While there are many methods of detecting fake precious metals, there are realistically only two options available for verifying the marked fineness of metal as being reasonably accurate: assaying the metal (which requires destroying it), or using X-ray fluorescence (XRF). XRF will only measure the outermost portion of the piece of metal, so it may get fooled by thick plating.
This becomes a concern because it would be possible for an unscrupulous refiner to produce precious metals bars that are slightly less pure than what they mark the bar. A refiner doing $1 billion of business each year that marked .980 pure bars as .999 fine would make about an extra $20 million in profit. In the United States, the actual purity of gold articles must be no less than .003 less than the marked purity (e.g. .996 fine for gold marked .999 fine), and the actual purity of silver articles must be no less than .004 less than the marked purity.
A piece of alloy metal containing a precious metal may also have the weight of its precious component referred to as its fine weight. For example, 1 troy ounce of 18 karat gold (which is 75% gold) may be said to have a fine weight of 0.75 troy ounces.
Troy mass of silver content
Fineness of silver in Britain was traditionally expressed as the mass of silver expressed in troy ounces and pennyweights ( 1⁄20 troy ounce), in one troy pound (12 troy ounces) of the resulting alloy. Britannia silver has a fineness of 11 troy ounces, 10 pennyweights, or about 95.833% silver, whereas sterling silver has a fineness of 11 troy ounces, 2 pennyweights, or exactly 92.5% silver.
- Colored gold
- Gold as an investment
- Gold coin
- Platinum coin
- Silver as an investment
- Silver coin
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