Troy weight
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Troy weight is a system of units of mass customarily used for precious metals, black powder, and gemstones.
Named after Troyes, France, the troy system of weights was known to exist in medieval times, at the celebrated fair at Troyes in northeastern France.
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Relationship to British coinage
The troy system was the basis for the pre-decimalisation British system of coinage introduced by Henry II of England, in which the penny was literally one pennyweight of silver. One pound sterling was equal to twenty shillings, with each shilling equal to twelve pennies. Thus, one pound sterling equals 240 pennyweights, or one troy pound of sterling silver.
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The troy ounce (ozt) is a unit of imperial measure. In the present day it is most commonly used to gauge the weight and therefore the price of precious metals. One troy ounce is equivalent to 31.1034768 grams. There are 32.1507466 troy oz in 1 kg. One troy ounce is equivalent to 1.09714 avoirdupois ounces.
The troy ounce is part of the troy weights system derived from the Roman monetary system. The Romans used bronze bars of varying weights as currency. An aes grave weighed equal to 1 pound. One twelfth of an aes grave was called an uncia, or in English an "ounce". Later standardization would change the ounce to 1/16 of a pound (the avoirdupois ounce), but the troy ounce, which is 1/12 of a troy pound (note that a troy pound is lighter than an avoirdupois pound), has been retained for the measure of precious metals. At 480 grains, the troy ounce is heavier than the avoirdupois ounce, which weighs 437.5 grains. A grain is 64.79891 milligrams (mg); hence one troy ounce is 31.1034768 grams (g) (exact by definition), about 10 percent more than the avoirdupois ounce, which is 28.349523125 g (exact).[1][2]
To maintain purity standards and common measures across time, the troy ounce was retained over the avoirdupois ounce in the weighing and pricing of gold, platinum, silver and gunpowder. Likewise, the grain, identical in both the troy and avoirdupois systems, is still used to measure arrow and arrowhead weights in archery along with projectile (bullet) and propellant (powder) weights in ballistics. The troy ounce and grain were also common to the apothecaries' system long used in medicine, but have been largely replaced by milligrams.[3]
[edit] History
The term "Troy" ounce probably originated in early middle ages as a weight unit used at fairs of Troyes, Champagne, France. The Troyes fair was one of the most famous trading places in the world (at the time) with merchants from many different countries attending -- see Champagne fairs. The name is therefore most likely not a reference of ancient Troy of Anatolia, ancient Greece, however it is interesting to note that the Roman pound was also divided into twelve equal ounces of (approx) 27.264g.[4]
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History
carob seed ~200 mg
barley grain ~65 mg
wheat grain ~50 mg
At least since antiquity, grains of wheat or barley were used by Mediterranean traders to define units of mass; along with other seeds, especially those of the carob tree. According to a longstanding tradition, 1 carat (the mass of a carob seed) was equivalent to the weight of 4 wheat grains or 3 barleycorns.[5] But since the weights of these seeds are highly variable, especially that of the cereals as a function of moisture, this is a convention more than an absolute law.[6]
The history of the modern troy grain can be traced back to a royal decree in 13th century England:
By consent of the whole Realm the King's Measure was made, so that an English Penny, which is called the Sterling, round without clipping, shall weigh Thirty-two Grains of Wheat dry in the midst of the Ear; Twenty-pence make an Ounce; and Twelve Ounces make a Pound.
—Henry III of England, [6]
The traditional reading of this text is that it refers to the troy pound, and that the reference to sterling pennies is purely symbolic. According to a more recent reading, however, the pound in question is the Tower pound, and it talks about the actual mass of real sterling pennies. The Tower pound, abolished in 1527, consisted of 12 ounces like the troy pound, but was 1⁄16 lighter. In any case, with both readings one needs to substitute 24 barley grains for the 32 wheat grains of the text, according to the general convention of a 4:3 equivalence, for it to make sense. The weight of the original sterling pennies was 22½ troy grains, or 24 "Tower grains" if the Tower pound was divided in the same way as the troy pound.[6] Regardless of which pound this text originally referred to, a (troy) ounce still equals 20×24 = 480(troy) grains, and a pound consists of 12×20×24 = 5760 grains.
Originally the troy pound was only "the pound of Pence, Spices, Confections, as of Electuaries", and the merchants used different standards, which had to be compatible with those used abroad.[6] One such standard, the avoirdupois pound, was later fixed officially at exactly 7000 troy grains. It consists of 16 avoirdupois ounces of 437½ troy grains each.[1]
