Troy weight is a system of units of mass customarily used for precious metals, black powder, and gemstones.
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Units
The Troy
Named after Troyes France, the troy was known to exist in medieval times. It was not until 1758 that it was established as the standard unit from which other weights derived. The standard troy was adopted as being the weight of a cubic inch of distilled water, 252.458 grains, when the temperature was 62 degrees Fahrenheit and the barometric pressure 30 inches. (Wightman's Arithmetical Tables). Today the grain is 64.79891 milligrams but the grain in 1758 was the weight of a seed from the middle of an ear of barley.
Troy ounce
A troy ounce is 480 grains,[1] somewhat heavier than an avoirdupois ounce (437.5 grains). A grain is exactly 64.798 91 mg; hence one troy ounce is exactly 31.1034768 g, about 10 percent more than the avoirdupois ounce, which is exactly 28.349523125 g. The troy ounce is the only ounce used in the pricing of precious metals, such as gold, platinum, and silver. The grain, which is identical in both the troy and avoirdupois systems, is used to measure arrow and arrowhead weights in archery and bullets and powder weights in shooting. In troy weight, there are 12 ounces in a pound,[1] rather than 16 as in the more common avoirdupois system. The troy ounce may be abbreviated to ozt. In the normal pound that is used now in the United States, there are 14.58 troy ounces. The above figures apply to England and Wales. In Scotland the Incorporation of Goldsmiths of the City of Edinburgh used a system in multiples of sixteen (See Assay-Master's Accounts, 1681-1702, on loan from the Incorporation to the National Archives of Scotland): Thus there were 16 drops to the troy ounce, 16 ounces troy to the troy pound and 16 troy pounds to the troy stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for silver and gold.
Troy pound
A troy pound is 5760 grains (about 373.24 g, 12 troy ounces), while an avoirdupois pound is 7000 grains (about 453.59 g).
Conversions
| Unit | Grains | Grams |
|---|---|---|
| Pound (12 ounces) | 5760 | 373.241 721 6 |
| Ounce (20 pennyweights) | 480 | 31.103 476 8 |
| Pennyweight | 24 | 1.555 173 84 |
| Grain | 1 | 0.064 798 91 |
The Troy pound and ounce were also used in the Apothecaries' system, but with different further subdivisions.
Relationship to British coinage
The troy system was the basis for the pre-decimalisation British system of coinage introduced by King Henry II, in which the penny was literally one pennyweight of silver. A pound (pound sterling) consisted of 20 shillings, each of which consisted of 12 pennies. A pound sterling thus weighed 240 pennyweights, or a troy pound of sterling silver.
