Troy & Tower Weights Pennyweights and Carats

Troy weights were first used in England in the 1400s.

In 1527, the troy ounce became the official standard measurement for gold and silver in Britain, and the US followed suit in 1828.

Prior to the troy ounce, the tower pound and tower ounce (based on a standard scale stored at the Tower of London) were used to measure the purity of monetary pounds, ounces and pennyweights from about 800AD.


One Tower Pound = 240 Tower Pennyweights = 349.914114 grams = 5400 barleycorns or 7680 (wheat) grains

One Tower Ounce = 20 Tower Pennyweights = 29.1595095 grams = 450 barleycorns or 640 (wheat) grains

One Tower Pennyweight = 1.457975475 grams = 22.5 barleycorns or 32 (wheat) grains

But by 1527, the pennyweight no longer bore any semblance to the silver content of the English penny.

Today all coins consist of copper.

In the Troy system, 

One Troy Pound = 240 Pennyweights = 373.241722 grams = 5760 grains
One Troy Ounce = 20 Pennyweights = 31.1034768 grams = 480 grains
One Troy Pennyweight = 1.55517384 grams = 24 grains

Note, one Grain (or barleycorn) =  64.79891 Milligrams


While troy weights were used for precious metals, avoirdupois weights were used for everything else.

One Avoirdupois Pound = 16 ounces = 453.59237 grams = 7000 grains
One Avoirdupois Ounce = 28.349523125 grams = 437.5 grains


Now for Carats

When weighing diamonds, the modern carat weighs 200 milligrams.

It started back in 300 AD in Rome when, until about 1000 AD in Constantinople (today Istanbul), the solidus gold coin that was issued to soldiers (known as the soldo in Italy and the sou in France), contained about 4.536 grams of gold, and was equal to 24 carats or "carob seeds". A typical carob seed thus weighed 189 milligrams.

Later when measuring purity in gold coins, carats referred to level of fineness. 24 carat meant 99.9% fine and 1 carat became equal to 4.1666% of the whole.


Therefore

23 carat = 95.8333% fine
22 carat = 91.6666% fine
21 carat = 87.5% fine
20 carat = 83.3333% fine etc

In England the carat could be divided into four grains, and the grain could be divided into four quarts.
So a gold alloy of fineness 99.2 per cent purity could be described as being 23-carat, 3-grain, 1-quart gold
where 3-grain (12/16ths) and 1-quart (1/16th) equalled 13/16 of 4.1666% or 3.3853625%,
and when added to 95.83%, equalled 99.2% rounded.

Today in the US the word for fineness is normally spelt as "karat", to differentiate it from "carat" meaning weight.

** End notes