The Troy Ounce

The set of brass troy ounce weights shown here was provided to the Goldsmiths’ Company in 1588. It was one of 58 sets that were copies of a new standard legalized by royal proclamation and distributed to various towns throughout the country.

Brass weights continued to be used in the vaults of the London bullion market until quite recently, though they have now been superseded by stainless steel. The main problem with brass weights is that their weight increases with time due to tarnishing.

Although there is some uncertainty about the origin of the troy ounce, it is usually considered to be named after the weights used in the famous mediaeval fair in the market town of Troyes, which is located on the river Seine, some 150 km south-east of Paris.

The troy ounce and its relatives, the grain and the pennyweight, have been used for weighing precious metals in England since the 12th century or earlier. The smallest measure was the grain, supposedly equal to the weight of a grain of wheat from the middle of the ear. The relationships between the three units were (and are):

• 1 pennyweight = 24 grains

• 1 troy ounce = 20 pennyweights

In 1527, the verdict of the Trial of the Pyx (the annual test of the country’s coinage) during the reign of Henry VIII was a key moment in the history of the troy ounce in England. It included a ruling that the basis of the country’s weights and measures for gold and silver would change from the previously used ‘towre’ pound to the troy pound of 12 troy ounces.

With the advent of the metric system in the early 19th century, the definition of the troy ounce was agreed as 1 troy ounce = 31.1034768 grams (exactly). It is worth noting that the conversion factor traditionally used in the London bullion market for the number of troy ounces in a kilogram (calculated from the reciprocal of this figure) is an approximation, namely 1 kg = 32.1507465 troy ounces.