Since 1282
coinage issued in England has gone on
trial by jury to determine that it is of the required
purity. A small sample of the coins issued by the
Royal Mint is set aside in a box called the
Pyx, and once a year brought out for
assay by freemen of the
Worshipful Company of Goldsmiths.
Originally the sample was of one coin per journeyweight, the weight of coins that could be struck (by hand) per day: 15 pounds troy of gold, and 60 pounds troy of silver. Today no silver nor gold is used in coins, and the numbers are 1 in some 5 to 20 thousand depending on the material of the coin.
King Edward I introduced the Trial of the Pyx from France, and it was held every three months until the reign of Edward VI. Formerly it was conducted in the Chapel of the Pyx in Westminster Abbey, which still holds remains of the test plates used as standards of purity. These were cut into irregular shapes and divided among the interested parties to ensure they could be matched for authenticity. Today it is held in the Hall of the Goldsmiths' Company in London, as they are an independent livery company, beholden neither to the Crown, which issued the gold, nor to the Master of the Mint, who made the coins and might keep parings of gold above the amount needed. (Safekeeping of money with the Goldsmiths gave rise to modern banks.)
These days of course the actual fineness, weight, and size of mass-produced coins are checked by machine, and there is no precious metal worth skimming off; but the Trial is still one of the great ceremonies of London, wreathed in high and ancient splendour.
The jury of freeman goldsmiths is presided over by an official called the Queen's Remembrancer on behalf of the Treasury. They open the pyx and count, weigh, and examine selected coins to ensure they fall within the remedy (tolerance) allowed. The coins of the pyx are then sent off for detailed scientific examination at the National Weights and Measures Laboratory, and returned for the jury to declare its verdict in writing to the Queen's Remembrancer.
The remedy allowed for gold coins was very small: a sovereign (pound coin) could vary by only 0.2 grains in weight and 0.2% in fineness.
C. Chamberlain, Guide to Numismatics, English Universities Press, 1960
www.legislation.hmso.gov.uk/si/si1998/19981764.htm
www.royalmint.com/about/trialofpyx.asp
www.royalmint.com/museum/trialplates.asp
www.thegoldsmiths.co.uk is unfortunately down today, but last time I looked it had splendid pictures of the ceremonial flummery.