Trial of the Pyx
The Trial of the Pyx is the procedure in the United Kingdom for ensuring that newly minted coins conform to required standards. Trials have been held from the twelfth century to the present day, normally once per calendar year; the form of the ceremony has been essentially the same since 1282 AD. They are trials in the full judicial sense, presided over by a judge with an expert jury of assayers. Trials are now held at the Hall of the Worshipful Company of Goldsmiths; formerly, they took place at the Palace of Westminster. Given modern production methods, it is unlikely that coins would not conform, but this has been a problem in the past—it was tempting for the Master of the Mint to steal precious metals.
In medieval times the Master of the Mint was ordered to save for trial one coin for every ten pounds of silver minted. A trial was normally conducted every three months.
There is also a Pyx Chapel (or Pyx Chamber) in Westminster Abbey, which was once used as secure storage for the Pyx and related articles.
The presiding judge is the Queen's Remembrancer (or King's Remembrancer when the sitting monarch is male), the Senior Master of the Queen's Bench. It is his or her responsibility to ensure that the trial is held in accordance with the law, and to deliver their final verdict to Her Majesty's Treasury. Where and when a trial is to take place is at the Treasury's discretion, though there must be a trial in any year during which the Royal Mint issues coins.
Coins to be tested are drawn from the regular production of the Royal Mint. The Deputy Master of the Mint must, throughout the year, randomly select several thousand sample coins and place them aside for the Trial. These must be in a certain fixed proportion to the number of coins produced. For example, for every 5000 bimetallic coins issued, one must be set aside, whereas for silver Maundy money the proportion is one in 150.
The jury is composed of at least six assayers from the Company of Goldsmiths. They have two months to test the provided coins, and decide whether they have been properly minted. Criteria are given for diameter, chemical composition and weight for each class of coinage.
Statutory basis for the Trial of the Pyx is given by the Coinage Act 1971, the latest in a long series of similarly named Acts of Parliament. Specific procedures are established by Order in Council, the most recent being the Trial of the Pyx Order 1998, which was amended by The Trial of the Pyx (Amendment) Order 2005 and more recently by The Trial of the Pyx (Amendment) Order 2012. It is not required for a new Order to be issued for each Trial: this is mandated (to occur) only with regulatory revision.
A Trial of the Pyx is called for in Neal Stephenson's fiction novel The System of the World, where it is revealed that Jack Shaftoe raided the Tower of London specifically to tamper with the Pyx, Viscount Bolingbroke and Charles White use the event as leverage for political attack against Whigs, specifically Isaac Newton.
- Royal Mint description, retrieved 23 March 2009
- Company of Goldsmiths description
- Pyx Chamber at Westminster Abbey, retrieved 23 March 2009
- Blog report by an observer of the Trial
- Report of Trial of the Pyx 2013. 
- Official text of the The Trial of the Pyx Order 1998 as in force today (including any amendments) within the United Kingdom, from the UK Statute Law Database
- Official text of the The Trial of the Pyx (Amendment) Order 2005 as in force today (including any amendments) within the United Kingdom, from the UK Statute Law Database
- Official text of the The Trial of the Pyx (Amendment) Order 2012 as in force today (including any amendments) within the United Kingdom, from the UK Statute Law Database