It was Edward I who first passed a statute requiring all silver to be of sterling standard – a purity of 925 parts per thousand – ushering in a testing or assay system that has survived for over 700 years.
The statute made it the responsibility of the Wardens of the Goldsmiths' Guild to mark all items of sterling standard with a leopard's head stamp.
This mark has become known as the Incuse Duty Mark.
From 1786 to Victoria's reign, the King's head Duty Mark always faces right and is embossed like all other hallmarks.
Dublin’s assay office has been operating since the middle of the 17th century and silver is still marked there.