Investigation of the 'rouge clair' glass on the Royal Gold Cup

The Royal Gold Cup is unique in many ways, and is a particularly fine example of Medieval enamelling on gold, including use of the transparent red enamel, sometimes known as ‘rouge clair’ or ‘ruby glass’. This colour was so highly regarded at the time that it was considered worthy of mention in Medieval inventories.

The cup itself dates to 1370–1380 while the stem seems to be a later Tudor addition. It is one of the rare survivors of the Commonwealth period in British history (1649–1660) when so many gold objects were melted down. It is also the product of a very high standard of craftsmanship and a superb example of basse-taille enamelling.

A scientific study of the transparent red enamel was carried out to establish how it was coloured. Ruby glass is coloured by metallic nano-particles (one millionth of a millimetre) of copper, silver or gold. While the earliest ruby glasses are found in the Roman period (the Lycurgus cup in the British Museum is one of the most famous examples and is a gold ruby glass), the knowledge of gold ruby glass appears to have been lost and only rediscovered in the late seventeenth century by the German alchemist Kunkel. The first copper ruby glasses were produced during medieval times and used for red stained-glass, but the knowledge of how to make this red glass was lost by the eighteenth century.

Ruby glass is difficult to use as an enamel. It has to be melted to apply it to metal, which can change the size and distribution of the small metallic particles, spoiling the colour of the glass. It can change either to an opaque liverish colour or to black. The sixteenth century goldsmith Benvenuto Cellini found it was impossible to enamel red onto silver or copper, though it could be done on gold.

The Royal Gold cup has large areas of brilliant red enamel. Examination in a scanning electron microscope of tiny fragments of this transparent red enamel from the cup and its stem confirmed that both are copper ruby glass.