Scientists rediscover how to make ancient Roman colour-changing glass

The fourth-century Lycurgus Cup, held by the British Museum, shifts from jade green in ordinary light to ruby red when illuminated from behind. Its optical effect has puzzled historians and glassmakers for centuries. Researchers at the University of Sheffield have now reproduced it more closely than earlier attempts, using a recipe informed by a fragment of the Roman glass. They heated glass above 1,000°C and, as it cooled, embedded gold and silver nanoparticles within it. The particles interact with light: viewed through the glass, they suppress blue and green wavelengths so reds and oranges pass through; viewed by reflected light, they scatter it to create the opaque green appearance.

The researchers’ replica gives experts a better sense of the Romans’ technical achievement and the difficulty of controlling it. Emeritus professor Ian Freestone said the glass’s colour depended heavily on its position in the furnace, suggesting Roman makers may have achieved the precise effect by chance. The cup’s carved scene shows Dionysus overcoming King Lycurgus, and its red-green colours may have evoked ripening grapes or been suited to ceremonial drinking. Sheffield has commissioned glass artist Elliot Walker to create a modern sculpture inspired by the cup; it is due to be shown at Sheffield’s Millennium Gallery before entering the Turner Museum of Glass.