Sheffield researchers recreate colour-changing effect of Roman Lycurgus Cup

Mixed · 2Scientists at the University of Sheffield say they have replicated the optical behaviour of the Lycurgus Cup, a fourth-century Roman glass vessel in the British Museum, using an ancient recipe derived from a fragment of the original. 1 2 Mixed · 2The cup looks jade green in normal light and ruby red when lit from behind, an effect known as dichroism that has puzzled historians and glass-makers for centuries. 1 2 Mixed · 2Dr Lisa Hollands, a technician in the university's school of chemical, materials and biological engineering, did the initial research with Prof Caroline Jackson, a leading Roman glass expert. Hollands called the work "a labour of love", said "people have tried it before with limited success" and described the result as "magic". 1 2

Mixed · 2The challenge was embedding gold and silver nanoparticles, each about a thousand times smaller than a grain of sand, into glass. The team made a thick molten glass, and as it cooled from well over 1,000C, gold and silver nanocrystals grew around particles within it. 1 2 Mixed · 2Hollands explained that light shone through the glass has its blue and green wavelengths blocked by the particles, so only reds and oranges pass. Light reflected off the surface is scattered by the particles, which gives the opaque green. 1 2 Mixed · 2The original carries a mythological frieze, carved in deep relief, of the god Dionysus triumphing over King Lycurgus, who is ensnared by a vine. An international panel named it one of the seven glass wonders of the world in 2022. 1 2

Mixed · 2Ian Freestone, emeritus professor of archaeological materials at University College London, said a similarly coloured glass was made in the 1960s, but the Sheffield work is much closer to the cup's actual green and red shades: "This is more ambitious and more precise." He said it will help show how Roman craftsmen worked. 1 2 Left · 1Freestone said the colour depended heavily on where the glass sat in the furnace, which suggests the Romans would have struggled to control it. In the Guardian's account he added, "with the Lycurgus glass, they hit the jackpot". 1 Mixed · 2Freestone also cited a theory that the red-green shift symbolised the ripening of grapes, and that the cup may have been used in a drinking ceremony. A letter attributed to the emperor Hadrian refers to "parti-coloured cups that change colour" for use at banquets on feast days. 1 2

Mixed · 2Sheffield commissioned the glass artist Elliot Walker to make a contemporary sculpture using the effect. It will be unveiled at the Millennium Gallery during the university's Festival of the Mind, 6-15 November, and will then go on permanent display at the Turner Museum of Glass. 1 2