Scientists recreate the colour-changing glass of an ancient Roman cup

The Lycurgus Cup is a glass vessel about 1,600 years old that sits in the British Museum. Under ordinary room light it looks jade green. With a light placed behind it, it turns ruby red. For centuries nobody could work out how the Roman glassmakers managed it. A team at the University of Sheffield has now made glass that behaves the same way, working from a recipe derived from a fragment of the original.

A cup with two colours and a story carved into it

The cup is a fourth-century original. The scene carved into it in deep relief comes from the sixth book of Homer's Iliad and shows the god of wine, Dionysus, defeating King Lycurgus, who is caught in a vine. The glass is described as "dichroic", which simply means two-coloured: jade green in ordinary light, ruby red when a light is placed behind it. In 2022 an international panel named it one of the seven glass wonders of the world.

What the tiny specks of gold and silver do

The colour comes from specks of gold and silver inside the glass, each about a thousand times smaller than a grain of sand. Dr Lisa Hollands, a technician at the university's school of chemical, materials and biological engineering, worked on the recipe with Prof Caroline Jackson, a Roman glass expert. They made a thick molten glass and let it cool from well over 1,000C. As it cooled, the gold and silver grew into tiny crystals inside it.

Hollands explains the effect with a rainbow. "White light is made up of many different colours and you see this when there is a rainbow in the sky," she said. The specks are far smaller than the waves that light travels in, and in the right conditions they block certain colours. Shine light through the glass and the gold and silver block blue and green, letting only reds and oranges pass, so the glass looks ruby red. Light bouncing off the surface is scattered instead, which gives the green. Replicating all of this was, in her words, "a labour of love". "To me, it's magic," she said.

Why the colour was so hard to control

Ian Freestone, emeritus professor of archaeological materials at University College London, said a similar-coloured glass was made in the 1960s, but that Sheffield's project is much closer to the actual green and red shades of the Lycurgus Cup. "This is more ambitious and more precise," he said. He added that the colour depended heavily on conditions in the furnace: glass that went in at a different position came out a different colour. That suggests the Romans would have found the colour very difficult to control. "It makes me think that, with the Lycurgus glass, they hit the jackpot."

Freestone also noted a theory that the red and green stood for the ripening of the grape, since the carved scene shows grape vines pulling Lycurgus into the underworld. A letter supposedly written by Hadrian to his brother-in-law in Rome mentions "parti-coloured cups that change colour", presented to him by the priest of a temple, which he wanted used at banquets on feast days.

Sheffield commissioned the glass artist Elliot Walker to make a new sculpture in homage to the cup's "ancient twin", combining the Lycurgus story with the Farnese Atlas sculpture and replacing the globe with a cup. "For me, it's a connection with the Roman makers," he said. It will be unveiled at the Millennium Gallery during the university's Festival of the Mind, which runs from 6-15 November, before a permanent display at the Turner Museum of Glass.