‘It’s magic’: scientists rediscover how to make ancient Roman colour-changing glass

The Romans' proficiency in glass-making has produced some of antiquity's wonders, but few creations match the 1,600-year-old masterpiece housed in the British Museum. Depending on the light source's position, the Lycurgus Cup changes color—from jade green under normal lighting to ruby red when illuminated from behind—an astonishing feat of optical engineering that has confounded historians and glass-makers for centuries.

Ancient Recipe Recreated

Now, scientists from the University of Sheffield have replicated its remarkable optical behavior using an ancient recipe derived from a fragment of the Roman 'dichroic' (two-colored) original. The fourth-century cup bears a mythological frieze depicting King Lycurgus from Homer's Iliad, showing Dionysus triumphing over Lycurgus, who is ensnared by a vine.

Nanotechnology Secrets

The researchers faced the challenge of embedding into glass nanoparticles of gold and silver—each a thousand times smaller than a grain of sand. They created a thick molten glass that, as it cooled from over 1,000C, allowed gold and silver nanocrystals to grow around particles within it.

Hollands explained the science: 'White light is made up of many different colors... Nanoparticles are much smaller than the wavelengths of light and in certain conditions they block certain colors due to the light affecting the electrons in the atoms. So for this glass, if light is shone through it, the gold and silver particles block blue and green light, only allowing reds and oranges to pass through.'

Modern Interpretation

Sheffield commissioned glass artist Elliot Walker to create a contemporary sculpture that pays homage to the cup's 'ancient twin' by combining the Lycurgus story with the Farnese Atlas sculpture, replacing the globe with a cup. 'For me, it's a connection with the Roman makers,' Walker said.

The replica will be unveiled at the Millennium Gallery from November 6-15 as part of the university's Festival of the Mind before being permanently displayed at the Turner Museum of Glass.

Historical Significance

Ian Freestone, emeritus professor at UCL, noted that while similar-colored glass was replicated in the 1960s, Sheffield's project more closely matches the actual shades of the original. 'This is more ambitious and more precise,' he said. 'I was excited because, for the first time, I was getting a feel of just how difficult it was to get these colors.'

Freestone suggested the red-green colors may symbolize grape ripening, given the depicted scene of grape vines pulling Lycurgus into the underworld. A letter supposedly written by Hadrian to his brother-in-law mentions 'parti-coloured cups that change color' used at banquets on feast days, indicating these objects were prized in Roman society.