Scientists solve secret of colour-changing Roman cup

British researchers have recreated the glass used in the 1,600-year-old Lycurgus Cup, explaining why it appears green in ordinary light but glows ruby red when lit from behind, ERR reports.

The fourth-century cup, held at the British Museum, is carved with a mythological scene featuring King Lycurgus and Dionysus, the god of wine. Historians believe its changing colours symbolised grapes ripening in the sun and that it was used at special feasts.

Researchers at the University of Sheffield recreated the ancient glass in a laboratory using molten glass heated to more than 1,000 degrees and incorporating tiny gold and silver crystals.

The metal particles cause the optical effect. When light passes through the glass, they block blue and green wavelengths, allowing red and orange light through and producing a deep red glow. When light reflects off the glass surface towards the viewer, the particles scatter it differently, creating an opaque green appearance.

Working with glass artist Elliot Walker, the researchers made a beaker that behaves like the Lycurgus Cup. They said ancient craftspeople would have needed considerable skill and some luck, as the final colour depended largely on where the glass was placed in the furnace.