After 1,600 years, how Romans made color-changing glass has been unraveled

The Roman Lycurgus cup appears jade green in normal light but turns ruby red when backlit. Researchers from the University of Sheffield replicated this optical effect using an ancient recipe based on the cup's glass composition. The fourth-century cup features a mythological frieze depicting King Lycurgus entangled in a vine due to his defiance against the wine god Dionysus. In 2022, it was included in a selection of seven glass world wonders. Scientists incorporated nanoparticles of gold and silver, which are a thousand times smaller than a grain of sand, into the glass. When heated to over 1,000 degrees, these particles formed nanocrystals that selectively block certain colors of light, resulting in the cup's unique color changes.