The Lycurgus Cup, a 1,600-year-old masterpiece kept in the British Museum, has amazed historians for centuries because of an unusual visual trick. Under normal room light, the Roman glass cup looks jade green, but when light shines through it from behind, it glows ruby red. Now, scientists from the University of Sheffield have successfully recreated this colour-changing effect.
To remake the glass, Dr Lisa Hollands and Professor Caroline Jackson embedded tiny nanoparticles of gold and silver into hot molten glass. These metal particles are a thousand times smaller than a grain of sand. As the glass cooled from more than 1,000 degrees Celsius, tiny crystals formed inside it. When light passes directly through the glass, these particles block blue and green light, letting only red pass through. When light bounces off the surface instead, the particles scatter the light to produce an opaque green.
Researchers say the achievement will help them understand how ancient Roman craftsmen worked. Ian Freestone, a professor of archaeological materials at University College London, explained that the colours depend heavily on heat and position inside the furnace. Because slight differences in furnace conditions completely change the resulting colour, controlling the process was extremely difficult, suggesting the ancient makers may have "hit the jackpot" when creating the original cup.