Dating back to around 400 CE and made of dichroic glass, the Lycurgus Cup is notable for exhibiting different colors depending on the light. (It gets its name from the scene depicted on its surface, of King Lycurgus of Thrace.) Light it from the front, and the cup looks green; light it from behind, and the color changes to a deep red. University of Sheffield scientists have recreated the original recipe to produce a contemporary glass sculpture with the same optical effect.

The cup is considered one of the seven glass wonders of the world, one of just 40 glass cage cups that survived. Exactly why the unusual color effects of the Lycurgus Cup occurred was a mystery until 2007, when a team of scientists discovered the color shifts were due to the presence of nanoparticles in the glass—the result of Roman artisans adding finely ground gold and silver into the mix. Those nanoparticles change how the electrons in the cup vibrate in response to light so that the color one sees shifts with the observer’s vantage point.

As we reported in 2019, it’s a similar phenomenon to the one responsible for the shifting colors of 19th-century daguerreotypes due to the scattering properties of nanoparticles. The Romans added salt when they melted the silicon (sand) to make the glass, and those salts contained gold atoms. In the melting process, those atoms aggregate and produce nanoparticles, which in turn produce the colors. It’s how any stained glass is made. It's also similar to how the precise lattice-like structure of photonic crystals produces iridescent colors in nature, like the wings of butterflies, or opals.

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