In the 1950s, Victor Rothschild sold the British Museum an artifact that had been passed down in his family for generations – the Lycurgus cup. The Roman artifact, created around 4 AD, baffled scientists at the time with its ability to change color from deep red to jade green – depending which way the light struck it. It’s now understood that the cup is made with dichroic glass, which contains multiple micro-layers of metal.
In the case of the Lycurgus cup, the glass is fused with nanoparticles of gold and silver. The disbursement of the particles is so perfect, and the particles so small, that the cup is centuries ahead of its time. Either the cup’s craftsman was a metallurgical and glassblowing genius, or more likely had a very lucky accident while making the cup. The quantity of gold and silver in the cup is so infinitesimal (even clumped together, it would not be visible to the naked eye), that the Romans would have had no way to add it deliberately. What likely happened was that traces of gold leaf were accidently added to the glass mixture, and then further diluted with more molten glass.
The cup depicts the enraged king Lycurgus bound by vines and mocked by Bacchus, the Roman god of wine and revelry. When lit from the front, the glass is green. When lit from behind, the glass becomes red. This is because of the way the nanoparticles of gold and silver diffuse the light. Coincidentally, the colors are fitting of the depiction. The change from green to red could represent the ripening of wine grapes, or of Lycurgus’ growing rage.
Currently on display at the British Museum, it’s the only Roman artifact of its kind and a shining example of refining technique.