Lycurgus Cup: A Piece of Ancient Roman Nanotechnology

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In the 1950s, the British Museum came into possession an ancient glass chalice called the Lycurgus Cup, so named for its depiction of Dionysus’s triumph over King Lycurgus of Thrace, who is shown entangled in grape vines, on the cup’s outer surface. The craftsmanship is excellent — the inside is smooth while the outside has been painstakingly cut and etched to create a decorative cage-like structure around the inner cup. This class of Roman vessels are known as cage cups, and they were mostly made during the 4th century CE. About fifty cups or so, mostly in fragments, have survived, with only a few in near-complete condition. The Lycurgus Cup is one of the best preserved Roman cage cups.

Cage cups were clearly very difficult to make, and no doubt very expensive, but this particular specimen stands apart because it exhibits a strange optical phenomenon that had stumped experts for decades. Under normal lighting, the glass appears jade green, but when lit from behind, it turns ruby red. Initially, experts weren’t sure whether the cup was made of glass, or was a gemstone. It wasn’t until 1990, that researchers figured out how the color changers were brought about.
It appears that the glass contains trace amount of gold and silver particles that have been ground up so finely that they are only about 50 manometers in diameter, or less than one thousandth the size of a grain of salt. The quantities involved are so tiny (330 parts per million of silver and 40 parts per million of gold), that researchers speculate that the glass might have been accidentally contaminated by gold and silver dust, and that glass-makers may not even have known that these particles were involved. Yet, the discovery of other glass pieces with the exact same composition shows that the mixture was deliberately fabricated. Somehow, the ancient Roman glassmakers had figured out that when light hits glass embedded with the tiniest particles of gold and silver, it alters the color of the glass. Modern science has a name for this effect —dichroism, and the glass that exhibits this phenomenon are known as dichroic glass.

But that’s not the end of the story. Simply adding ground-up gold and silver to glass would not produce these unique optical properties. For that the gold and silver particles need to form minute submicroscopic crystals or colloids. It is these colloids that give rise to the light scattering phenomena that result in dichroic effects.

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