Lycurgus Cup: A Piece of Ancient Roman Nanotechnology

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.

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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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Photo credit: The History Blog

The addition of metals or metal oxides to color glass was not unfamiliar to Roman glassmakers. For example, opaque red and brown glasses were produced by the addition of copper. However, coloring glasses using gold and silver was far from routine and something of a hit and miss affair. There were a large number of factors to control including the concentration of the metals and the particle size, oxidation states of certain elements, the time and temperature of heating and probably the atmosphere during heating. It is unlikely that the Romans were able to figure this all out precisely 1,600 years ago when technology was very restricted.

The inability to control the coloring process explains why the technology never developed beyond the fourth century AD. Among the few piece of glasses they were able to produce, Lycurgus Cup is one outstanding example and is among the most technically sophisticated glass objects produced before the modern era.

There are a few other examples of nanotechnology in ancient history. The Maya people produced a corrosion resistant azure pigment known as Maya Blue in 800 AD, that was discovered to contain clay with nanopores into which indigo dye was combined chemically to create an environmentally-stable pigment. Damascus steel swords, known for their impressive strength, shatter resistance and exceptionally sharp cutting edge, contain nanoscale wire-and-tube-like structures. These swords were produced in the Middle East between the 3rd and 17th century.

The presence of nanoparticles in these materials, however, doesn’t mean that the ancient people knew about nanotechnology.

Ian Freestone at the Institute of Archaeology at University College London, who studied the Lycurgus cup, thinks that these ancient craftsmen “were highly skilled but they were not nanotechnologists.”. “They did not know that they were working on the nanoscale," he says.

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