The Lycurgus Cup is a fourth-century Roman cage cup, now in the British Museum’s collection (it is the museum’s best-known piece of ancient glass, per the cup’s catalogue history), that changes colour with the direction of the light: lit from the front it sits jade green, backlit it glows ruby red. The effect is not a pigment or a coating — it is built into the glass itself, and the recipe for building it was lost for roughly a millennium and a half. Scientists at the University of Sheffield have now produced a consistent, replicable version of that glass, Ars Technica reported, and the first large object made from it goes on public display next month.
Fifty-nanometre metal in ancient glass
Analysis of the cup’s glass found the cause: silver-gold alloy nanoparticles, roughly 50 nanometres across, dispersed through the melt in a silver-to-gold ratio of about 7:3 with around 10% copper, according to the compositional study by British Museum scientists Ian Freestone and Nigel Meeks and colleagues. Particles that size — a fraction of the 400–700 nm wavelength band of visible light — support a surface plasmon resonance: the conduction electrons at the metal’s surface oscillate with incoming light and absorb and scatter particular colours rather than letting the full spectrum pass uniformly. The result is a glass with two colours at once:
white light in → Ag–Au nanoparticles (~50 nm) in the glass ├ scattered back toward the viewer: green └ transmitted through the wall: red
The optics are the easy half. The manufacturing is not: particle size, alloy ratio and dispersion all depend on how the melt is run — furnace temperature, the oxygen content of the atmosphere, how long the metals spend dissolved before re-precipitating as particles of the right size. Add the metals wrong and you get grey glass.
A recipe rebuilt by trial
In 2020, Dutch researchers reproduced the dichroic effect in a 3D-printable nanocomposite by embedding silver and gold nanoparticles of controlled size and shape — proof the physics was understood, but not a glass recipe anyone could repeat. The Sheffield work, by Lisa Hollands and Roman-glass specialist Caroline Jackson, attacked process rather than physics: they varied furnace types, materials and oxygen levels, at one point adding sugar to the mix. “I knew that making a glass of this type was likely to be a complex task, as it is not just a case of adding nanoparticles to glass,” Hollands said, adding that after 25 years of working with glass, “if the Romans could have seen me experimenting, they’d probably laugh and ask: ‘Why does she not know how to make this?’” The output was a melt that flips green in reflected light and red when backlit — the cup’s signature, produced on demand.
From lab melt to sculpture
Hollands’s first piece was exhibited at LightFest, part of Molecular Photonic Breadboards, a UK project developing materials that harness light for sustainable technologies. The project then commissioned UK glass artist Elliott Walker — winner of series 2 of Netflix’s Blown Away — to blow the recreated glass into a contemporary homage. “You can really get an idea of how the glass makers of the past would have adapted to this sort of glass; it’s like shaking their hands,” Walker said. “The piece is a blend of the Lycurgus story and the Farnese Atlas sculpture, with the globe replaced by a cup.”
The sculpture is unveiled at Sheffield’s Millennium Gallery next month during the university’s Festival of the Mind, then goes on permanent display at the university’s Turner Museum of Glass. The deliverable that matters beyond the gallery is the word “replicable”: every earlier experiment worked from a single 1,600-year-old sample that cannot be remelted, and the next numbers to come out of this — particle sizes, furnace conditions, optical spectra across production batches — can finally be measured against a recipe rather than a relic.