University of Sheffield researchers figured out how to recreate the 1600-year-old glass recipe.
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.
There is a key difference, however: The nanoparticles are embedded into the glass of the cup, whereas with a daguerreotype, they are sitting on top of a substrate, adding some reflection that does not occur with an artifact like the Lycurgus Cup. So the scattering of the particles has a strong dependence on the substrate. Also, if one coats the surface of a daguerreotype with a layer of gold, the colors shift more to the red side of the spectrum. That’s in keeping with a 19th-century trick called “gilding,” in which the photographer would apply a solution of gold salt to the image to achieve warmer tones and sharper contrast.
An elusive recipe
In 2020, Dutch researchers reproduced the green/red dichroic effect of the Lycurgus Cup in a 3D printable nanocomposite material by embedding small amounts of silver and gold nanoparticles, of the proper size and shapes. But nobody had yet devised a consistent, replicable recipe. University of Sheffield scientists Lisa Hollands and Caroline Jackson—the latter an expert on Roman glass—worked together to come up with one, experimenting with different furnace types, materials, oxygen levels, and at one point including sugar in 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,” said Hollands. “I’ve studied and worked with glass for over 25 years, so I used the knowledge and experience I’ve gained, coupled with a modern scientific understanding of what I was trying to achieve—although if the Romans could have seen me experimenting, they’d probably laugh and ask: ‘Why does she not know how to make this?’”
The experimentation paid off when Hollands found a mix that produced glass with the desired optical effects, appearing green in reflected light, red when backlit. Her initial glass piece was exhibited at LightFest, which is part of Molecular Photonic Breadboards, a UK project geared toward designing new materials that harness the power of light for sustainable technologies. Project leaders next decided to commission UK glass artist Elliott Walker—the S2 winner of Netflix’s competition—to create a contemporary sculpture to pay homage to the original Lycurgus Cup.
“It is a very rare thing to have the opportunity to work with glass developed from ancient mixtures and recipes, and it is always a very exciting one,” said Walker. “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. I wanted to keep the sculpture quite classical in form and to carry through the original story of Lycurgus shown on the cup, but we needed a blown element within this to really show the dichroic glass. The piece is a blend of the Lycurgus story and the Farnese Atlas sculpture, with the globe replaced by a cup.”
The public will get its chance to see Walker’s sculpture in person when it is unveiled at the Millennium Gallery next month during the University of Sheffield’s Festival of the Mind. After that, it will have a permanent home on display at the university’s Turner Museum of Glass.