The glass that changes colour

Sand, soda, fire and a trace of precious metal. Roman glassmakers made a cup that changes colour when the light moves, and for sixteen centuries nobody could make that glass again. Until now.

Roman glassmakers worked a kind of alchemy, a word that, like chemistry after it, may go all the way back to Kemet, the ancient name for Egypt. They took sand, soda and the faintest trace of precious metal, pushed it through fire, and every so often pulled out something that behaved like magic. The finest piece to survive sits in a case in the British Museum: a cup that is one colour, until the light moves and it becomes another. For sixteen centuries nobody could quite make that glass again. Now, in a furnace in Sheffield, somebody has.

A king caught in a vine

The cup was made in the 4th century AD, probably around 290 to 325, possibly in Rome or Alexandria. It is a cage cup, or diatretum: the maker started with a single thick blank of glass and spent a very long time cutting and grinding it away until the figures stood almost free of the bowl, held on by thin bridges of glass. Only fragments of most cage cups survive. Lit from the front, the cup is a dull, opaque jade green. Put a light behind it and the same glass glows a deep, translucent red. The Lycurgus Cup is the only complete Roman object made from this colour-changing glass, and the art historian Donald Harden called it the most spectacular glass of the period.

The carving tells the story of Lycurgus, a king of the Edoni in Thrace who attacked the followers of the god Dionysus. When he went after Ambrosia, one of them, the earth turned her into a vine that wrapped around him and held him fast while Dionysus, Pan and a satyr closed in. On the cup the king is caught mid-struggle in the vine's coils. Ian Freestone, emeritus professor of archaeological materials at University College London, has suggested the shift from green to red may itself be part of the story, like grapes ripening into wine for a god of wine.

The Lycurgus Cup in the British Museum, a Roman glass cup carved with the figure of King Lycurgus caught in a vine — The Lycurgus Cup, Roman, 4th century AD. British Museum, London. Photo: Johnbod, CC BY-SA 3.0, via Wikimedia Commons.

Gold and silver, ground smaller than light

The cup belonged to the Rothschild family from at least the 1850s, and Victor, Lord Rothschild sold it to the British Museum in 1958 for £20,000. Scientists started on the colour almost at once. Analyses published in the 1960s found traces of gold and silver in the glass, which pointed to precious metals rather than the usual glass colourants. The full answer came in 1990, when David Barber and Ian Freestone looked at a tiny fragment under a transmission electron microscope and found particles of a gold and silver alloy, with a little copper, mostly between 50 and 100 nanometres across. A nanometre is a millionth of a millimetre, so these are far too small to see with an ordinary microscope. The glass holds only about 330 parts per million of silver and 40 parts per million of gold.

At that size, metal behaves strangely with light. The free electrons in each particle are set ringing by the light waves passing through, an effect physicists call surface plasmon resonance, and particles of this size and mix ring most strongly with the blue and green part of the spectrum. Light shining through the cup loses its blues and greens on the way, so what reaches your eye is red and orange. Light bouncing off the front is scattered back by the particles instead, and that scattered light looks green. It is a cousin of the effect that turns sunsets red, explained in Why the sky is blue and sunsets are red.

Knowing why is not knowing how

Understanding the physics did not make the glass easy to copy. Gold and silver stirred into molten glass do not automatically form particles of the right size, shape and number, and the result depends on the chemistry of the batch and on what happens inside the furnace. Glass with a similar colour change was made in the 1960s, and in 2020 researchers in the Netherlands led by Lars Kool and Floris Dekker printed a small green and red cup on a 3D printer, but theirs was a plastic composite with silver and gold particles in it rather than glass.

The Sheffield project began as a challenge. Christina Metcalfe, a project manager at the university, asked Lisa Hollands, a glass technician in the School of Chemical, Materials and Biological Engineering with more than 25 years of experience, to make something like the cup's glass for a public exhibition about light in 2025. Hollands worked with Caroline Jackson, a professor and specialist in Roman glass, using published studies of the cup and of Roman glassmaking to put together a likely starting recipe from modern versions of the ancient raw materials. Then came the trial and error: different furnaces, different starting materials, different amounts of oxygen, even added sugar, which changes the chemistry of the melt. The glass was melted at well over 1,000°C, and as it cooled the metal particles grew inside it. In their paper Hollands, Jackson and John Parker report that they got a glass with an effect similar to the cup's the day before the exhibition opened. Hollands has described the change in the finished glass as "magic".

Did the Romans know what they were doing?

Freestone says the Sheffield glass comes much closer to the cup's actual green and red than the 1960s attempts did. He also points out how touchy the colour was: simply moving the glass to a different spot in the furnace changed the result. If a modern lab with a chemistry textbook found it that hard to control, a Roman workshop working by eye must have been fighting the same uncertainty, and the Lycurgus Cup may be the batch where everything happened to come right.

How the metals got into the glass is still argued over. The amounts are so small that some researchers think they came in by accident, with silver and gold dust or scrap from metalworking in the same workshop. Others, Freestone among them, have argued that the proportions are consistent enough to suggest the glassmakers added precious metal on purpose, perhaps as finely divided metal or as waste from refining, even if they could not have known about the particles they were making. The handful of other Roman fragments that change colour do so less dramatically, which suggests that whoever cut the cup was working with an unusually lucky piece of glass and knew it.

A new cup for Sheffield

Sheffield then asked the glass artist Elliot Walker, winner of the second series of Netflix's glassblowing contest Blown Away, to make a sculpture from the new glass. His piece borrows from another ancient work, the Farnese Atlas, a Roman marble of the Titan Atlas carrying the heavens on his shoulders, and swaps the celestial globe for a cup. It goes on show at the Millennium Gallery during the university's Festival of the Mind, from 6 to 15 November 2026, where the light moves around it so the glass can be seen turning from green to red. After that it will go on permanent display at the university's Turner Museum of Glass.

People have always wanted to play with light and colour, and they still do. It is the same itch that sent Stuart Semple into the lab to make LOVETONE and PHANTOM, paints that shift as the light moves across them.