Few Roman creations better a 1,600-year-old masterpiece which sits in the British Museum. Depending on where a light source is positioned, the Lycurgus Cup changes colour, from jade green under normal lighting to ruby red when lit from behind, a feat of optical engineering that has confounded historians and glass-makers for centuries.
Scientists from the University of Sheffield have now replicated its remarkable optical behaviour, using an ancient recipe derived from a fragment of the Roman "dichroic", or two-coloured, original.
Dr Lisa Hollands, a technician at the university's school of chemical, materials and biological engineering, said that replicating it has been "a labour of love" and that "people have tried it before with limited success". She added: "To me, it's magic."
The fourth-century original bears a mythological frieze depicting the legend of King Lycurgus from the sixth book of Homer's Iliad. Carved in deep relief, it shows the triumph of the god of wine, Dionysus, over Lycurgus, who is ensnared by a vine. In 2022 it was recognised as one of the seven glass wonders of the world by an international panel.
Hollands worked with Prof Caroline Jackson, a leading Roman glass expert, on the initial research. Then came the challenge of embedding into glass nanoparticles of gold and silver, each a thousand times smaller than a grain of sand. A thick molten glass was created and, as it cooled from well over 1,000C, gold and silver nanocrystals started to grow around particles within it.
Hollands explained the effect in terms of wavelength. "So, for this glass, if light is shone through it, the gold and silver particles block blue and green light, only allowing reds and oranges to pass through. If light is reflected off the surface, the particles have the effect of scattering the light, giving the opaque green colour we see," she said.
Ian Freestone, emeritus professor of archaeological materials at University College London, said that although a similar-coloured glass was replicated in the 1960s, the Sheffield project is much closer to the actual green and red shades of the Lycurgus Cup. "This is more ambitious and more precise," he said.
Freestone also described what the work revealed about the difficulty of the original. "The colour was quite dependent on conditions in the furnace. If the glass went in a different position in the furnace, then the colour came out differently," he said. "The fact that the colour is so sensitive to the position in the furnace suggests that it would have been very difficult for the Romans to control the colour. It makes me think that, with the Lycurgus glass, they hit the jackpot."
Sheffield commissioned Elliot Walker, a glass artist, to bring the effect to life in a contemporary sculpture that combines the Lycurgus story and the Farnese Atlas, with the globe replaced by a cup. It will be unveiled at the Millennium Gallery as part of the university's Festival of the Mind from 6 to 15 November, before going on permanent display at the Turner Museum of Glass.
A letter supposedly written by Hadrian to his brother-in-law in Rome reads: "I have sent you parti-coloured cups that change colour, presented to me by the priest of a temple ... I would like you to use them at banquets on feast days."
Two findings sit side by side in this work, and the second is the more interesting. The colours are now reproducible in Sheffield, and on the furnace evidence they were probably not reproducible in Rome. The method, stated honestly, was to put the batch somewhere in the heat, take it out and see.
That is a technique in the sense that a raffle is a technique. The Lycurgus Cup is accordingly not a triumph of Roman optical engineering. It is a Roman shelf position, carved afterwards in deep relief by people who had every reason to believe they had meant it.
This newspaper wishes to be fair to the fourth century. Reproducing an accident under controlled conditions is harder than reproducing a method, and Sheffield has done the harder thing. What the laboratory has recovered is not the Roman procedure but the Roman luck, now available on demand.
The implication for the seven glass wonders of the world is uncomfortable. A production run whose output varied with position in the furnace must have produced a great many cups that came out the wrong colour, and those cups are not in the British Museum. The surviving example has been admired for sixteen centuries on the strength of being the one that was placed correctly.
The Department of Unnecessary Research has written to the international panel asking whether the unsuccessful batches are also wonders, assessed on the same evidence and found to have been equally skilfully made.
Until a reply arrives, the position stands. Sixteen hundred years of admiration have been awarded to the cup that happened to be facing the right way.