Scientists at the University of Sheffield have recreated a Roman-era glass that changes colour from green to red depending on the light. According to The Guardian, their model was the Lycurgus Cup, a vessel some 1,600 years old held at the British Museum.
Under ordinary lighting the cup appears jade green, but when lit from behind it turns ruby red. The researchers reconstructed the recipe from a fragment of the original. It turned out that gold and silver nanoparticles are responsible for the colour change. To produce the effect, the scientists melted the glass at a temperature above 1,000 degrees. As it cooled, nanocrystals of these metals formed within it. When light passes through the glass, the particles absorb the blue and green parts of the spectrum, letting red and orange through. When light is reflected from the surface, the particles scatter it, and the glass looks green.
Similar effects were achieved before, as far back as the 1960s, but the Sheffield team managed to reproduce the shades of the Lycurgus Cup far more precisely. Emeritus Professor Ian Freestone of University College London, commenting on the work, noted that the result was extremely sensitive even to the glass's position in the furnace.
If the glass was placed in a different spot inside it, the colour came out differently. This sensitivity to position in the furnace suggests that the Romans would have found it very hard to control the colour. I think with the Lycurgus Cup glass, they hit the jackpot.
— Ian Freestone, Emeritus Professor, University College London
The Lycurgus Cup was made in the 4th century. Its relief depicts the myth of the victory of Dionysus, god of wine, over King Lycurgus, who is entangled in a vine. According to one interpretation, the shift from green to red may have symbolised the ripening of the grapes, and the vessel was used at ritual feasts.
Glass artist Elliot Walker has realised this effect in a contemporary sculpture. It will be shown at the Millennium Gallery in Sheffield as part of the university's festival, running from 6 to 15 November. The work will then join the permanent collection of the Turner Museum of Glass.
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