A 1,600-year-old Roman cup appears green when lit from the front. But when light passes through it, it turns red. Researchers have now succeeded in reproducing this astonishing effect.
This cup, known as the Lycurgus Cup, is kept at the British Museum in London. Made in the 4th century, it is decorated with figures sculpted into the glass. Its colors puzzled historians for a long time: how could Roman artisans create an object capable of changing appearance in this way?
Roman Lycurgus Cup.
Image Wikimedia
Today, we know the scientific explanation. The glass contains tiny particles of gold and silver, so small that they are invisible to the naked eye. These particles alter the way light passes through the glass or bounces off it. As a result, it appears green under front lighting and red when lit from behind.
The most difficult part was making this glass again with a reliable result. The amount of metal, the temperature and the firing conditions change the size of the particles, and therefore the colors produced.
At the University of Sheffield in the United Kingdom, Lisa Hollands, a glass specialist, conducted numerous trials. With the help of Caroline Jackson, an expert in Roman glass, she prepared a mixture close to the materials that ancient artisans might have used. She then modified the furnaces, the ingredients and even the amount of oxygen during production.
Modern reinterpretation of the remarkable color-changing dichroic glass of the 4th-century Roman Lycurgus Cup, based on glass developed and made by Dr Lisa Hollands and Professor Russell Hand of the School of Chemical, Materials and Biological Engineering.
Credit: David Heath, University of Sheffield
After numerous trials, she obtained glass that displays the same color changes as the ancient cup. The team now has a method it can reproduce, rather than merely a small sample that happened to succeed. Glass artist Elliot Walker used this material to create a modern sculpture inspired by the Lycurgus Cup.
No one knows exactly what ingredients the Romans used or how they controlled their furnaces. Other experiments had already reproduced similar effects, but the Sheffield researchers succeeded in obtaining this result in glass made to order.
The new sculpture is to be presented to the public in Sheffield in November 2026. Beyond its historical and artistic interest, this achievement helps scientists better understand how very small particles can alter light, a property that could be useful in developing new materials.