Roman glass cup that turns from green to red is remade
A 1,600-year-old Roman cup looks green in one light and red in another, and researchers have now worked out a repeatable recipe for its glass.
The Lycurgus Cup is a Roman glass cup dating to around 400 CE, and it does not hold still. Lit from the front, the cup looks green. Lit from behind, the same glass turns a deep red. Researchers at the University of Sheffield have now worked out a recipe for glass that does the same thing, and a glass artist has used it to make a new sculpture.
A cup that changes colour with the light
The cup is made of dichroic glass, which means it shows different colours depending on how light passes through it or bounces off it. It takes its name from the scene carved on its surface, which shows King Lycurgus of Thrace. The cup is considered one of the seven glass wonders of the world, and it is one of just 40 glass cage cups that have survived.
For a long time, nobody could explain the colour change. That lasted until 2007, when a team of scientists found the answer: tiny particles of gold and silver, far too small to see, were mixed into the glass.
Gold and silver, hidden in the glass
Roman glassmakers added salt when they melted sand to make glass, and that salt contained gold atoms. During melting, those atoms clumped together into the tiny particles that give the cup its colours. The particles change how the electrons in the cup respond to light, so the colour seen shifts with the observer's vantage point.
The same basic idea colours stained glass. It is also close to the way the precise internal structure of some materials produces shimmering colours in nature, such as the wings of butterflies or opals.
There is a difference between the cup and a 19th-century photograph type called a daguerreotype, which can also shift colour. In the cup, the tiny particles sit inside the glass. In a daguerreotype, they sit on top of a surface, which adds a reflection that the cup does not have.
Making the glass again
In 2020, Dutch researchers reproduced the green-and-red effect in a material that could be 3D printed, by embedding small amounts of silver and gold particles of the right size and shape. But nobody had yet devised a consistent, repeatable recipe.
Lisa Hollands and Caroline Jackson at the University of Sheffield set out to find one. Jackson is an expert on Roman glass, and Hollands has studied and worked with glass for over 25 years. They experimented with different furnace types, different materials and different oxygen levels, and at one point they included sugar in the mix. Hollands said that if the Romans could have seen her experimenting, they would probably laugh and ask why she did not know how to make it.
The work paid off when Hollands found a mix that produced glass with the desired optical effect, appearing green in reflected light and red when backlit. Her initial glass piece was exhibited at LightFest, which is part of Molecular Photonic Breadboards, a UK project aimed at designing new materials that harness the power of light for sustainable technologies.
Project leaders then commissioned UK glass artist Elliot Walker, the second-season winner of the Netflix competition Blown Away, to create a contemporary sculpture paying homage to the original cup, using glass made in the Sheffield melting shop.