Sheffield Recreates Roman Glass, Mystery Remains

The University of Sheffield has recreated the colour-changing effect of the Lycurgus Cup, a 4th-century Roman glass artefact held at the British Museum, but researchers still haven't worked out every detail of how Roman glassmakers made it.

A recipe, not a complete answer

The cup is made from dichroic glass: it appears jade green in reflected light and ruby red when light passes through it. The effect depends on where the light comes from and how it reaches the glass. Only 40 surviving glass cups from the period exist, and the Lycurgus Cup is the most complete.

For centuries, historians and glassmakers have puzzled over the process behind its shifting colours. Experts believe Roman makers added tiny amounts of gold and silver. Under the right conditions, those metals formed a layer of light-absorbing nanoparticles, smaller than the wavelengths of light. The particles absorb light strongly, with the colours they display depending on their size and shape.

The effect had not previously been widely reproduced through a repeatable glassmaking recipe. The Sheffield team says its experiments have now produced glass with the same basic colour-changing behaviour, not a full account of the original Roman manufacturing process.

Lisa Hollands, a glass technician from the university's School of Chemical, Materials and Biological Engineering, described the work as more complicated than simply mixing nanoparticles into glass. “I knew that making a glass of this type was likely to be a complex task, as it is not just a case of adding nanoparticles to glass.”

Hollands worked with Caroline Jackson, a leading Roman glass expert, to review what was known about glass of the period and develop a base glass using modern materials. She tested different furnace types, starting materials and oxygen levels. The resulting recipe produced glass that looked opaque green in reflected light and red when light shone directly through it. Under the right conditions, nanoparticles thousands of times smaller than a grain of sand formed.

What the light reveals

When viewed normally, the Lycurgus Cup looks green because light bounces off the glass. When light passes through, the gold and silver nanoparticles interact with it differently: they absorb some colours more strongly than others. The emerging light is left richer in red, making the cup glow like a ruby.

The University of Sheffield announced the successful reproduction on Monday. The report was published on Tuesday, 27 October 2026. The gap between reproducing an effect and explaining the original process remains. Researchers say they haven't yet established every detail of Roman production.

From laboratory recipe to sculpture

Project manager Christina Metcalfe said the team approached Elliot Walker, one of the UK's finest glass artists and winner of season two of Netflix's Blown Away. Walker accepted the challenge and was commissioned to create a contemporary sculpture using Hollands' recipe.

Researchers said a better understanding of the experiments could lead to new photonic materials and advanced glasses for radioactive waste management. Those are possible applications, not results reported from the work itself. The immediate result is narrower: a modern recipe has reproduced the cup's contrasting green and red appearance, while the ancient makers' full method remains unresolved.