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In Sheffield, England, University of Sheffield technician Lisa Hollands has managed to reproduce the colour-changing effect seen in the Lycurgus Cup, a Roman glass vessel roughly 1,600 years old. According to the university, the process relies on dichroic glass, which appears jade green in reflected light and ruby red when light passes through it. An artwork made using this recipe will be unveiled next month.
A Roman cup with two colours
The Lycurgus Cup is a Roman cage cup, a type of glass vessel whose outer wall forms a sculpted lattice almost detached from the body of the vessel. According to the University of Sheffield, it shows a jade-green tint when lit from the front and a ruby-red hue when light shines through it.
This phenomenon, known as dichroism, makes the object unique among surviving ancient glass. The decoration depicts the Thracian king Lycurgus, a figure from Greek mythology who opposed Dionysus and was trapped in a vine, a scene that has long intrigued specialists. The glass, cut and ground, was worked to make this image appear sharply.
The secret of gold and silver nanoparticles
According to the explanation given by the university, the Romans introduced tiny fragments of gold and silver into the body of the glass. The size and shape of the resulting nanoparticle layers allow the glass to absorb certain wavelengths of light and display different colours depending on the viewing angle.
In other words, simply mixing these metals into molten glass is not enough: the very fine distribution of the particles determines the final effect. This technical demand explains the rarity of these objects and the difficulty of reproducing them today, since every firing parameter can change the result.
A recipe obtained through trial and error
Lisa Hollands first sought to understand the process, aware that the task would be complex. She consulted Caroline Jackson, a specialist in Roman glass, before carrying out many experiments in a patient process of refinement.
She tested different types of furnaces, several materials, varying oxygen levels and other ingredients until she obtained a working formula. This approach, which combines experimental archaeology and materials chemistry, helps us better understand the know-how of ancient glassmakers, of whom no detailed treatises survive.
From experiment to artwork
Once the material was perfected, glassmaker Elliot Walker was commissioned to create a work with this new material. According to the university, he chose to keep a fairly classic form and to take up the story of Lycurgus depicted on the original cup.
He nevertheless felt it necessary to incorporate a blown element to fully showcase the dichroic glass, as blowing allows volumes that cutting alone cannot achieve. The piece is due to be unveiled next month before joining the university's collection, which will thus keep a tangible trace of this research.
Why this work matters for archaeology
Reproducing an ancient object is not merely a stylistic exercise. When a recipe works, it validates hypotheses about the materials and temperatures used nearly two millennia ago, and it clarifies what remains uncertain in reconstructions.
This kind of experimentation complements the study of originals held in museums, including the Lycurgus Cup, which is attributed to the late Roman period. It also paves the way for new research on other known cage glasses, some of which are the subject of comparative studies published in the journal Archaeology.
Frequently asked questions
What is the Lycurgus Cup?
The Lycurgus Cup is a Roman cage cup, about 1,600 years old, whose glass changes colour depending on the light. It depicts the Thracian king Lycurgus caught in a vine. It stands out for this exceptional dichroic phenomenon.
Why does the glass of the cup change colour?
According to the University of Sheffield, the Romans added small fragments of gold and silver to the glass. The size and shape of the resulting nanoparticles allow the glass to absorb light and display different colours.
How was this dichroic glass recreated?
Technician Lisa Hollands consulted specialist Caroline Jackson, then tested several furnaces, materials and oxygen levels. She eventually found a recipe that produces the desired effect.
Where can you see Roman glasses of this type?
Roman cage glasses are held in major museums across Europe and Britain. Check each institution's website for current exhibitions and possible loans.
Sources
- Rare Roman Glass Re-created — Archaeology Magazine
Original summary written with the help of artificial intelligence from the sources cited, checked automatically, then translated from the French original. Information and quotations belong to their authors. Read the French original.
