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The Lycurgus Cup, a Roman Empire glass goblet held at the British Museum in London, fascinates researchers for its ability to change colour. According to Live Science, the vessel shifts from jade green to ruby red depending on whether light shines through it from the front or the back. Despite several studies over more than half a century, specialists still do not know whether Roman craftsmen deliberately sought this effect.
A major piece acquired by the British Museum
The Lycurgus Cup stands about 15.9 centimetres tall and weighs nearly 700 grams. According to Live Science, the British Museum acquired it in 1958 from the Rothschild banking family, who had owned it since the mid-19th century. It takes its name from the scene it depicts: King Lycurgus, who opposed the cult of the god Dionysus.
In the myth, this figure is king of the Edonians, a Thracian tribe. He chased the god into the sea, but one of Dionysus's priestesses, Ambrosia, fought back by transforming into a vine that entangled him, while other priestesses tormented him. For an object meant for a god of wine, this punishment carried a particular resonance.
A "cage-cup" goblet
According to a 1959 study by archaeologists D. B. Harden and Jocelyn Toynbee, the cup belongs to the family of "cage-cups", of which only a handful of examples survive. The decoration was obtained by cutting and grinding thick glass, leaving small bridges of material to hold the motifs in place, hence the cage-like appearance.
The two researchers dated its manufacture to the 4th century, probably in Italy. They also noted that the bronze mount forming the foot and rim is probably a 19th-century addition, a reminder that the object was altered over the course of its modern history.
Dichroic glass, an ancient mystery
Dichroic cups, which change tint depending on the lighting, are mentioned in Roman sources. A 2nd-century letter, possibly written by the emperor Hadrian to his brother-in-law, refers to the gift of two such cups intended for banquets. A 2007 study nevertheless points out that very few fragments of this mysterious glass have survived.
The mechanism was gradually explained. A 1963 study revealed the role of microscopic gold and silver nanoparticles, smaller than grains of sand, present in the glass. Lit from the front, the material appears milky green; lit from behind, it becomes almost translucent and red. It is the interaction of light with these metallic particles that creates the perception of the colour change.
Reproductions and new experiments
Attempts at reproduction have multiplied. According to the 2007 study cited by Live Science, a replica made by Corning Glassworks in the 1960s shifted from green to amethyst. In 2020, researchers incorporated silver and gold nanoparticles into a 3D print in order to reproduce the properties of this glass.
More recently, experts at the University of Sheffield, in the United Kingdom, reproduced dichroic glass by reconstructing the "recipe" from the original. They then commissioned a glass artist to create an object inspired by the Lycurgus Cup, showing that a lost craft skill can be recovered through experimentation.
A question still open
Experts still do not know whether Roman craftsmen consciously mastered this effect. One hypothesis holds that traces of gold, available in an ancient workshop, accidentally slipped into the molten glass, giving rise to a rare material that the glassmakers themselves would not have understood.
This uncertainty makes the Lycurgus Cup one of the most discussed pieces of Roman glass art. It also illustrates the technical sophistication of antiquity, capable of manipulating, without necessarily understanding, optical phenomena that modern science only described in the 20th century.
Frequently asked questions
Why does the Lycurgus Cup change colour?
The glass contains microscopic gold and silver nanoparticles, added during the blowing process. Depending on the direction of the light, they alter the way the glass scatters and transmits rays, making it appear green or red.
Where can the Lycurgus Cup be seen?
The Lycurgus Cup is held at the British Museum in London, where it was acquired in 1958. It is displayed among the museum's Roman collections.
Who was King Lycurgus?
According to the myth, Lycurgus was king of the Edonians, a Thracian tribe. He opposed the cult of Dionysus and was punished by a priestess who transformed into a vine that entangled him.
Sources
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.
