In ordinary museum light, the Lycurgus Cup looks like opaque, jade-like glass in a greenish-yellow tone. Switch the light on behind it and the same glass glows ruby red. It is a late Roman vessel in the British Museum, and it has been puzzling people since it first appeared in print in 1845.
The color change is real, and so is the science behind it. The story that the Romans were “nanotechnologists” is only partly right, and what we do not know is the most interesting part.
What the cup is
The Lycurgus Cup belongs to a small class of Roman glass called cage cups, or diatreta. The decoration is carved in open work, standing clear of the cup wall and joined to it by thin bridges. According to a 2007 study by Ian Freestone and colleagues at the British Museum and Cardiff University, cage cups are generally dated to the fourth century AD, and probably only 50 to 100 examples are known. Most are decorated with simple lattices of linked circles. The Lycurgus Cup has a figural scene that is more elaborate and better preserved than any of the others.
The scene shows the myth of King Lycurgus of the Edoni in Thrace, who attacked the god Dionysus and his followers and was destroyed by them. The British Museum record describes a green and red glass drinking-cup with a silver-gilt rim band and foot; with its modern metal mounts it stands 16.5 cm tall and is 13.2 cm across. The British Museum acquired it from Lord Rothschild in 1958 with help from the National Art Collection Fund.
Green in the light, red in the dark
Freestone and his colleagues describe the glass as dichroic. In reflected light it looks like jade, with an opaque, greenish-yellow tone. When light shines through it, it turns a translucent ruby red. The craftsman also seems to have used the effect: the body of Lycurgus himself is cut from a patch of glass that looks more violet in transmitted light and more yellow in reflection. Behind the figures, the inside of the cup was hollowed or bored out so that light would pass through the bodies and the walls in similar amounts.
What is actually in the glass
The 2007 paper reports that analytical transmission electron microscopy found tiny metal particles in the glass, typically 50 to 100 nanometres across. X-ray analysis showed they are an alloy of silver and gold, with silver to gold in a ratio of about 7 to 3, plus roughly 10 percent copper. The authors conclude that the dichroic effect is caused by this colloidal metal.
Before anyone knew that, there was a debate over whether the cup was glass at all. The paper notes that its optical behaviour made some people doubt it could have been produced with known ancient glassworking. The Keeper of Mineralogy at the British Museum (Natural History) concluded that it was glass rather than opal or jade, and X-ray diffraction confirmed that in 1959.
Did the Romans know what they were doing?
The best reading of the evidence is: partly. Roman glassmakers did add metals and metal oxides to color glass. They made opaque reds and browns with copper, for example. Freestone and colleagues suggest that metallurgical by-products such as slag or dross may sometimes have been added to glass, and that this could explain how the Lycurgus effect was first discovered. They also say there are other possibilities, including accidents in making glass with gold-leaf decoration.
What the paper does not claim is that Romans understood nanoparticles. It says the nanoparticle effect is something we understand now, and that the inability to control the process meant relatively few glasses of this type were made and even fewer survive. In the authors’ words, understanding of how this glass was produced remains unclear.
How it was cut
The usual view is that cage cups were made by cutting and grinding the open work out of a thick-walled blank, leaving small glass bridges. The authors say a replica blank produced by the Corning Glassworks in the 1960s suggests the walls of the original blank were about 15 mm thick. Under the microscope, a cut-work fragment from the cup shows faint tool marks that suggest rotary abrasion and polishing, which the paper takes as evidence for cutting and grinding rather than moulding of soft glass. It also notes that another researcher has argued that diatreta like this may have been moulded rather than cut, so the method is not entirely settled.
What we do not know
- Where it came from. The cup was first mentioned in print in 1845, and the Rothschild family is thought to have acquired it soon after. Its earlier history and its find spot are unknown.
- Whether the color was planned. The red-green change may have been a deliberate effect, but the paper treats the discovery of the process as possibly accidental.
- How the glass was made. The method for producing the alloy particles in the glass is not fully understood.
- How many others existed. Relatively few glasses of this type survive, which makes it hard to know how rare the technique was originally.
Why it matters
The 2007 study calls the cup the outstanding example of this glass technology and a unique record of it. It is a case of an old object teaching new science, and a reminder that skilled makers can use a material well without having our words for it. For more on objects like this, see our guide to archaeology, objects and museum collections, or read how a corroded lump of bronze turned out to hold a Greek gear train.
Common questions
Why does the Lycurgus Cup change color?
The glass is dichroic. Analysis found tiny silver–gold alloy particles, about 50 to 100 nanometres across, which cause it to look greenish in reflected light and ruby red in transmitted light.
How old is the Lycurgus Cup?
It is a late Roman cage cup. Cage cups are generally dated to the fourth century AD.
Where is the Lycurgus Cup now?
It is in the British Museum in London, which acquired it from Lord Rothschild in 1958 (object 1958,1202.1).
Did the Romans know about nanotechnology?
Not in the modern sense. Roman glassmakers knew how to color glass with metals, but the researchers who studied the cup say the process was hard to control and that how this glass was made remains unclear. The nanoparticle explanation is a modern one.
Sources & Further Reading
- Ian Freestone, Nigel Meeks, Margaret Sax and Catherine Higgitt, “The Lycurgus Cup — A Roman Nanotechnology,” Gold Bulletin 40 (2007), pp. 270–277 — open-access study by British Museum and Cardiff researchers. Source for the dating, dimensions, acquisition, dichroism, particle size and composition, and the cutting and discovery discussion.
- British Museum collection record, object 1958,1202.1 — catalogue entry: glass, green and red, scenes of the death of King Lycurgus, with a silver-gilt rim band and foot.
- Wikimedia Commons, photograph by Chappsnet (front view) — CC BY 4.0.
- Wikimedia Commons, photograph by Chappsnet (top view) — CC BY 4.0.
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