The Lycurgus Cup
Later explainedSummary
The Lycurgus Cup is a fourth-century Roman glass cup in the British Museum that looks green when lit from the front and red when lit from behind. The cause is minute particles of a silver-gold alloy dispersed through the glass, identified by electron microscopy in 1990.
It is a cage cup, its figures cut and ground from a thick glass blank, and it shows the death of the mythical King Lycurgus. Chemists linked its colours to traces of gold and silver in the 1960s; the 1990 study showed the form the metal takes. How deliberately the Romans achieved the effect is less clear than the popular story says.
What is documented
The object. The British Museum dates the cup to the fourth century AD, Late Roman. It is 158.8 millimetres high and 132 millimetres across at its widest, and weighs 700 grams. Its figures, representing the death of King Lycurgus, stand out from the body of the cup as an openwork cage. Cups of this kind were made, as Ian Freestone and colleagues put it, by cutting and grinding the openwork decoration out of a thick-walled blank of cast or blown glass. The silver-gilt rim band and the openwork foot of vine leaves are thought to date from the eighteenth or nineteenth century. The foot was separated from the cup in 1958 and reunited with it in 1973.
The colours. In reflected light the glass is green. Lit from behind, it turns a translucent ruby red. Other Roman glass with a similar two-colour effect survives, but, in the words of the 2007 study, "even the colours of the other dichroic glasses do not replicate the Lycurgus effect exactly".
Where and when it was made. The fourth-century date rests on style and on comparable pieces. D. B. Harden and J. M. C. Toynbee, who published the cup in 1959, thought it probably Italian, with Alexandria also possible.
Its history. The early history of the cup is unknown, and so is its find spot. It was exhibited at the Society of Arts in London in 1850, according to the British Museum's record, which cites the Illustrated London News of 13 April 1850 and the exhibition catalogue. It was acquired by Baron Lionel Nathan de Rothschild (1808 to 1879), and the museum bought it from Victor, 3rd Lord Rothschild, in 1958, with a contribution from the Art Fund, then the National Art Collections Fund. It is on display in Room 41.
The chemistry. Analysis at the laboratories of the General Electric Company (GEC) showed the cup to be a soda-lime-silica glass like most Roman glass, with a small fraction of trace elements that included silver and gold. Work by the glass chemist R. H. Brill, later in collaboration with GEC, on the cup, on other samples and on experimental glass melts, linked the two colours to minute amounts of gold and silver in the glass. Brill's paper appeared in 1965; an analytical study of the cup by Chirnside and Proffitt had appeared in 1963.
The 1990 study. D. J. Barber and I. C. Freestone examined the glass by analytical transmission electron microscopy. They found minute particles of metal, which X-ray analysis showed to be an alloy of silver and gold with some copper, dispersed through the glass, together with particles of sodium chloride. Their paper appeared in Archaeometry in 1990.
The 2007 study. Freestone, Nigel Meeks, Margaret Sax and Catherine Higgitt reviewed the evidence in Gold Bulletin in 2007. They concluded that "the reduction of previously dissolved silver and gold during heat treatment of the glass" produced the dispersion of silver-gold nanoparticles responsible for the colour, and that antimony in the glass was a likely agent of that reduction.
Leading explanations
Silver-gold alloy nanoparticles in the glass. Established. The colours come from metal nanoparticles dispersed through the glass: seen by reflected light the cup looks green, and light passing through it emerges red. The particles were observed directly in 1990, and their composition is consistent with the earlier chemical findings.
How the particles got there. Proposed in 2007, consistent with the chemistry. On the 2007 study's account, the silver and gold were first dissolved in the glass and came out of solution as nanoparticles when the glass was heat-treated, with antimony helping the reaction. The particles were formed in the glass, not added to it ready-made.
How deliberate it was (open). The same study judges it "quite likely that the glassmakers were unaware that gold was the critical colourant", since most of these glasses are richer in silver, and calls the colouring of glass with gold and silver "far from routine and something of a hit and miss affair".
What the popular version gets wrong
"The Romans ground silver and gold into particles 50 nanometres across." Smithsonian magazine wrote in 2013 that Roman artisans "impregnated the glass with particles of silver and gold, ground down until they were as small as 50 nanometers in diameter". On the 2007 study's account they were not ground at all: the particles formed inside the glass, from silver and gold already dissolved in it, when the glass was heat-treated.
"The Romans knew exactly what they were doing." The same article says the exact mixture of metals suggests the Romans knew what they were doing. The 2007 study, co-written by the researcher the article quotes, concludes that the makers were quite likely unaware that gold was the critical colourant, and that the process was hit and miss. "Roman nanotechnology" describes the size of the particles, not the makers' understanding.
"The mystery was not solved until 1990." The link between the colours and small amounts of gold and silver had been made by chemical analysis in the 1960s. What the 1990 microscopy added was the form the metal takes: nanoparticles of silver-gold alloy.
Current status
Later explained. The colour change is explained by silver-gold alloy nanoparticles, observed directly and consistent with the glass's chemistry. It stays in the registry because its colours were a genuine puzzle until they were analysed, and because the explanation is more interesting than the legend. What remains open is historical rather than physical: where the cup was made, and how far its makers controlled the process.
Sources
- British Museum. Collection record 1958,1202.1, "The Lycurgus Cup": dimensions, date, mounts, acquisition, exhibition history and bibliography.
- Freestone, I., Meeks, N., Sax, M. and Higgitt, C. (2007). "The Lycurgus Cup: A Roman nanotechnology." Gold Bulletin 40(4), 270 to 277.
- Barber, D. J. and Freestone, I. C. (1990). "An investigation of the origin of the colour of the Lycurgus Cup by analytical transmission electron microscopy." Archaeometry 32, 33 to 45. Known to the registry through Freestone et al. (2007).
- Harden, D. B. and Toynbee, J. M. C. (1959). "The Rothschild Lycurgus Cup." Archaeologia 97, 179 to 212. Known to the registry through Freestone et al. (2007) and the British Museum record.
- Brill, R. H. (1965). "The chemistry of the Lycurgus Cup." Proceedings of the 7th International Congress on Glass, Brussels, paper 223. Known to the registry through the same sources.
- Merali, Z. (2013). "This 1,600-Year-Old Goblet Shows that the Romans Were Nanotechnology Pioneers." Smithsonian magazine, September 2013.
- Kool, L., Dekker, F., Bunschoten, A., Smales, G. J., Pauw, B. R., Velders, A. H. and Saggiomo, V. (2020). "Gold and silver dichroic nanocomposite in the quest for 3D printing the Lycurgus cup." Beilstein Journal of Nanotechnology 11, 16 to 23.
Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.