The Lycurgus Cup is a Roman glass goblet that changes from jade green to ruby red depending on whether light shines on its front or its back, Live Science reported. Experts are still unsure where the cup was found, where it was made, or whether the glassblower who made it created that color-changing effect on purpose. The British Museum acquired it in 1958 from the Rothschild banking family, which had owned the artifact since the mid-19th century. The glass is about 6.3 inches (15.9 centimeters) tall and weighs around 1.6 pounds (700 grams). It is a well-preserved example of a Roman "cage-cup," according to a 1959 study by archaeologists D. B. Harden and Jocelyn Toynbee. Only a handful of cage-cups still exist. Experts believe they were made by cutting and grinding decoration into thick glass, leaving small bridges so the decoration stays attached, like a cage. Harden and Toynbee dated the cup to the fourth century, likely in Italy, and noted that the bronze frame forming the stem and rim is probably a 19th-century addition.
The decoration shows the punishment of Lycurgus, king of the Thracian Edoni. In the myth, Lycurgus disapproved of the worship of Dionysus and drove the god into the sea with an ax. One of Dionysus's priestesses, Ambrosia, turned herself into a vine and wound around Lycurgus so he could not escape, while other priestesses tormented him. Because Dionysus was a god of wine and merriment, that scene may have been a fitting subject for a drinking cup. The shift from green to red may also have been meant to mimic grapes ripening on the vine, though that, too, is interpretation rather than a documented instruction from the maker.
Color-changing, or dichroic, cups are mentioned in Roman records. A second-century letter possibly penned by Emperor Hadrian to his brother-in-law mentions a gift of two such cups for banquets. Very few fragments of this glass survive. For decades, researchers have tried to work out how Romans produced it, and some have replicated the effect. A 1963 study confirmed that the key appears to be microscopic nanoparticles of gold and silver added during glassblowing, which make the glass look milky green when lit from the front and nearly translucent red when lit from behind. The metal particles are smaller than grains of sand and interact with light to create the perceived color change. A replica made by Corning Glassworks in the 1960s went from green to amethyst, a 2007 study reported, and in 2020 researchers included silver and gold nanoparticles in 3-D printing to copy the properties. More recently, experts at the University of Sheffield reverse-engineered a "recipe" from the original cup and commissioned a glass artist to make a Lycurgus-inspired design.
Because so few examples exist, specialists are still unsure whether the color shift was a planned piece of ancient nanotechnology or a lucky accident, with traces of gold in a workshop slipping into molten glass and mystifying even skilled blowers. The verified object is in the British Museum, the chemistry is reasonably well described, and the intention of the maker remains an open question.