The Lycurgus Cup is a 4th-century Roman glass goblet made of dichroic glass that changes color depending on whether light passes through it: red when illuminated from behind and green when illuminated from the front. It is the only complete Roman glass object made of this type of glass, and the one that exhibits the most impressive color changes; it has been described as "the most spectacular glass of the period, appropriately decorated, that we know existed."
The bowl is also a very rare example of a complete Roman cage bowl, or diatretum, where the glass was carefully cut and sanded to leave only the decorative "cage" on the original surface level. Many parts of the cage have been completely cut out. On most of the cage cups, the cage has a geometric abstract design, but here there is a composition with figures depicting the mythical king Lycurgus, who (depending on the version) tried to kill Ambrosia, a follower of the god Dionysus (Bacchus for the Romans). She transformed herself into a vine, which wrapped itself around the enraged king, restraining him, and finally killed him. Dionysus and his two followers taunt the king. The bowl is the "only well-preserved example" of a caged bowl.
The dichroic effect is achieved by making glass with tiny proportions of gold and silver nanoparticles dispersed in colloidal form throughout the glass material. The process used remains unclear, and it is likely that it was not well studied or controlled by the manufacturers, and was probably discovered as a result of accidental "contamination" by tiny gold and silver dust. The makers of the glass might not even have known that it was gold, as the amount is so negligible; it could have come from a small proportion of gold in the silver (most Roman silver contains a small proportion of gold), or from traces of gold or gold leaf accidentally left in the workshop, as residue on tools or as a result of other work. Very few other surviving Roman dichroic glass fragments vary significantly in the two colors.
It has been estimated that 330 parts per million of silver and 40 parts of gold were added to the normal composition of Roman glass flux: "These particles precipitated as colloids and formed an alloy of silver and gold. When viewed in reflected light, the tiny metal particles are large enough to reflect enough light without excluding light transmission. In transmitted light, the fine particles scatter the blue end of the spectrum more effectively than the red end, resulting in the transmission of red, and that is the color observed." Since it is impossible that the Roman craftsmen were able to add these incredibly low levels of silver and gold to the volume of glass used to make the vessel intentionally, these levels were probably added at higher levels to a larger volume of molten glass, and increasingly diluted as more glass was added." The particles are only about 70 nanometers across and are embedded in the glass, so they cannot be seen with optical microscopy, instead requiring a transmission electron microscope. At this size they approach the size of the wavelengths of visible light, and a surface plasmon resonance effect occurs.
The inner surface of the bowl is mostly smooth, but behind the main figures in the glass, recesses are made far beyond even the main outer surface, so that they have the same thickness as the main outer surface, which gives an even color when light passes through. This is a unique feature of the surviving goblets; Harden suggests that they were "reworkings." The area around the torso of Lycurgus is different in color from the rest of the glass; it may have been an accident of manufacture, but the glass cutter took advantage of it "to make Lycurgus' fury glow even more." After a very long stage of cutting, the fine polished look was achieved by a process called "flame polishing," which risked the total loss of the object. The suggestion in 1995 that a mixture of molding and cutting was actually used in this and other cage bowls met with little support.
The figure of Lycurgus, bound by a vine and naked except for his boots, is complemented on the left by a crouching Ambrosia, depicted on a much smaller scale. Behind her, one of Dionysus the Satyr (depicted in normal human form) stands on one leg, preparing to throw a large stone at Lycurgus. In his other hand he holds a pedum or shepherd's staff. To the right of Lycurgus appears first the figure of Pan, then at his feet a rather fanged panther, the traditional companion of Dionysus, whose face is missing, but presumably she snaps at the king, and then the god himself, taunting him with his right arm outstretched in an angry gesture. Dionysus carries a thyrsus, a special staff of the god and his followers, and his clothing has an eastern, perhaps Indian, coloration, reflecting what the ancient Greeks generally believed (perhaps erroneously) about the origins of his cult. The calf of one leg is lost. A streamer hanging behind him from the thyrs overlaps the raised leg of the satyr with the rock, completing the circle of the bowl.
It has been suggested that this not very common scene was a reference to the defeat in 324 of Emperor Constantine I by his co-emperor Licinius, who was killed in 325 after a period spent under close guard. Another suggestion is that the color change from green to red was taken as a reminder of the ripening of red grapes, so that the depiction of the scene with the god of wine was particularly appropriate. Perhaps the goblet was intended for use during the celebration of Bacchic cults, which were still characteristic of Roman religious life around 300. A letter presumably from the emperor Hadrian (d. 138) to his brother-in-law Servianus, quoted in a biography in Historia Augusta, speaks of a gift of two dichroic goblets which the fourth-century author saw: "I have sent you multicolored goblets which change color, given to me by a priest of a temple. They are specially dedicated to you and my sister. I would like you to use them at banquets on feast days."