This extraordinary cup is the only complete example of a very special type of glass, known as dichroic, which changes colour when held up to the light. The opaque green cup turns to a glowing translucent red when light is shone through it. The glass contains tiny amounts of colloidal gold and silver, which give it these unusual optical properties.
The cup is also the only figural example of a type of vessel known as a ‘cage-cup’. The cup was made by blowing or casting a thick glass blank. This was then cut and ground away until the figures were left in high relief. Sections of the figures are almost standing free and connected only by ‘bridges’ to the surface of the vessel.
The scene on the cup depicts an episode from the myth of Lycurgus, a king of the Thracians (around 800 BC). A man of violent temper, he attacked Dionysus and one of his maenads, Ambrosia. Ambrosia called out to Mother Earth, who transformed her into a vine. She then coiled herself about the king, and held him captive. The cup shows this moment when Lycurgus is entrapped by the branches of the vine, while Dionysus, Pan and a satyr torment him for his evil behaviour. It has been thought that the theme of this myth – the triumph of Dionysus over Lycurgus – might have been chosen to refer to a contemporary political event, the defeat of the emperor Licinius (reigned AD 308–324) by Constantine in AD 324.