A dichroic glass cup with a mythological scene
It has been suggested that this not very common scene was a reference to the defeat in 324 by the Emperor Constantine I of his co-emperor Licinius, who was killed in 325 after a period under close guard.Another suggestion is that the colour change from green to red was understood as evoking the ripening of red grapes, so that there was a particular suitability in depicting a scene with the god of wine. The cup may have been intended for use at Bacchic cult celebrations, still very much a feature of Roman religious life around 300. A letter suppposedly from the Emperor Hadrian (d. 138) to his brother-in-law Servianus, quoted in a biography in the Historia Augusta, records the gift of two dichroic cups, which the 4th century author had seen: "I have sent you particoloured cups that change colour, presented to me by the 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."
Other depictions of the story tend to either depict Lycurgus attacking Ambrosia, often with a double-headed axe, while her companions rush to her aid, or Lycurgus alone, entangled in the vine. The closest parallel to the scene on the cup is one of the apse mosaics in the triconch triclinium at the Villa del Casale, Piazza Armerina, which may also refer to Licinius. There is also a mosaic at Antioch on the Orontes, and a group on a 2nd century sarcophagus at the Villa Parisi in Frascati.There is also a floor-mosaic from Vienne, now in the museum at Saint-Romain-en-Gal, with Lycurgus alone inside the vine.The preceding scene of Lycurgus attacking Ambrosia, is on a floor-mosaic at Brading Roman Villa on the Isle of Wight.Of this and similar mosaics, Martin Henig says: "In cases such as this, we are not concerned with simple, popular paganism but with recondite knowledge. This is the sort of esoteric religion which the Emperor Julian, Symmachus, Praetextatus, Macrobius and Proclus relished. The religious thought behind these floors is probably deeper and more complex than contemporary Christianity and many of the keys to understanding it have been lost."
The dichroic effect is achieved by making the glass with tiny proportions of minutely ground gold and silver dust. To a conventionally composed Roman glass flux 330 parts per million of silver and 40 of gold were added: "These particles were precipitated as colloids and form a silver-gold alloy. When viewed in reflected light the minute metallic particles are just coarse enough to reflect enough of the light without eliminating the transmission. In transmitted light the fine particles scatter the blue end of the spectrum more effectively than the red end, resulting in red transmission, and this is the colour observed. Since it is impossible that the Roman artisans managed to add these incredibly low levels of silver and gold to the volume of the glass used to make the vessel deliberately, the levels were probably added at higher levels to a larger volume of glass-melt, and increasingly diluted by adding more glass." The particles are only about 70 nanometers across, and embedded in the glass, so they cannot be seen by optical microscopy, and a transmission electron microscope is needed instead.At this size they approach the size of the wavelengths of visible light, and a surface plasmon resonance effect takes place.The interior of the cup is mostly smooth, but behind the main figures the glass has been hollowed out, well beyond even the main outer surface, so that they are of similar thickness to the main outer surface, giving an even colour when light passes through. This is a feature unique among surviving cups; Harden suggests they were an "afterthought".An area around the torso of Lycurgus is a rather different colour from the rest of the glass; perhaps an accident of manufacture, but one exploited by the glass-cutter "so that he could make Lycurgus's rage glow even more strongly".After the very lengthy cutting stage the fine polished appearance was achieved by a process called "flame polishing" that risked the complete loss of the object.A suggestion in 1995 that in fact this and other cage cups used a mixture of moulding and cutting has met with little acceptance.
The cup was probably designed for drinking from at feasts, or more specifically Bacchic cult celebrations,where the lack of a foot, also a feature found in other cage-cups, may mean it was passed around, as elaborate cups often were in medieval cultures. Alternatively other cage cups were almost certainly used, suspended, as oil lamps, where the dichroic effect of this cup would show to advantage.