In the 1950s, the British Museum came into possession an ancient glass chalice called the Lycurgus Cup, so named for its depiction of Dionysus’s triumph over King Lycurgus of Thrace, who is shown entangled in grape vines, on the cup’s outer surface. The craftsmanship is excellent — the inside is smooth while the outside has been painstakingly cut and etched to create a decorative cage-like structure around the inner cup. This class of Roman vessels are known as cage cups, and they were mostly made during the 4th century CE. About fifty cups or so, mostly in fragments, have survived, with only a few in near-complete condition. The Lycurgus Cup is one of the best preserved Roman cage cups.
Cage cups were clearly very difficult to make, and no doubt very expensive, but this particular specimen stands apart because it exhibits a strange optical phenomenon that had stumped experts for decades. Under normal lighting, the glass appears jade green, but when lit from behind, it turns ruby red. Initially, experts weren’t sure whether the cup was made of glass, or was a gemstone. It wasn’t until 1990, that researchers figured out how the color changers were brought about.
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