A Cup That Shifts Colors
The Lycurgus Cup, made in the 4th century, is a Roman glass vessel that astonishes by changing color: green when viewed in reflected light and red when illuminated from behind. It is the only complete surviving example of Roman dichroic glass, and its preservation makes it one of the most spectacular artifacts of ancient craftsmanship.
Myth Carved in Glass
This vessel is a rare cage cup, or diatretum, where the outer glass was painstakingly cut away to leave a decorative cage. Unlike most with geometric patterns, this cup depicts the myth of King Lycurgus, who attacked Ambrosia, a follower of Dionysus.
She transformed into a vine that trapped him, while Dionysus and his companions looked on. It is the only well-preserved figural cage cup known from the Roman world.
Nanoparticles Behind the Glow
The cup’s shifting hues come from nanoparticles of gold and silver embedded in the glass, each about 70 nanometers across. These scatter light to produce the color change. Scholars believe the effect was discovered accidentally through contamination in workshops.
Roman artisans may not have known gold was present, since most silver at the time contained tiny traces of it. Modern electron microscopy confirmed how this interaction of metal and light creates the dazzling effect.
Survival and Rediscovery
The Lycurgus Cup measures 16.5 by 13.2 centimeters and likely involved several workshops: one for producing the thick dichroic blank, and another for the cutting. Flame polishing completed the finish, though it risked shattering the piece.
Its early history is unknown, but by the mid-19th century it was in the Rothschild collection. In 1958, it was acquired by the British Museum, where it remains today. Modern experiments, from Corning Glass Works to 3D-printed nanocomposites, have recreated its color-changing effect, proving Roman glassmakers achieved something remarkably advanced.
Shift the light, and a 4th-century Roman cup swings from jade green to blood-red.
This is the Lycurgus Cup—the only complete Roman dichroic glass known—turning science into theatre and luring you into a tale of gods, glass, and a mystery forged in fire…🧵👇 pic.twitter.com/dcze14dJHu
— Fascinating True Stories (@FascinatingTrue) September 22, 2025