Excuse me, have I just drunk from a red or green cup?

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ROME, 4th Century A.C., an entertaining souvenir or a probe for royal poisons? This way or another, a Lycurgus cup remained a secret for scientist until the last decade of the 20th century. This chalice changes color: from green if lit from the front and scarlet if lit from behind. Craftsmen in Rome have grounded silver and gold till they got a nano-particle, a thousandth of a grain of a table salt!

Particles were then added to glass. Here is an explanation of Gang Logan Liu, an engineer of University of Illinois: "When hit with light, electrons belonging to the metal flecks vibrate in ways that alter the color depending on the observer’s position. When various fluids filled the cup, Liu suspected, they would change how the vibrating electrons in the glass interacted, and thus the color."

Drinks, anyone?

Full article can be found on The Smithsonian Magazine website.