Nanotechnology in the Roman Empire

Lycurgus Cup

Sometime around the year 300, artisans in the Roman Empire created the Lycurgus Cup, using technology that was not understood for almost 1,700 years.

The glass cup has properties that baffled glass-makers and scientists alike: When lit from the front, the cup is green; however, when lit from the rear, the cup is red. The color depends on the relationship between the viewer and the light source -- in other words, two different viewers could see the cup as two different colors at the same time. In ancient times, simply pouring different liquids into the cup would also change the color of the glass. How can this be so? More importantly, would Romans drink green wine?

The cup was lost to history until the mid-19th century, and the British Museum eventually acquired it from Lord Rothschild in 1958. Scientists analyzed it for decades before discovering the secret in 1990 -- gold and silver. There are 40 parts/million of gold (.004%), and a whopping 330 parts/million (.033%) of silver. However, that was not the only secret: The Roman artisans had ground the gold and silver into nanoparticles only 50-70 nanometers in diameter -- about a thousand times smaller than a grain of salt. The size of the particles is so small, and the amounts so infinitesimal, that they cannot be detected by standard optical microscopes. Their presence was not discovered until the use of a transmission electron microscope. How were they able to grind the metal so small? We haven't figured that part out yet.

How does it work? Localized surface plasmon resonance. Look it up. The glass is a homogeneous continuum material described by a frequency-dependent relative permittivity between the external medium and the surface, the materials' dielectric constant, which is complex permittivity. Ha! I was just showing off -- even though I've just described much of the process, I really have no idea what that means. Let's put it this way: When light hits the metal nanoparticles embedded within the glass mixture, their electrons vibrate in ways that change the color of the glass, based on the angle of observation. When liquids were poured into the cup, they also would affect how the metals' electrons would vibrate, thus changing the cup's color even when the light is constant.

As a result of the science discovered in the cup, modern scientists are now using this same technique -- employing gold and silver nanoparticles -- to determine the presence of salt and other contaminants in liquids at a level that is a hundred times more sensitive than current commercial sensors.

The Lycurgus Cup is incredible for reasons beyond its color-shifting abilities. In fact, it is the only existing complete Roman object made with color-shifting glass (dichroic glass). It also is one of only a few surviving examples of a Roman diatretum, or "cage cup." A diatretum is created by taking a solid piece of glass and carving the outside surface until is resembles a "cage" that holds in place a "cup" -- which is the rest of the glass piece hollowed from within, so the sides are very thin. In this case, the metal stem and rim were added at a later date. Finally, the Lycurgus Cup is the only complete Roman diatretum in existence that depicts figures instead of a geometric pattern. The Lycurgus Cup is named after the scene carved into its exterior: King Lycurgus wrestling with vines.

It is likely that the Romans did not understand nuances of plasmon resonance or complex permittivity (in fact, neither do you and I). However, even though they might not understand the term "nanoparticle" (although it's derived from Greek, it wasn't used until 1983) they certainly comprehended its significance, and developed techniques in processing and using them that we still do not understand to this day.