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The stunning magic of ancient Roman nano-alchemy: a technology 1,700 years ahead of its time but completely lost in history?

Published on 22 Sept, 2021


Archaeologists know that glass has been around since Late Bronze Age in the Near East, predating the rise of Rome by almost a millennium. However, the Romans were so renowned for their skills of glass-making throughout history that many firsts in glass-manufacturing technologies and applications of glass went to the Romans. The Romans were the first to use glass panels for making windows, they were the first to invent double-glazed glass window to insulate heat and sound, they also made beautifully decorated and colored glassware that earnt international reputation.

In fact, the craftmanship of Roman glassware was well-known not just in Europe, the Mediterranean world and the Middle East, they were even exported to the far East where they became highly prized luxury items. Ancient Chinese author Du Huan, a Tang Dynasty prisoner-of-war captured by the Arabic Army, has left us with an important eyewitness account of his contemporary Western Eurasia from a rare East Asian perspective. In his literary work "Record of Travels" (Jingxingji), he described Roman glass (Byzantine glass at his time) as "the finest glassware unrivaled anywhere on Earth". However, Roman glass had earnt its reputation in China long before Du Huan. Aristocrats of the Six Dynasties (222-589 CE) were craving for insanely expensive Roman and Persian glassware, similar to how Roman aristocrats craved for Asian silk. Glass was one of the most important Roman exports to Asia, as silk and spices flowed in the opposite direction in the Roman-Asian trade.

Roman glassware discovered in 5    Century Japanese grave in Nara


Chemical analysis showed that the blue dish has a chemical signature identical to 2    Century Roman glassware made in the Mediterranean region, however, the blue paint was likely applied in a workshop in Ctesiphon, the capital of Parthian and Sassanid Empires.  Source:


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In antiquity, maritime and overland trades flourished and created some form of proto-globalization in the Eurasian continent and North Africa. Roman glass had reached as far as Japan, a historical fact demonstrated vividly by the recent discovery of unmistakably Roman glassware among the grave goods in ancient Japan. The Fifth-Century grave postdated the production date of the glassware by some three Centuries, which could be explained by an indirect trade through Central Asia and China.

Roman glass earnt its reputation through its sheer sophistication and artistic value, and just like Asian silk in Roman Empire, fine Roman glassware was valued more than Gold of the same weight in East Asia. However, it is not in Asia where archaeologists found the finest product of the once world-known Roman glass industry. The most mysterious, beautiful, and stunning Roman glass artifact is in fact exhibited in the British Museum, London.

The Lycurgus Cup

In the Late Antiquity to Medieval Period  Europe (300-1,100 CE) area, exhibition hall 41 of the British Museum, lies a seemingly humble glass cup. With a height of 16.5cm and a diameter of 13.2cm, the Roman glass cup is no larger than an ordinary modern glass wine cup. However, this cup once held one of the greatest mysteries of Roman technological history. Until fairly recently, no one in the entire world could fully understand what was the cup made of, and how it was made.

The Lycurgus Cup reliefs at different angles

The Lycurgus Cup reliefs retold a story from the Homeric Epic


If we need to understand why this cup is so special and mysterious, we first need to take a closer look at the cup itself. The cup depicted a scene recorded on Iliad Book VI. Lycurgus, the Thracian King, prohibited his citizens from worshipping Dionysus, the Greek deity for wine, and his action had enraged Dionysus. This resulted in a war between the gods and a mortal. On the cup, relief shows Lycurgus was attacking Dionysus and his follower Ambrosia, the latter called upon Gaia to help. Gaia turned Ambrosia into a grapevine and it entangled Lycurgus, rendering him immobile. Dionysus took this opportunity to stage a counteroffensive with his tiger and staff, finally defeating Lycurgus and ending his "tyranny" against the wine-lovers of Thrace.

The glass cup has been named "The Lycurgus Cup" after the main antagonist depicted on its reliefs: King Lycurgus of Thrace.

Considering the fact that the cup was made in early 4    Century and its high value, it is highly likely that it was a custom-made product tailored for the most powerful man in the Empire: Constantine the Great. Constantine probably ordered the cup to be made to celebrate his triumph over Licinius (Eastern Roman Emperor from 308 to 324 CE) and the reunification of the Roman Empire. The pagan iconography of the cup echoed Constantine and the Christian God's triumph over Licinius, and the traditional Roman polytheistic religion.

The reliefs of the cup shows remarkable sophistication: human figures are so lively, the details are extremely rich and complete. The hopeless, distorted face of King Lycurgus is striking even at first glance. How the cup was made had been up to speculations since its rediscovery and public exhibition. But this is not its most remarkable feature. The Lycurgus Cup appears dark green when there is external lighting, however, its color changes dramatically to translucent purplish-red, like a full cup of red wine, if the light source is moved inside the cup. How is that possible? How the reliefs on the cup was made? And what makes the cup change color when the lighting condition changes?


The Lycurgus Cup changes color when lighting condition changes

When the cup is subjected to external or natural lighting, its color appears solid dark green, however, when the light source is moved inside the cup, it appears translucent purplish-red.

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How the reliefs were made?

The Lycurgus Cup belongs to a special type of luxurious Roman glassware called "cage cup". Surviving examples of cage cups are rare, approximately 50-100 examples exist worldwide. Most cage cups were discovered in Western Germany in the Rhine region. Most cage cups have an exterior glass layer of geometric patterns like a wire cage wrapping around the glass cup itself, and hence they are called "cage cups". Archaeologists believe that cage cups were once expensive luxurious items used by the super-rich and politically influential.

What makes the Lycurgus Cup stand out at first glance is its "cage" layer. Most cage cups have a cage layer composed of repetitive geometric patterns like this:

Cage cup on exhibition

Roman-Germanic Museum, Cologne (Römisch-Germanisches Museum, Köln)

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Ordinary cage cups required laborious skilled work to create, yet the Lycurgus Cup has a rich pattern significantly more sophisticated than ordinary cage cups. The human resources and efforts invested in the creation of Lycurgus Cup must have been unimaginable. But the key unanswered question is: how it was made.


Three competing hypotheses were proposed to explain the creation of the outer layer of the Lycurgus Cup:

  1. lost-wax casting or clay mold

  2. casting the two layers separately and then rejoin

  3. casting a thick-walled glass cup and then carve the reliefs on it


 Among the three hypotheses, lost-wax casting or the clay mold hypothesis could not explain the empty spaces behind the figures which maintain consistent light transmittance throughout the cup. Archaeologists also did not find any evidence of melting necessary for joining the two layers together after casting them separately. This left us with only one possibility. However, if the Lycurgus has always been one-piece, how the Romans were able to carve such a complicated pattern on such a hard surface? After all, glass is 6-7 on a hardness scale. It could not be effectively cut using metal tools available at the time. Also, this method required extreme focus and skills, as one wrong cut and the whole cup was finished. How was it possible for the Roman artisans to possess the skills necessary for making the Lycurgus Cup?

Surface of a fragment of glass from the Lycurgus Cup

Backscattered electron image taken in the scanning electron microscope of the cut-work fragment from the Lycurgus Cup, showing coarse and fine abrasion striations on the back of the fragment. (Source: The Trustees of the British Museum)

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Scientists from Corning Glass, manufacturer of the Gorilla Glass used on modern smartphones, were able to obtain a fragment from the cup from the British Museum to do scientific analysis. They determined that the Lycurgus Cup was casted in one piece and the reliefs were meticulously carved mechanically using a rotating blade 6-12mm in diameter. The rotating blade was likely a metal cylinder with sapphire or diamond attached at its tip. The use of diamond blade to cut other hardstones was well-documented by Pliny the Elder (23-79 CE). Fine quartz powder was also used as an abrasive material. The process left countless microscopic parallel trenches on the glass surface which was detected by modern scientists when they magnified the glass fragment with a high-power electron microscope. It was also determined that the cup was likely made in a specialized glass workshop in Rome or Alexandria.

A glass cup that can change color

The crafting of the cup is remarkable enough, however, it is the material science itself that has attracted most scholarly attentions and intensive researches. As I mentioned before, the Lycurgus Cup can change color when the light source changes in location. This type of glass has a formal name: dichroic glass. The physical principle is simple: the cup appears dark green when external light source is applied because it reflects green light while allowing red and blue light to pass through. It appears purplish-red when the light source is put inside because only red and blue light can penetrate the glass to reach the observer, while reflecting green light back and trapping it inside the cup. Until recently, the explanation of this spectacular yet bizarre optical phenomenon had eluded the best material scientists in the world. 

Actually, scientists even debated whether it was indeed glass before it was confirmed by X-ray scattering analysis in 1959, because back then, no known glass possessed this physical property. However, chemical analysis in 1959 did tell us something. The glass from Lycurgus Cup contains 1% of Gold and Silver, plus 0.5% of Manganese. Material scientist B. S. Cooper from the General Electric researcher team speculated that the trace metals inside the glass could be causing the complex scattering that gave rise to its observed optical property.

Since the cup was made in a tightly controlled environment, scientists believed that the cup was a product of accident. It was not until later when more Roman glass cups that could change color were discovered, confirming that the Romans indeed possessed the technology to make this type of special glass almost 2,000 years before us.

Other examples of color-changing glass cups made in ancient Rome

Another Roman cage cup that can change color. Even though its color change is not as dramatic as the Lycurgus Cup, its discovery nevertheless confirmed that the Lycurgus Cup is not a product of pure accident. (Source: The Trustees of the British Museum)

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The magic of ancient Roman nano-alchemy

In order to confirm or falsify this hypothesis, the British Museum sent samples to Corning Glass for further analysis in 1962. Dr. Robert Brill from Corning Glass worked closely with the GE team, and they successfully isolated 40ppm (parts per million) of Gold and 300ppm of Silver from the glass fragment. However, the presence of Gold and Silver alone did not explain the color change. Brill speculated that trace among of Gold and Silver were added into the glass in molten state, and the glass was heat treated with reducing agents, causing the Gold and Silver ions to form nanoscale colloids as they were reduced back to its element form. It was the diameter of the colloids that determined the optical property observed.


We got some hints from the experiments in the 1960s. Unfortunately, Brill's experiment did not prove the existence of colloids in the Lycurgus Cup. Also, even the best technologies available at that time were used, the Lycurgus Cup was still tight-lipped about its last secret, as it had been doing for more than one and a half millennia, until 20 years later...

In the late 1980s, advances in the field of electron microscopy finally enabled material scientists to hunt for the lost alloy colloids in the Lycurgus Cup. Analysis results immediately shocked the entire academia. This 1,700 years old Roman glass cup indeed has metallic nanoparticles with an average diameter of 70 nanometers, smaller than the most advanced mass-production computer chip transistor at the time, which was 1 micron (1,000 nanometers). X-ray scattering analysis done on these nanoparticles confirmed that they were indeed Gold-Silver alloy, with a Au:Ag ratio of 3:7, with an additional 10% Copper. Reduction reaction caused the ions to reduce back to their element form, and the two metals formed an alloy that dispersed in the molten glass as nanoparticles. The Gold in the particles allowed red light to pass through, halogens of the salt added into the glass enhanced the effect, while the Silver part scattered green light.


The Lycurgus Cup finally revealed its last secret: its color can change because the nanoparticles in the glass was causing surface plasmon resonance. The final puzzle of the Lycurgus Cup's mystery was finally pieced together when the research result was peer-reviewed and published on the Journal Archaeometry in 1990.

Silver-Gold nanoparticles found inside the Lycurgus Cup

The culprit of the spectacular optical phenomenon observed in the Lycurgus Cup: the ancient Romans were able to use advanced chemical and physical treatment processes to make nanoscale metal alloy colloids to selectively transmit/reflect light with different wavelengths. (Source: The Trustees of the British Museum)

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Physical principles of surface plasmon resonance

Surface plasmon resonance is used extensively on modern biochemical and chemical analytical equipment. (Source: Nanophotonics and Metrology Laboratory, EPFL Lausanne, Switzerland)

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The magic of ancient Roman nano-alchemy

The fact that ancient Romans were able to make dichroic glass immediately sent shockwaves across the academia worldwide. Its implications shook the very foundation of our understanding and the then-academic consensus regarding Roman science and technology. Traditionally, the commonly-held belief was that the Romans were not so innovative and they copied things from other civilizations most of the times, with little original innovation. However, the decipherment of the Lycurgus Cup's secret changed everything.

Even though gold was used to make red glass in 16-17    Century European Cathedrals, the physical phenomenon linking redox treatments using metal compound solutions and the generation of red color had not been investigated until the 19    Century. Dichroic glass itself was reinvented only by NASA for remote sensing satellite filters in the latter half of the 20    Century. Lycurgus Cup was made of true dichroic glass, but it predated the reinvention of this type of glass by almost 1,700 years. It also means that the cup must be genuine, because when the cup was first exhibited, we even could not replicate it using the best technology available at that time.


The production of Lycurgus Cup required manipulating something on a nanoscale and it is obvious that the temperature, duration of reaction and the use of redox agents were tightly controlled and ingeniously designed by the glassmith and workshop that made it. The mind-boggling sophistication in the complex physical and chemical treatments that led to the creation of this ancient technological marvel was no small feat in the world almost 2,000 years in the past, and it still inspires awe, appreciation, and respect many Centuries later in the modern world.




References and footnotes

  1. Barber D. J. and Freestone I. C. (1990) An investigation of the origin of the colour of the Lycurgus Cup by analytical transmission electron microscopy. Archaeometry 32, pp.33-45.

  2. Freestone I. et. al (2007) The Lycurgus Cup – A Roman Nanotechnology. Gold Bulletin 40(4), pp.270-277.


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