Did an ancient Roman inventor come up with flexible glass? That’s one possible interpretation of a curious anecdote told by several Roman sources.
There is no archaeological evidence for flexible Roman glass; nothing like it has turned up in any excavation. All the evidence we have is literary, three mentions from various sources. Here is what we have (my own translations):
In the reign of Tiberius, a kind of glass was invented that was concocted in such a way that it was flexible, but the entire workshop of its inventor was destroyed so that the price of bronze, silver, and gold would not be brought down (a rumor that has for a long time had more repetition than credibility).Pliny, Natural History 36.66
There was once an artisan who made a glass drinking up that was unbreakable. When he was given an audience with the emperor to show off his invention, he made the emperor hand the cup back to him, then hurled it to the stone floor. The emperor could not have been more alarmed. The man picked the cup up off the ground, and it was dented just like a bronze cup, but he produced a small hammer from his pocket and with very little effort he made the cup good as new. With this performance, he thought he was in the throne of Jupiter.
The emperor then asked: “No one else knows how to make glass like this, do they?”
Now, look what happened. When the man answered “No,” the emperor ordered him beheaded, because if knowledge of this invention got out, we would treat gold like mud.Petronius, Satyricon 51
[An engineer comes up with a novel way of renovating a collapsing building, for which the emperor Tiberius jealously exiles him.] Later this man came to the emperor as a supplicant and deliberately let a glass drinking cup fall to the floor in front of him, and although the cup was somehow damaged, after rubbing and beating it with his hands on the spot he showed the emperor that it was unbroken. He was aiming to get himself a pardon, but the emperor ordered him executed.Cassius Dio, Roman History 57.21
Could it be true?
There are a few reasons to think these stories might be true, if not in all details then at least in the most interesting one: that some Roman crafter figured out how to make a material that looked like glass but didn’t shatter like it.
The fact that we have this story from three different sources gives it some credibility, especially since two of those sources, Pliny and Petronius, are roughly contemporary with the emperor Tiberius under whom the unbreakable glass was supposed to have been invented.
Flexible kinds of glass exist today, but they are recent developments. It is unlikely that a Roman glassmaker, even if they had stumbled on the right chemical formula, would have had a furnace capable of high enough temperatures with precise enough control to have achieved the same result. It is more possible to imagine that a Roman artisan came up with something like modern plastic. Early plastics developed in the 1800s used materials that would have been available to the Romans, such as cellulose from wood, the resin of the sweetgum tree, and proteins derived from milk, eggs, and blood. Some of the plastics derived from these materials are translucent and flexible, and might have appeared to onlookers unfamiliar with their source as flexible glass.
Despite these considerations, though, there are much stronger reasons to think that nothing like flexible glass was ever created in antiquity.
First of all, we have to look at our sources critically. None of them is very good as evidence. Pliny straight out tells us that he doesn’t believe the story he is relating. Petronius puts the story into the mouth of a boorish and narcissistic fictional character, far from a reliable narrator. And Cassius Dio was writing about two centuries later and seems to have garbled this story with the tale of a later emperor, Hadrian, and his jealousy of a famous architect. Although it is interesting that we have versions of this story from three different sources, all that means is that, as Pliny notes, it was a tale widely told, not necessarily that there was any truth to it.
The fact that this story is connected with Tiberius also points to it being unreliable. Pliny, Petronius, and Cassius Dio were all part of the Roman elite, who generally disliked Tiberius. As the second emperor of Rome after the beloved Augustus, Tiberius had big sandals to fill and little of his predecessor’s charisma and social grace. The accounts of Tiberius as emperor that have come down to us describe him as tactless, cynical, cruel, and prone to paranoia. He also ruled Rome during a time of economic hardship, and his pragmatic concern for financial stability (including worrying about things like the prices of commodities and the steadiness of the gold and silver supply) came off as small-minded stinginess to the rest of the Roman upper crust. The idea of Tiberius responding to a wondrous new invention by destroying both it and the inventor appealed to existing prejudices about him, which helped the story spread. Romans like Pliny and Petronius already believed that Tiberius was cruel when he should have been magnanimous, tight-fisted when he should have been generous, and quick to apply violence to those who did not deserve it. The story of the wondrous glass cup not only made these qualities manifest, it served as a cautionary tale about the foolishness of such behavior. It was, in short, a good story, and good stories spread easily even when they aren’t true.
If there is any kind of truth behind the tale, it may be something less revolutionary. Glassmaking is a skilled art, and in antiquity it practitioners carefully guarded their secrets. To an uninitiated observer, the malleability of hot glass in a glass-blower’s workshop may have seemed quite wondrous, and the story may have spread from there without the crucial understanding that glass only flows so easily when it is fresh from the furnace. Additionally, around the time of Tiberius, new kinds of mold-blown glass were coming onto the Roman market that imitated the shapes of metal vessels. To the average Roman aristocrat shopping for luxury housewares, the idea that a material might exist combining the translucency of glass with the malleability of metal might not seem so far-fetched. If these ideas were already circulating in Roman literary circles, it is not strange to imagine that someone put them together with the existing negative perceptions of Tiberius and concocted a “What if” story that took on a life of its own as gossip and political mudslinging.
In the end, it is unlikely that any Roman artisan ever figured out how to make flexible glass. As interesting as the story is, it tells us more about the perception of Tiberius than it does about any fabulous ancient discoveries.
Champlin, Edward. “Tiberius the Wise.” Historia Bd. 57, H. 4 (2008): 408-425
Keller, Vera. “Storied Objects, Scientific Objects, and Renaissance Experiment: The Case of Malleable Glass.” Renaissance Quarterly 70, no. 2 (2017): 594-632.
Stern, E. Marianne. “Ancient Glass in a Philological Context.” Mnemosyne 4th ser. 60, f. 3 (2007): 341-406.
Image: Roman drinking glass (not flexible), photograph by Marie-Lan Nguyen via Wikimedia (Domvs Romana, Mdina, Malta; 1st c. BCE-2nd c. CE; glass)
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