There are three sources that concern the story of the “flexible” glass of first century Rome: Pliny’s Natural History, Petronius’ Satyricon, and Dio Cassius’ Roman History. Petronius (d. 63 A.D.) published the story before Pliny, who completed his encyclopedia in 78 A.D. Dio Cassius’ story is much later (he died in 235 A.D.).

As Petronius (Sat. 51) tells it, a glassmaker was granted an audience with Emperor Tiberius (reigned 14-37 A.D.), and presented him with a phiale (a shallow drinking vessel). The glassmaker asked the emperor to give it back, and then threw it to the floor. It did not break, but was dented, like a bronze vessel. The glassmaker took out a hammer and removed the dent. Tiberius asked him if anyone else knew how to make this kind of glass and, the glassmaker said “No.” The emperor promptly had him beheaded.

Pliny (NH 36, 66) makes it pretty clear that he does not believe in vitrum flexile. He starts his short account by stating “They say” or “There is a story.” The only thing he says about the glass is that it was flexible. He then says that the glassmaker’s entire workshop was destroyed so that the value of copper, silver, and gold wouldn’t suffer [because people would acquire flexible glass instead]. Pliny comments that the story is more frequently told than it is reliable.

Dio Cassius (RH 57, 21) has a different version of the story. An architect re-erected a fallen portico in Rome and the feat aroused the jealousy of Emperor Tiberius, who exiled him. The architect obtained an audience to ask for clemency, in the course of which he threw a glass drinking vessel on the floor, causing it to be dented or broken. He then repaired the damage with his bare hands.

Not surprisingly, no example of Roman vitrum flexile is known to exist.