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Flexible glass is a legendary lost invention from the time of the reign of the Roman Emperor Tiberius Caesar (between 14–37 AD). As recounted by Isidore of Seville, the craftsman who invented the technique brought a drinking bowl made of flexible glass before Caesar who tried to break it, whereupon the material dented, rather than shattering. The inventor then repaired the bowl easily with a small hammer. After the inventor swore to the Emperor that he alone knew the technique of manufacture, Tiberius had the man beheaded, fearing such material could undermine the value of gold and silver.
The story of the sad fate of the inventor of unbreakable glass (vitrum flexile) at the hands of Tiberius (42 BC–37 AD) was first related by two more-or-less contemporary compilers, namely Petronius (c. 27–66 AD, Satyricon 51) and Pliny the Elder (23–79 AD, Naturalis Historia XXXVI.lxvi.195). Pliny apparently did not believe in vitrum flexile, remarking that the story is, "more frequently told than it is reliable". Cassius Dio (c. AD 150–235, Historia Romana 57.21.7) also told a similar story, writing that ‘an architect whose name no one knows’ whom Tiberius had exiled out of jealousy for his skill, ‘approached him to crave pardon, and while doing so purposely let fall a crystal goblet; and though it was bruised in some way or shattered, yet by passing his hands over it he promptly exhibited it whole once more. For this he hoped to obtain pardon, but instead the emperor put him to death.’ The story was picked up and retold by Isidore of Seville (c. 560–636, Etymologiae XVI.16.6, ‘De vitro’). Subsequently the 13th century collection of technical recipes by the pseudo-Heraclius included this story, in the section treating glass recipes, using Isidore’s own words (De coloribus et artibus Romanorum III.vi ). This is thus an example of ‘technical knowledge’ wending its way from historical gossip (Petronius), to a technical encyclopaedia (Pliny, with reservations), and then via a philological text (Isidore) to a collection of practical craft recipes (pseudo-Heraclius).