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 acquired flexible glass instead]. Pliny comments that the story is more frequently told than it is reliable.
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Not surprisingly, no example of Roman vitrum flexile is known to exist.
Apparently, Pliny himself was skeptical.