For centuries, Venetian glassblowing was world famous. By the 13th century, glass had become a primary export for Venice. Everyone wanted it—and everyone wanted to know how it was made.

But Venice was intent to keep the process secret.

In 1271, a law banned foreign workers from participating in the glass industry, ostensibly so that only Venetian citizens would know the technique. Even that was not enough. Twenty years later, Venice required that all glass production move to the more isolated islands of Murano. Though the reason given was that glassblowing furnaces presented a fire hazard, the law was almost certainly about protecting Venetian trade secrets, as a subsequent ordinance demonstrates: in 1295, Venetian glassblowers were entirely banned from traveling abroad. Disobeying came with the risk of death.

The video above depicts William Gudenrath, a glassblower and scholar who, after years of research, has demonstrated how Venetians made their beloved glass.