Clear and blue glass bowl on foot

From the early Middle Ages on, Venetian craftsmen specialized in glassblowing, developing procedures for blowing very thin and refined glass. The hot glass furnaces in the middle of the city posed a fire hazard, and in 1291 the glass industry was therefore moved by decree of the city government to the island of Murano. In this way, the city was protected, and more important: the secrets of the magnificent Venetian glass were protected. Glassblowers were not allowed to leave the island, under penalty of  death, and were in fact imprisoned on Murano. The techniques and composition of the various types of glass, including the colorless and wafer-thin cristallo which was discovered in the fifteenth century, had to remain a Venetian secret. Despite these measures, glass blowers did leave Venice and settled throughout Northern Europe.