In 1291, the Venetian Republic ordered every glass furnace moved off the main islands to Murano — officially to spare the city from fire, in practice to lock a trade secret behind a lagoon. The decree worked. For three centuries the maestri of Murano held a near-monopoly on clear glass, and the Republic guarded them accordingly. Glassmakers were granted privileges no other artisan class enjoyed: their daughters could marry patricians, and their names entered the Libro d'Oro. Leaving the lagoon with the formulas was treated as treason. Out of that closed system came cristallo, the colourless glass perfected by Angelo Barovier in the fifteenth century, along with lattimo, aventurine, and the filigree canes still twisted by hand in workshops along the Fondamenta S. Giovanni dei Battuti. A murano glass blowing factory tour today follows that same short walk from the vaporetto stop to the furnace door. What survives on the island is less a museum than a working district. The furnaces run at roughly 1,000 degrees Celsius and are rarely allowed to cool, since restarting one is slow and costly. The tools have barely changed: the borselle for shaping, the pontello for holding the piece, the blowpipe itself, unchanged in form since Roman Syria. Each fornace di vetro a Murano keeps its own recipes for colour, which is why the blue of one workshop never quite matches the blue of another. Murano glassblowing demonstrations are consequently not staged reconstructions but abbreviated versions of ordinary production, compressed into a few minutes so a visitor can follow the whole arc from gather to annealing oven. The island itself repays attention. Santi Maria e Donato, consecrated in the twelfth century, holds a Byzantine mosaic floor of porphyry and glass tesserae — the earliest local evidence that Murano was already working in colour before it was working in secret. The Museo del Vetro, housed in the Palazzo Giustinian, traces the technical lineage from Roman fragments to Ercole Barovier's mid-century pieces. Between them run the canals lined with low brick workshops and chimneys, a Venetian lagoon landmark shaped by industry rather than by ceremony. Decline came, as it usually does, from imitation. Bohemian and English lead crystal undercut Murano in the eighteenth century, and Napoleon's suppression of the guilds in 1806 finished the old order. Revival began in the 1850s under Antonio Salviati. The murano glass blowing factory tour tours running today are the descendants of that revival: an industry that survived by letting outsiders watch what it once killed to protect.