In the 15th century, workshops in Venice perfected the technique of making plate glass. The craftspeople were then moved to the island of Murano to keep the secret of their craft away from foreign spies. The guildsmen were sworn to uphold trade secrets on penalty of death. At this time mirrors were rare and valuable items—the cost of one Venetian mirror being comparable to the cost of a naval ship. The French king Louis XIV persuaded some artisans to come to Paris and share their knowledge. In the late 1600s, Gerolamo Barbini contributed to the realization of the Hall of Mirrors in Versailles. The Barbini family still crafts mirrors on Murano, and Front collaborated with them to make the Secret Mirror.