Venetian glassmaking grew in importance to the city's economy. Around 1271 the local glassmakers' guild made rules to help preserve glassmaking secrets. It was forbidden to divulge trade secrets outside of Venice. If a glassworker left the city without permission, he would be ordered to return. If he failed to return, his family would be imprisoned. If he still did not return, an assassin would be sent to kill him. Additional rules specified ingredients used for making glass and the type of wood used as fuel for the furnaces.