Laboratory condensers are usually made of glass for chemical resistance, for ease of cleaning, and to allow visual monitoring of the operation; specifically, borosilicate glass to resist thermal shock and uneven heating by the condensing vapor. Some condensers for dedicated operations (like water distillation) may be made of metal. In professional laboratories, condensers usually have ground glass joints for airtight connection to the vapor source and the liquid receptacle; however, flexible tubing of an appropriate material is often used instead. The condenser may also be fused to a boiling flask as a single glassware item, as in the old retort and in devices for microscale distillation.