Sir Carl William Siemens invented the regenerative furnace in which gas, used for fuel and air for supporting combustion, is pre-heated. The process proved more economical on fuel. Two other innovations, which also helped to streamline the sheet glass industry, were the Beivez 13-hr cooling oven, which reduced annealing time from eight hours to 30 minutes (1870) and the Siemens tank furnace (1873), which replaced the traditional pot furnace, making glass melting a continuous process.