During the mid-19th century, when thermos-dynamic law became understood, procedures for heat regeneration that provided much higher heat transfer and lower friction were established. Developed by brothers Friedrich and William Siemens around 1860, regenerator-equipped pot furnaces consumed only about one-tenth of the fuel of the old furnaces. In 1867 Friedrich Siemens, working in the family factory in Dresden, Germany., successfully converted a day tank to a continuous cross-fired furnace equipped with regenerators. The furnace had three chambers—melting, refining, and working—separated by dividing walls. A need to obtain higher resistance to chemical attack by molten glass was recognized: clays of a high alumina-to-silica ratio, with minimal impurities, were recommended for areas of glass contact as well as the furnace crown. Seventy-five years later, in 1942, electric-arc fusion-cast refractories became commercially available—particularly the ZAC refractory (35 percent zirconia, 53 percent alumina, and 12 percent silica) developed by Gordon Fulcher at Corning Glass Works in New York. These refractories displayed extremely high resistance to corrosion during continuous contact with glass at elevated temperatures and paved the way to the current technology.