flour would turn rancid. Minneapolis milling companies solved this problem by inventing the middlings purifier, which made it possible to separate the husks from the flour earlier in the milling process. They also developed a gradual-reduction process, where grain was pulverized between a series of rollers made of porcelain, iron, or steel. This process resulted in "patent" flour, which commanded almost double the price of "bakers" or "clear" flour, which it replaced. The Washburn mill attempted to monopolize these techniques, but Pillsbury and other companies lured employees away from Washburn and were able to duplicate the process. Although the flour industry grew steadily, one major event caused a disturbance in the production of flour. On May 2, 1878, the Washburn "A" Mill exploded when grain dust ignited. The explosion, known as the Great Mill Disaster, killed eighteen workers and destroyed one-third of the capacity of the milling district, as well as other nearby businesses and residences. By the end of the year, though, seventeen mills were back in operation, including the rebuilt Washburn "A" Mill and others that had been completely rebuilt. The millers also took the opportunity to rebuild with new technology such as dust collection systems. The largest mill on the east side of the river was the Pillsbury "A" Mill, built in 1880–1881 and designed by local architect LeRoy S. Buffington. The Pillsbury Company wanted a building that was beautiful as well as functional. The seven-story building had stone walls six feet thick at the base tapering to eighteen inches at the top. With improvements and additions over the years, it became the world's largest flour mill. The Pillsbury "A" Mill is now a National Historic Landmark, along with the Washburn "A" Mill. In 1891, Northwestern Consolidated Milling Company was formed, consolidating many of the smaller mills into one corporate entity. Between 1880 and 1930, Minneapolis led the nation in flour production, earning it the nickname "Mill City". Minneapolis surpassed Budapest as the world's leading flour miller in 1884, and production stood at about annually in 1890. In 1900, Minneapolis mills were grinding 14.1 percent of the nation's grain, and in 1915–16, flour production hit its peak at annually. Hydroelectric power In 1882, the first central station (supplying multiple users) hydroelectric power plant in the United States was built at the falls on Upton Island. The Brush Electric Company, headed by Charles F. Brush, supplied the equipment,