Built in 1906-1907, Carrie Blast Furnaces 6 and 7 are the only remaining pre-World War II era blast furnaces in the Pittsburgh District, the nation's largest iron and steel production district for much of the nineteenth and twentieth centuries. Moreover, they are two of only a handful of remaining modern, pre-World War II blast furnaces in the United States that retain a large percentage of their original equipment. The existing resources show how furnace technologies were adapted to changing engineering theories, raw material supplies, and increasingly mechanized operations during the pre-World War II years. The advances in ironmaking technologies that are reflected at Carrie 6 and 7 are inherently linked to their significance in American engineering and industrial history, and reflect the dominant technological concepts of American ironmaking during the first half of the twentieth century. These advances were critical to the development of mass production in the American steel industry that made Pittsburgh the leading iron and steel manufacturing district in the world. - NPS