During the 19th century, Americans in New Jersey and the Adirondack region of New York brought the ancient bloomery process for the direct reduction of iron to a high state of technological development. Using this process, they were able to make iron as good as the best Swedish grades. Rich magnetite ore was used; low fuel consumption was achieved by preheating the air blast, and labor productivity was maximized by ore preparation and hearth design that speeded the reduction process. Magnetite grains were reduced to particles of sponge iron in the upper part of the hearth, fell with liquid slag, and agglomerated on rims of iron formed around pieces of charcoal to nucleate the bloom. The hearth was manipulated to form a pool of liquid slag on top of the bloom that served as a trap for descending sponge iron particles. The iron in the bloom was often partially carburized by entrapped charcoal particles. Although dismissed by some historians as primitive, the American bloomery process was a sophisticated adaptation of an ancient technology to local resources and economic conditions and was capable of producing grades of iron for special applications not easily made in other ways.