Cyanotypes

"Cyanotypes or blueprints, another plain paper process, were introduced in 1842 by Sir John Herschel. They did not become popular however, until the 1880s, when their low cost and easy processing appealed to amateur photographers. . . [C]oating well-sized paper with a light-sensitive solution produced images in tones of blue and white." From Photographs: Archival Care and Management, Ritzenthaler and O'Conner, 2006.