The cyan tone of these snapshot images is a product of the cyanotype process, which uses ferric ferrocyanide (Prussian Blue) to form an image. These prints are mounted back-to-back on a green piece of construction paper, suggesting the prints were likely in an album at one point.
Sir John Herschel invented the cyanotype process in 1842. The process was briefly used in the 1840s to make camera-less prints, or photograms, most notably for scientific purposes. The process experienced a resurgence in the late nineteenth and early twentieth centuries corresponding with the introduction of roll film. Both the cyanotype and roll film appealed to a new class of amateur photographers. The images shown here were made around 1910 with a roll film camera that shoots a 2 1/4 x 2 3/4 image. Cyanotype paper was commercially available and did not require any processing chemicals, only a thorough wash in water after exposure. | src Graphic Atlas