Cyanotype, also known as blue printing, was invented by Sir John Herschel in 1842. A year later, Anna Atkins, one of the few women who worked in photography in the mid-1800s, published the first book using cyanotype imagery, titled ‘Photographs of British Algae: Cyanotype Impressions, Cyanotypes of British and Foreign Flowering Plants and Ferns.’
The chemical process of cyanotype is non-toxic. Two iron salts are used: potassium ferricyanide and iron ammonium citrate. These, diluted and mixed in equal measure in distilled water, produce an emulsion photosensitive to ultraviolet rays. The prepared mixtures must remain in the dark for at least 24 hours. They will then be distributed evenly on the paper that will then be exposed to direct sunlight together with the objects that we want to impress and covering everything with a glass plate. The development takes place through a washing in running water. The solution not exposed to light is rinsed off the support while the exposed part remains indelible and slowly assumes a cyan blue color. Development under water takes about 10 minutes, after which the prints will be dried in the dark.
The most suitable paper is watercolor paper or for graphic arts, possibly 100% or at least 50% cotton. You can choose the texture of the paper between smooth or rough, depending on the effect that you want to get with the print. Photos on smooth paper will be quite sharp, while those on rough paper will be soft and almost dreamy.
Note also the absorbency of the paper: if it is too absorbent, the image will be blurred, otherwise puddles of emulsion will accumulate and the photo will be exposed unevenly.