Cyanotype is an early hand-made photographic process that produces prints with a distinctive Prussian blue color. Cyanotypes are created by painting a light-sensitive solution onto watercolor paper or fabric, in this case, then plant material is layered on top and then exposed to the sun or other UV light source. The print is then washed and dried. No two are alike, even if the same materials are used. Through additional manipulation, the process can be created in various brown tones as well.
The cyanotype process was first developed in 1842 by the scientist and astronomer Sir John Herschel. It has been used since then for printing from large negatives (and now digital negatives), for architectural blueprints, and many forms of photograms like these (objects placed directly on the paper).
Anna Atkins, the daughter of the director at the Royal Botanic Gardens at Kew, in London, and an accomplished botanist in her own right, knew Mr. Herschel and used his technique to create cyanotypes of plants. She used this method to create the very first book of photographs in 1843-1853: Photographs of British Algae: Cyanotype Impressions - over 400 pages in 3 volumes.