The "blueprint" process was invented about 1839 by John Herschel, the astronomer son of the famous Herschel who made many discoveries, including Uranus, the first planet that couldn't be seen by the naked eye, and ground his own lenses for his telescopes - aided by his sister the amazing Caroline, who is now getting more recognition for her achievements. (btw on 12 July this year Neptune, the planet beyond Uranus, discovered in 1846, completed its first solar orbit since its discovery.)
Anna Atkins was quick to use the process and published her cyanotypes of ferns as a book in 1842, soon after invention of the process. They are sublime. (Image from here.) They bring to mind the plant studies of Blossfeldt, but this post is about cyanotype ... so let's leave Blossfeldt for another time.
More contemporarily, these bottles work rather well as an image -
Cyanotype uses two chemicals, potassium ferricyanide and ferric ammonium citrate; the related blueprint process (discovered in 1861) uses ferro-gallate in gum. In both, exposure to ultraviolet light reduces the iron, turning it grey. Blueprint needs no washing, but in cyanotype washing the print rinses off the unexposed iron, leaving the prussian blue colour.
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