Cyanotype
The English scientist and astronomer Sir John Herschel discovered the cyanotype procedure in 1842. Though the process was developed by Herschel, he considered it as mainly a means of reproducing notes and diagrams, as in blueprints.
The technique was soon adopted for photographic printing, and was further developed by chemist Mike Ware in 1995 and 2019.
The improved chemistry invented in 1995 on the left, and the “classic” on the right. Images printed from the same negative.
Print from digital negative produced from 6x6 medium format film.
Street photography in blue. The good one.
Cyanotypes can be toned in a wide range of liquids and will take on different tones. Tannic acid is a popular one, but one can go crazy with less scientific approaches like beer, wine, soaking water from nuts or even berry juice. Any high tannin liquid will do.