Cyanotype

The cyanotype was developed by John Herschel in 1842 and became his most commercially successful photographic process. It requires only two chemical compounds: ferric ammonium citrate and potassium ferricyanide, which produce Prussian blue, an intense blue pigment used by artists before photography. The two solutions are created separately and mixed together before sensitizing the paper. The iron in the mixture is the light sensitive property, so once the mixture is brushed onto the surface of the paper, it is ready to be exposed. Fixing the image requires only water, and although the pigment is highly insoluble in water, much of it is lost during processing. Nonetheless, the water brings out the rich blue color and makes the image permanent.

The cyanotype, unfortunately, was not terribly popular in Herschel's time. It had slightly limited tonal range and the strong Prussian blue color wasn't approved of for portraits or other popular photographic subjects. Proper exposure could require 30 minutes or more and the "bleeding" of the Prussian blue during fixing can stain the highlights. The low cost and availability of the chemicals, however, made it easy for amateurs to use, and the cyanotype was able to survive into the 20th century.


Ware, Mike. Cyanotype: The history, science and art of photographic printing in Prussian blue
Schaaf, Larry J. Sun Gardens: Victorian Photograms by Anna Atkins

http://www.alternativephotography.com/wp/processes/cyanotype/cyanotype-classic-process

http://www.mikeware.demon.co.uk/cyano.html

We created solution A with 100ml of distilled water and 25g of ferric ammonium citrate (green) and solution B with 100ml of distilled water and 10g of potassium ferricyanide. The two solutions were mixed 1:1. The solutions, however, must be stored separately when not being used.