cyanotype technique

Cyanotype or blueprint is a technique to print photos invented in 1842 by John Herschel. The Prussian blue color is caused by the exposure of cyanide to ultraviolet light. The color cyan, as we know it from CMYK from the printing world, is therefore actually an incorrect name for that color blue.

Mix ammonium iron(III)citrate, potassium ferricyanide and water. Apply it to paper and let it dry. Place this together with a 1:1 negative in the sun or under a UV lamp for about 2 to 5 minutes, depending on the UV strength. By then rinsing the exposed paper with water, you wash away the unexposed parts of the yellow iron solution and the insoluble Prussian blue remains on the paper. The blue color darkens as it dries, but the effect can be accelerated by placing the print in a water bath with hydrogen peroxide.