Cyanotypes

Process:

A solution of potassium ferricyanide and a solution of ferric ammonium citrate are mixed. The contrast of the sensitizer can be increased by adding about 6 drops of 1% solution potassium dichromate for every 2 ml of sensitizer solution

The sensitizer solution is then added to either paper or cloth and is dried in a dark place. It can be printed on any surface capable of soaking up the solution.

The print can be made from large format negatives and lithography. A positive image is produced by exposing it to a source of UV light as a contact print through the negative. 

The combination of UV light and the citrate reduces the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) complex with ferricyanide. The result is an insoluble, blue dye (ferric ferrocyanide) known as Prussian blue. The extent of color change depends on the amount of UV light, but acceptable results are usually obtained after 10–20 minute exposures on a dark, gloomy day.

After exposure, developing of the picture involves the yellow unreacted iron solution being rinsed off with running water. The water-soluble iron(III) salts are washed away, while the non-water-soluble Prussian blue remains in the paper. This is what gives the picture its typical blue color. The highlight values should appear overexposed, as the water wash reduces the final print values.

Examples: