Invented by Sir John Herschel in 1841, this simple process gives a continuous-tone image of Prussian Blue using a sensitising solution of Ammonium Ferric Citrate and Potassium Ferricyanide. These iron salts, when exposed to natural or artificial ultraviolet light, are reduced to their ferrous state, producing a high contrast blue image when oxidised. Oxidation is hastened by immersion in running water, which also washes away the unused iron salts.
Mix up two solutions: Solution A 65 g ammonium ferric citrate (green) in water to make 250ml total and solution B 23 g potassium ferricyanide in water to make 250ml total. Both solutions should be stored separately in brown glass bottles, away from the light. When you are ready to coat your paper mix both in equal proportions A to B, in subdued tungsten light.
Using a coating rod or sponge brush evenly coat the paper and allow to dry. This, again, should be done in subdued tungsten light. Ideally leaving paper to dry in darkness.
When paper is dry place your film negative, digital negative or objects on top. A piece of glass will keep the negative or objects in place and create a sharper print. Place the paper in sunlight or under an ultra-violet light source until the shadows look bronzed. Allow at least one stop over-exposure to compensate for loss of density during processing.
Next, immerse in running water and wash until the green chartreuse stain of the ferric ammonium citrate has completely disappeared, usually about 5 minutes. Take care to avoid excessive washing as this will wash out the image.
Finally, leave your print to dry. The image will darken and highlights will brighten in the drying process.