Cyanotype
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.

The Process
Solution A 65 g ammonium ferric citrate (green) in distilled water to make 250ml total.
Solution B 23 g potassium ferricyanide in distilled water to make 250ml total.
Note : distilled water is more commonly known as 'purified water'.
Store solutions separately in brown glass bottles, away from light. Filter before use, and mix in equal proportions A to B.

Printing
Paper: For all alternative processes high quality art paper such as those used in printmaking or watercolour work are most suitable. Atlantis Silversafe 200g/sq m., Somerset Satin and Waterford Satin are good papers to start with, however it is advisable to make tests to find a paper to suit your requirements.
Coat in subdued tungsten light, and dry away from light and heat. Print by contact (in a printing frame with a full sized negative) in sunlight or by 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.

Development
Immerse in running water, and wash until the chartreuse stain of the ferric ammonium citrate has completely disappeared. Take care to avoid excessive washing as this will to wash out the image.