This photographic printing process is named after the cyan color it develops. The pigment is Prussian blue which has the highest color saturation among inorganic pigments was first synthesized by Diesbach, a color fabricant, in 1706. After several early photographic processes ware invented, Sir John Herschel developed the cyanotype as the only acceptable silver free process in 1842. Even the used chemicals and their products are nontoxic and allowed to be drained, but never use excessive heat or strong acids as toxic hydrogen cyanide may develop!
The basic cyanotype process
You need two chemicals: Potassium ferricyanide (CAS: 13746-66-2) and Ammonium ferric citrate (CAS: 1185-57-5)
Solve 4g Potassium ferricyanide and 10g Ammonium ferric citrate in 50ml water. (absolute amounts are uncritical, just keep the stoichiometric correct ratio)
Paint the yellowish solution on soak resistive paper or other ground, even wood and let dry. Use a plastic brush to prevent a reaction with iron particles.
Place contact negative or other depicting material onto the paper and expose to light source. ½ hour sunlight or tanning light is sufficient. Only the ultraviolet portion matters.
Exposed areas darken and turn blue:
Now ‘develop’ the print by washing the yellow unexposed layer away. The unsolvable Prussian blue will stay, which is not photosensitive to further exposure:
Some recipes recommend an additional immersion in 1% Hydrogen peroxide to achieve higher image contrast.
Second try, on wood: