Cyanotype

Cyanotype is an old photographic procedure which typically admits a cyan-blue colour.

The English scientist and astronomer Sir John Herschel discovered this procedure in 1842. It uses two chemicals:

Ammonium iron (III)
Potassium ferric cyanide

They result in an photo-sensitive solution when solved in water, which is used to coat a material (usually paper), then a positive image can be produced in by exposing it to a source of Ultraviolet light (such as sunlight) with a negative.

The developing of the picture takes place by flushing it with flowing water. The water-soluble Iron (II) salts are washed away, while the non-water-soluble Iron (III) salt crystalls remain in the paper. These give the picture its typical blue color.