Cyanotype printing is one of the earliest photographic processes, invented in 1842 by Sir John Herschel, and is best known for its distinctive Prussian blue images.
The process itself is simple and largely unchanged today: a solution of iron salts – typically ferric ammonium citrate and potassium ferricyanide dissolved in distilled water and then combined in equal amounts – is brushed onto paper or fabric and allowed to dry in the dark.
Objects or negatives are then placed on the coated surface and exposed to ultraviolet light – either direct sunlight or using lamps – causing the exposed areas to chemically reduce and form an insoluble blue pigment.
After exposure, the print is washed in water, which removes the unexposed chemicals and fully reveals the image.
Avoid hanging Cyanotype prints in direct sunlight.