Cyanotype

Cyanotype is an old monochrome photographic printing process that gives a cyan-blue print.

The English scientist and astronomer Sir John Herschel discovered this procedure in 1842. Though Herschel is perhaps the inventor of the cyanotype process, it was Anna Atkins, a British scientist, who brought the process into the realm of photography. She created a limited series of cyanotype books that documented ferns and other plant life. By using this process, Anna Atkins is regarded as the first woman photographer.

The process uses two chemicals:

* Ammonium iron(III) citrate
* Potassium ferricyanide

They result in a photo-sensitive solution when dissolved in water, which is used to coat a material (usually paper). A positive image can be produced by exposing it to a source of ultraviolet light (such as sunlight) with a negative. The UV light reduces the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) complex with ferricyanide. The result is an insoluble, blue dye (ferric ferrocyanide) known as Prussian blue.

The developing of the picture takes place by flushing it with flowing water. The water-soluble iron(III) salts are washed away, while the non-water-soluble Prussian blue remains in the paper. This is what gives the picture its typical blue color. The process was popular in engineering circles well into the 20th century. The simple and low-cost process enabled them to produce large-scale copies of their work, referred to as blueprints.