Editing Cyanotype (section)

=== Herschel's formula and method ===
In a typical procedure, equal volumes of an 8.1% (w/v) solution of potassium ferricyanide and a 20% solution of ferric ammonium citrate are mixed. The overall contrast of the sensitizer solution can be increased with the addition of about 6 drops of 1% ([[w/v]]) solution [[potassium dichromate]] for every 2&nbsp;ml of sensitizer solution.{{citation needed|date=January 2016}}

This mildly [[photosensitive]] solution is then applied to a receptive surface (such as paper or cloth) and allowed to dry in a dark place. Cyanotypes can be printed on any support capable of soaking up the iron solution. Although [[watercolor]] paper is a preferred medium, cotton, [[wool]], and even [[gelatin]] [[sizing]] on [[Porous medium|nonporous]] surfaces have been used. Care should be taken to avoid alkaline-buffered papers, which degrade the image over time.

An image can be produced by exposing sensitised paper to a source of [[ultraviolet]] (UV) light (such as sunlight) as a [[contact print]]. The combination of UV light and the citrate [[Reduction reaction|reduces]] the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) with ferricyanide. The result is an insoluble, blue pigment (ferric ferrocyanide) known as [[Prussian blue]].<ref name=WDL>{{cite web|title=General View of Niagara Falls from Bridge|url=http://www.wdl.org/en/item/285|publisher=[[World Digital Library]]|access-date=11 February 2013}}</ref> The exposure time varies widely, from a few seconds in strong direct sunlight, to 10–20&nbsp;minute exposures on a dull day.

After exposure, the paper is [[photographic developing|developed]] by washing in cold, running water; the water-soluble iron(III) salts are washed away. The parts that were exposed to UV turn blue as the water-insoluble Prussian blue pigment remains in the paper, which gives the print its typical blue color.<ref name=WDL /> The blue color darkens upon drying.