Editing Cyanotype (section)

=== Improved formula ===
The ingredients for the process have remained mostly unchanged since its inception in 1840.<ref>{{Cite book |last=Thornthwaite |first=W. H. |title=Guide to Photography |publisher=Horne, Thornthwaite and Wood |year=1851 |location=London |oclc=316441617}}</ref> In 1994, [[Mike Ware (photographer)|Mike Ware]] improved on Herschel's formula with ammonium iron(III) oxalate, also known as [[ferric ammonium oxalate]], to replace the variable and unreliable ammonium ferric citrate.<ref name=":2">{{Cite book |last=Ware |first=Mike |title=Cyanomicon |publisher=www.mikeware.co.uk |year=2014 |location=Buxton}}</ref> It has the advantage of being made up as a convenient single stock solution with a good shelf-life that does not nourish mould growth. The solution is well-absorbed by paper fibres, so it does not pool on the surface or result in a tackiness, which may adhere to negatives. The paper better retains the pigment, with little of the Prussian blue image being lost in the washing stage, and exposure is shorter (ca. 4-8 times) than the traditional process. The cyanotype solution, even once its excess is washed off with water, remains photo-sensitive to some degree. A print that has been stored or displayed in bright light will eventually fade, the light causing a chemical reaction that changes the Prussian blue of the cyanotype to white. Storing the cyanotypes in darkness reverses the process,restoring them to their original vibrancy.<ref name=":1">{{Cite book |last=Burns |first=Nancy |title=Cyanotypes: Photography's Blue Period |publisher=Worcester Art Museum |year=2016 |isbn=978-0-936042-06-0 |location=Worcester}}</ref>

Different composition levels of ferric ammonium citrate (or oxalate) and [[potassium ferricyanide]] result in a variety of effects in the final cyanotypes. Mixtures of half ferric ammonium citrate and half potassium ferricyanide produces a medium, even shade of blue that is most commonly seen in a cyanotype. A mix of one-third ferric ammonium citrate and two-thirds potassium ferricyanide produces a darker blue, and a more high-contrast final print.<ref name=":0">{{Cite book |last=Anderson |first=Christina Z. |title=Cyanotype: The Blueprint in Contemporary Practice |publisher=Focal Press |year=2019 |isbn=978-0-429-44141-7 |location=New York |pages=11–18}}</ref>

Disadvantages of the Ware formula are a higher cost, more complicated preparation, and a level of toxicity.<ref name=":8">{{Cite book |last1=Hirsch |first1=Robert |url=https://www.safaribooksonline.com/library/view/title/9780240803623/?ar?orpq&email=%5Eu |title=Photographic Possibilities, 2nd Edition |last2=Valentino |first2=John |date=2001 |publisher=Focal Press |isbn=978-1-136-09053-0 |location= |language=en|oclc=1103262926}}</ref>