Editing Cyanotype (section)

== Process ==
=== 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.

=== 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>

=== Printmaking ===
{{multiple image
 | total_width = 360
 | image1 = Paprikazweig, Photogramm, Cyanotypie, belichtet.jpg
 | image2 = Paprika, Zweig, Zyanotypie, entwickelt.jpg
 | footer = A paprika branch (Chilie) with leaves, cyanotype as a photogram, exposed (left) and developed (right).
}}

The simplest kind of cyanotype print is a [[photogram]], made by arranging objects on sensitised paper. Fresh or pressed plants are a typical subject, but any opaque to translucent object will create an image. A sheet of glass presses flat objects into close contact with the paper, resulting in a sharp image. Otherwise, three-dimensional objects or less-than-perfectly flat ones create a more- or less-blurred image depending on the incidence and breadth of the light source.

[[Chemigram]]s are variants of photograms. The cyanotype solution is applied, poured, or sprayed irregularly. A variant of action painting results from repeated washing and application, placing objects on top.

More sophisticated prints can be made from artwork or photographic images on transparent or translucent media. The cyanotype process reverses light and dark, so a [[Negative (photography)|negative]] original is required to print as a positive image. Large-format photographic negatives or [[Digital negative (transparency)|transparent digital negatives]] can produce images with a full tonal range, or [[lithography|lithographic film]] can be used to create high-contrast images.

The cyanotype may be combination-printed with gumoil,<ref>{{Cite book |last=Koenig |first=Karl |url=https://www.worldcat.org/oclc/1016062767 |title=Gumoil Photographic Printing, Revised Edition |date=1999 |publisher=Taylor & Francis |isbn=978-1-136-09029-5 |language=en|oclc=1016062767}}</ref> or with a [[gum bichromate]] image, in which, for full-colour imaging from colour separations, it may form the blue layer; or it may be combined with a hand-painted or hand-drawn  layer.<ref>{{Cite book |last=Anderson |first=Christina Z |url=https://www.taylorfrancis.com/books/e/9781317326427 |title=Gum printing: a step-by-step manual, highlighting artists and their creative practice |date=17 June 2016 |publisher=Taylor & Francis |isbn=978-1-138-10150-0 |location=New York |pages=69, 73–77, 121, 149, 168, 266 |language=en|oclc=1047865042}}</ref>

=== Toning ===
[[File:Cyanotypie Testbild für Tonen.jpg|thumb|Cyanotype, toned with tannin, tea, and coffee; left side - bleached with washing soda before toning, picture of flower made with Image Creator]]
In a cyanotype, blue is usually the desired color, but a variety of alternative effects can be achieved. These fall into three categories - reducing, intensifying, and toning.<ref name="toning">{{cite web |url=http://www.berk-edu.com/HYB_subsite/PDF_hyb/Cyanotype_Toning_v2.pdf |title=Hybrid Photography – Cyanotype Toners |first=Steven |last=Berkowitz }}</ref> It is common to bleach prints before toning them, but also possible to achieve different effects by toning prints without bleaching.<ref>{{Cite book |last=Golaz |first=Annette |url=https://www.worldcat.org/oclc/1291715929 |title=Cyanotype toning using botanicals to tone blueprints naturally |date=2022 |publisher=Routledge |isbn=978-0-367-55356-2 |language=en|oclc=1291715929}}</ref>
* Bleaching processes are ways of decreasing the intensity of the blue. [[Sodium carbonate]], ammonia, borax, Dektol photographic developer, and other chemicals can be used to do this. Household bleach is also effective, but tends to destroy the paper base. How much and how long to bleach depends on the image content, emulsion thickness, and what kind of toning is being used. When using a bleaching agent, controlling the bleaching process by washing in clean water as soon as the desired effect is achieved, to prevent loss of detail in the highlights is important.<ref name="mpaul">{{cite web |url=https://mpaulphotography.wordpress.com/2011/04/01/cyanotype-toning-the-basics/ |title=Cyanotype toning: the basics |date=2011-04-01 |access-date=2015-09-14 |website=mpaulphotography }}</ref>
* Intensifying processes strengthen the blue effect. Chemicals used are [[hydrogen peroxide]] or mild acidic substances - citric acid, lemon juice, vinegar/acetic acid, etc.<ref name="toning" /> These can also be used to speed up the oxidation process that creates the blue pigment.
* Toning processes are used to change the color of the iron oxide in the print.<ref name="toning" /> The color change varies with the reagent used. A variety of agents can be used, including various types of tea, coffee, wine, urine, tannic acid, or pyrogallic acid, resulting in tones varying from brown to black.<ref>http://www.blog.alexisrago.com/residency-2019-cyanotype-workshop/ |Alexis Rago</ref> Most toning processes will to some extent tint the white parts of a print.

=== Long-term preservation ===
One of the most robust of Victorian print technologies, cyanotypes are quite stable on their own, but in contrast to most historical and present-day processes, the prints do not react well to [[base (chemistry)|basic]] environments.<ref>{{Cite book|url=https://books.google.com/books?id=Kd5cAgAAQBAJ&q=cyanotype+prints+do+not+react+well+to+basic+environments&pg=PA361|title=Encyclopedia of Nineteenth-Century Photography|last=Hannavy|first=John|date=2013-12-16|publisher=Routledge|isbn=978-1-135-87327-1|language=en}}</ref> As a result, it is not advised to store or present the print in chemically buffered [[museum board]], as this makes the image fade. Another unusual characteristic of the cyanotype is its regenerative behavior: prints that have faded due to prolonged exposure to light can often be significantly restored to their original tone by simply temporarily storing them in a dark environment.<ref>{{Cite journal |last=Ware |first=Mike |date=1999 |title=Cyanotypes: their history, chemistry, and conservation |url=https://www.worldcat.org/oclc/610989224 |journal=Care of Photographic Moving Image & Sound Collections |language=en|pages=115–123 |oclc=610989224}}</ref><ref>{{Cite journal |last=Aloi |first=Giovanni |date=2021 |title=Unearthed: Photography's Roots |journal=Antennae: The Journal of Nature in Visual Culture |volume=53 |pages=66–77}}</ref>

Cyanotypes on cloth are permanent but must be washed by hand with non-phosphate soap<ref>{{cite web |url=http://www.blueprintsonfabric.com/instructions.php |publisher=blueprintsonfabric.com |title=Washing instructions for cloth}}</ref> so as to not turn the blue to yellow.