Cyanotype

Written by: Geza Csosz

 

The discovery of cyanotype

Johann Wolfgang Döbereiner discovered the photosensitivity of iron oxides as early as 1831 and published the theoretical basis of this photochemical reaction, which provided important knowledge for the later discovery of the cyanotype process. Cyanotype is a traditional contact printing method. On 15 June 1842, the English astronomer Sir John Frederick William Herschel (1792-1871) published the procedure in the Philosophical Transactions of the Royal Society under the title “On the Action of the Rays of the solar Spectrum on Vegetable Colours, and on some new Photographic Processes,” part of which was read to the society on the 16th. It was only available in print from September onwards. Herschel has been using the term “cyanotype” since 16 August 1842, as a positive photographic process. In 1844, the researcher and writer Robert Hunt (1807-1887) perfected the cyanotype and named it “Chromo-Cyanotype,” which he made public in his writing “Researches on Light.” The process has been used in industry since the second half of the 19th century and is still in use today.

Image Source: en.wikipedia.org

Chemical basis of cyanotype

Ferric ammonium citrate Other names: ammonium iron citrate, which is a mixed salt of trivalent citric acid. This compound has a non-stoichiometric composition and can be described by the approximate formula Fe(NH4)C6H5O*. It contains varying amounts of water of crystallization. It is soluble in water but poorly soluble in ethanol. It decomposes upon exposure to moisture and light. Its solution is not stable and decomposes during precipitation. It occurs in brown and green variations, with the green one recommended for photographic use. The commercially available material is categorized as follows: reddish-brown color – 16.5-22.5% Fe content, brown – 14.5% Fe, blood-red – 28% Fe, green – 12.85-16% Fe content. Ferric ammonium citrate is the main component of cyanotype along with potassium hexacyanoferrate. The Fe3+ ions in the compound are reduced to Fe2+ ions by the UV light, forming the blue precipitate.

3Fe2+ + 2[Fe(CN)6]3- = Fe3[Fe(CN)6]2

(Turnbull blue) In addition to the main reaction, a redox reaction also takes place:

Fe2+ + [Fe(CN)6]3- <==> Fe3+ + [Fe(CN)6]4-

and the hexacyanoferrate(II) ions also form a precipitate with the iron(III) ions (Prussian blue). Therefore, a mixture of these two precipitates is generally formed. The side reaction can be eliminated by adding fluoride ions, which complex the Fe ions. This compound likely plays a role in absorbing light within the appropriate wavelength range for photography and ensuring sufficient sensitivity. It is also used for toning photographic papers to a blue colour.

Potassium hexacyanoferrate(III)

Other names: red prussiate, potassium ferricyanide K3[Fe(CN)6], M: 329.24, red crystalline substance. Solubility: soluble in water at 334.77,5100, insoluble in ethanol. It decomposes under light, the crystals turn yellow, and the compound is converted to potassium ferrocyanide (K4[Fe(CN)6]*3H2O), known as yellow prussiate. Its light green solution is also photosensitive, and upon decomposition, it forms iron(III) hydroxide (Fe(OH)3), which is why both the solid material and its solution should be stored in brown glass. It is a strong oxidizing agent, and during oxidation, Fe3+ ions are reduced to Fe2+ ions. If we add a solution containing Fe2+ ions to its solution, we can observe the precipitation of a blue compound. This is Turnbull blue (Fe3[Fe(CN)6]2), which is actually identical to the precipitate known as Prussian blue due to the ongoing redox processes. The preparation of cyanotype is based on this reaction, where potassium hexacyanoferrate is one of the main components. The mixture of ferric ammonium citrate and potassium ferricyanide solution is reduced by light, and the resulting ferrous ions (Fe ) react with potassium ferricyanide to form Turnbull blue precipitate. The precipitation reaction is highly sensitive. It dissolves metallic silver in complex salt form in the presence of thiosulfate ions, which is why it is used in photography in a solution called Farmer’s reducer and in bleaching during color development. The first step is a redox process in which the silver metal Ag is oxidized to Ag+ ions, which then dissolve in complex form:

4[Fe(CN)6]3- + 4Ag ——> Ag4[Fe(CN)6]2 + 3[Fe(CN)6]4 –

Ag4[Fe(CN)6]2 + 8S2O3 ——> 4[Ag(S2O3)2]3- + [Fe(CN)6]4-

It is also used in blue dyes. At high temperatures or under the influence of strong acids, it decomposes, releasing hydrogen cyanide gas, which is highly toxic. When ingested orally, it undergoes the latter reaction due to the action of stomach acid, making the compound also toxic.

Potassium hexacyanoferrate(II)

Other names: potassium ferrocyanide, yellow prussiate. K4[Fe(CN)6]*3H2O, M: 422.39, yellow crystalline substance. Solubility: soluble in water at 27.81-2, 90.63-6, insoluble in ethanol. Upon exposure to light, it forms iron(III) hydroxide from its aqueous solution. It forms a precipitate of Berlin blue with Fe3+ ions: 4Fe3+UV+ 3[Fe(CN)6]4- ——> Fe4[Fe(CN)6]3 This precipitate is dissolved by oxalic acid, unlike Turnbull blue, and it does not precipitate in the presence of fluoride ions. It can appear as a contaminant alongside Turnbull blue, causing a fog-like effect in cyanotype.

 

The method of preparation

We are familiar with several descriptions, and here is a well-functioning recipe that I regularly use as well. We prepare two solutions: one by dissolving 9g of potassium hexacyanoferrate(III) in 100ml of distilled water, and the other by dissolving 25g of ferric ammonium citrate in 100ml of distilled water. Each solution can be stored separately for a long time in the refrigerator. The two solutions are filtered, then combined in a darker place, filtered again, and left to stand for one hour. The resulting photosensitive solution is evenly applied with a wide brush on alkaline-free paper, silk, wood, etc., and then dried. All these steps can be carried out in a darkened room under the light of a 40-watt incandescent bulb. The sensitized paper has a greenish-yellow color and can be stored in a dry, dark place for weeks. It is recommended to make multiple copies using a copying frame and expose them to sunlight or UV light, periodically checking the progress. The exposed areas will turn grayish. Wash the finished image in running water until a yellowish liquid is released. At this point, the areas affected by light will have formed insoluble Prussian blue, while the unreacted potassium hexacyanoferrate(III) and ferric ammonium citrate will dissolve. To deepen the shades, soak the print in a 1:20 dilution of hydrochloric acid for a few seconds, rinse, and then dry. If you want to make the images more vibrant, you can use a 20% solution of ammonium dichromate. In an alkaline environment, the image disappears, but it can be made visible again in a sulphuric acid or hydrochloric acid solution.

Representatives of cyanotype

I would like to introduce some photographers who use cyanotype technology. Naturally, they also employ other photographic processes, just like contemporary artists today, as they choose from the colourful palette of historical photographic techniques. In the appendix, I present one image from each of the artists listed here.

Sir John Herschel, the developer of the process, has already been introduced.

Anna Atkins (1799-1871), an English botanist and photographer, was the first person in the world to publish a book illustrated with photographs. Atkins’ father was a friend of Sir John Herschel, and she used the process described in 1842 to create botanical prints. Her first book, “British Algae: Cyanotype Impressions,” was self-published in 1843 and featured nearly 400 images that she sent to her friends, researchers, and, among others, William Henry Fox Talbot. The album contained negative impressions (photograms) of British algae. She also published a second volume titled “Cyanotypes of British and Foreign Flowering Plants and Ferns,” which featured botanical prints.

Henri Jean-Louis Le Secq (1818-1882), a French painter and photographer, created cyanotypes from paper negatives. Commissioned by the French government, he photographed the masterpieces of French architecture.

Several American photographers also used the cyanotype process, including Edward S. Curtis (1868-1952), an American photographer, Thomas Eakins (1844-1916), an American painter, sculptor, and photographer, and Charles F. Lummis (1859-1928), an American journalist, historian, poet, and photographer.

After the discovery of its positive version in Hungary (1877, Pellet, see the attachment), it started to spread. Dr. Vince Wartha (1844-1914), who was a professor at the Hungarian Royal Joseph University of Technology, followed the development of photochemistry with interest due to his profession and engaged in the production of these papers. Several cyanotypes have been preserved from Ferenc Veress (1832-1916), who created cityscapes and family photos on paper and canvas. Vidor Tafner (1881-1966), based on his own experiments, combined cyanotype and gum printing, giving his process the name “cyanochromotype.”

Cyanotype in the 20th and 21st centuries

Cyanotype is a rediscovered and unique branch of fine art photography today. For example, if we transform a soft, tonally rich image into something very crisp using a certain technique, its character changes completely, almost becoming a graphic work: a new image is born. The application of the photosensitive material with a brush lends a dramatic frame to the finished work. The spontaneity of brush strokes dynamically connects with the compositions. Even today, there are several photographers willing to delve into the old techniques, as the originality of the resulting works is often astonishing. Among contemporary Hungarian artists, many use the cyanotype technique. It’s worth mentioning Magda Vékás (1956-), who has been involved in creating special process images since 1985, following the Gödöllő Photo Technique Camp led by Károly Kincses. She is interested in techniques that can be applied to dipped paper and works on various supports. The texture of different materials adds extra meaning to the image and the artwork.

Magdolna Vékás cyanotype by Geza Csosz