The cyanotype (from Ancient Greek: κυάνεος, kyáneos 'dark blue' and τύπος, týpos 'mark, impression, type') is a slow-reacting, photographic printing formulation sensitive to a limited near-ultraviolet and blue light spectrum, the range of 300 nm to 400 nm, known as UVA radiation. It produces a monochrome, blue-coloured print on a range of supports, and is often used for art and reprography in the form of blueprints. For any purpose, the process usually uses two chemicals – ferric ammonium citrate or ferric ammonium oxalate, and potassium ferricyanide, and only water to develop and fix. Announced in 1842, it is still in use.
History
The cyanotype process was discovered and named by Sir John Herschel. Herschel's investigation of the effects of light on iron compounds was published in 1842. In this investigation Herschel expected that photochemical reactions would reveal the infrared extreme of the electromagnetic spectrum, which had been detected by his father William Herschel, and the ultraviolet or "actinic" rays, that had been discovered in 1801 by Johann Ritter, in a form visible to the human eye.
Johann Wolfgang Döbereiner had published earlier in 1831 in German on the light-sensitivity of ferric oxalate, of which Herschel became aware during his visit to Hamburg, his reaction created too lightly toned images and would require a second reaction to make a permanent print.
Alfred Smee had in 1840 used electrochemistry to isolate a pure form of potassium ferricyanide from potassium ferrocyanide, which he sent to Herschel, whose innovation was to use the ammonium iron(III) citrate or tartrate, then commercially available as an iron tonic (meant to remedy iron deficiencies) and also introduced to him by Smee, for photographic purposes. Today, green ferric ammonium citrate is used instead. He mixed the ammonium ferric citrate in a 20% aqueous solution, with 16% of the potassium ferricyanide, to make the sensitizer for coating plain paper. Upon exposure to sunlight, the ferric salt is reduced, and then combines with the ferricyanide to yield ferric ferrocyanide – Prussian blue (also known as Turnbull's blue, or Berlin Blue in Germany). Intensifying and fixing is achieved simply by rinsing the print in water in which unexposed sensitizer and reaction products are readily soluble.
Anna Atkins, a friend of the Herschel family, over 1843–61 and with the assistance of Anne Dixon, hand-printed several albums of botanical and textile specimens, especially Photographs of British Algae: Cyanotype Impressions, effectively the world's first photographically illustrated books. After the Antarctic Ross expedition (1839–1843), John Davis, artist and naturalist on the expedition, made or commissioned some cyanotypes in 1848 from seaweeds collected on the voyage. Also in the Antipodes, Herbert Dobbie, in imitation of Atkins, produced a book New Zealand ferns: 148 varieties, but with double-sided pages of cyanotype prints, in 1880.
John Mercer in the 1850s used the process for printing photographs onto cotton textiles, and discovered means of toning the cyanotype violet, green, brown, red, or black.
As with all of his photographic inventions, Herschel did not patent his cyanotype process. Chemist George Thomas Fisher Jr. quickly disseminated information on the new medium internationally in his popular 50-page manual Photogenic manipulation in 1843, containing plain instructions in the theory and practice of the arts of photography: calotype, cyanotype, ferrotype, chrysotype, anthotype, daguerreotype, and thermography, which the following year was translated into German and Dutch. The medium was immediately taken up and perfected by notable photographic practitioners of the time, including William Henry Fox Talbot and Henry Bosse. The latter, in making fine presentation albums of bridges and structural steel, foresaw an appropriate effect in colour; the intense blues of his refined cyanotypes from large glass plates were printed on fine French paper 37 cm x 43.6 cm, and watermarked Johannot et Cie. Annonay, aloe's satin and leather bound.
Commercial use came only in 1872, the year after Herschel's death. Marion and Company of Paris were first to market the cyanotype, under the proprietary name of "Ferro-prussiate", for reprography of plans and technical drawings and to advantage due to its low cost and simplicity of processing, which required only water. In this application and with the manufacture of blueprint papers, it remained the dominant reprographic process until the 1940s. During the 217-day Siege of Mafeking of the town of Mafeking (Mafikeng) in South Africa during the Second Boer War from October 1899, the process was used to print stamps and banknotes. Unlike many nineteenth-century photographic processes that became obsolete, cyanotype remained in continuous use through the twentieth century due to its widespread adoption for architectural and engineering blueprints and its later revival among artists and alternative-process photographers.
The simple technology of the cyanotype, though, remained accessible in the nonindustrial realm and contributed to folk art; Francois Brunet notes the cyanotypes on cloth used by American home quilt-makers after 1880, and Geoffrey Batchen cites 30 or more early cyanotyped family snapshots on cloth, sewn into pillow slips or quilts, in the collection of Eastman House. Sandra Sider perpetuates this tradition in her own quilt-making and as a proponent for increased museum acquisitions of art quilts.
The cyanotype produced negatives, reversing the darks and lights of the image or object exposed on it, but Herschel also contrived a version, though more complex, to produce positives, which he hoped would aid in his ambition to achieve images of full natural colour. Its difficulties were overcome by Henri Pellet in 1877 in his gum arabic iron cyanofer direct positive photographic tracing method, which he commercialised.
Process
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 ml of sensitizer solution.
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, anodized aluminium, and even gelatin sizing on 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 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. The exposure time varies widely, from a few seconds in strong direct sunlight, to 10–20 minute exposures on a dull day.
After exposure, the paper is 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. An acidified rinse using citric acid or vinegar is sometimes used after exposure to help clear residual unexposed ferric compounds, reducing staining and improving highlight separation. The blue color darkens upon drying.
The ingredients for the process have remained mostly unchanged since its inception in 1840. After 1897, the green form of ammonium ferric citrate introduced by Austrian chemist Eduard Valenta quickly replaced the earlier brown form because it increased the light sensitivity of cyanotype materials, although the final chemical composition of cyanotype images made with either form is identical.
In 1994, 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. 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. Some formulas include ammonium dichromate to increase contrast and sensitivity; however, ammonium dichromate is toxic, carcinogenic, and an environmental hazard. 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.
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.
Disadvantages of the Ware formula are a higher cost, more complicated preparation, and a level of toxicity.
Printmaking
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.
Chemigrams 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 original is required to print as a positive image. Large-format photographic negatives or transparent digital negatives can produce images with a full tonal range, or lithographic film can be used to create high-contrast images.
The cyanotype may be combination-printed with gumoil, 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.
Toning
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. It is common to bleach prints before toning them, but also possible to achieve different effects by toning prints without bleaching.