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Cyanotype process - Discovery

Potassium ferricyanide (also known as potassium hexacyanoferrate (III)) was first discovered in the 1820s, but was only easily obtained pure by the use of electrolytic oxidation on the related compound potassium ferrocyanide by Dr Alfred Smee in 1840. Smee sent samples of the material to Sir John Herschel in 1842, and almost immediately he set about experiments making use of its sensitivity to light that resulted in the cyanotype process.
Herschel bushed a solution of the ferricyanide onto paper, dried it and exposed part to light. When the paper was treated with an ferric chloride solution, the exposed parts produced a deep blue colour. As Mike Ware has pointed out in his exhaustive monograph on the process, Smee had already observed the same reaction in a test tube, but had apparently not thought to apply it to making prints.
Ware also points out that it was Smee who supplied Herschel with the remaining key to the cyanotype and other iron-based processes. Smee was a surgeon and aware of the latest compounds made for pharmacy, which included those now commonly known as ferric ammonium citrate and ferric ammonium oxalate. These substances were considerably more sensitive to light than potassium ferricyanide
Within weeks, Herschel had developed what became the traditional method of preparing cyanotypes, coating the paper with a mixed solution of potassium ferricyanide and ferric ammonium citrate, exposing to light and developing by washing the exposed paper with water. Soon after he also proposed the name 'cyanotype' for all the variations of the process.
Although the cyanotype still uses potassium ferricyanide, it depends on the greater light sensitivity of the ferric ammonium citrate. Under the action of light, this compound undergoes an internal decomposition in which the ferric iron is reduced to ferrous iron and the citrate is oxidised to carbon dioxide and acetone. The ferrous irons then react with the ferricyanide to give an intense blue compound, which we once used to call 'Turnbull's Blue' but is now known to be ferric ferrocyanide or 'Prussian Blue'. (The final compound is actually the same as in Herschel's first experiment, although it was long thought to be different.)
Herschel produced a great number of specimen prints, which were made by contact printing from engravings and thus contained no intermediate tones. The process was also taken up by others for making what we would now call photograms, placing objects on the coated paper and exposing to sunlight.
The best known of these early efforts are undoubtedly the work by Anna Atkins, a family friend of the Herschels, who in 1843 produced the first photographically illustrated book with her hand produced 'British Algae: Cyanotype Impressions'. At least 13 copies of the work still exist, although they differ. Atkins sent out the work in 12 parts, and those in different copies show considerable differences in both the number of plates and also in the exact composition of the images.
Other botanical works illustrated in a similar way (including later work by Atkins) were also produced, both at around the same date and much later.
The cyanotype was little used for pictorial processes in the following 50 years, but enjoyed a comeback following the introduction of the dry plate, which made photography considerably more popular. Its simplicity and the cheapness of the materials used made it popular as a proofing process (including some fine images by photographers including Edward S Curtis and Arthur Wesley Dow), and for some applications in record photography in the period 1890-1920. Its sole drawback as a straightforward printing process was the colour of the prints - blue seldom seems a sympathetic colour for most purposes.
It was particularly useful for photographers working under difficult conditions, such as Charles Loomis, who recorded the exploration of the American West, and Henry Bosse along the Mississippi in the 1890s. During the siege of Mafeking, South Africa, in the Boer War in 1900, military commander Robert Baden-Powell had a local photographer make cyanotype bank notes and postage stamps. Her Majesty Queen Victoria was presumably impressed by his resourcefulness, but may have been less amused when she learnt this his head took the place of hers for these special issues.
Blue-printing continued in technical use, particularly for the reproduction of large technical drawings from around 1875 to the late 1960s, when it was largely replaced first mainly by diazo methods and later by electrostatic copying. It was also marketed as a scientific toy, giving many children their introduction to photographic printing. With a growing interest in alternative printing processes in the 1970s and 80s, many photographic artists also returned to the process, if only as a cheaper and easier way of learning some of the skills required for platinum and other processes.
Mike Ware made a significant advance in the chemistry with his 'New Cyanotype' process in 1994. This gave some advantages over the traditional method, including greater speed and a claimed better gradation, along with less loss of highlight detail on washing. The solution also keeps better, not growing the mould that can be a problem with the traditional formula, although this is almost certainly linked to the toxicity that makes the new cyanotype unsuitable for use with children. It is also more difficult to prepare, and I found it also seemed to be more sensitive to different paper types. Both methods are in current use.
More recently, Terry King and Michael Maunder have also experimented with variations on the process, at least some based on re-reading Herschel's original laboratory notes. In one variation the paper is coated with a ferric oxalate solution containing a little oxalic acid, dried and exposed and then treated in a developer of 3% potassium ferricyanide. King calls his process Cyanotype Rex, and if you feel rich and treat the exposed paper with gold chloride in place of the ferricyanide you get a Chrysotype Rex.
These methods are similar to those I experimented with ten years previously when I was working with Terry, using a 'universal' ferric oxalate sensitiser to produce - with suitable developers - kallitype (silver) prints, platinum and platinum/silver images - and rather weak gold prints.

© Peter Marshall

Toning of salt prints is possible, but the results are often fugitive. Another method to moderate the colour of the image is to use multiple printing with other methods such as salt printing, kallitype and gum bichromate.

Blue salt
© Peter Marshall

Chemical interactions occur between processes, the order is vital. In some cases methods can only be combined by the careful application of some of the solutions to parts of the paper only.

Blue salt K
© Peter Marshall

This section is courtesy of Peter Marshall
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