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Herschel's Chrysotypes

Herschel's Chrysotypes

Invented in 1842 at the same time as the cyanotype, in the course of investigating the photo-chemical properties of substances other than silver, the chrysotype was perhaps Sir John Herschel's most promising new photographic process. For all the later and lasting success of the cyanotype, which owed much to its ease of manipulation, the chrysotype fulfilled more perfectly Herschel's quest for a process capable of registering fine detail without loss of perfect contrast (strong dark areas and clean white highlights); and he was always captivated by a beautiful colour.

A latent or semi-latent (very faintly visible) image was formed on paper treated with light-sensitive iron salts such as ferric ammonium citrate (as used in the cyanotype). The image was then 'brought out' or developed with gold chloride solution, from which the affected iron salt precipitated metallic gold over the image. It was fixed with 'spring water' and weak potassium iodide solution, and then washed.

The resulting image was a strong purple, a colour which was to become familiar to photographers through the practice of gold toning, much used with albumen prints (for an example see 30607). The extraordinary contrast and clean whiteness of the highlights in the best chrysotypes can be seen for example in 22081 and 30155, albeit perhaps too dark in the dark areas; while the clarity of detail can be seen in 22573 and 30155. When the darkness of the purple could be restrained a very pretty and delicately detailed bluish-purple could be achieved (22573).

Herschel thought it 'a process no wise inferior in the almost magical beauty of its effect to the calotype process of Mr. Talbot'. Giving it the name chrysotype, which he said was deliberately meant to sound like calotype, indicates Herschel's expectation that it might have a practical future, especially as a process for making negatives. Although slower, it might even have proved a competitive alternative to Talbot's invention, as things stood in 1842. But Herschel's primary interest being in the scientific aspects rather than the application, the rapid adoption of the calotype as the standard paper process left the chrysotype un-developed. It was taken up later by Hunt and others; and still fascinates practitioners of revived alternative processes, who sometimes produce very beautiful results.

A sister process was also described by Herschel, and later called 'argentotype', in which silver nitrate instead of gold chloride developed the latent iron-based image. In other words, the silver nitrate was not used in its usual role of basic photo-sensitive agent: as with the gold, it was applied after exposure and reacted with the iron salt to form a strong and very finely detailed image - in this case purply brown in colour. As with all silver processes, it was fixed with hypo. The surviving specimens that have been recognised (19112, 82025), however, while preserving sections of extremely fine detail, are defective either in fixing or from some other cause.

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