Unlike black and white photography that is based on the chemistry of salts of silver, cyanotype is based on the photochemical reduction of iron (III) citrate salts, and is a technique used essentially to produce copies on paper or other supports.
This photo printing process was discovered by the scientist and astronomer English, Sir John Frederick William Herschel, in 1842, and has the merits of being cheap, easy, and substantially non-toxic. In the original formulation of Sir John Herschel, ferric ammonium citrate and potassium ferricyanide are used. On the basis of the blue color print is the formation on the sheet of an insoluble "prussian blue" precipitate.
To perform this tecnique we need two solutions of iron and a source of ultraviolet radiation (sun or as UV lamp) and to make an image, the paper is prepared with a mixture in equal parts of the two iron solutions and allowed to dry out from the light. The impression is made by contact in a press, placing the negative image on the paper and exposing the whole to ultraviolet radiation (sunlight or a powerful ultraviolet lamp).
So...let's go!:-)
Two stock solutions are required; their concentrations need not be exact and they are easily made up as follows. These solutions must be kept in brown bottles, stored in the dark.
Sensitizer chemicals needed
25gr Ammonium iron(III) citrate ('green' variety)
10gr Potassium ferricyanide
200cc Distilled water
1. Dissolve 25gr of the solid ammonium iron(III) citrate in 100cc of distilled water at room temperature
2. Dissolve 10gr of the solid potassium ferricyanide in 100cc of distilled water at room temperature
Mixing and coating
Shortly before coating, mix equal volumes of stock solutions A and B to make a total volume of sensitizer appropriate for the amount of coating to be done.
As support paper we must use papers for techniques of gouache or watercolor, of heavy weight (300 mg); there is no need for particular concern as to paper being acidic (cyanotype in alkaline medium fades). The coat technique does not need to be done in the dark: a 40 W lamp at 3 m distance does not cause damage. Once the material is sensitized, allow to dry in a dark place and use within 48 hours (after that time, the natural oxidation of the solution will render its use impracticable).
Since it is a contact printing process, the negative must be the same size as the desired copy.
Printing exposure and contrast
Expose to sunlight or a UV lamp: the image prints out fully, with the uncovered regions of sensitizer first darkening to blue, but then reversing back to a pale blue-grey. The exposure time depends on the type of paper, the density of the negative, and the intensity of sunlight (cyanotype has an iso of about 0.05)
Wet processing and drying
Immerse the exposed print in running water until the yellow sensitizer has entirely disappeared. Substantial amounts of the blue image pigment will leach out and disappear with the washing water, so the washing necessary to clear the paper fully will greatly 'reduce' the image.
My first Cyanotype - March 14, 2013
My album, on Flickr:
A very good and very useful website:
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