The Traditional Cyanotype Process

The 'traditional' recipe was invented by Sir John Herschel in 1842, and has the merits of being cheap, easy, and substantially non-toxic. It is suitable for newcomers to alternative processes and can be placed in the hands of children, but an experienced printer wishing to get the best result out of a fine negative should find the slightly more complicated New Cyanotype process worthwhile.

Sensitizer chemicals needed

Ammonium iron(III) citrate ('green' variety) 25 g
Potassium ferricyanide 10 g
Distilled water 200 cc

Ammonium iron(III) citrate is also known as ammonium ferric citrate or ferric ammonium citrate; its composition varies enormously and the 'green' form, with an iron content between 14% and 18%, is preferable to the 'brown' form (18-28% iron), although the latter will work, more slowly.

Potassium ferricyanide is also known as potassium hexacyanoferrate(III).
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.

Preparation of sensitizer stock solution A

Dissolve ca. 25 g of the solid ammonium iron(III) citrate in ca. 70 cc of distilled water at room temperature and make up to 100 cc.

Within a week or two in many environments, the surface of this solution will be covered by a thick growth of mould. This can be inhibited at the outset by adding a few crystals of thymol, which float on the surface without dissolving, and should simply be avoided in extracting a sample of the solution. (N.B. Thymol is a harmful chemical.)

Preparation of sensitizer stock solution B

Dissolve ca. 10 g of the solid potassium ferricyanide in ca. 80 cc of distilled water at room temperature and make up to 100 cc.

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. The mixed sensitizer only has a short life (days). It is best to dedicate a separate, labelled syringe or pipette to each solution, and to use a third one for applying the mixture, in order to avoid cross-contamination. As a guide to the volume needed for coating, about 1.5 cc should suffice for a 10 x 8 inch print, if coated by rod; but brush coating may consume possibly twice this.

Although it is not part of the traditional formula, I would strongly recommend adding one or two drops of a 20% solution of Tween 20 wetting agent to each 10 cc of sensitizer before coating, to improve its absorption by the paper fibres; the appropriate amount depends on the chosen make of paper and should be found by experiment.

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 image shadows should be distinctly reversed, giving it a solarized look, and the high values should be green (the effect of colour-mixing the Prussian blue image with the residual yellow sensitizer). UV lamp exposure times may be around 20 to 30 minutes. The exposure is sufficient when the image appears quite 'blocked up'. The reason for this gross over-exposure is that about half the tonal scale of your print will disappear in the wet processing, as most of the Prussian blue image pigment in the high values is washed out. This loss in tonal gradation reduces the negative density range that can be successfully printed to only about 1 or 1.2, which is half that (ca. 2.4) of the 'new cyanotype' and the other iron-based processes. The traditional method therefore best suits those who only have negatives made for standard silver-gelatin (or gum bichromate) printing.

Wet processing and drying

Immerse the exposed print in gently running water (or several changes of static water), face down, until the yellow sensitizer has entirely disappeared from highlight areas (20 minutes should suffice). 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. It is important that the wash water should not be at all alkaline; addition of dilute citric acid, or even dilute hydrochloric acid, to the first wash bath may improve the image.

The 'reversed' shadow tones will slowly regain their densities during the drying, due to aerial oxidation of Prussian white to Prussian blue. If you wish to see the full density immediately, then immerse the print for about half a minute in dilute (0.3%) hydrogen peroxide during the final wash. This does not make any difference to the final dried-down densities, however.

More about the history and practice of this process may be found in my Cyanomicon which can be downloaded here.