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CYANOTYPE
HISTORIC OVERVIEW
Cyanotype was invented by the English Astronomer Sir John Herschel, in 1842 during his search for a method of copying his calculations and other memoranda. He coated paper with an organic ion salt and after exposure to UV light under objects, the image was developed with potassium ferricyanide which he called Blue Prints or Cyanotypes.
Herschel used organic salts of ion as the bases of this light sensitive printing process. The ion salt processes are based on the reduction of ferric salts of organic compound (citric, oxalic, etc.), to a ferrus state by exposure to ultraviolet light. Once in the ferrus state it will combine with potassium ferricyanyde to form Prussian Blue, also called Turnbull's Blue.
Herschel also used gold and silver to develop his images, calling them respectively Gold prints or Chrysotypes, and Silver Prints or Argentotypes.
When dichromate is combined with other metal salts it reduces them to metalic form, this gave way to a number of processes using organic ion salts, so called 'ION PROCESSES', such as the Kallitype, Vandyke Process, Pellet Process, Mercurio Sepia Print, and later in the 19th century the Platinum and Palladium processes.
Cyanotype was taken a brother use rather then just office copying by Mrs. Anna Atkin who, from 1843 to 1953, produced photographs of British algae, "Cyanotype Impressions", a set of 411 plates that ilustrated a series of British seaweed by contact printing onto sensitised paper. After Mrs. Atkin's achievements, the use of Cyanotype declined until aproximately 1881. Then Marion & Comp. reintroduced it as the "Ferroprussiate Process". Paper was then sold already coated for architects and shipbuilders for making quick and inexpensive reproductions.
The following is an example of a Cyanotype sensitiser formula:
Solution - A
Ferric ammonium citrate green31g
Diatilled water 116ml
Solution - B
Potassium ferricyanide 15.5g
Distilled water 116ml
The individual solutions keep very well if kept in tightly capped brown bottles and stored in a cool place.
To use, mix equal parts of solution A and B. Once mixed, the sensitiser will not last longer then 24 hours.
An updated version of the Cyanotype has been recently developed by Dr. Mike Ware. This newly designed Cyanotype sensitiser is a one solution only, with an excelent keeping quality, and very good image quality. Also has a lower risk of cross contaminating negatives. It certainely made it easier and convinient to have available in the market a one solution sensitiser.
EXPOSING CYANOTYPES
Prepare the work station as discribed on the page " General Information - Coating".
Cyanotypes are printed by contact under the sunlight or other UV light source (see "General Information - Exposure"), and exposure is judged visualy, therefore a contact printing frame is very useful. The contact frame allows to inspect your exposure without moving the paper and negative from register. Two pieces of glass can be used with success, providing the paper and negative are taped in register to allow for inspection. The only draw back is that you will see the marks of the tape in the end.
Mike Ware's formula is faster then the traditional ones. The exposure time, obviously will depend on the intensity of the ultra violet radiation. As a rule of thumb, the exposure is carried out until the high lights show a green detail, the midtones are blue and the shadows are gray blue.
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PROCESSING
Cyanotypes can simply be processed with nothing more then a few changes of water, although a better gradation can be obtained if prints are treated initially in a citric acid bath at aproximatly 4% strength (40 gm of citric acid in 1 ltr of tap water). One litre of citric acid bath will process aproximately ten 8x10 ins prints. The colour of the cyanotype will intensify quickly within the first 15 to 20 scds and the print should be removed imidiately otherwise it will start fading. Remove the print to a washing tray and let it wash facing down. The yellow from the sensitiser should clear compleatly from the unexposed areas. Washing will take long if an excessive amount of sensitiser was used to coat the paper.
Contrast control can be achieved by adding a few drops of a 2.5gm per liter solution of ammonium dichromate to the sensitiser (start with 1 or 2 drops per ml of sensitiser).
Adding a few drops of 5% solution of citric acid to per ml of sensitizer improves image quality and ease of clearing, but shortens its storage life (start with 2 or 3 drops ).
