Cyanotype

contact print from old b&w 5×7" glass plate negative - artist unknown. image by Uwe Kulick (Image rights)

  

The cyanotype process is one of remarkable simplicity, producing a blue and white print on paper or cloth. It was invented in 1842 by Sir John Herschel.

From 1870 to sometime in the 20th century, it was commonly used as the "blueprint" method to copy technical drafts. The used chemicals aren't silver-based as those of most other photo-chemical photographic processes, and their sensitivity is mostly to ultraviolet, such as the UV in sunlight. The paper can be prepared under incandescent light. Solutions of the method's iron-based chemicals are applied to paper surfaces or cloth, which can then be printed on when they are dry. The chemicals involved are ferric ammonium citrate and potassium ferricyanide, AKA pot ferri.

It is a printing-out process: the image formed by the light is visible, not latent, and so there is no strict "development" stage. Instead, excess emulsion is washed away by running water. Toning can be done to increase or decrease the intensity of the blue and/or the contrast.

A common use is to make photograms: opaque objects with interesting shapes are laid on the sensitized paper, and then the paper is put in direct sunlight or under an ultraviolet lamp, for a time usually measured in minutes (quality of sunlight or brightness of the UV lamp is a factor.)

Of course, this can also be done with large or medium format negatives, or ink on tracing paper. The latter is how blueprints were made when the process was in vogue. A drafter would draw the plans on paper in pencil, then trace it in ink on tracing paper. This would then be used as a negative, to produce positive copies made of white lines on a Prussian Blue background. This was done for several reasons, the main being that it was cheap, could make any number of copies, and was very difficult to alter once printed. In the digital age, a hybrid process is possible, my making digital negatives, using a printer and either transparency film or paper.

Digital negative used for contact print of cyanotype image by Pablo Coronel (Image rights)

Cyanotypes can be toned using several methods, a common one is to use tannins, such as the ones found in tea or coffee. This has the effect of increasing the permanence of the images.

As cyanotype is a cheap way of making contact prints, it still survives to this day under the heading of an "alternate process." Unfortunately, it is extremely difficult to make enlargements on cyanotype: most enlarger lenses are not designed to transmit ultraviolet light, and most enlargers do not take any common ultraviolet bulb. Thus, it is somewhat limited to the realm of contact prints and photograms.

supply makers

• Astromedia (ready-to-go cyanotype paper)
• Fotospeed (process kit & sensitiser)
• Photographer's formulary (process kit)

Links

• "New Cyanotype Process", (toxic) aspects of modern methods to improve the old process. 1996/2004 by Mike Ware
• flickr group "Cyanotypes"