Cyanotype is a photographic printing process that produces a cyan-blue print. Engineers used the process well into the 20th century as a simple and low-cost process to produce copies of drawings, referred to as blueprints. The process uses two chemicals: ammonium iron(III) citrate and potassium ferricyanide.
In a typical procedure, equal volumes of an 8.1% (w/v) solution of potassium ferricyanide and a 20% solution of ferric ammonium citrate are mixed. This mildly photosensitive solution is then applied to a receptive surface (such as paper or cloth) and allowed to dry in a dark place. Cyanotypes can be printed on any surface capable of soaking up the iron solution. Althoughwatercolor paper is a preferred medium, cotton, wool and even gelatin sizing on nonporous surfaces have been used. Care should be taken to avoid alkaline-buffered papers, which degrade the image over time.
A positive image can be produced by exposing it to a source of ultraviolet light (such as sunlight) through a contact negative (which can be produced by any suitable means, e.g. a conventional photographic negative or a print on acetate film made using photo-processing software to invert a positive monochrome digital image). The UV light reduces the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) complex with ferricyanide. The result is an insoluble, blue dye (ferric ferrocyanide) known as Prussian blue.
Exposure to ultraviolet light reduces the iron in the exposed, turning the paper a steel-grey-blue color. The extent of color change depends on the amount of UV light, but acceptable results are usually obtained after 10–20 minute exposures on a dark, gloomy day. The highlight values should appear overexposed, as the water wash reduces the final print values. Prints can be made with large format negatives and lithography film, or everyday objects can be used to make photograms.
Van Dyke Brown is an early photographic printing process. The process was so named due to the similarity of the print color to that of a brown oil paint named for Flemishpainter Anthony van Dyck
Printing with Van Dyke Brown requires the use of a negative in the size of the desired print, a suitable substrate for coating and subsequent printing, and an ultraviolet light source, either sunlight or suitable bulbs. The substrate can be almost anything to which the solution will adhere. Watercolor paper is a good first choice, but trickier substrates such as metal, glass, or tile can be first 'sized' with gelatin or arrowroot to facilitate coating. The substrate is coated with solution under tungsten light, air-dried, and coated a second time, if desired, for a stronger image.
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