Cyanotype
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Cyanotype is an old monochrome photographic printing process which gives a cyan-blue print.
The English scientist and astronomer Sir John Herschel discovered this procedure in 1842. It uses two chemicals:
They result in an photo-sensitive solution when dissolved in water, which is used to coat a material (usually paper). A positive image can be produced by exposing it to a source of Ultraviolet light (such as sunlight) with a negative. 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.
The developing of the picture takes place by flushing it with flowing water. The water-soluble Iron (II) salts are washed away, while the non-water-soluble Prussian Blue remain in the paper. This is what gives the picture its typical blue color. The process was popular in engineering circles well into the 20th century. The simple and low-cost process enabled them to produce large-scale copies of their work, the so-called blueprint.
Long term preservation
In contrast to most historical and present-day processes, cyanotype prints do not like basic environments. So it is not a good idea to store or present the print in chemically buffered museumboard. This will cause the image to fade. Another, quite unique characteristic of cyanotypes is its regenerative behaviour: prints that have faded due to prolonged exposure to light can often significantly be restored to their original tone by simply storing them in a dark environment for a while.
References
Ware,M. (1999) Cyanotype: the history, science and art of photographic printing in Prussian blue. Science Museum,UK
Cyanotype by Mike Ware (http://www.mikeware.co.uk/mikeware/Traditional_Cyanotype.html)
Mike Ware's New Cyanotype (http://www.mikeware.co.uk/mikeware/New_Cyanotype_Process.html) a new version of the cyanotype that address some of the classical cyanotypes short comings.
Mike Ware's web site (http://www.mikeware.co.uk/mikeware/main.html)ja:青写真 zh-cn:卡罗法