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Invented by Herschel in 1842, Cyanotype produces characteristic Prussian Blue images through the combination of iron salts with potassium ferricyanide. Once coated, the paper can either be left to dry by air in a darkened room or heat dried with a hair dryer. The image is formed by contact printing using the sun, but because the process cannot resolve fine detail, working from a line negative is recommended. Once exposure is complete, wash the print in cold running water for around 30 minutes until all yellow is gone. To brighten the highlights, rinse the print briefly in a dilute chlorine bleach bath, or to lighten specific areas, use a brush and bleach diluted 1:32. As well as paper, Cyanotype prints can be made onto heavy cotton or canvas, but you should avoid exposing finished images to bright light, or they will fade.