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uses a photosensitive ferric compound. The best-known version utilises ammonium ferric citrate and potassium ferricyanide. The paper of the prospective copy is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron. The image is then developed using a solution of potassium ferricyanide forming

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