Duotone (sometimes also known as Duplex) is a halftone reproduction of an image using the superimposition of one contrasting color halftone over another color halftone. This is most often used to bring out middle tones and highlights of an image. Traditionally the superimposed contrasting halftone color is black and the most commonly implemented colors are blue, yellow, brown, and red. There are, however, many varieties of color combinations used.
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Duotone in the context of Gum bichromate
Gum printing is a way of making photographic reproductions without the use of silver halides. The process uses salts of dichromate in common with a number of other related processes such as sun printing.
Gum prints tend to be multi-layered images sometimes combined with other alternative process printing methods such as cyanotype and platinotype. A heavy weight cotton watercolor or printmaking paper that can withstand repeated and extended soakings is best. Each layer of pigment is individually coated, registered, exposed and washed. Separation negatives of cyan, magenta, and yellow or red, green, and blue are used for a full-color image. Some photographers prefer substituting the cyan emulsion in the CMYK separations with a cyanotype layer. A simple duotone separation combining orange watercolor pigment and a cyanotype can yield surprisingly beautiful results.