Relief printing

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Relief printing ( Relief for short ) is a collective name in photography for those fine printing processes with the help of which gelatine reliefs are produced on the one hand and which are used for photomechanical copies in the form of cinema films, paper prints or slides on the other.

A distinction is made between two groups of relief processes: wash-out reliefs and source reliefs .

Printing principle

The printing principle consists in picking up suitable printing ink with the gelatine on a matrix film or a matrix plate (light-sensitive coated glass plate) and allowing this to be sucked off by the gelatine on a target film or a target plate. Historically, the technology is based on oil transfer .

The various relief processes were of great economic importance until the introduction of photographic color films of acceptable quality around 1935 ( Kodachrome , Agfacolor -Neu), as they could be used to produce colored paper prints and slides. Some processes retained their importance beyond this change because they have technical advantages for special applications, such as large slide projection, or are of artistic interest up to the present day.

Wash-out reliefs

They use the property of some photochemical developer substances to harden the gelatin of a silver halide emulsion to different degrees according to their respective exposure. The unhardened gelatin can then be washed out with a water-based solvent at a certain temperature; the gelatin remaining on the template is used to absorb the printing ink.

Uvatypia and Technicolor 4 belong to the group of wash-out relief procedures .

The term dye transfer can actually be used for both relief processes, but because of the widespread practice that was practiced for decades in the 20th century, it generally means one or the other form of washout relief. The prints produced in this way are called dye transfer prints .

Source reliefs

In contrast to the wash-out reliefs, the matrix films and plates in the case of source reliefs are sensitized with chromium salts , usually potassium or ammonium bichromate , instead of silver salts . During development, the gelatin swells to different degrees depending on its exposure and then shows a more or less pronounced ability to take on color proportional to the exposure, which in turn is sucked off by the gelatin of the target film or target plate. Similar techniques are collotype and heliogravure .

The group of source relief methods includes, for example, the Pinatypie and Erwinotype .

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