Pigment printing process
Photographic images are referred to as pigment prints , in which the image is created by mineral or artificial color pigments on a carrier material. These color pigments are usually lightfast and the images are therefore stable. While pigment prints in historical photography were produced by washing out unexposed components, pigment prints of the modern type are mostly produced by inkjet printers .
Historical procedure
The pigment printing process , (including coal method or pressurized coal called) is one of the precious printing process . It is based on the fact that if gelatine is exposed to light with a chromic acid salt ( potassium dichromate or ammonium dichromate ), it becomes insoluble ("tanned") in water. If a dye ( pigment ) is added to it, the insoluble areas hold it back mechanically. Because coal dust was often used as the pigment, the name carbon printing is also used for this process .
If paper is coated with a mixture of gelatin bathed in chromate and exposed under a negative, an image can be obtained by washing it out with hot water. However, since the effect of light begins on the surface and extends more or less deeply through the thickness of the light-sensitive layer, individual gelatin particles lying directly on the paper will remain soluble in the areas that have become insoluble in the light, and they will dissolve in hot water and rob the overlying "semitones" of their hold.
In order to avoid this, the initially invisible image lying on the surface of the exposed layer is lifted off by a transfer printing . For this purpose, a piece of paper coated with tanned gelatine is pressed on - the so-called transfer paper. This then sticks to the surface. If one now treats the compressed papers with hot water, all areas not hit by the light are loosened; the first paper, which only served as a carrier for the photosensitive gelatin layer, peeled off, and the image areas, which consist of colored gelatin that has become insoluble, adhere to the transfer paper.
If the transfer paper is rubbed with a fine layer of resin, the adhesion is only loose. If you then press on a second piece of gelatine paper, the image adheres more strongly to the second surface than to the first and can thus be transferred for the second time. The "pigment image" obtained with the first transfer is wrong, ie it appears as a mirror image of the object; the image transmitted twice, however, is not laterally correct.
These pigment images can also be transferred to glass, thereby creating transparent window images. Because the pictures are made of lightfast pigment, they do not fade like the silver pictures, but are easily mechanically damaged.
From 1903 to 1911 the Neue Photographische Gesellschaft Berlin-Steglitz brought the advanced NPG pigment process onto the market for the production of reflective or transparent images .
Modern process
After an image has been processed in the computer, the artist prints it onto the desired substrate using an inkjet printer and special pigment-containing ink. This can be paper, fabric, canvas or another carrier material. This artistic technique is also known as giclee and belongs to the field of digital art . C-prints are also used as proofing processes in the printing industry. With color pigments in the basic colors (CMYK system), they produce true-to-color and true-to-color originals. B. Cromalin . More recently, this technique has also been referred to as digital pigment printing.
literature
- Wolfgang Autenrieth: New and old techniques of etching and fine printing. From witch's meal and dragon's blood to the photopolymer layer. Tips, tricks, instructions and recipes from five centuries. (An alchemical workshop book for erasers). Version 6.3. W. Autenrieth, Krauchenwies 2010, ISBN 978-3-00-035619-3 (→ excerpts online) .
