Technical Notes

Brief Explanation of The Dye Transfer Process

     The principles of color separation have been known since the middle of the nineteenth century, and dye transfer is a good example of how they work. Once the basics of this process are understood, it is easier to understand other color processes.

     If one intends to use photographs in a publication it is usually better to begin with slides, and it is cheaper and easier to sort large numbers of slides. Here we will only discuss the process of making prints from slides. The slide is placed in an enlarger which is bolted into a solid wall at several points for stability. I use a special point light source designed for making microfilm images. The point light source makes for a very sharp image, but is also reveals every scratch. On a photographic enlarger the carrier is the glass and metal tray that holds the film which contains the image. If a color slide is put in a very clean carrier and immersed in an oil (castor oil) with the same incidence of refraction as the clear plastic of the slide, then for optical purposes, the surface of the slide becomes as perfect as the glass of the carrier. Oil immersion eliminates flaws on the film base, scratches on the film.

     The slide is exposed through red, green, and blue filters onto three different black and white film sheets. These are the SEPARATIONS. I make 8"x10" separations, "seps" in the jargon of Dye Transfer. An exposure is made through a blue filter to get a negative of the yellow image, for example. If this black-and-white negative of the yellow image is put in a much bigger enlarger, and exposed onto another black-and-white film called MATRIX film, the result will be a positive of the yellow image. The matrix film has two layers: a clear, dimensionally stable plastic base and a mixture of silver halide emulsion, gelatin layer, and a yellow dye. The yellow dye is used to keep blue light to which the film is sensitive from penetrating too deeply, to cause it to penetrate more deeply proportional to its intensity. When the silver has been reduced by development with a tanning developer, the gelatin is hardened adjacent to points on the image where the silver turned black. The rest of the gelatin and the yellow dye are washed down the drain with repeated baths of hot water after the film is developed and fixed. What is left is a relief image in gelatin. Remember, there are at least three such images. The slide is exposed through blue to get a negative of yellow, and that is exposed onto matrix film for a positive image of yellow. The slide is exposed through green to get a negative magenta image, then that is used to make a positive magenta image, red to get negative cyan, then positive cyan. The result is a set of three positive images (cyan, magenta and yellow) each in a gelatin relief; these are the MATRICES.

     In the presence of acetic acid, gelatin absorbs dye The matrices sit for a minimum of ten minutes in trays of dye on a motor driven rocker table that gently moves the dye across the surface. Each matrix is rinsed and rolled onto wet white paper which has been rolled down on a flat stone surface. This is a plain paper with a gelatin layer and chemical mordant to keep the dye from moving around once it hits the paper. The paper is less acid than the dye. In less than ten minutes the DYE TRANSFERS into the gelatin on the paper from the gelatin relief on the matrix. It has migrated from acid to base. That matrix is peeled away, and the next color is rolled down. The colors blend together. After all three colors are applied in this way, the print is dried on a heated drum.