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Related to Fixing: spot fixing, Nitrogen fixing
in photography, the process of dissolving the silver halides in an exposed photosensitive emulsion that were not reduced during photographic development; as a result, the image becomes resistant to the effects of light and does not change during prolonged storage.
The most common fixing solutions use sodium thiosulfate, usually in the form of a 25-percent aqueous solution. An acid, such as sulfuric or acetic acid, is sometimes added to the solution to neutralize any developing agents remaining in the photographic emulsion after completion of the development process. Usually, however, neutralization is an independent stage—the stop bath—since the oxidation products of developing agents can saturate the fixer and cause a yellow tinting of the emulsion. The strength of the emulsion gelatin may be increased by the addition of substances that double the gelatin, such as aluminum alum or chromium potassium sulfate. Ammonium chloride, typically in a concentration of 50 g per liter of simple fixer, is used in high-speed fixers, which yield fixing periods from one-half to one-third as long as normal.
The duration of fixing depends on the concentration and temperature of the fixing bath, the intensity of agitation, the thickness of the emulsion layer, the concentration of silver halides, and the size of the microcrystals of silver halides in the emulsion. An increase in temperature accelerates fixing, but at temperatures above 22°–23°C the photosensitive gelatin layer may undergo excessive swelling, slip off the backing, or disintegrate. The fixing of images on fine-grain emulsions proceeds more rapidly than on coarse-grain emulsions, since the overall surface area of the silver halide crystals is considerably greater. The average duration of fixing is 15–20 minutes for coarse-grain emulsions and 8–10 minutes for fine-grain emulsions. Furthermore, it is necessary to distinguish between the time in which visible traces of silver halides disappear and the total time of fixing, which is approximately twice as long. Disappearance of silver halide traces does not indicate the completion of fixing.
In the rapid processing of photographic materials, fixing is often replaced with a less protracted operation called stabilization, which consists in the following: (1) rapid drying of the photosensitive layer after development, (2) neutralization of the alkaline developing agent remaining in the layer by treatment with an acid, (3) treatment with a solution of potassium iodide, and (4) extremely short-term processing with an acid fixer. Stabilization makes it possible to examine the photographic image before fixing. If the image is to be preserved, it is then fixed.
Other widely used processing techniques include high-speed combination methods for simultaneous development and fixing by means of special solutions that produce finished, fixed images; however, such processing methods result in a reduction in the photosensitivity of photographic materials.
REFERENCESBliumberg, I. B. Tekhnologiia obrabotki fotokinomaterialov, 2nd ed. Moscow, 1967.
Mees, C., and T. James. Teoriia fotograftcheskogo protsessa. Leningrad, 1973. (Translated from English.)