Because of the reversal of dark and bright areas from the latent image to the visible image, the visible image is often called a negative. The most common method of making the image visible is to bathe it in a chemical developer that reduces the unstable silver halide grains to black metallic silver, which forms the image. In addition to the reducing agent, which is generally an organic compound such as a phenol or an amine, the typical developer contains additives that cause development to go on at a desired rate, prevent the reducing agent from being destroyed by the air, and keep unexposed silver halide from fogging the film. Each developer is generally designed to be used with particular film emulsions and to produce certain desired effects, such as fineness of grain in the finished image.
After development the negative must be stabilized, or fixed, so that it will no longer be sensitive to light. In fixing, the unexposed silver halide grains are removed by immersion in a water solution of sodium or aluminum thiosulfate. Between the developing and fixing processes the negative may be placed in an acid bath to neutralize excess alkali left by the developer. After fixing, the negative is washed and dried. Next the negative may be subjected either to intensification, a process in which additional silver is deposited in exposed areas to increase the contrast in the image, or to reduction, a process in which silver is removed to decrease the contrast. Toning is a process in which a photographic image is treated to change its color, as by changing the deposited silver to silver sulfide or causing a colored metal salt to form along with the silver.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.