Development

The rate of development of the individual grains in an emulsion is affected by so many factors, such as the rate of diffusion of the solution through the gelatin matrix, the adsorption of the developing agent, the solution of the silver halide, oxidation products of the developing agent and the accumulation of restraining by-products, that exact analysis of it is difficult. The time of appearance of a visible image is, within limits, a reliable indication of the rate of development. It varies with different emulsions and is quite different with different developing agents, but the variation with temperature, dilution and pH is almost directly related to the variation in the rate of development.

The rate of development, as determined from the change in the optical density of the developed image, is complicated by the fact that density increases in two different ways: (1) by the increase in the amount of silver as the grains develop and (2) by an increase in the number of grains in the process of development. Density grows rapidly at first and then slows down until development is complete and no further growth in density takes place. Prolonged development would, of course, increase overall density through the development of unexposed grains (fog).

Halting Development - The Stop Bath

Once the exposed image has been developed to the desired degree, it is necessary to halt the chemical process quickly to prevent over development and the production of fog. The solution used to that end is referred to as the "stop" bath. Since developers function a relatively high pH’s, the typical stop bath is simply a solution of acetic or some other weak acid. The action of the acid is so rapid it usually requires only seconds for the process to be effectively halted.