Introduction

Discuss the use and theory of photographic chemicals. Explain briefly how developer brings out the latent images on photographic paper and film and how fixer makes the image permanent.

The photographic process, from the manufacture of the light-sensitive materials to the processing of the final print, is based on a series of chemical reactions. Some of the reactions are comparatively simple; others are quite complex. All require careful control and reagent purity and reaction conditions.

Photographic material has a mechanical support, such as film, glass, or paper, with a light-sensitive coating or "emulsion" made up of minute silver halide crystals (usually silver bromide) suspended in gelatin. Exposure to light in a camera, printer, or enlarger gives no visible effect, but there is an invisible change which produces a "latent image." To obtain a visible, usable image, the exposed material must be developed, fixed, and washed.

When exposed photographic material is placed in a developer solution, the developer attacks the exposed grains, which contain the latent-image material, freeing the silver from its compound and depositing it as tiny, irregular grains of metallic silver. Multitudes of these minute grains form the black silver image. The developer will also attack unexposed grains, but much more slowly so that only a relatively small amount of silver (fog density) is formed in the unexposed areas during normal development.

After development, the undeveloped silver halide crystals in the emulsion must be removed to keep them from darkening and obscuring the image. This "fixing" is done by treating the emulsion in a solution of sodium or ammonium thiosulfate. Either forms a soluble compound with the silver halide, but has practically no effect on the silver image under normal conditions.

After the developed silver halide has been dissolved, the emulsion is still saturated with chemicals of the fixing bath. If the hypo remained, it would slowly decompose and attack the image, making it discolor and fade. The hypo is removed by washing.

Instruction

Discuss and demonstrate proper methods of selecting, mixing and storing chemicals. (See instruction sheets packaged with chemicals.) Emphasize the need for cleanliness and care when working with chemicals. Contamination of the chemicals leads to poor picture quality. When mixing chemicals, wear safety glasses to protect your eyes and an apron to protect clothes.

Discuss the need for safety in the darkroom -- care in use of chemicals and safe placement of sharp objects and obstructions to avoid accidents in the dark.

Have each student mix a batch of each chemical needed for the course and store it properly in a clean, cool place. Developers have a shorter life than other chemicals, so it is wise to mix a smaller batch.

Assignment

Have students study the proper methods of selecting, mixing, and storing chemicals.