Dark Room Chemicals in Photography

Process:
The process of developing film and preparing it for printmaking has changed little since the 19th century. Film that has been exposed to light is developed, washed, then fixed so the emulsion is no longer sensitive to light. This process has multiple steps---each require a different set of chemicals. There is little difference between developing color and black and white film except for temperature control. Developing film can be accomplished with chemicals readily available at a photography supply store.

Developer:
Developer acts on the film's emulsion, which is a mix of silver, gelatin and light-sensitive dyes. When submerged in developer and agitated, the dyes in color film are activated and the silver removed from areas of the film that have been exposed to light.
Photographic developers are normally solutions of phenidone or metol, combined with an alkaline agent such as borax, sodium carbonate or sodium hydroxide. These chemicals react with the silver halide crystals exposed to light to create metallic silver. Leaving the film in the developer for the correct length of time is critical; too short of a time is "pulling" the film and will result in a lighter printed image, too long of a time is called "pushing" the film, resulting in a darker printed image. This can be used to correct exposure issues or for artistic effect.

Stop Bath:
This chemical is the second step of the photographic developing process. A stop bath is used to stop the action of the developer, preventing the process of converting silver halide to metallic silver from continuing. This ensures that the film's opacity will create an accurate image when reversed in the printing process.
Stop bath is an acid, which neutralizes the alkaline base of the developer. It is a diluted concentrate of acetic or citric acid, although plain water is sometimes used. By neutralizing the base chemical, this stops the reaction between the silver halide and the developer.

Fixer:
Fixer is a chemical used to remove the remaining silver halide to prevent changes to the image upon further exposure to light. This keeps the film's image stable when it is removed from the light proof developing canister. Without fixer, exposure to light would cause the silver halide to have the same reaction as when the film was initially exposed, destroying the latent image.
Fixer, which is sometimes called hypo, is ammonium thiosulfate salt or sodium thiosulfate.

Film Rinse:
An optional step, film rinses add a protective coating to the final, processed film strip. This step protects the finished strip from dust and scratches. This clear coat is added after the final water bath, which removes the fixer from the strip, then the film is rinsed again and hung in a drying cabinet or other dust free area until dry. A film rinse can also speed the drying of the film, which will help to avoid spotting and water streaks in the finished print.

Water:
The most crucial of any chemical used in photographic processes, water is used in nearly every step of the process. A steady supply of running water is crucial to darkroom practices for rinses; it is also used in the dilution and preparation of the chemical concentrates for each step of the process.