Chemicals for processing B&W film

Some are essential; some are optional (but worthwhile)

In addition to fresh water, the two chemicals that are essential to the development of film are developer and fixer. Other chemicals that are optional, and which most people use to save time and to extend the life of the chemicals and of the film, are stop bath, hypo clearing agent and wetting agent.

ESSENTIAL CHEMICALS

Developer - This is the chemical that converts the latent image on exposed film into a visible image by changing silver halide crystals that have been affected by light into black metallic silver. There is more than one type of developer for black and white film. No one developer is suitable for all film conditions. Normally-exposed film requires a normal developer. But, film exposed under poor lighting may need an active developer to extract as much of the image as possible. Your choice in developer depends upon the type of film, the conditions under which it was exposed, the type of negative that is desired and the developing time that is ideal for your method of development. (See Types of B&W film developer - click here or on the "Developer" link at the bottom.)

Fixer - When film has just been developed, it contains a visible image but it cannot yet be exposed to light because not all the silver halide crystals were reduced to black metallic silver. If you did expose it to light, the images would become obscured (fogged) by the silver halide in the emulsion because they are still light-sensitive. Fixer is the chemical that dissolves undeveloped silver halides from the film's emulsion (making them water soluble), and that makes the developed image permanent. When film is placed in fixer, it is said to be in a fixing bath. The main ingredient in fixer is commonly either sodium thiosulfate or ammonium thiosulfate, also known as hyposulfite and often referred to simply as "hypo". (See more about Fixer - click here or on the "Fixer" link at the bottom.)

Washing in water - Fixing, as we just saw, converts silver salts (silver halide crystals) into water-soluble salts. These salts must be removed from the film, and the fixer itself must also be thoroughly removed, otherwise it would combine over time with the black metallic silver image in the negative, resulting in brownish-yellow stains of silver sulfide. The image would also fade. The salts and the residual fixer are removed by washing in plain water. (Interestingly, sea water may be used to wash negatives, too, and does the job in about two-thirds the amount of time. The sea water's salt, however, must be removed from the film by a final film washing in fresh water.) (For more information on washing film, click here or on the "Washing film" link at the bottom. For information on drying film, click here or on the "Drying film" link at the bottom.)

OPTIONAL CHEMICALS

Stop bath or Acid stop bath or Acid rinse - This chemical is used after development but before fixing the developed film. When the developer is removed from contact with the film at the appropriate time, a small amount of it remains in the film's emulsion and on the surface of the film. Since you no longer want development to continue, you must neutralize or remove this residual developer. That is where Stop bath comes in; it stops the action of the developer.

Note that some people commonly do not use stop bath, but will instead quickly rinse the freshly-developed film in plain water, known as a "Water rinse bath." This bath slows down on-going development by washing residual developer from the surface of the film, but does not completely stop it since the bath does not remove developer that is in the film's emulsion. By treating the washed film with fixer, development then becomes stopped. A water rinse bath is not ideal. Using stop bath is better than using only a water rinse bath. (See more about Stop bath - click here or on the "Stop bath" link at the bottom.)

Hypo clearing agent - This chemical speeds up the washing process, clearing hypo (the main ingredient in fixer) from the film more quickly than simply washing in plain water.

Two relatively short water washes are usually carried out - one before the wash with hypo clearing agent, and one after. Some manufacturers will tell you that the water rinse after the fixer is not essential, but that it will increase the capacity of films for a given volume of hypo clearing agent solution, so it is wise to use it. A 30-second water rinse after fixing is recommended, then one to two minutes (with moderate agitation) of hypo clearing agent, followed by a five-minute wash in fresh running water that has at least one complete change of water in that time.

Wetting agent - Surface tension can cause wash water to remain in beads on the surface of the processed negatives, and water can drain off the film in an erratic manner so that some parts of the film dry more quickly than others, creating unwanted water spots. Furthermore, when irregular drying occurs, parts of the film can shrink unevenly, changing the density of the image. Wetting agents are chemicals that lower surface tension and cause the film to dry faster and more evenly.

Follow the manufacturer's instructions, which generally require immersing the negatives in the properly-diluted solution for thirty seconds, then allowing them to drain before hanging to dry.