After film has been developed and fixed, it must be washed in water to remove the thiosulfate ("hypo") that remains in the emulsion. Thiosulfate binds to gelatin and is difficult to remove. If it is allowed to remain it will react with atmospheric sulfur to stain, bleach, and degrade the image over time.
A continuous, rapid change of water at the surface of the film is the most effective way to wash. Thiosulfate is removed from the emulsion by diffusion as long as the concentration of thiosulfate in the gelatin is greater than the concentration in the surrounding water. As fresh water flows past the surface of the film, thiosulfate ions migrate to the region of lowest concentration. Washing is very rapid at the outset and slows exponentially as the wash proceeds and the gelatin contains progressively less and less thiosulfate.
A half hour wash in running water is usually enough to remove all the thiosulfate that can be removed. The amount remaining is so small that it has no practical effect on the emulsion's longevity.
When it is desirable to use the minimum amount of water, it is possible to use sequential changes of water, with agitation. The exact timing is not particularly important. Fill the tank with water, agitate continuously for a one to three minutes, then drain the tank and repeat. Ten or twelve changes of water, agitating the film for two to three minutes per change, removes as much thiosulfate as the same amount of time in a running water bath, but uses much less water.
Whether you use a running water bath or sequential changes of water, the use of a sulfite wash aid reduces the total time required for a wash to not more than five minutes, which cuts down markedly on water use.
You can buy wash aids, or make your own. To make a wash aid, make up a 2% solution of sodium sulfite in water (20 grams of sulfite in a litre of solution).
After removing the film from the fixer, rinse it thoroughly in water to remove fixer from the surface, then immerse it for about a minute in the sulfite solution, agitating continuously. Sulfite ions in the solution will react with thiosulfate ions in the emulsion, forming a complex that is easily washed from the gelatin.
Then a five-minute wash in running water, or five or six consecutive changes of water (with agitation, at least one minute per change) removes virtually all of the thiosulfate-sulfite complex.
If the film dries with water spots remaining on its surface, these are likely to cause physical distortion of the gelatin, making dimples that are visible as defects when the film is printed. To avoid water spots, give your film one final rinse in a bath of distilled water to which a wetting agent has been added. Be sure to use a wetting agent especially formulated for photographic purposes (such as Bluefire SpotStat).