What is the minimum time required for final wash in film development?
According to Massive Dev App it's around 10 minutes, which I feel quite long:
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Washing photo materials to remove the processing chemicals has been well studied. Both photo film and photo papers are based on a binder consisting of gelatin. In other words, dispersed in gelatin is the metallic silver image (black & white) or dye image (color). Gelatin is a long chain polymer; under the microscope, it resembles transparent spaghetti. Now photo chemicals and potions are mainly water. When photo materials are plunged into water, the gelatin coat swells. This action parallels what you see when a dry sponge is wetted. It is the swelling of the gelatin that opens the structure of the emulsion allowing fluids to freely percolate about. Later when these materials dry, the gelatin shrinks and thus returns to its customary thickness.
When films and papers are processed, likely the final major step is a bath in a fixer solution. The fixer, which is either sodium or ammonium thiosulfate, renders the photographic image permanent. The bad news is that fixer readily breaks down to form sulfur compounds. In the case of black & white, this sulfur attacks the metallic silver image by converting it to silver sulfide. This action stains the image and likely this stain will be mottled. In the case of color images, these are based on organic dyes that are “fugitive”, they are easy altered by environmental conditions. In other words, they fade.
The key to image stability is a thorough washing to rid the emulsion and the underlying base of residual chemicals. The total time to remove all of fixer is a variable, temperature, and agitation and gelatin hardness are the key factors. Well studied in the laboratory, the average time for complete fixer removal is about 30 minutes. Double-weight fiber papers can take over an hour to be completely chemical free.
During World War II it was discovered that photo materials washed in seawater (2.6% salt), followed by a short fresh water rinse, washed in one-fifth to one-tenth the time. This discovery is the basis for the modern fix (hypo) clearing rinse agents. These are solutions of various salt compounds, some containing peroxide. Use of a washing aid promotes permanence plus reduces the amount of water required to do the job. Keep in mind, if water is plentiful and time is unimportant, rinse aids are not a requirement.
The purpose of the final wash is to get all of the chemistry (really, the fix) out of the film (or paper). If you don't do this then your film will be damaged by the chemistry over time. It's hard to tell how much chemistry is still in the film, and washing is a very easy process to do, so people tend to be very conservative.
Ilford did some research on this a while ago, actually measuring what was left in the film, and discovered that washing by changing the water in the tank & inverting a bunch of times was very effective. They have some instructions (for their chemistry!) here (PDF link).
What I do is an exaggerated version of the Ilford thing: I do 20, 50 & 100 inversions changing water between each set & then leave the film to stand in another change of water for 10 minutes, finally using yet another change with photo-flo in it. This is excessive and I feel bad about using a lot of water, but I really want to know my film won't get eaten by the fixer, and I've seen too many old prints which have been. And of course you can do other stuff in the final 10 minutes (like rinse & dry all the graduates, put the chemistry away &c &c).
The absolute minimum time is 0 to get a usable image. The process, however, is incomplete and the residual chemicals in the emulsion will degrade the image over time. The longer you wash (the more hypo you remove from the emulsion) the greater the archival stability of the image.
For rapid access (processing breaking news film before electronics), we would process and broadcast. Then, we would return the film to tanks to finish processing wash and drying the footage.