It's not a power washer.
Some people mistakenly believe that washing film under running water is like using a power washer on their driveway. It's not. The running water is not blasting away the residual thiosulfate (fixer).
The thiosulfate is actually embedded in the emulsion and can only be removed by diffusion (or dispersion, we'll let the chemists argue over the difference). While the chemistry behind this gets really complex really quickly, the concept is simple. It's more like soaking a dirty pan in the sink to remove the baked-on grease.
Basically, the universe likes things in equilibrium. Thus the concentration of fixer molecules in the emulsion wants to match the concentration in the wash water.
Once the wash water reaches the same concentration as the film's emulsion, it won't absorb any more thiosulfate. By providing constantly running water, it is hoped that the concentration never reaches equilibrium and that eventually all the fixer is removed.
First, you can't remove all the fixer. You can only hope to leave so little that it won't affect the film over the long term.
Second, water doesn't always flow downhill. Take a look at any stream or spillway, not all the water tumbles over the rocks and heads downstream. Just watch the foam or debris, you'll observe some of it trapped in eddy currents or miniature whirlpools.
The same effects can take place in your film tank when you're rinsing. Depending on a thousand different variables, some parts of the tank may not get as much fresh wash water as the rest. Thus the film in those areas isn't as free of fixer as other areas.
This is why we're now recommending the FAD technique: Fill, Agitate and Drain. Ilford proposed this decades ago and it not only helps ensure an even wash, but it probably uses less water.
Our version is just slightly different:
- After draining the fixer, fill with clean water, slosh it around a minute and drain. (This gets rid of any fixer trapped in the tank or clinging by surface tension.)
- Do it again because we're a little paranoid.
- Now refill, cap the tank and agitate gently for 30 seconds. Drain.
- Refill, cap and agitate for 60 seconds, then drain.
- Repeat step four two more times.
- Refill with distilled water, add a wetting agent (our H2O flow is recommended); cap and agitate for 30 seconds. DO NOT DRAIN.
- Remove the lid, carefully remove the film from the holders while submerged. Hang to dry.
Agitating helps ensure that the particle concentration in the wash water stays uniform and that all areas of the film are treated equally. We've added the time requirement since diffusion is time dependent.
Of course, your results might vary and we recommend testing your negatives to make sure they meet your archival requirements.