As the lazy and impatient person I am, today I thought, "Why do I have to stop agitating while fixing and wait all this time between agitations?" I can't imagine the compensating effect with intermittent agitation as in the developer is desirable when fixing. Is there any harm in agitating all the way through the fixer? Can fixing times be reduced by doing so?
Fixation is a diffusion controlled process. This means that agitation, while necessary, does not have that much effect on the completion of the processe. So follow the manufacturer's instructions as to temperature, agitation and time. Not doing so may effect the permanence of your negatives.
Agitation during fixing is mainly to do with making sure all the film has been reached evenly by the fix than getting fresh fix on the film all the time. You can demonstrate it by the normal method of testing if your fix needs replenishing. Put a spot of fix on an end of roll film leader and leave it for ten minutes, the spot will clear a patch on the film. Then drop the whole leader into a cup of fix and time how long it takes for the rest of the film to match the cleared spot. That time is doubled and that is your fixing time, longer than the manufacturer says and your fix is getting exhausted. But you will see that not agitating the fix still clears the film.
[QUOTE=250swb;1466326. Put a spot of fix on an end of roll film leader and leave it for ten minutes, the spot will clear a patch on the film. Then drop the whole leader into a cup of fix and time how long it takes for the rest of the film to match the cleared spot. That time is doubled and that is your fixing time,
It may be that your choice of 10 mins for clearing time was merely to illustrate the twice clearing time principle but in fact if it takes 10 mins to clear and therefore 20 mins minimum fixing time then I think users should be seriously questioning their fixer's efficacy. At these times the silver content in the fix has, I believe become saturated and should be dumped. Using 250mls of correctly diluted fix for 135 films I find that the silver test tells me that the fixer is exhausted after 5 films so I dump after 4 to be on the safe side
Do the clip test Steve outlines above. Get a benchmark clearing time for your film in fresh fix. However, be sure to discard the fixer when the clearing time is double that in fresh fix (much, much shorter than 10 minutes!).
Originally Posted by 250swb
Before each batch, determine your fixing time for that batch by tripling the clearing time in the used fix (yes, I know most say doubling is enough, but with the presence of silver iodide in many modern emulsions, tripling is necessary to ensure proper fixation for these emulsions). I add 10% to account for fixer exhaustion during fixing. I also never fix for shorter than the manufacturer's recommended minimum time, even if the time indicated by the clearing test is shorter. And, FWIW, I like to use two-bath fixing for film too, which is likely overkill for many. At least do the other things :)
Oh yes - the agitation. Just agitate as you do in the developer; more frequent agitation won't speed up the process, as Gerald points out above.
Hmmm... when I started in a darkroom over 40 years ago I was taught, or read, that fixing agitation should be continuous. And that's how I've done it ever since. Maybe that's overkill, but at least I know I'm do all I can to fix out the developer. Perhaps less would be called for if using a staining developer? Just a question...
I have used continuous agitation for all baths ever since I started with the Jobo system. There is no downside to continuous agitation. I have no comment as to how much time you might be able to save.
I agitate during fixing in my inversion tank because I have a theory that any dust that might have found its way into the tank would be less likely to stick to the film. I don't know if it is really true, but I'm happy with the results.
Why would you wish to do this and no.
Originally Posted by NolanGalbreath