Fix clearing test for film: agitate or not?
I've read about the clearing time test for old fix, but I need to have some details clarified.
The test is simply to drop a strip of undeveloped film into the fixer, and see how long it takes to clear completely. Apparently, my fixing time should then be at least twice that time.
Now, do I agitate the film when I do this test, or do I leave it alone? I would expect agitation to speed up the clearing time of a piece of film compared to just resting on the bottom of a tray or beaker.
How rigid is that factor 2? Do different films need different fixing times, or are they all the same?
While I don't agitate, there's no rule says you can't.
T grain films like Tmax and maybe Delta require longer fixing.
I usually go more than twice the clearing times because some films can clear in 60seconds.
I always fix at least 5-6 minutes to be safe.
Depending on the fixer type and film used.
I haven't noticed any appreciable speed improvements using agitation when it comes to TMax 100, TMax 400, Delta 100, and to a lessor degree, the rest of the Ilford films which are "super" hardened emulsions. It simply takes time to penetrate the hardened layers of the emulsion, and particularly so with sodium thiosulphate rather than ammonium thiosulphate. Agitation doesn't seem to make that process any faster though you might be fooled into thinking so by an incomplete examination of the negatives. If it's not a thorough fix you may not see it until magnified with a loop or through your enlarger.
I'm mostly shooting 4x5. I find generally 7 to 10+ minutes is required for TMax's and Delta's. 7 minutes if it's really super fresh fixer. 10+ minutes if it's not fresh. When the fixing time required goes closer to 12 minutes, I abandon the fixer even if it's not exhausted.
I got this education through trial and error. I have some negatives a few years ago that weren't fixed enough and now they are cloudy in the highlights. I finally started doing clearing tests with film that hasn't been developed, but I haven't done this experiment with unexposed film. Rather it's been fully exposed in daylight practically. I wonder if there's a difference in fixing speeds over that factor. hmm...
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Some agitation seems wise - otherwise the test might be distorted by local exhaustion of the fix.
In addition, if the purpose of the test is to determine how long you should fix film, it seems prudent to use the same agitation scheme (frequency and duration) as you are using for the film itself.
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I have had problems in the past with T-Grain films such as a magenta cast. I reuse my developer and fixer. I now mix fresh fixer every two months and do the clearing test. Set the timer to 10 minutes, agitate the fixer, check after 2 minutes and see if it clears. If it does, then pulled out at 5 minutes. Not making a cocktail here but some agitation does help. This ended all of the faults I was encountering. Thank You Apugers, Steven.
"The test is simply to drop a strip of undeveloped film into the fixer, and see how long it takes to clear completely. Apparently, my fixing time should then be at least twice that time."
Clearing time is more apparent if you first put a drop of fixer on the emulsion of a film strip and wait for it to clear. Then dip the strip into the fixer. The full clearing of the strip will be more easily read when comparing it to the fully cleared dot.
Various fixers work differently, completing different parts of the process at different rates. For example, I believe TF-4 and TF-5 work a little differently from each other, and different parts of the fixing operation occur at different rates. The short of it is that you should fix for the recommended time, which might be longer than twice the clearing time.
Visual inspection of the clearing time is greatly increased in accuracy by putting a drop of fixer on the emulsion as described above and waiting for 30 seconds before fixing the rest of the film. The spot formed by the drop will "disappear" when clearing is complete.
In order for a clip test like this to be relevant, it should have the same conditions as the actual fixing of film. That means a pre-soak and similar agitation.
A bit of film will not exhaust the fixer, but a large quantity will, which means that the actual clearing time when fixing a roll or several sheets of film in a given amount of fixer will be a bit longer than your clip test.
Anchell and Troop recommend 3x the clearing time, due to the fact that silver iodide compounds in modern films fix more slowly than the more common bromides and chlorides. I'm not sure if this is true, but it can't hurt.
Haist, Gudzinowicz and others recommend two-bath fixing for optimum permanence of film and to increase capacity, and a minimum of 2x of the clearing time total. Gudzinowicz points out that film is not hard to wash and that extra fixing up to a point will do no harm. He recommends 2x the clearing time in both baths for a total of 4x.
I tend to do the latter. I toss the first fix when the clearing time reaches 2x that in fresh fixer. This, in my estimation, is the real advantage of doing the clip test, i.e., knowing when the fixer is exhausted.
Fixing longer than just 2x the clearing time by a bit to be on the safe side is certainly a good practice in my opinion. If you fix batches or rolls of film successively in the same fixer, you need to do a clip test before each batch/roll to see how the fixer is doing and to make sure your fixing time for the fixer at that level of exhaustion is adequate. As mentioned above, I end up with 4x clearing time.
Doing residual silver tests on your film after you have decided on a regime is the only reliable way to see if your particular processing method is working. Test several times toward the end of the fixer's life to find where the effective cut-off point is for you and keep track of the clip test clearing times as well. This exhaustion point can then be correlated to the clip test clearing time (maybe you need to toss the fix before 2x the time in fresh fix is reached). Once established, you can fix film confidently with just the clip test for a guide.