Mr Bill's given you detailed and authoritative responses. I'll just add one thing: You are making an error in your premises which is leading you to conclude that two-bath fixing for film doesn't increase capacity. You state that the first fixing bath needs to be replaced when the clearing time for a film is double that of fresh film. This, for a two-bath fixing regime, is wrong.
The whole point of using two-bath fixing is to use the first bath well past the capacity of a single bath. Indeed, the first films through are fixed completely, but as more film is put through bath one, it reaches and exceeds the amount of dissolved silver allowable for single-bath fixing, resulting in a lot of underfixed negatives. These, however, get the rest of their fixing in bath two, so no problem.
What remains to be determined, however, is just how far you can push bath one. For fiber-base papers, this is pretty clear, since there is a lot of literature on the technique. For film, you'd have to do your own testing. I can imagine, however, that you can increase the capacity of bath one to double that of a single bath (i.e., forget about clearing time and double the manufacturer's throughput recommendation). For example, Ilford gives 24 rolls or 8x10-sheet-equivalents per liter as the capacity for its Rapid Fixer. With two-bath fixing then, double that capacity for bath one, finishing all the films in bath two. Then, test the last film through for residual silver using the Kodak ST-1 test or selenium toner. In order for this to be economical, you'd have to test the last negative/film to go through all the four changes of fix-2 to fix1. What you don't want to do is push the system to its limits, since bath two needs to be fresh enough to replace bath one; the object is to find a workflow that saves you money and time.
For most of us, saving a bit of money on fixer isn't high on the priority list. FWIW, I do often use two-bath fixing for film, but it's not for economy's sake, rather to ensure that my negatives are optimally fixed. I don't feel so bad about exceeding the capacity of bath one a bit when I've got a bath two backing it up; cheap insurance IM-HO.
For fiber-base prints, two-bath fixing and regular testing for residual silver is SOP for me.
Best,
Doremus
The capacity of a liter of fixer in a one bath system is 20 8x10 prints per liter, the capacity of a liter of fixer in a two bath system is 20 8x10 prints per liter! It is the same, but the fixation per print is much more consistent and the fixers by products are much easier to wash away when you use the two bath system. So both methods cost the same per print fixed but your prints will probably wash with less water (but TEST!) and your prints will probably last much longer. Over using any fixer or fixer scheme is a sure fired recipe for disaster.
Keep in mind that when you use two-bath fixing, you have two baths! The capacity calculation is made using the volume of the first bath only; the second bath is extra. Example: 1 liter of fixer fixes 24 rolls of film. Two liters of fixer divided into two baths allows us to double the capacity of bath one to 48 films per liter, but it has to go through that liter of bath two as well. In other words, you're using two liters of fixer to fix 48 rolls of film. At this point, there is absolutely no difference in the amount of fixer used per unit of film. However, you have ensured optimum fixation for your film; no negatives have been fixed in fixer that is close to the end of its capacity.
The economy only comes in when you replace bath 1 with bath 2, mix a fresh bath two and fix 48 more rolls of film. Now, you've fixed 96 rolls of film in three liters of fix, still ensured optimum fixation and saved a bit on fixer. Carry this to the next step, replace bath 1 with bath 2 again and fix another 48 rolls. Now, we've used four liters of fix for 144 rolls of film. And so on, through four cycles (actually, Kodak recommended seven cycles). What we're doing here is utilizing the fixer more efficiently, keeping the second bath as fresh as possible to complete the last stages of fixation. That's where the economy comes in, but only after you've used the two-bath regime through several cycles. If you just fix that 48 rolls of film through two separate 1-liter baths of fix, you end up with exactly the same economy as if you had done two separate 24-film fixing batches with the one-bath regime.
Bottom line: the capacity increase (and the resulting economic advantage) with two-bath fixation only appears after you've cycled bath 2 to bath 1 a few times.
I should mention that two-bath fixation for fiber-base papers is approached a bit differently, since there are varying requirements for fixation depending on how permanent you want the resulting prints to be. Because complex silver compounds from anything but very fresh fixer can bond with the fibers in the paper base and be difficult to wash out, fixer capacity for optimum permanence for fiber-base papers is relatively low when compared to capacities for film and RC paper. Some don't need to process fiber-base papers to "archival" standards, so manufacturers give less-stringent fixing guidelines for "commercial" or "general-purpose" processing. Processing to less-than-optimum permanence used to be expedient for newspapers, magazines, etc. It is less practiced today. In any case, for film and RC prints, which don't have exposed paper bases, much higher levels of dissolved silver in the fix are acceptable and there is usually only one "standard."
Best,
Doremus
I already suspected there was an issue here, hence my note at the end of post #28 assuming that bath #2 is virtually fresh. This is now solved with your note.
Another point is the fixing time.
I guess my fixing time is twice the clearing time, measured with a clip test on bath #1, and then halved to get the times for bath #1 an #2. So when twice the capacity is used, these times will be quite long in the end isn't it?
I also found some older posts from you about fixing but I must still read them.
Many thanks for your help
About fixing time for film. The usual recommendation is to fix twice the clearing time. However, many recommend longer times for modern films, especially T-grain films with more silver iodide in the emulsion. Since fixing film somewhat longer than the minimum fixing time needed does no harm up to around 10 minutes (in a rapid fixer), I almost always fix longer than 2x the clearing time. My standard is 3x +10% of the clearing time (the extra 10% to compensate for fixer exhaustion during fixing).
As for "quite long" times: Most conventional films clear in 30 seconds or so in fresh fix; T-max and Delta films in 45-60 seconds. Even at 3x of the longest time, you'll only end up with around 6 minutes (6'30" if you add my 10%). That's not too long in my opinion.
Best,
Doremus
As far as that maximizing the use of fixer and getting every last bit of use, the more films you fix per liter the more silver there is in you fixer, I do silver recovery with steel wool before proper disposal of my fix and find that it's easier with a less silver satuarated solution.
Mike, the main problem with this idea is that steel wool cartridges all "leak" a certain amount of silver, concentration-wise.
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