How about the capacity of two-bath fixing for B/W film

My question is about the capacity of two-bath fixing for B/W film.

I have never used two-bath fixing but it has some interesting features so I want to give it a try.
The procedure is clear and I understand that the main purpose of two-bath fixing is a good depletion of silver to get an archival film.

Ilford says two-bath fixing is "an extremely efficient method of fixing film or paper", whatever that means. It could mean it is indeed efficient in removing silver, but there are also hints from some members that the capacity is increased, which is interesting from an environmental point of view.

I searched posts on Photrio but I still don't get sufficient answers on this latter point.

In my understanding the procedure is as follows:

1. We have bath #1 and bath #2.
2. The initial clearing time from bath #1 is measured with a clip test.
3. Usually it is suggested to apply a total fixing time T about twice the clearing time.
4. For two-bath fixing a film is first fixed in bath #1 with 1/2 T.
5. It is then fixed in bath #2 with 1/2 T.
6. Bath #1 can be used until the measured clearing time (clip test) is twice the initial clearing time, and then disposed (I know you can also measure the silver concentration).
7. Bath #2 becomes bath #1 and a new bath #2 is made.
8. This can be repeated max 4 times (Ilford) and both baths are disposed.

When we look at the capacity we see that the measured clearing time for bath #1 is first doubled to get a total fixing time, and then halved again to get the fixing time for each bath.
So, my conclusion is that bath #1 is theoretically doing 100% of the fixing and bath #2 almost nothing which implies that you can't expect any increase in capacity.

Of course, it also matters if you are measuring clearing time or silver concentration, but I like to focus here on clearing time to keep the discussion simple.

So, my questions are:

1. Did I make a mistake in my reasoning here?
2. Is it indeed feasible to get an increase in capacity with two-bath fixing?

The reason for two bath fixing is issues fixing Fibre based papers where once the silver level builds up in the fixer semi soluble silver/thiosulphate complexes form weak bonds with the cellulose in the fibres in the base making them very difficult to wash out, ultimately causing staining or fading (image bleach). The fixing process is a series of equilibrium reactions and by using a fresh second bath the balance changes and the semi soluble complexes become soluble and wash out.

Films and RC papers don't have the same problem and fixing can complete even with significantly higher silver levels in the fixer, have a look at Ilford's data-sheets. So two bath fixing isn't really needed for films, capacity is far less of an issue.

Ian

I should have added that with films (and RC papers) you're right 100% of the fixing will be in the first fixer bath with films, the second won't hurt and will ensure better archival permanence but there is a little carry over of soluble silver in the emulsion.

With two bath fixing of FB papers there is far more carry over of silver into the second bath in the emulsion as well as more in the fibre base itself.

Ian

Thanks Ian, I'll first wait for some more reactions to get my act together.

I am puzzled by point 8 which states that the 2 bath method can only be used 4 times. Any one of the two baths is only ever used twice then disposed of, isn't it? So isn't the 2 bath method a self perpetuating system whereby there is no "end" or maximum usage. Lets assume that at the start you have two fresh baths called 1 and 2 and they are used in that order. Bath 1 does the most or all of the fixing, say. Once bath 1 is close to exhaustion it is dumped and bath 2 becomes bath 1 and is then dumped so the original bath 2 is used twice only and thereafter each bath is used twice only before dumping. If I have understood the sequence correctly no bath is ever used more than twice so I am puzzled about where the 4 times maximum comes from?

pentaxuser.

pentaxuser said:

I am puzzled by point 8 which states that the 2 bath method can only be used 4 times. ..... pentaxuser.

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I have that from a quote here: https://www.photrio.com/forum/index.php?threads/two-bath-fix-question.136703/#post-1787067

saying that this is mentioned in Ilford's 'PROCESSING B&W PAPERS' publication, but actually I can't find it back there (I guess there have been various versions).
I have seen it elsewhere too, but I don't know where. I have also read somewhere on Photrio that Ilford recommends max 4 cycles and Kodak 7 cycles.

pentaxuser said:

I am puzzled by point 8 which states that the 2 bath method can only be used 4 times. .... pentaxuser.

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I have found it, it's in this Ilford PDF, 'Processing b&w paper', for FB Paper:
http://www.estudiocasanueva.com.ar/archivo/IlfPQfb.pdf

There is also a similar publication for RC papers but is has a different text.

Edit: I guess this doesn't apply to film.

The current data sheet for Ilford Rapid Fixer is here:

https://www.ilfordphoto.com/amfile/file/download/file_id/2012/product_id/711/

It discusses two-bath fixing but does not mention a maximum number of cycles.

photog_ed said:

The current data sheet for Ilford Rapid Fixer is here ... It discusses two-bath fixing but does not mention a maximum number of cycles.

Click to expand...

Yes, that's correct, it is also the publication I quoted in post #1.
Interestingly the 'Processing b&w paper' publication (post #7) for FB papers explicitly says:

"The capacity of a fixer can be significantly increased, while still obtaining optimum permanence, by using a second fixing bath."

But Ian Grant probably has hit the nail on it's head, saying that this is all relevant for Fibre based papers and not for film.

Edit 25/09/17 - Quote extended.

Well it was simply the logic of your point 8 that I was questioning which if it is drawn from an Ilford publication mean I am questioning Ilford's logic. Unless I have totally misunderstood what 2 bath fixing involves or my logic is faulty I cannot see how in the 2 bath method you can get to 8 times. It is rather like an "ad infinitum replenishment system whereby if I change a certain portion of the liquid at regular intervals then my current liquid bears no relationship to what I started with.

pentaxuser

The "discard after X cycles" probably flows from the fact that as you repeat the cycles, the second bath (which will eventually become the first bath) has the tendency to have slightly more and more retained silver and other by-products of processing left in it.
You can avoid that by switching the baths more frequently, but that reduces the total capacity, and as two bath fixing of films is probably most useful for increasing the capacity of commercial processing lines, the manufacturer's recommendations are conservative.

Yes Matt, that sounds logical.

But still, the important point for me is to know how to increase the capacity with a two-bath process (for film), also quoted from Ilford. But until now I fail to see how that would work.

The purpose of my question is how to reduce the amount of waste by exploiting the fixer capacity to a maximum.

Billy Axeman said:

Yes Matt, that sounds logical.

But still, the important point for me is to know how to increase the capacity with a two-bath process (for film), also quoted from Ilford. But until now I fail to see how that would work.

The purpose of my question is how to reduce the amount of waste by exploiting the fixer capacity to a maximum.

Click to expand...

There are two factors that limit the "capacity" of fixer.
The chemical activity capacity, and the build-up of silver and byproducts of development in your fixer solution.
You can reach the practical limit of one, while the other isn't at the limit.
And the silver and byproduct issue may very well vary tremendously both with type of film, and the nature of the exposures.
I expect that the biggest reason to use two bath fixing with film is that you are more likely to reach both types of limits to the capacity at roughly the same time. So in that way, you don't waste any more fixer than is absolutely necessary.
FWIW, the Kodak materials relating to Kodak Rapid Fixer used to indicate that capacity could be increased through use of replenishment.

Matt, you may be right, but it is difficult to quantify the effect. As you describe it here it looks a subtle process with an uncertain outcome, but Ilford is saying in the PDF 'Processing b&w paper' for FB papers:

"The capacity of a fixer can be significantly increased, while still obtaining optimum permanence, by using a second fixing bath." (my italic).

If such a big improvement is possible, one would expect a clear and unambiguous reason, but strangely nobody seems to know how this exactly should work (until now).

Perhaps the best way is simply to try it out, but it is in my nature to find out the exact course of a process because it is easier to optimize.

Thanks everybody for the discussion!

I'm going to ditch my plan for two-bath fixing because I don't think it will work for film, that is, if I want to use it to increase capacity. There are more direct ways to achieve that goal.

Billy Axeman said:

My question is about the capacity of two-bath fixing for B/W film.

I have never used two-bath fixing but it has some interesting features so I want to give it a try.
The procedure is clear and I understand that the main purpose of two-bath fixing is a good depletion of silver to get an archival film.

Ilford says two-bath fixing is "an extremely efficient method of fixing film or paper", whatever that means. It could mean it is indeed efficient in removing silver, but there are also hints from some members that the capacity is increased, which is interesting from an environmental point of view.

I searched posts on Photrio but I still don't get sufficient answers on this latter point.

In my understanding the procedure is as follows:

1. We have bath #1 and bath #2.
2. The initial clearing time from bath #1 is measured with a clip test.
3. Usually it is suggested to apply a total fixing time T about twice the clearing time.
4. For two-bath fixing a film is first fixed in bath #1 with 1/2 T.
5. It is then fixed in bath #2 with 1/2 T.
6. Bath #1 can be used until the measured clearing time (clip test) is twice the initial clearing time, and then disposed (I know you can also measure the silver concentration).
7. Bath #2 becomes bath #1 and a new bath #2 is made.
8. This can be repeated max 4 times (Ilford) and both baths are disposed.

When we look at the capacity we see that the measured clearing time for bath #1 is first doubled to get a total fixing time, and then halved again to get the fixing time for each bath.
So, my conclusion is that bath #1 is theoretically doing 100% of the fixing and bath #2 almost nothing which implies that you can't expect any increase in capacity.

Of course, it also matters if you are measuring clearing time or silver concentration, but I like to focus here on clearing time to keep the discussion simple.

So, my questions are:

1. Did I make a mistake in my reasoning here?
2. Is it indeed feasible to get an increase in capacity with two-bath fixing?

Click to expand...

Well, I don't care about fixer capacity so much;I'm much more concerned about negative longevity. That's why I didn't reuse fixer and only use fresh;Now, with two-bath fixing, only the 2nd bath is fresh and then is demoted to be 1st.So, I figure, it saves me 50%fixer.

Any one who is doing this commercially tests regularly for complete fixing.
Here is the old Kodak documentation: http://125px.com/docs/techpubs/kodak/j11-1985_04.pdf
If you test regularly, you can stretch out capacity.

RalphLambrecht said:

Well, I don't care about fixer capacity so much;I'm much more concerned about negative longevity. That's why I didn't reuse fixer and only use fresh;Now, with two-bath fixing, only the 2nd bath is fresh and then is demoted to be 1st.So, I figure, it saves me 50%fixer.

Click to expand...

Yes, the purpose of two-bath fixing is to improve negative longevity.

I don't see how you get a saving of 50% though. You are actually fixing with bath #1 and bath #2 is doing virtually nothing. So, from a capacity standpoint, so to say, I can also keep bath #2 in the cupboard and only get it when bath #1 is depleted. That's not different from straightforward one-bath fixing.

MattKing said:

Any one who is doing this commercially tests regularly for complete fixing.
Here is the old Kodak documentation: http://125px.com/docs/techpubs/kodak/j11-1985_04.pdf
If you test regularly, you can stretch out capacity.

Click to expand...

Thanks Matt, I'll look at it.

Billy Axeman said:

You are actually fixing with bath #1 and bath #2 is doing virtually nothing.

Click to expand...

This isn't fully accurate.
Fixing isn't a linear process. It is more like an exponential process. The last 10% of the job takes way more than 10% of the time, and is much more affected by the fixer nearing exhaustion.
The addition of fresh fixer in the second bath greatly improves the efficacy of that last 10%, and allows the first bath to do a lot more examples of the first 90%.
(10% and 90% chosen for illustrative purposes - the exact percentages are most likely slightly different).

Billy Axeman said:

Thanks everybody for the discussion!

I'm going to ditch my plan for two-bath fixing because I don't think it will work for film, that is, if I want to use it to increase capacity. There are more direct ways to achieve that goal.

Click to expand...

Kodak claimed you could get 25 8x10 fiber prints from 1 quart of fixer (standard F-5 powder) using it as a single bath. If you did 2 bath you could increase that to 40 / qt. Now this would have you standing and rocking and interleaving your prints for 4 minutes/ tray. However you could improve capacity by 68%, fix 250 prints with 6 quarts of fixer.

Or as just about everyone does now stick to 20 8x10's / liter of straight ammonium sulfate film strength fixer 1 minute for fiber base prints .

TMAX film uses so damn much fixer I'm not sure if 2 bath is such a bad idea. I've been shooting more Ilford HP-5 lately, it's great in fresh Rapid fix it's darn near clear in 45 seconds, I usually use the old twice as long as it takes to clear.

With T-grain films , I let it run on the Jobo for 5 minutes with fresh fix, then I use Hypo clearing agent to get out the purple.

I still fool around making Kodak F-6 from scratch when I want to channel my inner Ansel. It's great fixer and it hardens the emulsion so the prints don't stick to dryers or blotters. You really have to like rocking trays
Best Mike

Billy Axeman said:

Yes, that's correct, it is also the publication I quoted in post #1.
Interestingly the 'Processing b&w paper' publication (post #7) for FB papers explicitly says:

"The capacity of a fixer can be significantly increased, while still obtaining optimum permanence, by using a second fixing bath."

But Ian Grant probably has hit the nail on it's head, saying that this is all relevant for Fibre based papers and not for film.

Edit 25/09/17 - Quote extended.

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Hi, it is not correct to say that two-bath fixing is "not for film." One main reason for going to two-bath (or more) is that a given amount of fixer can do upwards of a couple times more film or paper. I can't speak much for the intricacies of fiber (vs RC) paper, but have a great deal of film and RC (color) paper experience.

Here is the quick explanation. We normally consider the fixer to be "exhausted" when the silver concentration in the final bath exceeds some specific concentration. (For "commercial" work a higher final concentration is specified than for "archival" use.) Since most photographers don't have a good way to measure silver concentration, it is typical to either use published film/paper capacity info, or to measure the increase in fixing time (this works only when fairly high silver concentration is allowable).

If you use a single bath fixer, as soon as you exceed the aim concentration for silver, say 1/2 gram/liter as an example, the fixer is at its "end of life." But if you use a two-bath system you might find that the first bath reaches, say, 2 g/L before the final tank reaches the 1/2 g/L limit. So obviously (?) the two-bath system has much greater capacity than the single bath, right? (If you are loading your silver up to 2/g/L before discarding then clearly you'll be using less total fixer than if was discarded at 1/2 g/L.) You can probably expect to get more than double the film/paper through a given amount of fixer.

Additionally, there,is a large benefit if you do this "industrially" and recover your silver. You can get a higher proportion of the silver as high quality electrolytic "flake," as opposed to low grade sludge.

I went through some calculated cases in the thread below, comparing some hypothetical situations. In the real world, it makes a lot of difference how much solution "carryover" you get when transferring the film/paper from tank to tank; industrial-scale photofinishing always uses multiple squeegees to help control this.

https://www.photrio.com/forum/index.php?threads/buffering-neutral-fixer.101115/page-2#post-1339077

Ps, I don't think I was saying it at the time, but I spent years as the QC Department manager at a large processing lab where we ran miles of film every day, and vastly more color RC paper (using bleach-fix as opposed to just fixer). My department included an onsite chem lab capable of silver analyses, etc., so we hardly bothered with clearing tests. I personally did much of the process configuration design whereby we used multiple tanks with countercurrent-flow replenishment, so I have some experience with this sort of thing. Bottom line is that if you let the the silver concentration in the final fix tank set the limits for fixer "exhaustion," then a two-stage fixer system can significantly reduce fixer usage. The real issue, on a small scale system, is whether the chemical savings outweigh the extra work you have to put into it.

Ps, I don't see any reason why the system has to be limited to some number of tank swaps. There may well be a good reason; I just don't know what it would be. My best guess is that at some point in the past an Ilford or Kodak customer ran into some sort of odd problem, and the manufacturer decided that the easiest way to deal with such things in the future was to simply set a limit for reuse cycles.

Billy Axeman said:

Yes, the purpose of two-bath fixing is to improve negative longevity.

I don't see how you get a saving of 50% though. You are actually fixing with bath #1 and bath #2 is doing virtually nothing. So, from a capacity standpoint, so to say, I can also keep bath #2 in the cupboard and only get it when bath #1 is depleted. That's not different from straightforward one-bath fixing.

Click to expand...

Fwiw I think you are looking at this in a very sensible manner.

The generally accepted reason why it works is this. When fixer is only lightly used the silver is almost all in a fairly simple silver-thiosulfate complex (thiosulfate is the main working component of fixer) which can be washed out of the photo material fairly easily. But as the fixer's silver concentration increases, more complex compounds are formed, which additionally are harder to wash out. (So this is a main reason, for "archival" use, to consider the fixer "exhausted" at a low silver concentration, even though it still seems to fix just fine.) Anyway, if the photo material that is loaded with the "hard to wash" compounds is passed through a nearly fresh fixer bath, then these complex compounds seem to be mostly converted into the simpler silver thiosulfate compounds that are easier to wash out.

I don't have first-hand knowledge of these things, or how chemists have determined what the compounds are, but this is the generally accepted explanation in reputable photographic literature. In my large-scale finishing experience we've done this more as an "effluent control" measure than for chemical savings. If you have a high silver concentration in your final fixer tank, then a higher amount of silver ends up in the wash water, and this can be a big problem for commercial photofinishers.

MattKing said:

This isn't fully accurate.
Fixing isn't a linear process. It is more like an exponential process. The last 10% of the job takes way more than 10% of the time, and is much more affected by the fixer nearing exhaustion.
The addition of fresh fixer in the second bath greatly improves the efficacy of that last 10%, and allows the first bath to do a lot more examples of the first 90%.
(10% and 90% chosen for illustrative purposes - the exact percentages are most likely slightly different).

Click to expand...

That's a neat observation. That probably also explains why the advise is to apply a fixing time twice the clearing time, measured with a clip test, because the tail of the exponential curve is difficult to see/follow visually even with a drop to compare with.

I actually had reservations about my own procedure (the list in post #1) because I didn't fully grasp the purpose of the second bath when the fixing time of bath #1 is identical to the clearing time. But with this addition it is looking a more reasonable process.

That said, I think we must be careful not to squeeze everything into a standard process because the various types of paper and film are behaving quite differently.

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