How much film could a fixer fix? ( if a fixer could fix film...)

[Will Frostmill]

I was doing some reading on fixer capacity, and I'm trying to get a handle on real-world usage. I've read that with most fixers, the chemistry has an enormous capacity for reacting with silver, provided one is willing to wait longer and longer times between films. This seems to lead to a lot of variability for what capacity people report. With TF-3, for example, I've read that you can fix between ten and twenty films.

I'm genuinely curious, but also a cheapskate.

Thanks,
Will

[Doremus Scudder]

Will,

The capacity of a fixer has less to do with the viability of the fixing agent than the build-up of complex silver compounds in the fixing solution; these are by-products of the fixing process. After a certain concentration of these, the fixing process no longer manages to completely change the silver halides into soluble compounds. These residual silver compounds stay in the emulsion, since they can't be washed out. And, it is this residual silver that can later degrade to cause damage (yellowing, mottling, etc.)

Also, you should know there are different amounts of residual silver tolerated in different photographic materials and for different uses. "Commercial" or "general" standards for materials that do not need a long lifespan allow more residual silver than "archival" or "optimum permanence" standards. This (and the inherent inaccuracy in using throughput as an indicator of capacity) is the reason for the often confusing and contradictory capacity recommendations.

So, the amount of fixer capacity you have depends on the type of material you are fixing, the amount of silver halide per unit of area that needs to be fixed (this varies with the amount that has been developed out to metallic silver), and the standard to which you are fixing.

The best and easiest way for normal photographers without access to a research lab to determine proper fixation is with the Kodak ST-1 residual silver test or with a selenium toner test. The procedure is to put a drop of the test solution on a fixed but unexposed part of the print or negative (or separate test strip processed together with the material in question) and note the change in color after a given time. These tests have been discussed thoroughly here and on APUG, so I'll not go into detail here.

There was (is still on the web) comparison strips with patches of different colors that would allow one to estimate the amount of residual silver left. In practice, though, many of us just use the "anything darker than a barely discernible cream color indicates inadequate fixing" standard.

If you want to know the capacity of a given fixer for a given material, you can easily do the test yourself using one of the above tests. I usually test the last print through a batch of fixer for both residual silver and residual hypo (HT-2 test). In lieu of testing, follow the manufacturers' recommendations.

Finally, fixer is relatively inexpensive compared to ruined prints, so your thriftiness here may be misplaced. It is better to err on the side of caution when it comes to fixer capacity.

Best,

Doremus

[jnantz]

it also depends on what sort of film you use.
if you use traditional black and white film
it is single sided ... some xray films are double sided
and will fill your fixer with silver at a faster rate

[Peter Lewin]

While testing is the best answer, does anyone have a sense of how reliable the manufacturer's quoted capacities are? For example, Photographer's Formulary suggests that TF-4 can fix 30 8x10 fiber based prints per liter. It also suggests that this may be a conservative estimate. If its conservative, then following the estimate should be safe? And a second question: since 4 sheets of 4x5 film are equal in area to one 8x10 print, can one safely use that as a "print equivalent" in following the manufacturer's capacity guidelines?

[Oren Grad]

A few other points:

Film, RC paper, and FB paper have different washing characteristics and probably respond differently to progressive fixer exhaustion. There may be substantial variation within these categories as well, as a function of varying emulsion and base characteristics - for example, my own tests in prior years found that some FB papers worked well with the Ilford accelerated fix/wash sequence, while some did not. The capacity estimates provided by Ilford/Harman for their fixers are radically different for FB vs RC; similarly, it is probably a mistake to assume that capacity expressed in terms of film surface area will match that for paper, of either type.

The amount of silver that needs to be removed by the fixer depends on how heavily exposed the print or negative is; any given capacity number assumes an "average" print or negative, whatever that means.

Again, the bottom line is that unless you are going to carefully test every batch of prints or negatives for adequate fixing, it is best to err on the side of caution.

[Will Frostmill]

This is probably the best possible answer I could have hoped for. Thank you Doremus. I know exactly what to do.
Will


Will,

The capacity of a fixer has less to do with the viability of the fixing agent than the build-up of complex silver compounds in the fixing solution; these are by-products of the fixing process. After a certain concentration of these, the fixing process no longer manages to completely change the silver halides into soluble compounds. These residual silver compounds stay in the emulsion, since they can't be washed out. And, it is this residual silver that can later degrade to cause damage (yellowing, mottling, etc.)

Also, you should know there are different amounts of residual silver tolerated in different photographic materials and for different uses. "Commercial" or "general" standards for materials that do not need a long lifespan allow more residual silver than "archival" or "optimum permanence" standards. This (and the inherent inaccuracy in using throughput as an indicator of capacity) is the reason for the often confusing and contradictory capacity recommendations.

So, the amount of fixer capacity you have depends on the type of material you are fixing, the amount of silver halide per unit of area that needs to be fixed (this varies with the amount that has been developed out to metallic silver), and the standard to which you are fixing.

The best and easiest way for normal photographers without access to a research lab to determine proper fixation is with the Kodak ST-1 residual silver test or with a selenium toner test. The procedure is to put a drop of the test solution on a fixed but unexposed part of the print or negative (or separate test strip processed together with the material in question) and note the change in color after a given time. These tests have been discussed thoroughly here and on APUG, so I'll not go into detail here.

There was (is still on the web) comparison strips with patches of different colors that would allow one to estimate the amount of residual silver left. In practice, though, many of us just use the "anything darker than a barely discernible cream color indicates inadequate fixing" standard.

If you want to know the capacity of a given fixer for a given material, you can easily do the test yourself using one of the above tests. I usually test the last print through a batch of fixer for both residual silver and residual hypo (HT-2 test). In lieu of testing, follow the manufacturers' recommendations.

Finally, fixer is relatively inexpensive compared to ruined prints, so your thriftiness here may be misplaced. It is better to err on the side of caution when it comes to fixer capacity.

Best,

Doremus