I used to think that the best way to get into an argument with someone was to discuss politics. I think there may be a close second: Talking about exhaustion of darkroom chemicals — at least if reading photography forums is any indication. But I’d prefer not to argue about it, and instead offer a way to track the exhaustion of a key chemical in the development process: fixer.
As you may already know, fixer is critical to the development of both photographic films and papers, because it removes unexposed silver halides, leaving behind the metallic silvers that form the resulting image. If the silver halides are not fully removed, they’ll darken and fog the image over time, making proper and complete fixing essential for good, stable, and lasting film and print results.
Because the fixer chemistry becomes increasingly saturated with the silver halides as it’s reused, it reaches a point where it can’t do its job anymore — this is what everyone refers to as exhaustion.
Knowing when your fixer is exhausted (or nearly so) is important so you can replace it in a timely manner, and ensure your negatives and prints are stable over time.
Strategies for ensuring that you have a good and complete fix include:
- Using a double fix bath, each of which are replaced at alternating intervals;
- In the case of film fixing, performing a clearing test with a snippet of film;
- Tracking fixer use and staying within manufacturer-recommended limits; and/or,
- Periodic testing of the fixer with a “hypo check” chemical.
Some people use multiples of these strategies. For me personally, the first two are too tedious; I don’t want to keep two bottles of mixed fixer around, and I don’t have room for dual fixer trays for fixing prints when I use my enlarger. And while the clearing test is straightforward, it’s just too involved. That leaves me with the other two.
Manufacturer-recommended fixer capacities are conservative by nature, meaning they’re well-tested and proven. You can exceed the capacities, and many people seem to like to do that (based on forum posts), but I have assumed that it’s a good metric to use for a reason, and time and experience have seemed to validate that. Additionally, as a backstop, I still use a hypo check chemical to make sure the fixer remains in usable condition when (based on my tacking) it’s nearing the end of its stated capacity.
The problem with fixer tracking is knowing what to track. The data sheet for Ilford Rapid Fixer, the fixer I personally use, provides capacities for just three things:
- 35mm, 36-exposure film
- 8×10 RC (resin-coated) paper
- 8×10 FB (fiber) paper
What about 120 film? Or 4×5 sheet film? Or 11×17 sheets of paper? If you process only one thing in your own darkroom, and it’s one of those three, then the capacity information is helpful. But if, like me, you process films in differing sizes, as well as making prints in different sizes, it can be a lot of confusing guesswork to figure-out how to track how much capacity you’re using-up with each print, or each sheet or roll of film. Which brings us to the point of this article.
Calculating Capacities for Other Photosensitive Materials
At the end of the day, what we want to track is the surface area of the materials we’ve run through the fixer of each type (film, RC paper, FB paper), and map that against the published capacities. Of course, how much a fixer tires with each use is a combination of total surface area, and image density (darkness). Since we can assume that images average-out over time to a medium gray, this leaves us with surface area as the main factor. I am not, however, getting out a tape measure every time I process a roll of film or print.
So, what I wanted was a predictable and reasonably accurate method to track consumption for various film types, lengths and print sizes that conforms as closely as possible to Ilford’s limited published information.
I’m sure I’m not the first person to think of it — I take no credit at all for this idea — but I thought a points-based system would be easiest to deal with. I picked 100 as my base; I’d track each roll or sheet of film, and each print that I fix, and when I reached 100 points, I’d dispose of my fixer. (I also hypo check periodically after I reach about 85 points.)
To come-up with the points, then, I did the following:
- Used published capacity equivalencies from other sources for film types, lengths and/or sizes to cover for the fact that Ilford didn’t include this in their data sheet (specifically, I used the equivalencies that Unicolor and others provide in their C-41 kits, as they’ve already done the relative comparison work, and their capacities would be based on the same surface area relativity between film sizes);
- Calculated square inch measurements for various print sizes that I make in my darkroom; and,
- Used some basic math against these numbers and the ones Ilford provides for their Rapid Fixer to arrive at all the equivalencies that I needed so that I could, in turn, figure out capacities, and ultimately, my “scores.”
These were done in the form of, “a fresh 1 liter batch of Rapid Fixer will process X number of Y film or paper” — just like Ilford does in the data sheet:
As an example, a 24-exposure roll of 35mm film is two-thirds the number of frames, and thus (roughly) two-thirds the surface area of a 36-exposure roll. Thus, you can fix about 50% more 24-exposure rolls than 36-exposure rolls. As another example, a roll of 120 film is (according to Unicolor) about the same from a chemical consumption perspective as a 36-exposure roll of 35mm film — so they’re considered equivalent. And finally, as a paper example, I knew that 4×5 test prints are one-quarter of an 8×10 — hence the fixer can fix 4x the quantity. Other numbers were arrived at using similar strategies.
To calculate the points, I divided 100 by the quantity capacity for each. For instance, 100 divided by 24 rolls of 35mm 36-exposure film = 4.2 points.
The resulting values are solely for Ilford Rapid Fixer, mixed at 1+4 dilution with a total of 1 liter of mixed fixer — which can be used with both films and papers, and is the chemical, dilution and quantity I use for all my fixing needs. You’ll need to scale accordingly for different quantities, or even calculate your own if you use a different dilution, or choice of fixer.
The Resulting Points
So, here’s the net result, with notes on how to use it.
| Film or Paper | Quantity Capacity | Points |
|---|---|---|
| 35mm 36-exposure film roll (per roll) | 24* | 4.2 |
| 35mm 24-exposure film roll (per roll) | 36 | 2.8 |
| 35mm 20-exposure† film roll (per roll) | 43 | 2.3 |
| 110 film roll (per roll) | 108 | 0.9 |
| 120 film roll (per roll) | 24 | 4.2 |
| 126 film roll (per roll) | 48 | 2.1 |
| 127 film roll (per roll) | 41 | 2.4 |
| 4×5 film sheet (per sheet) | 96 | 1.0 |
| 4×5 RC paper** (per sheet) | 320 | 0.3 |
| 5×7 RC paper** (per sheet) | 183 | 0.6 |
| 8×10 RC paper** (per sheet) | 80* | 1.3 |
| 11×17 RC paper** (per sheet) | 34 | 2.9 |
** FB (fiber) paper capacities are half that of RC. As a result, simply double the points shown when using a fiber paper.
† I frequently bulk roll 20-exposure rolls for my older cameras that have specific markings for this formerly common frame count.
Each time I mix a liter of Rapid Fixer, I start with a running total of zero on a piece of note paper. For each roll or sheet I run through the fixer, I just add the corresponding points to the running total.
For example, two rolls of 35mm 36-exposure film, one roll of 35mm 24-exposure film, a roll of 120 film, and two 8×10 RC prints would give me a running total of 18:
4.2 + 4.2 + 2.8 + 4.2 + 1.3 + 1.3 = 18.0
That means my fixer would be about one-fifth (20 points = one-fifth of 100) of the way to exhaustion after fixing these six items.
As I said earlier on, when I get to around 85 points or so, I’ll start using hypo check periodically to make sure I’ve not made a mistake along the way, or for some other reason reached chemical exhaustion earlier than the math suggests. I also don’t exceed 100 — no matter what the hypo check says. Whether I get to 100, or the hypo check clearly shows a precipitate forming, the fixer gets replaced (and my hypo clear along with it) with freshly mixed chemicals.
As “scientific” as it all might seem, this is all still just an approximation, although a relatively conservative one, and it’s close enough for me. It also provides a pretty concrete way to track fixer use without a bunch of guesswork or assumptions.
More importantly: Experience and time have proven to me that this method is effective. I keep a printed copy of the table above in my darkroom closet for easy reference.
As I often say, “Your mileage may vary,” meaning that your experience could be different, but hopefully it’s something that you find useful. Of course, if you prefer to do clearing tests with film clips, or use dual fixer baths, or any other technique — as long as you’re getting the results you want, pretend you didn’t read this article. 🙂
And regardless — just keep shooting film.
A Closing Thought on Storage
One item I didn’t explicitly mention is the effect of storage. Darkroom chemicals that sit around deteriorate simply from oxidation. Follow the published guidelines for your preferred fixer would be my general advice.
I personally use “canned air” — technically, difluoroethane — to clear room air out of the top of my chemical storage containers, as I described at the bottom of this article on C-41 processing. I’ve had mixed, partially used fixer in storage in my darkroom for as along a four months so far, and have not experienced oxidation issues. Fixer oxidizes by precipitating sulfur; if you happen to notice yellowish crystalline precipitate in your fixer container, replace it with freshly mixed fixer.
