Here's a coincidence. When Mike wrote his column about the craft of black-and-white darkroom printing two weeks ago, to the applause of film photographers who felt they were becoming depraved on account of being deprived, I was already in the midst of writing this column on the craft of black-and-white darkroom printing. Great minds running in the same gutter, I suppose.
Then Mike made me an offer I wasn't about to refuse (two weeks off) and so here we are.
One of the modest burdens that photography writers and editors have is revisiting basic topics. It always annoys advanced readers to see yet another article covering Something 101. I know it annoyed me. But it's necessary, because everyone doesn't absorb the message the first time, there are always new people coming into the field (yes, even black-and-white film photography today), and every single person on the planet does not read every single word we write.
Yes, I know that last one is a real shocker. Don't we have the ear of the entire universe?
Anyway, this is one of those basic topics.
A reader of my book Post Exposure (available for free here) wrote me asking if it was important to have a print washer whose water entered from the top and exited from the bottom, because fixer was heavier than water and would sink to the bottom of the print washer.
A lot of people have thought that, including some otherwise extremely knowledgeable darkroom folks.
It doesn't work that way.
Yes, fixer has a higher density than water (density is what counts, not weight). Pour it very carefully into pure water and it will sink to the bottom. It won't stay there. There are countervailing forces at work. Gravitational potential energy drags the heavier stuff down. But diffusion/entropy/statistics favors mixing everything up uniformly. They compete. On a molecular scale, diffusion almost always wins.
I recapitulated an experiment from elementary school science (at least, when I was in elementary school). I squirted a heavy liquid into a jar of water. I used dish detergent, but sugar syrup will work just as well. I was careful not to move it much so there was very, very little water circulation going on. The left-hand picture shows what it looked like at the start. The middle picture shows the jar after it sat on my desk for four days, and the right after 10 days. Notice the direction of motion is not downward.
Conversely, if I had started with a mixed solution, the denser stuff wouldn't have settled out*. As I said, the same trick will work with sugar syrup. I don't think any of you have noticed that if you open a long-standing container of soda it tastes bitter at the top and excessively sweet at the bottom, because the high fructose corn syrup settled out.
You'll notice this took a fairly long time; diffusion is a very slow process. Still, it was enough to overwhelm gravity. Imagine what would happen if I stirred the container at all. Even, say, once a minute with just one twirl with a swizzle stick. Very quickly, all that dense detergent would go into solution. Even the most modest circulation of water works much faster than diffusion or gravity work all by themselves.
Which is the whole point behind a flowing-water print washer. The idea is to keep fresh, relatively uncontaminated water near the surface of the paper so that fixer can diffuse out of the paper as quickly as possible. It doesn't matter which way the water is flowing, up, down, or sideways. Even slow flow is much faster than all the competing processes.
You can wash prints, even fiber-base ones, in (mostly) stationary water, and people who have to be extremely water conscious or don't have a running tap in the darkroom can do this. You give the print a quick rinse in a holding tray, maybe 15 seconds sloshing it around; that gets rid of perhaps 90% of the fixer, all of the relatively concentrated carryover. Then you move the print through a series of standing trays of water, half a dozen is sufficient**, agitating the trays on occasion. Just let the fixer diffuse out of the paper into the surrounding water. My recollection is that it takes two to three times longer than vigorous water changes in a flowing water print washer, but it's a very conservative of water. Taken to the extreme, you can thoroughly wash a print in ounces of water rather than gallons. No, I don't have a good citation at hand for the precise procedure and it's not in my book, but I'm sure one of the readers here can come up with a pointer to a tutorial.
Back to the science for a bit. This is one of those instances where size matters. Mix gravel into water and it sinks immediately. Mix sand into water and it sinks very quickly. Silt will take days. Fine rock flour has small enough particles that some of them will remain in suspension forever. Like those fixer molecules.
Not so irrelevantly, if you change the balance of the forces, you get different results. Make gravity much stronger, and you can overwhelm the countervailing diffusion effect. Behold the centrifuge. A centrifuge not only will make stuff fall out of solution faster, it can extract stuff that would stay in suspension indefinitely under one gravity.
Ctein, author of well over 500 published articles for many different photography magazines, writes a weekly column for TOP that appears on Wednesdays.
*[Ed. Note: This was the direction of Phil Davis's experimentation when he sought to disprove the same myth. He simply mixed equal parts fixer and water, shook the jar, then let it stand on the shelf in his darkroom for a year. Then he carefully sampled the contents at the top and bottom of the jar. No difference—the solution was still uniform.]
**[Ed. Note: I believe David Vestal's experiments proved that three—or was it four?—changes of water is sufficient.]