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.