That's consistent with what I saw when I experimented with using less soda (trying to control the problem with edge overdevelopment). I don't have the tools to measure the pH, so I was just tracking the amount of soda, and there seemed to be a fairly wide range within which the results showed no significant change, followed by a sudden crash in activity when the level got below a critical value.
My thinking was that a pH that was too high might make for a "hyperactive" developer in which local overdevelopment happened easily, but the results I saw didn't really bear that out.
I assume we're talking about the same thing, though I wouldn't describe it as "banding". One theory suggested when I brought it up on APUG is that we're getting more development at the edges because the dynamics of fluid travelling over an edge make the edges effectively get more agitation than the rest.
That was why I started reducing the soda, since it seems like lower developer activity would reduce such an effect. I've also tried tweaking my agitation regime in a bunch of different ways, but so far no luck.
-NT
It is good to know I am not alone! Actually, it sounds like we went through a similar thought process (that it must have something to do with the flow of developer around the reel on the edges). So, I tried pretty much everything you did. I started by agitating less and less - even stand development - but it made no difference. Then I went in the opposite direction with continuous development - still, no difference. Then I came up with the idea to add a spacer in the tank so that the reel couldn't "slosh" during inversion. But again, no difference.
Last night I came up with a completely different idea. Instead of the usual inversion agitation, I tried rolling the tank to agitate it, the idea being to try to avoid any top to bottom fluid motion and instead allow only motion around the tank axis - i.e., following the spiral. I don't really know what the fluid dynamics are like inside the tank when rolled, but I had high hopes for this technique. Unfortunately, what I found was that this also didn't really seem to help.
My latest crazy hypothesis is that maybe there is something about caffenol-c that gives it some affinity for the metal of the reel, such that it concentrates nearer the edges of the film, where it sits in the reel. Maybe there is some kind of separation that happens? If this is true then I suppose those who develop with plastic reels would never see the effect we are talking about (and maybe that's why many say they have never seen it?). Ok, I know that explanation is rather "out there," but I am running out of ideas! Perhaps I am just missing something simple. I hope so!
By the way you're right, edge "banding" isn't the best description, as that could easily be confused with other effects. Something like "edge overdevelopment" is fine as long as it is not actually the opposite which is going on - i.e., underdevelopment in the center! Perhaps "transverse differential development" would be more succinct, but it certainly isn't catchy.
