After introducing our Rev 2 film holders, we started getting occasional reports of "blotches" on the film that matched up with the vertical bars of the film holder. (About 3% of users reported it.) Here's a sample (we played with the tint in photoshop to make it more visible):
Some users also reported that the film was "stuck" to the holder and had to be pried off.
Over time, we discovered that there were, in fact, two radically different defects being reported:
1. If the blotches are black or dark gray, then most likely the negative was loaded in backwards (emulsion toward the film holder). Obviously, with the emulsion pressed against the film holder, it didn't develop correctly. The blotches may also be almost clear, indicating that the film stuck during the development cycle but broke free during the fix. Unfortunately, there's nothing you can do about, just be more careful next time.
2. If the blotches are green, blue or whatever color the back of the unprocessed film is, then it's just anti-halation dye that didn't' get washed out.
Background: manufacturers coat both sides of sheet film with gelatin to prevent curling (nothing to do with Canadians chasing stone jugs on ice). They also impregnate this gelatin with an anti-halation dye to help prevent light from bouncing off the film holder in the camera and generating halos on the negative.
Fortunately, rewashing the film in a tray usually removes the dye and saves the negative.
At first, we blamed warped film holders, theorizing that the film holder was contacting the back of the film and preventing the dye from washing out. We tested with extremely warped holders and even added shims to force the film against the holder. We could never duplicate the problem in our lab.
We also suspected agitation. But even with very limited and extremely gentle agitation, we could not duplicate the issue. Besides, we know of users doing stand development with no problems.
The only quasi-solid data we had was:
- It seemed to occur more often with Ilford film.
- Many of the users were pre-soaking1.
- A particular user might not see the problem for months and then have it crop up again, only to disappear the next week.
Even after processing thousands of sheets film, we couldn't duplicate the issue. We even sent film tanks to Ilford for their evaluation; they couldn't duplicate the problem either. (In fairness to Ilford, the issue has occurred with Kodak and Foma as well. This may be more related to market share than anything else.)
The Rev 4 holders almost solved this, reducing the incidence to around 1%. (Note: the real motive for the redesign was a production issue but we hoped to solve the sticking issue at the same time.)
An old rule of engineering: if you can't break it, you can't fix it. We were probably more frustrated than the users with the blotchy negatives.
Anyway, after eliminating any mechanical mechanism, we started investigating chemical mechanisms. We asked our team of chemists "what would make the film stick?" and we think we've found the answer: softened water.
By processing with softened water, we can reproduce the problem almost at will! Indeed, doing a presoak with softened water will usually cause the film to stick to the holder as if glued! Sometimes you'll get lucky and it's fine; but usually, it sticks. Here's a negative after a four minute presoak (no other processing), in softened water:
Note that we said "softened" water, not soft water. Quick definitions:
- Soft water has very little calcium and magnesium in it.
- Hard water has high levels of calcium and/or magnesium.
- Softened water has been treated to remove the calcium/magnesium, often using an ion exchange process. This process replaces the calcium/magnesium with sodium (salt).
If you've ever showered in softened water, you know it makes soap behave quite differently than regular water. Granted, we don't completely understand what's going on and there could be other situations that cause the same problem. Even so, we're pretty certain that the issue is related to the water being used.
It appears that the ions in the treated water soften the gelatin enough that it will stick to the holders (even the textured ones.) This prevents the dye from washing out.
Of course, we asked our chemists "why?" They started rambling on about "hard ions", "soft ions", amino acids ... we're not expecting a complete answer anytime soon.
To add to the complexity, even tap water can contain the "soft" ions that can affect the gelatin. And the quality of your tap water can change drastically over a short period of time. This can explain why some users see the problem one week but not the next.
Another variable: every manufacturer uses a different, usually locally sourced, gelatin. Gelatin is actually an extremely complicated compound. Combined with all the variations in water quality and you can see why the problem is so sporadic.
So while our team looks for the root cause, the solution is simple: if in doubt, use distilled water.
1. Ilford (and other manufacturers) recommend against pre-soaking for black and white films. Yes, we know Ansel pre-soaked. That was then, this is now.