Elevated fog on a curve

Lifted from the characteristic curve and process control with control strips. The register keeps the curve-shape signatures with Parts XIII and XV, and both pages are written.

What you see

The whole floor of the curve has lifted. The unexposed end, which should sit at the film’s base plus fog, reads higher than aim; the toe starts from a higher place; and the highlights — which sit on that nearly flat foot — have lost separation while the mid-tones and shadows look untouched.

That last asymmetry is the signature and it is worth stating plainly. Fog does not make a curve grey all over. It raises the bottom. A small uniform exposure added everywhere is negligible beside what a mid-tone already received and decisive beside what a highlight received, so the damage lands where the curve is shallowest.

ILFORD treat D-min as one of the three quantities on a film control chart, alongside speed and contrast, which is the manufacturer’s way of saying the same thing: a drift in the floor is a fault signal in its own right, not a nuisance to be subtracted away.

Likely causes

1. The film. Age, warm storage, or proximity to a radiation source. Each has its own entry in this atlas and each raises D-min before it does anything else.
2. The development. Too long, too warm, or a developer short of its restrainer — see chemical fog.
3. Light that should not have been there. A leak, or a safelight that is not safe for this material for this long — see safelight fog and light-leak fog.
4. Contamination. Kodak list solution contamination among the causes of an out-of-control process and name its routes: unclean mixing equipment, dry chemical going airborne during mixing and settling into an adjacent solution, and splashing.

What is happening: the chemistry and physics

Kodak’s workbook names the two contributions to D-min and they behave differently. The support — the base and, on many films, a dye in it — is fixed, and does not drift from run to run. Chemical fog, the crystals that develop although no light reached them, is the part that moves.

Everything on this list is a route to developing an unexposed crystal. Age and heat move grains towards developability without light. A radiation source does it by a different physics and leaves a different pattern. A developer left too long, run too warm, or short of bromide crosses the threshold from the other side: it becomes energetic enough to reduce grains that carry no latent-image centre worth the name. And a small uniform light exposure simply gives every grain a little of what the image gave a few.

!The confirming test: an unexposed strip, and where it was

Process an unexposed strip from the same cassette alongside the exposed one. That is the course’s convention on every sensitometric page and it is what makes this diagnosis possible, because it separates what the material and the chemistry did from what the exposure did.

Then split the remaining causes with a second strip that never leaves the box until it is in the tank:

• Fog on a strip that was never out of the box is the film or the developer.
• Fog only on strips that were handled in the room is light, and the safelight test is the next step.

Kodak add the trap in this test: excessive safelight exposure may show up only in the image area, because that area receives additional exposure from the enlarger as well. So a coin-on-paper fog test can pass while the working material still fogs.

Diagnostic questions

• What is the aim, and is it the same film batch? D-min is a property of a coating. A batch crossover is an event to annotate on the chart, not a fault to chase.
• Where has the film been? Heat and time are the two commonest answers and neither is reversible.
• Did fog move with contrast, and in which direction? Fog up with contrast up is over-development. Fog up with contrast flat or down is more likely light or material. Fog down with speed down is bromide restraint, which is the opposite fault and reads quite differently.
• Was anything else in the room lit? Kodak name lighted dials on equipment controls and light escaping from an enlarger head in the same breath as the safelight itself.
• When was the developer mixed, in what, and after what? Contamination is the hypothesis of last resort and the test for it is a fresh set of solutions in clean vessels rather than a further reading of the chart.

Corrective action

Nothing recovers a fogged strip, and that is worth saying rather than implying. Density that was added uniformly cannot be subtracted selectively; the highlight separation it destroyed was never recorded. A chemical reducer trades one loss for another and the course has no assessed reducer to offer.

What the reading is for is the next run. Replace the single most likely cause and process again — a fresh film batch, or a fresh developer, or the same film handled entirely in the dark — and read the same three numbers.

If the fog came with the film, and the film is what you have, the working answer is to measure your speed point against the measured D-min rather than the nominal one, because a criterion defined as a fixed density above base-plus-fog moves with it.

Prevention

Cold, dry, dark storage, and use the film before the base has drifted. Age fog and heat fog are the two causes on this list with no cure at all.

A control chart with a D-min row, which is ILFORD’s own arrangement. The floor moving is the earliest warning most processes give.

Test the safelight for the material and the time you actually use, not for the material the filter was sold for; the safelight test SOP is the procedure.

Separate mixing vessels for developers and fixers, which is Kodak’s own remedy for the contamination routes they list, and one that costs a second jug.