Base fog

Owned by Fog and Where It Comes From, which separates the three words this entry has to keep apart — base density is the material, fog is unintended developed silver, and base plus fog is the single number an instrument reads — and which settles the threshold question: a floor is not a defect until it departs from one you measured, because none of the four film datasheets in the corpus publishes a value to compare against. This entry was built earlier from commissioning the pinhole camera, whose control sheet defines the floor, and from Break/Fix — emulsion coating defects, entry 11.

What you see

Density where nothing was exposed — across the whole sheet including the strips under the paper stops, and on the rebate of a film.

In its mild form it does not look like fog at all. It looks like a general loss of contrast, which is how the safelight literature warns you it presents, and it is therefore missed by people looking for a grey veil.

Likely causes

Four families, and the point of this entry is that they are separable.

1. The material. Age, heat, over-ripening during the make, or a hand-coated emulsion whose digestion ran long or warm.
2. Light that reached the sheet outside the camera — a safelight, the storage box, or the handling routine.
3. The developer. Too much alkali gives chemical fog; a contaminated bath can give it too.
4. Radiation and storage, which act over months rather than in a session.

What is happening: the chemistry and physics

Development is selective because the barrier to reducing silver at a bare crystal is higher than at one carrying a latent-image speck. Fog is what happens when that selectivity is eroded — because the crystal already carries a developable centre it should not have, or because the developer has become energetic enough to clear the higher barrier as well.

That is why the causes are so different in kind and the appearance is so similar. Light, heat, age and alkali all end at the same place, which is silver reduced where no picture asked for it.

The consequence for reading a negative is the one that matters. Fog raises the floor under the shadows, so it compresses the bottom of the scale, and a constant addition is proportionally largest where the picture delivered least. It costs contrast before it costs anything you would call fog.

Diagnostic questions

• Process an unexposed control. One sheet from the same box, handled identically, that never went in the camera or under the enlarger. It carries the box, the safelight and the developer and nothing else. Clean control means the fault is downstream of them.
• Process an unexposed strip developed and fixed, and one fixed without development. The difference between them is your base fog, and it separates material fog from chemical fog in a single operation.
• Which family? Four tests, one each.

Candidate	The test that isolates it
Safelight	The four-step fog test at 0, 1, 2 and 4 minutes, run on your own material
Over-ripening, or a make run warm	Compare an unexposed strip from this batch against one from a shorter ripening
Keeping	Develop an unexposed strip coated today beside one coated three weeks ago
Contamination	Was the vessel ever used for fixer, or for a developer?

📓The one published number the course can offer

Baker gives an acceptance criterion for hand-made material — fog on a trial coating on glass should not exceed 0.02 density — and the same 0.02 figure as the extra fog allowed after a ten-day oven keeping test at 105 °F below 65 per cent relative humidity. Those are the course’s only sourced numbers for this fault, and they are for a coated plate rather than for a commercial film.

Corrective action

Where the fault is the developer, a bromide restrainer at the published dose lowers the floor — and pay for it in speed, because a restrainer moves the threshold down the wedge as well as the fog.

Where the fault is the material, there is no correction. Fog is developed silver.

Where the fault is light, fix the light. The correction is to the room and not to the sheet.

Prevention

Keep an unexposed control sheet in every session. It is one sheet of paper and it separates the whole class of faults that happen outside the camera from the whole class that happen inside it, and it does so before any argument starts.

Re-run the safelight test whenever the material changes.

Store material cool and dry, and date the box. Age fog cannot be diagnosed from a box with no date on it.