Heat fog

Owned by fog and where it comes from.

What you see

Exactly what age fog looks like — a raised, even floor across the whole material including the rebate, shadows that will not separate, a base that will not stay white — but on a film or a box of paper that is nowhere near its date.

The identification is therefore not in the appearance at all. It is in the history of the box.

Likely causes

1. A vehicle. The inside of a parked car in summer reaches temperatures far above anything a manufacturer’s storage recommendation contemplates, and a camera bag on a back seat is inside it.
2. A loft, a conservatory, a windowsill, a shelf above a radiator, the top of a fridge.
3. A warm cupboard for a season, which is the version that catches careful people, because nothing dramatic happened.
4. Heat and humidity together. Both Kodak and ILFORD name the pair rather than temperature alone.
5. A hot posting or shipping route, which is outside your control and is a reason to test a batch that arrives in July.

What is happening: the chemistry and physics

The mechanism is age fog’s mechanism, run faster. A grain becomes developable when it acquires a sensitivity centre, and the chemical steps that build one without light are ordinary reactions in the emulsion whose rate rises with temperature. Warm the box and you have not changed what happens; you have changed how long it takes.

That is why an afternoon can cost more than a year: the same total effect is reached in a fraction of the time, and there is no recovery period. Fog does not go away when the film cools down.

The manufacturers’ storage figures are the practical statement of the same thing. Kodak ask for unexposed film at 24 °C or lower; ILFORD ask for 10–20 °C. Both frame the instruction as a warning about quality changes rather than as a hard limit, and neither publishes a rate — so the course states the direction and the recommended conditions and does not invent a figure for how much fog a given excursion costs.

✕Heat fog and hot development are different faults with the same word in them

Heat fog happens to the material before you ever open the box, and it is a property of the emulsion by the time it reaches you.

Chemical fog from a hot developer happens in the tank, and it is a property of the process. Kodak Limited’s antifog tablets were sold for both cases in one sentence — for development “carried out at high temperatures” and for materials that “have deteriorated through age or incorrect storage conditions” — but they are two faults and only one of them is fixed by cooling the bath.

The test separates them in one step. Develop an unexposed strip of the suspect film at the correct temperature, in a bath you trust. A raised floor convicts the material; a normal floor convicts the process, and you should be reading chemical fog instead.

Diagnostic questions

• Where has this box actually been? Not where it should have been. Ask about the car, the loft and the windowsill specifically, because nobody volunteers them.
• Is the film well inside its date? Age fog on an in-date film is heat fog until proved otherwise.
• Is it the whole batch or one roll? A batch points at a shared storage failure; one roll points at the roll that travelled.
• Was the film in the camera through the heat? Then the latent image has been decaying at the same time, and the negatives will be thin as well as fogged.
• What does an unexposed, developed strip read against fresh stock? The measurement, and the only one that settles it.
• Was the developer hot? If so you have the other fault, or both.

Corrective action

Nothing on the material. The fog is developed silver.

For the remainder of the batch, the options are age fog’s: more exposure to lift the shadows off the raised floor, less development to hold the fog down, a restrainer from Part VIII, or a decision not to use it for work that matters.

Do not refrigerate a heat-fogged film in the hope of recovery. Cooling stops further accumulation and returns none of what has already formed.

Prevention

Never leave film in a vehicle, and treat a camera bag as part of the vehicle.

Keep the stock indoors, on an inside wall, away from every heat source — and note that the top of a refrigerator and the cupboard above a boiler are both warmer than the room.

Refrigerate or freeze long-term stock, which both makers permit provided the film is allowed plenty of time to acclimatise before the package is opened, because condensation on cold film is a fault of its own.

Test a batch that arrived in hot weather before you commit anything to it. One strip, no exposure, your normal development, against a strip you trust.