Chemical fog

Owned by Fog and Where It Comes From, which gives the decisive signature — even across the whole film, rebate and leader included, because the developer reached all of it equally and none of it needed exposing — and the four things that push development past selectivity: too much alkali, too little restrainer, too long or too warm, and a contaminated bath. This entry was assembled from Break/Fix — the developer that went wrong and the activity series before that page existed, and agrees with it.

What you see

Density on the masked patch — the part of the strip that saw no light at all — or an even grey veil right out to the film edges on a roll.

It is read against clear, fixed, unexposed film from the same batch, and it is read as a rank rather than as a number unless you have a densitometer.

Likely causes

1. Too much alkali. Kodak’s 1928 primer pairs “too much gives chemical fog” with “too little is slow” in one sentence, because they are the same knob.
2. A stronger alkali substituted for the one the formula names. Two grams of sodium carbonate in place of two of borax in D-76 computes to about pH 9.4, half a unit up, which a teaching electrode would see easily.
3. Development carried far past the optimum, where fog is still rising after the image has stopped.
4. Contamination.

What is happening: the chemistry and physics

Development is selective because the energy barrier to reducing silver at a bare crystal is higher than at one carrying a latent-image speck. Raise the developer’s energy far enough and the higher barrier starts to be cleared too, and silver is reduced where no exposure asked for it.

The alkali is what sets that energy. Most developing agents cannot develop at all without an alkaline solution, and the energy depends on how much alkali is present — so the alkali is simultaneously the contrast control and the fog control, and it moves both in the same direction.

Both image and fog rise with development time, but if fog rises faster there is a time beyond which the extra fog costs more than the extra image is worth. That, rather than a rule about minutes, is what sets the useful end of a development.

⚗The restrainer, and why it is not simply less of everything

A halide restrainer suppresses fog sooner than it suppresses image. Across a published series the unexposed density fell and then stopped falling while the highest exposure was still building density at the same concentration.

That asymmetry is the whole commercial case for the ingredient. If restraint fell on fog and image at the same rate, adding bromide would be equivalent to shortening the development and would be pointless.

Hurter and Driffield established the limit of it as well — a restrainer retards rather than stops, and the image will appear in full force if sufficient time be allowed.

Diagnostic questions

• Rank the fog floor against clear fixed base, and against a control strip developed in a bath of known condition. A floor higher than the control’s is chemical fog; a floor lower than the control’s is an exhausted bath supplying its own restrainer.
• Measure the pH against the control bottle, at 20 °C, with the electrode’s stated accuracy written beside the reading. Clearly above the control means too much alkali. Within the electrode’s accuracy convicts nobody, and saying so is part of the diagnosis.
• Is the fog on the masked patch, or on the whole sheet including the edges? The masked patch is the developer’s own work. A veil that reached the sheet outside the tank is base fog or safelight fog instead.
• Was the development extended? A long development in a sound bath produces this legitimately.
• What was mixed, and from which tub? A carbonate weighed where borax was meant is a plausible route and the mixing record is the only evidence.

Corrective action

Nothing on the negative.

For the next film, either lower the alkali to the formula’s own figure, or add a restrainer at a published dose and accept the loss of speed that comes with it. Do not do both at once, or you will not know which worked.

Where the cause is a mis-mix, discard the bath. The fix is to the labelling and the shelf rather than to the chemistry.

Prevention

Weigh with the formula in front of you, label every solid with its full name and its hydrate state, and keep the alkalis apart on the shelf.

Run a process-control strip and rank its fog floor every time, so that a rise is caught as a trend rather than as a surprise.

Determine your own useful development time by watching where fog begins to rise faster than image, rather than by inheriting a number.