Dichroic fog

Owned by Fixer Faults, Washing Faults and the Stains They Leave, which files this fault with the fixing faults rather than the fogs because its cause is a solution rather than a light or a development event, and which sends the reader here for the historical removal treatments and the course’s position on them. Fog and Where It Comes From names it and hands it on for the same reason, keeping only the discriminator. This entry was assembled from Break/Fix — the exhausted fixer, set D, before either page existed.

What you see

A veil that is greenish or yellowish by reflected light and reddish-pink or violet by transmitted light. The two-colour behaviour is where the name comes from and it is the whole diagnosis.

Kodak’s 1928 primer and Wall’s 1912 dictionary describe it in the same terms, thirteen years and an ocean apart, which is about as good as agreement between historical sources gets.

Likely causes

1. Developer carried into the fixing bath without a rinse or a stop bath.
2. An old, exhausted fixer already carrying an excess of dissolved silver — Kodak’s condition is a bath that is “old and exhausted and contains an excess of dissolved silver salts”, or one that does not contain acid.
3. A silver-bromide solvent in the developer, which Wall reports as favouring its formation.

What is happening: the chemistry and physics

It is not undissolved halide at all. Kodak’s primer says so directly when discussing its removal - dichroic fog “consists of very finely divided silver”.

Put the sources together and the picture is coherent. A fixer dissolves silver halide into a complex. Carried-over developer is a reducing agent. Where the two meet inside the gelatin, some of the dissolved silver is reduced back to metal before it can diffuse out — physical development, happening in the fixer, in the wrong place. An old bath already carrying dissolved silver has far more to work with, which is why Kodak’s condition names exhaustion, and why the prevention is a rinse.

!The discriminator, and it is decisive

Wall — the plate “looks foggy and suggests that it has not been properly fixed, but prolonged immersion in hypo solution does not mend matters”.

Of course it does not. Metallic silver is not silver halide, and a fixer has nothing to say to it. That single sentence separates dichroic fog from under-fixing permanently, and it does so by naming a treatment that fails.

Diagnostic questions

• Look at the film, then look through it. Ten seconds, a lamp, and no other equipment. A veil that changes colour between reflected and transmitted light is dichroic and nothing else is.
• Rank the milkiness in the rebates by transmitted light. Under-fixing is a quantity and is ranked; dichroic fog is a class and is identified.
• Did the material go straight from developer to fixer? Kodak names an unrinsed film as one of the two conditions under which it appears, and a rinsed one as a condition under which it does not.
• What is the fixer’s clearing time as a ratio to fresh? An exhausted bath is the other named condition.
• Does refixing help? If it does, the diagnosis was wrong.

Corrective action

Not rescuable in this course, and the reason is worth stating rather than hiding.

Two historical removals are on record and both work by attacking finely divided silver in preference to the image’s own. The 1928 primer’s is a plain permanganate bath at about a quarter of a per cent, which it says has no appreciable action on image silver. Wall’s, credited to Lumière and Seyewetz, is ammonium persulphate acidulated with sulphuric acid and followed by a sulphite bath, and he rates it highest for fog that formed in the fixing bath itself.

The course records both and prescribes neither. Potassium permanganate has no entry in the chemical encyclopaedia and no hazard record anywhere in the corpus, and the persulphate route requires a concentrated mineral acid. A reducer whose reagents the course has not assessed is not something it will hand a reader as a procedure.

So the honest position is that the fault is identifiable, the historical treatments exist, and the course’s answer is prevention — which costs a thirty-second rinse.

Prevention

Rinse before the fixer. A stop bath is better, but even plain water removes most of the developer that causes this and most of the alkali that spends the fixer’s reserve.

Keep the fixer within its capacity and acid, per the exhausted fixer entry.

Keep the bath temperature moderate. Kodak’s stated condition under which it never occurs includes a bath held at 65 to 70 °F alongside the rinse and the fresh acid bath.