Developer contaminated by fixer or stop

Owned by chemistry in the bottle. The reverse direction — developer into the fixer — is fixer contaminated by developer, and it behaves quite differently.

What you see

With fixer in the developer: a film or print that is thin and lacks contrast, often with a raised fog floor and sometimes a stain, all at once, from a bath that is otherwise new. The combination is the tell — loss of activity and fog together is not what an old or cold developer does.

With stop bath in the developer: simply a slow developer. Prints come up late and never quite reach a full black in the published time; negatives are flat. No fog, no stain.

Both often announce themselves as “the new developer is no good”, which is why the bottle rather than the image is the place the diagnosis ends.

Likely causes

Kodak’s Z-133 gives four routes, and every one of them exists in a home darkroom:

1. Mixing equipment that has not been thoroughly cleaned — the graduate, the funnel, the stirring rod.
2. Dry chemicals that become airborne during mixing and settle in an adjacent solution.
3. Pipes and tanks made of material that reacts chemically with some solutions.
4. Solution splashed or dripped into another solution.

To which a small darkroom adds tongs used in two trays, a thermometer moved between baths without a rinse, a hand that had been in the fixer, a bottle refilled from the wrong jug, and a wet bench that carries the last spill to the next shelf.

What is happening: the chemistry and physics

The two contaminants do different things, and the asymmetry is the point of this entry.

Thiosulfate is a silver-halide solvent in a bath whose whole function is to reduce silver halide selectively. It dissolves halide that the developer is trying to develop, and it puts silver into solution where it can be reduced in the wrong places. The result is loss of activity, fog and stain together, and it happens at a concentration far below anything that would matter as a dilution. ILFORD put it in one sentence: “even a trace of fixer can contaminate the developer, and possibly ruin your next film.”

Acid, by contrast, does something proportionate and predictable. A developing agent works as its anion, and the fraction of it that is ionised is set by the pH. Kodak’s primer states the consequence directly: “the quantity of alkali governs the energy of a developer… if too little alkali is present, it will be slow in its action.” A splash of stop bath neutralises some of the alkali, the pH falls, and the developer slows in proportion. Ten millilitres in a litre is a small effect; a hundred is a large one.

!What the makers do about it, which tells you how they rate it

Kodak’s Z-133 troubleshooting table lists “developer contaminated with fixer or stop bath” among the causes of an abnormally light film. Its corrective action offers no adjustment at all:

Replace contaminated developer. Wash mixing equipment thoroughly before use.

That is a manufacturer declining to nurse the bath. The reason is the asymmetry above: for the acid case you could in principle correct the pH, but you cannot know how much went in; for the thiosulfate case there is nothing to correct.

And the course cannot tell you how small a trace matters. No source in this course’s corpus states a quantity of thiosulfate that measurably affects a developer. ILFORD say “even a trace”; Kodak say replace it. Neither publishes a threshold, and this entry will not invent one — which is itself the argument for the prevention below, because a practice that does not depend on knowing the threshold is the only safe one.

One more contaminant belongs here because it arrives by route 3 and is easy to forget: Kodak’s 1928 primer records that “tin, copper, and zinc, or alloys of these metals will usually produce bad fog and stain with photographic developers”. A galvanised bucket is zinc; a brass fitting is copper and zinc.

Diagnostic questions

• Is there fog as well as a loss of activity? Both together, from a new bath, points at thiosulfate. Loss of activity alone points at acid, or at the bath being cold, dilute or old.
• How new is the bath? A fresh bath behaving like an exhausted one is the signature.
• What was the pH, against the maker’s published figure? ILFORD publish fresh values for their stock developers — PERCEPTOL 7.68–7.82, ID-11 8.60–8.70, MICROPHEN 8.67–8.93 — and advise measuring your own fresh solutions for later comparison. This detects the acid case and not the thiosulfate one.
• What vessels were used, in what order, and were they washed?
• Are the tongs, the thermometer and the funnel shared?
• Was anything mixed near an open bath? Airborne dry chemical is Kodak’s second route.
• What is the bath in contact with? Metal fittings, a galvanised container, an unknown plastic.
• Does a clip test go to full black? The confirming test, and it costs a few centimetres of film.

Corrective action

Discard the developer. This is the manufacturers’ own answer and the course follows it.

Wash every vessel that touched it thoroughly, before it is used again. Kodak’s corrective action names this as the second half of the fix, and it is the half that stops the next bath going the same way.

Do not attempt to correct the pH of a contaminated developer. You do not know how much acid went in, and if the contaminant was fixer rather than stop, pH has nothing to do with it.

Prevention

Dedicated vessels per bath, in different colours. ILFORD’s own advice, and it is the whole answer for most people: “buy different colours, so that you can use a different colour for each solution.”

Make solutions in the order developer, then stop, then fixer, washing cylinders and stirrers in between — ILFORD’s SIMPLICITY instruction. Making the cleanest bath first means that whatever survives a rinse travels in the direction that costs least.

Wherever possible use dedicated equipment for making up developer, which ILFORD state on their chemical sundries sheet and Kodak state as “use separate mixing tanks for developers and fixers”.

One tong per tray, marked, and never moved.

Rinse the thermometer between baths, and rinse your hands: Kodak’s darkroom layout instruction is to divide the room into wet and dry areas and to keep a container of water for rinsing hands, drying them on a clean towel before touching film or paper.

Mix away from open baths, so that airborne powder has nowhere to land.

And label everything with what it is and when it was made. Two clear liquids on a shelf are indistinguishable at arm’s length.