Control chart: contamination

Lifted from process control with control strips. Of the four signatures that page sets out, this is the one where it states a limit rather than a rule, and this entry inherits that limit rather than closing it.

What you see

A movement that none of the other three signatures explains. Not the outlier of a temperature error, not the trend of exhaustion, not the compressed highlights of safelight fog. Speed, contrast and fog move in a combination the mechanisms do not predict, or move in different directions on successive runs.

Unpredictable is the signature. That sounds like an evasion and it is not: it is the honest description of a fault whose magnitude depends on how much of what got into where, and the course has no measurement of it.

Likely causes

Kodak name the routes, and they are four:

1. Mixing equipment that has not been thoroughly cleaned.
2. Dry chemical made airborne during mixing and settling into an adjacent solution.
3. Tanks and pipes of a material that reacts.
4. Solution splashed or dripped into another.

To which ILFORD add the commonest one in a home darkroom: a trace of fixer or stop bath carried into the developer, which they say leads to inconsistent results or, at worst, completely blank prints.

What is happening: the chemistry and physics

A developer is a reducing solution held at a chosen pH with a chosen halide concentration, and every one of those three is something another bath in the room can change.

Acid from a stop bath lowers the pH, and Part VIII owns what pH does to activity: a developing agent that is not dissociated does not develop. Thiosulfate from a fixer is a silver solvent, and Part XI owns that — a solvent in the developer attacks the image as it forms. Developer carried the other way, into the fixer, is the mechanism behind dichroic fog.

!Where the course stops, and it stops here deliberately

The course has found no manufacturer measurement of what a stated quantity of thiosulfate in a developer does to a characteristic curve. So it will not tell you how far fog rises per millilitre of carry-over, and this entry carries no number.

What the sources do say is that contamination is a named cause of an out-of-control process, that it is prevented by separate utensils, and that some stains have their own diagnosis — Kodak note that a severe magenta stain after fixing points at a fixer near exhaustion or an inadequate fixing time, which is a fixing fault and not a developer fault at all.

The confirming test is therefore procedural rather than sensitometric. When a chart moves in a way none of the first three signatures explains, contamination is the hypothesis, and the test is a fresh set of solutions in clean vessels rather than a further reading of the chart. If the chart returns, the diagnosis is made. If it does not, you have eliminated the hypothesis, which is worth as much.

Diagnostic questions

• Do the three rows move together in any sensible direction? If they do, one of the other three signatures probably fits and should be preferred.
• What was mixed in that vessel last, and was it washed? Kodak’s remedy for a developer contaminated with fixer or stop is to replace the developer and wash the mixing equipment thoroughly — both halves.
• Were dry chemicals weighed near open solutions? Airborne powder settling into an adjacent bath is a route most darkrooms never consider.
• Are the dishes distinguishable in the dark? ILFORD’s remedy is different colours for each solution, or clear labels, with chemicals stored in matching containers.
• Is there a rinse for hands between baths? Kodak’s AJ-3 keeps a container of water beside the trays for exactly this, with a clean towel.

Corrective action

Replace the developer and wash everything that touched it, which is Kodak’s own instruction rather than an extrapolation of it.

Then re-run the chart from a fresh set of solutions, and treat the previous aim as suspect: a chart whose aim was established on a contaminated bath has the contamination built into it.

Do not attempt to correct a contaminated bath by adjusting time. The change is not a change of degree of development, so a change of development will not undo it.

Prevention

Separate mixing vessels for developers and fixers. One extra jug, and it closes the commonest route.

Wet area and dry area, which is AJ-3’s arrangement and the same one Part II’s laboratory layout lesson teaches.

Two pairs of tongs, never interchanged, and a rinse for hands between baths.

Colour-code or label every dish, because the whole point of the fault is that it happens in the dark.