Under-fixing

Owned by Fixer Faults, Washing Faults and the Stains They Leave, which states the distinction this entry turns on — clearing is the disappearance of visible halide, fixing is carrying the complexes far enough that what remains can be washed out — and which gives the paper case, where the mild form shows nothing at the time and yellows months later. This entry was assembled from Break/Fix — the exhausted fixer, sets B and C, before Part XXVIII was written.

What you see

Milkiness, opalescence, a faint cloudy veil across the clear areas, easiest to see in the rebates and the clear film between frames — which ILFORD’s own guide says should be “almost clear” in a good negative.

This is not a symptom that resembles under-fixing; it is under-fixing. Kodak’s 1928 primer defines the endpoint of fixing in exactly these words — the time for the milkiness or opalescence of the unreduced silver salts to disappear — so its presence is the definition.

A magenta or pink stain, even across the frame, travels with it on some films. Kodak’s note is qualified and the qualification matters — a slight stain does not affect image stability, contrast or printing time and can be removed with a hypo clearing agent; a pronounced and irregular stain is the one that means refix.

On paper, the mild form shows nothing at all at the time and then yellows in the borders and highlights, which is its own entry.

Likely causes

1. Fixing time shorter than twice the clearing time.
2. A bath near exhaustion, so that silver leaves the grain as the sparingly soluble first complex.
3. A thick or unevenly coated layer, especially a hand coat, that the fixer could not penetrate — which hardening makes worse rather than better.

What is happening: the chemistry and physics

Two mechanisms, and the second is the one that matters.

The first is trivial. Silver halide that was never dissolved is still silver halide, it is still light-sensitive, and it scatters light — which is what the milkiness is.

The second is Kodak’s 1924 statement working exactly as advertised. Fixing proceeds through two complexes, and as a bath loses fixing power silver leaves the grain as the step-one mono complex more and more often, because the free thiosulfate that would carry it to step two is not there. That species is “almost insoluble in water”, it is invisible, and a material moved to the wash while it is present keeps it.

!Why the mild case is the dangerous one

What separates a badly under-fixed sheet from a mildly under-fixed one is only how much — and that is worth saying, because it makes the mild one worse.

A badly under-fixed negative announces itself. A mildly under-fixed one looks like a slightly flat negative, prints perfectly well, and declares its fault in the third year.

Diagnostic questions

• Transmitted light, rebates first. Light box, loupe, and the clear film between the frames. Rank against a piece of the same film that was never exposed and never processed — which is the maximum milkiness — and against a properly fixed sheet, which is the minimum.
• Reflected light at a glancing angle over a dark background, then transmitted light through the same area. A veil that changes colour between the two views is dichroic fog and nothing else is — and it does not answer to more fixer.
• Measure the clearing time of the bath on a fresh scrap and express it as a ratio to the fresh figure. This is the measurement that turns a ranking into a diagnosis.
• The sulfide test for paper. ILFORD’s procedure is 2 g of sodium sulfide in 125 ml of water, diluted 1+9 for use, a drop on a white area of a print known to be well fixed and washed to establish the reference tint for that paper, and any subsequent print showing a yellowing of the test spot is not properly fixed. Prints must be well washed before the test; it is not effective straight from the fixer.
• The capacity log, in films or 8 × 10 equivalents. If there is no log, that is itself the finding.

Corrective action

Rescuable, and worth doing today. Refixing, in full -

1. Measure the clearing time of the fresh bath first, on a scrap of the same film. You are about to use it as an instrument and you need its zero.
2. Soak the material in plain water for five minutes. It has been dry, and the gelatin has to swell before anything can diffuse into it.
3. Fix for twice the fresh clearing time, agitating properly.
4. Wash in full, as though it had never been washed — because as far as the retained compounds are concerned it has not.
5. Dry, and look again by transmitted light. The milkiness should be gone.

What refixing does and does not repair. It dissolves the halide that was never dissolved, and it converts the retained step-one complex into the step-two complex, which the wash can then remove. It does nothing whatever for silver already reduced to metal, for a stain already formed in a print, or for a negative that has sat in a sleeve for two years while the retained compound sulfided.

Prints are rescuable with a caution — a fibre-base print has an upper bound on total fixing time as well as a lower one, and a rescued print has now been fixed twice.

Prevention

Fix for twice the clearing time, measured on the bath you are actually using.

Use two baths for anything you care about. Kodak’s 1924 reasoning is the one to remember — the rotation exists to ensure that nothing leaves the fixer while the invisible insoluble compound is still in it.

Keep the capacity log, and test the clearing time on a schedule, not on suspicion — every quarter of the published capacity. By the time something looks wrong, the mild case has already happened.