Overview: Making the Image Permanent

Development reduced the exposed grains and left the rest alone, and the rest is a large fraction of the coated silver. Kodak’s method for estimating the silver recoverable from black-and-white material makes that explicit: you assume a percentage of exposure and take the remainder as the part the fixer removes, and Kodak’s own worked examples use thirty per cent for a graphic-arts film that is mostly dense and fifty per cent for radiography. A pictorial negative, most of whose area is thin, sits above both. Everything in that remainder is still a light-sensitive silver halide sitting in the gelatin, and until it is taken out the picture is not a picture but a delay.

One equilibrium dragging another

Fixation is a single trick applied at scale. Silver bromide dissolving in water is an equilibrium that goes essentially nowhere; thiosulfate taking silver ion into a complex is an equilibrium that goes essentially to completion; and because they share the silver ion, the second drags the first. Part III worked that out in full, numbered the stepwise complexes, and produced the combined constant. Part XI does not re-derive any of it. It asks the four questions that follow once you have it.

How fast, and why not instantly. A bath thermodynamically capable of dissolving forty grams of silver bromide per litre still takes two to five minutes on a film, and the reasons are the interesting part.

How far will a litre go. ILFORD publishes twenty-four 135-36 films per litre for its rapid fixer at 1+4, and a silver ceiling of 8 to 10 g/L for a film bath. Those two numbers are not the same kind of number, and the difference between them is most of what a capacity figure means.

What is left behind when it fails. Kodak’s 1924 primer is exact about this and worth having in mind from the first page of the part: two compound sodium silver thiosulfates exist, “one of them being almost insoluble in water, while the other is very soluble”, and only the soluble one forms while the bath still has fixing power. The insoluble one is invisible. A negative moved to the wash the moment it looks clear can carry it away in the gelatin, and no amount of washing removes it.

How you tell. By the clock and the leader, not by looking at the tray.

What is on the bench

Three fixers, mixed and then measured against each other.

Plain hypo — sodium thiosulfate and nothing else: Herschel’s 1839 reagent, easiest on the arithmetic, and the one that shows what the other ingredients are for by not having them.

An acid hardening fixer of the F-5 type — thiosulfate, sulfite, acetic acid and potassium alum, each present for a reason the page states. This family dominated the twentieth century and its failure modes fill Kodak’s troubleshooting pages.

A rapid fixer — ammonium thiosulfate, bought as a concentrate and diluted. It fixes in a third of the time, and the encyclopaedia entry is candid that the course could source the effect and not the mechanism.

What gets measured

Two quantities, and the first is free. Clearing time is the interval between putting a piece of undeveloped film in a fixer and the last of the milkiness going; ILFORD’s sheet gives the method in a paragraph and the rule in a sentence — fix for twice the clearing time, and discard the bath when the clearing time in used fixer exceeds twice the clearing time in fresh. Kodak’s 1928 primer gives the same rule, in the same words, from the other side of the Atlantic and eighty years earlier.

Capacity is what the experiment in this part measures for itself: clearing time against cumulative film area, run until the doubling point, plotted, and compared with what the maker publishes for the same bath. The curve is nearly flat and then it is not, and knowing where the corner sits is worth more than any published figure, because it is your bath.

⚠The three things this part asks you to be careful about

Acid meeting thiosulfate. This is the reaction on the incompatibility matrix that a darkroom is most likely to run by accident, because the two liquids are adjacent in the tray sequence and adjacent again in the waste corner. Kodak’s 1928 primer records the observation plainly: a few drops of a strong acid in a weak hypo solution decompose it and the solution turns milky as sulfur comes out. The lab in this part does it once, on purpose, at a scale of a few millilitres, so that you have seen it.

The alum. Potassium alum carries no agreed GHS classification, which is not the same as carrying none; HSE’s EH40 sets soluble aluminium salts a limit of 2 mg/m³ as an eight-hour average, and that is a control for dust that no hazard statement mentions.

The ammonium salt, and a correction. A bought rapid fixer does not smell of ammonia at working pH — CAMEO records ammonium thiosulfate solution at 60 per cent or less as an odourless colourless liquid. Ammonia appears when the bath is pushed alkaline, and Kodak’s silver-recovery publication puts a number on it: do not take an ammonium fixer above pH 7.8 to 8.0.

Every page states the level it earns and the criteria that set it, against the classification rubric.

What you need first

From Part III, complex formation, which owns the formation constants and the stepwise numbering this part uses without restating, and solubility and precipitation and diffusion and osmosis. From Part IV, the silver halides, whose solubility-product table is the one every calculation here reaches for. From Part VIII, how development works, and from Part X, why stopping works, because the fixer is what a stop bath is protecting.

The pages of this part

Part11 of 28Level2 — PractitionerPages6Estimated time10.8 hoursHighest safety levelLevel A

✱1 of 6 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 6 lessons in this part completed

• LessonHow Fixer Works75 min
• LessonFixer Formulations: Plain, Acid, Hardening, Rapid, Neutral, Alkaline60 min
• LessonFixer Capacity, Exhaustion and Residual Silver60 min
• LabLab: Mixing Fixers from ScratchA150 min
• ExperimentExperiment: Clearing Time and Fixer CapacityA180 min
• Break/fixBreak/Fix: The Exhausted FixerA✱ Darkroom120 min