Fixer rarely fails suddenly. It loses strength gradually as it is used, and a bath that still clears film, only slowly, can leave a negative that looks fine wet but yellows and browns over years as retained chemistry decomposes in the gelatin. The clearing-time test exists because that decline is invisible to the eye but measurable with a clock, and the residual-chemical tests below catch the failures the clock cannot.

Naming the Chemistry

Fixing dissolves the undeveloped silver halide left after development. Run Ilford Rapid Fixer, which is an ammonium-thiosulphate rapid fixer rather than the older sodium-thiosulphate hypo, and you fix film at 1+4 anywhere from 18 to 40C, with a working-strength pH of 5.0 to 5.5. The reaction does not go to a soluble product in one step. Silver bromide reacts first with thiosulphate to form silver monothiosulphate, AgS2O3, which is only sparingly soluble; in further fresh thiosulphate that intermediate converts to the soluble silver dithiosulphate complex, [Ag(S2O3)2]3-, which then diffuses out of the emulsion and into the bath.

The dangerous resident is that intermediate. As James M. Reilly’s account of fixation chemistry in The Albumen and Salted Paper Book explains, thiosulphate must remain in excess: “there must be more thiosulfate ions present than are needed to react with all the silver ions present, or else insoluble complexes are formed which cannot be washed from the image layer.” Reilly notes there are probably at least three different kinds of silver-thiosulphate complex, and that the troublesome one is soluble only in fresh thiosulphate. His text describes classic hypo; modern darkrooms run rapid fixer, but the trap is identical. A bath too tired to push AgS2O3 through to the soluble dithiosulphate leaves the intermediate lodged in the gelatin, where water cannot reach it.

Why Retained Chemistry Stains

There are two distinct permanence failures, and a tired bath causes both. The first is retained silver: monothiosulphate left in the emulsion is unstable and decomposes to silver sulphide, Ag2S, which discolours the image yellow to brown. This is the same Ag2S that sulphide toners create deliberately, only here it is uncontrolled and progressive. The second, as Reilly stresses, is retained thiosulphate: even leftover uncomplexed thiosulphate is itself unstable and decomposes to release elemental sulphur, which attacks the silver image. Fixing in a fresh bath addresses the first; thorough washing addresses the second. Neither alone is enough.

The Film-Clip Clearing Test, Worked

The test measures clearing time directly. Drop a scrap of undeveloped film leader of the same stock you are processing into the working bath and time the transition from milky to clear. Take a conventional film in fresh 1+4 Ilford Rapid Fixer at 20C clearing in 45 seconds. Set your minimum fixing time at twice that, 90 seconds, which sits comfortably inside Ilford’s 2 to 5 minute range for fresh fixer at 20C. Agitate as Ilford specifies: four inversions in the first 10 seconds, repeated in the first 10 seconds of each subsequent minute. Test the bath again as it ages; discard it once a leader of the same stock takes about 90 seconds to clear, twice the fresh time of 45 seconds.

Tabular-Grain Films Are Harder

Tabular-grain emulsions such as Kodak T-MAX 100 and 400 and Ilford Delta 100 and 400 carry a higher proportion of silver iodide. That iodide demands more fixer, fixes effectively only in an ammonium-thiosulphate rapid fixer rather than plain hypo, clears more slowly, and exhausts the bath faster. Kodak’s Technical Data F-32 for the T-MAX films calls for 5 to 10 minutes, or twice the clearing time, and tells you to check for clearing after 3 minutes in Kodak Rapid Fixer or 5 minutes in Kodak Fixer or Kodafix. A magenta or pink stain remaining after fixing is the warning sign: it means the fixer is near exhaustion or the film was under-fixed. Establish your reference clearing time with the actual stock in use, never a generic figure.

Capacity, in Numbers

A capacity claim with no figure is no claim at all. At 1+4, one litre of Ilford Rapid Fixer working solution handles 24 rolls of 135-36 film; a 5-litre concentrate bottle therefore covers around 600 such films. The print figures are lower and the permanence thresholds tighter. Ilford considers the bath spent for permanence when dissolved silver reaches about 2 g/L for fibre-base prints and 6 g/L for RC paper, but for maximum-stability prints the silver should not exceed 0.5 g/L, roughly ten 8x10in prints per litre. The same asymmetry shows in the molar exhaustion ratios for sodium-thiosulphate baths: about 1:17 silver-to-thiosulphate for film against 1:52 for prints. Film tolerates a heavier silver load, which is why print baths must be kept fresher than film baths.

The Two-Bath Method, Executable

Because the soluble dithiosulphate forms only where thiosulphate is fresh, splitting the work across two baths guarantees a fresh final stage. Mix two baths of equal volume. Fix for half your time in bath one and the remainder in bath two: on the worked example above, that is 45 seconds in each. Bath one does the bulk of the complexing and ages first. When bath one reaches its capacity, discard it, promote bath two to bath one, and mix a fresh second bath. Kodak’s practice runs roughly six to seven such cycles before both baths are remade. The second bath always sees lightly loaded film, so the intermediate monothiosulphate is reliably carried through to the washable complex.

Proving It Cleared

The clock confirms the bath is working; it does not confirm the negative is clean. For retained silver, use the Kodak ST-1 test: a stock of 2.0 g anhydrous sodium sulphide in 100 ml distilled water, used at 1+9. Place one drop on a clear film or paper margin, wait 2 to 3 minutes, then blot. Any yellowing beyond a faint cream tint, or a brown stain, signals retained silver, since the sulphide converts it straight to Ag2S. For retained thiosulphate, the Kodak HT-2 test (per 500 ml: 375 ml water, 62.5 ml 28% acetic acid, 3.75 g silver nitrate, water to 500 ml) gives a stain whose density read against the Kodak Hypo Estimator indicates how much hypo remains. ISO 18917:1999 specifies the methylene-blue and silver-sulphide methods for precise determination.

Washing Closes the Loop

Washing removes the unstable leftover thiosulphate before it can release sulphur. After Ilford Rapid Fixer, a film spiral can use the water-economy sequence: fill and invert 5 times, dump; refill and invert 10 times, dump; refill and invert 20 times, dump. Alternatively run 5 to 10 minutes of water within 5C of process temperature. Fibre-base paper is more retentive and needs about 60 minutes of running wash, or 5 minutes wash, 10 minutes in 1+4 Ilford Washaid, then 5 minutes wash. Fresh fixing and a proper wash are the two halves of a negative that still reads true years later.