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 over years. The clearing-time test exists because that decline is invisible to the eye but measurable with a clock.

How a Thiosulfate Bath Wears Out

Fixing dissolves the undeveloped silver halide that remains after development. Thiosulfate ions react with silver ions to form a series of soluble silver-thiosulfate complexes, which then diffuse out of the emulsion and into the bath. Because each frame consumes thiosulfate and loads the solution with dissolved silver, the bath becomes progressively slower and its capacity finite.

The critical point concerns the chemistry of those complexes. As James M. Reilly’s account of fixation chemistry in The Albumen and Salted Paper Book explains, thiosulfate 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.” Fixation proceeds through several silver-thiosulfate complexes, and one intermediate is soluble only in fresh thiosulfate.

Why Retained Complexes Stain

When a depleted bath cannot fully convert silver into the final soluble complex, the intermediate argentothiosulfate compounds remain lodged in the gelatin. Washing does not remove them, because they dissolve only in fresh fixer, not in water. Left in the emulsion, these compounds are unstable: they decompose to release elemental sulfur and silver sulfide, which discolour the image to yellow and brown. This is the mechanism behind the two-bath method, in which a fresh second bath dissolves the complexes the first bath could only partially form.

The Film-Clip Clearing Test

The test measures clearing time directly. A scrap of undeveloped film leader is dropped into the working bath; the time for it to pass from milky to clear is the clearing time. With fresh fixer at film strength this is typically on the order of 30 seconds to a minute, depending on the emulsion. The established rule is to fix for at least twice the clearing time, and to discard the bath once the clearing time of used fixer reaches twice that of a fresh batch. Modern tabular-grain films such as T-grain emulsions clear more slowly and benefit from the longer end of fixing-time recommendations, so a reference clearing time should be established with the same film stock in use.