Exhausted fixer

Owned by Fixer Faults, Washing Faults and the Stains They Leave, which defines exhaustion as product accumulation rather than reagent depletion and adds the consequence that matters most here: an overworked bath loads the material with the sparingly soluble complexes, so extending the time makes the washing problem worse while appearing to solve the fixing one. This entry was written before that page, from Break/Fix — the exhausted fixer and fixer capacity and exhaustion.

What you see

Often nothing at all in the bath. A used fixer may be slightly yellowed, or turbid, or entirely unremarkable, and appearance is not evidence.

What you see is downstream — under-fixed material, a magenta cast on some films, and, weeks or years later, a yellowing in the borders and highlights of prints.

The measurable symptom is the clearing time, and it is the only one that arrives before the damage.

Likely causes

1. More material through the bath than its published capacity allows.
2. Judging the bath by how it looks rather than by testing it.
3. Developer carried in without a rinse or a stop bath, which spends the acid and the sulfite reserve before the thiosulfate has done any work.

What is happening: the chemistry and physics

A fixer dissolves silver halide by complexing the silver, and the reaction consumes free thiosulfate and loads the bath with dissolved silver. Both effects push in the same direction — less free thiosulfate to carry the silver to the soluble second complex, and more dissolved silver already competing for it.

So a spent bath does not stop fixing. It fixes to the wrong endpoint, leaving the sparingly soluble step-one complex in the layer, which is invisible and which washing cannot remove. Kodak’s toning manual states the consequence directly — an exhausted bath contains insoluble silver compounds that will remain in prints and cannot be removed completely by washing.

!Why the log and the test are both needed

The clearing-time curve is flat for about two thirds of a bath’s life and then rises. So the running total is the early warning and the clearing time is the confirmation, and neither works alone — the log tells you where on the curve you are, and the test tells you when the curve has turned.

Diagnostic questions

• The clearing-time test. Drop a scrap of the film on the bath and time how long it takes to clear. ILFORD’s rule is to discard the bath when the used clearing time exceeds twice the fresh one. That threshold is published, unlike the borrowed one the course uses for developers, and it is the measurement this fault is defined by.
• What was the fresh clearing time? If you never measured it, the ratio cannot be computed and the test returns an opinion. Measure it on the day the bath is mixed and write it on the bottle.
• Kodak’s alternative threshold, from 1928, is absolute rather than relative — discard when the clearing time of a slow-fixing film exceeds 12 to 15 minutes. The two rules do not agree in general and the course records both rather than choosing.
• The capacity log, in films or 8 × 10 equivalents against the published rating.
• Was there a rinse or a stop bath before the fixer? An unrinsed film spends the bath faster and adds a second fault.

Corrective action

Discard it, and route it as silver-bearing waste rather than as general chemical waste — a spent fixer is where most of the silver in the darkroom ends up.

Refix anything processed in it that you would be sorry to lose, per the refixing procedure, while it is still recent.

There is nothing to be done for prints whose stain has already appeared, because it is in the paper rather than on it.

Prevention

Keep the log. Four columns — date, what went through, running total against the published capacity, and the clearing time when it was last measured.

Test on a schedule, not on suspicion. Measure the clearing time every quarter of the published capacity. By the time something looks wrong, a batch of mildly under-fixed material has already been made and put away.

Use two baths for anything you care about. Two-bath fixing exists precisely to ensure that nothing leaves the fixer while the invisible insoluble compound is still in it.

Rinse before the fixer. A stop bath is better, but even plain water removes most of the developer and most of the alkali that spends the fixer’s reserve.