The chemistry that makes a photograph last
Silver halides are the light-sensitive materials in a photographic emulsion — but after exposure and development, only some of them have done anything useful. The ones that were struck by light have been reduced to metallic silver and form the image. The rest remain unchanged in the emulsion: unstable, still sensitive, and capable of darkening over time until they destroy the picture entirely. Fixing is the step that removes them.
The agent is thiosulphate — sodium thiosulphate in older formulations, ammonium thiosulphate in most modern rapid fixers — and it works by forming soluble silver–thiosulphate complexes with the residual halide. Those complexes are then washed out of the emulsion. What the fixer cannot do is remove silver that has already been developed into the image, which is why it must be used at the right strength for the right duration: the fixer dissolves unexposed halide, not the developed metallic silver.
The sequence matters. A print or negative emerging from the developer goes first into a stop bath — typically a dilute acetic acid — which halts development instantly, preserves the fixer's working pH, and extends its useful life. Then it enters the fixer. Both time and exhaustion govern the outcome.
Underfixing leaves residual halide in the emulsion; it may not be visible immediately, but it will yellow and stain within months or years. Overfixing, in a depleted bath, produces its own problem: the silver–thiosulphate complexes can begin to break down and re-deposit silver in a form that is difficult to wash out and appears as a reddish-brown discolouration known as thiosulphate staining.
Testing is straightforward. The standard clip test involves cutting a small strip from the unexposed leader of a film, dropping it into the working fixer, and timing how long it takes to clear — to turn from milky white to transparent. That is the clearing time; fixing for twice that duration is a reliable minimum. For prints the same principle applies, and the test strip method remains the most direct check a printer can make on a bath that has been working for hours.
The thickness of the substrate changes the fixing time. Film base clears faster than fibre-based photographic paper, whose double-weight baryta layer — a barium sulphate coating beneath the emulsion — absorbs and holds both fixer and silver complexes tenaciously. Resin-coated papers, which carry the emulsion on a plastic-sealed base, fix and wash far more quickly precisely because the chemistry cannot penetrate the substrate itself.
Underfixing leaves residual halide in the emulsion; it may not be visible immediately, but it will yellow and stain within months or years.
For fibre-based paper, two-bath fixing is standard practice: a working bath that removes most of the residual halide, and a fresh second bath that finishes the job with a much lower silver loading. The baths are rotated when the first becomes exhausted, keeping the total silver level in the emulsion-contact solution low enough to wash out reliably.
What happens when fixer is not washed out completely is the subject of its own discipline. The residual thiosulphate left in a poorly washed print continues to react slowly with the image silver, producing yellowish silver sulphide — the familiar warm-brown discolouration found in old prints.
The washing and archival life of any photograph is therefore directly downstream of how well it was fixed: a badly fixed print cannot be saved by prolonged washing, and one that was never fixed at all — a daguerreotype, sealed behind glass, which has no paper fibre or binder layer to hold residual chemistry — survives precisely because its image sits on the surface rather than embedded in a permeable layer.
Temperature affects the rate at which thiosulphate acts, but most practitioners work at the same ambient temperature used throughout the darkroom process rather than heating fixer independently. Consistency between baths matters more than optimising any one variable, and a good fixing protocol is simply a disciplined one: known strength, known capacity, timed immersion, and a log of how many sheets or frames have passed through.
The chemistry is not subtle. Do it long enough, in a fresh enough bath, and the picture lasts. Cut it short, and time finishes what the fixer started.