Développement N&B · 2026-09-14

Stop Bath and Fixer: Don't Sabotage Your Film at the Finish Line

The developer gets all the attention. You time it to the second, dial in the temperature to within a fraction of a degree, debate dilutions at length. Then the stop bath arrives, dispatched in thirty seconds with whatever happens to be on hand, followed by a fixer you can't quite remember how many films ago you mixed up. That's where a lot of otherwise solid development runs come apart: milky negatives, residual silver halides, archival permanence out the window.

This guide is about those two neglected steps. Not to make them complicated, but to actually understand what they do, and to avoid the mistakes that cost you an entire roll.

What the stop bath does, and why it matters

Developer is an alkaline solution. When you pour it out of the tank, a residual amount stays trapped in the emulsion and keeps working. The stop bath neutralizes that residue quickly and cleanly. Without it, development continues for an unpredictable amount of time, especially with high-activity developers like concentrated Rodinal or XTOL.

But that's not the whole story. By knocking out the alkalinity carried over from the developer, the stop bath also protects the fixer that follows. A slightly acidic fixer will drift toward neutral or even alkaline pH if developer keeps accumulating in it, roll after roll. The result: it exhausts much faster. A proper stop bath is, in practical terms, also a way to get more mileage out of your fixer.

The four core steps of black-and-white film processing, and the protective relationship between stop bath and fixer.

Plain water: good enough or not?

You'll see it said everywhere that running water can stand in for a chemical stop bath, and there's truth to that. Two or three water changes over about a minute will dilute the developer enough that the fixer won't be under serious strain. For occasional use, with healthy film and a fixer you refresh regularly, it works fine.

Where water falls short is that it dilutes the alkaline residue rather than neutralizing it chemically. If you're working with a strongly alkaline developer, whether that's a carbonate-based or sodium hydroxide formula or a homebrew mix, dilution alone may not stop development as crisply as you'd want. With fine-grain films and short development times, even a few extra seconds of overdevelopment can show up in the highlights. In practice, for the majority of standard developers at normal dilutions, a water rinse is acceptable. It's just not the most rigorous approach.

Acetic acid or citric acid?

The classic stop bath is an acetic acid solution, diluted to around 2% in water. It neutralizes developer reliably, can be prepared from glacial acetic acid or commercial concentrates, and costs almost nothing.

The downside is the smell. Even diluted, acetic acid has a sharp vinegar odor that gets unpleasant in a confined, poorly ventilated space over a long session.

Citric acid is a widely used alternative, found in products like Berstop, which are noticeably less pungent and less irritating than acetic acid formulas. Both dilute at 1+19. You can also buy citric acid powder from grocery stores or cleaning supply shops for very little money. It works just as well as acetic acid for neutralizing developer and protecting the fixer. The one practical advantage Berstop has over plain citric acid powder is its color indicator, which tells you visually when the bath is exhausted.

Fixer: hypo, thiosulfate, a bit of chemistry worth knowing

The term "hypo" is still commonly used in the analog world, and it refers to sodium thiosulfate (Na₂S₂O₃). In film processing, the fixer is the chemical solution that completes development by dissolving the unexposed silver halides left in the emulsion, rendering the image stable and insensitive to light. Sodium thiosulfate is the classic fixing agent, found in what are generally called standard fixers.

Ammonium thiosulfate is the basis of rapid fixers, and it acts considerably faster. Where a standard fixer might need 8 to 10 minutes for film, a rapid fixer like Ilford Rapid Fixer, used at 1+4, will fix most films in 2 to 4 minutes. That difference is meaningful when you're running multiple rolls in a session.

Standard or rapid fixer: which to use?

For home use with a handful of rolls per session, either works well. A standard sodium thiosulfate fixer is cheaper, often available in powder form (Fomafix, for instance, handles around 15 rolls per liter at a 10-minute fixing time), and very stable on the shelf. The longer fixing time rarely matters if you're not running a production line.

Rapid fixer earns its keep when you're developing several rolls at a stretch, or working with thick-emulsion films or pushed stock that needs extended times in a standard fixer. Ilford Rapid Fixer at 1+4 handles around 24 rolls of 135-36 per liter of working solution. At 1+9, it's better suited to paper.

Fixer type	Fixing agent	Film fixing time	Capacity (per litre)
Standard (e.g. Fomafix)	Sodium thiosulfate	8–10 min	~15 rolls 135-36
Rapid (e.g. Ilford Rapid Fixer 1+4)	Ammonium thiosulfate	2–4 min	~24 rolls 135-36

The classic fixer mistakes

Exhausted fixer: invisible and treacherous

This is by far the most common error. Exhausted fixer doesn't change color, doesn't smell different, gives you no warning at all. Inadequate fixing leaves the negative milky and unstable. Films that come out of a spent fixer may look acceptable under the bathroom light, then begin to fog gradually over the following weeks as the residual halides break down.

You can spot a poorly fixed film by a faint milky or yellowish cast, most visible in the clear areas of the negative. At an earlier stage the film may look fine, but the residual halides are still there in quantities too small to see immediately.

Under-fixing from too-short times

The generally accepted rule: your fixing time should be at least twice the clearing time, meaning the time it takes for the film to go completely clear in the fixer. If your film clears in 3 minutes, fix for at least 6. With a fresh fixer and a typical film stock, 4 to 6 minutes for a rapid fixer and 8 to 10 for a standard fixer is a reasonable baseline.

The problem comes when you've been using the same batch for a while. Clearing time creeps up without you noticing, and if you keep fixing for the same duration out of habit, your safety margin quietly disappears.

Qualitative illustration: clearing time increases as the fixer ages. When it crosses the 2× fix-time threshold, your safety margin is gone. Values are indicative — always test your actual fixer.

Over-fixing: rarely catastrophic, but not harmless

Leaving film in the fixer too long won't destroy it in most cases with modern non-hardening fixers. But a very concentrated fixer can, over extended times, begin to attack the silver image itself, particularly in dense shadow areas. This is marginal at reasonable durations (15 to 20 minutes), but it's real. Don't leave your film soaking for an hour "just to be safe."

Testing your chemistry: simple and non-negotiable

The clearing test for fixer

This is the easiest one. Take a small piece of unexposed film leader (the bit before frame 1 on a roll) and drop it into your fixer. Time how long it takes to go completely clear. With a fresh rapid fixer, that should happen in well under a minute. If it's taking more than 4 to 5 minutes with a rapid fixer, or 7 to 8 with a standard one, it's time to mix a fresh batch. Whatever your clearing time is, double it for your actual fixing time.

The iodine test

This is a more precise way to assess a fixer's remaining capacity. You prepare a very dilute iodine solution, place a drop on a piece of unexposed film leader, then immediately immerse it in the fixer you're testing. If the iodine stain disappears quickly and completely, the fixer still has capacity. If the stain lingers or clears slowly, the fixer is on its way out. It's less common in home darkrooms than the clearing test, but more sensitive for catching a fixer that's nearly spent before it actually fails.

Keep a log

The simplest method is just to count your rolls. Stick a piece of tape on the bottle and mark off each film processed. Follow the manufacturer's stated capacity, with a small safety margin. Ilford's technical data sheets list 24 films per liter of working solution as the capacity for Rapid Fixer. Stopping at 20 and refreshing is a reasonable precaution.

Washing: the step that protects your negatives for decades

Residual fixer in the emulsion is the main enemy of long-term archival stability. Thiosulfate left behind reacts slowly with the silver image and causes characteristic brown staining, sometimes years later. Good washing is inseparable from good fixing.

The Ilford washing method using 5 changes of water (fill, 5 inversions; 10 inversions; 20 inversions, emptying completely between each stage) is effective and uses very little water. For films meant to last generations, a hypo-clearing agent speeds up the process by chemically displacing the thiosulfate from the emulsion before the final wash.

Going further

Citric acid powder as a homemade stop bath: available from grocery stores or pharmacies at minimal cost, easy to measure, virtually odorless. A concentration of 10 to 15 g/l is a good starting point, adjusted slightly depending on your developer.

Ilford Rapid Fixer at 1+4 for film: a reliable fixer with clear technical documentation and well-established capacity figures. The 1+9 dilution for paper is listed on the same bottle, making it a practical all-in-one option if you're also doing darkroom printing.

Manufacturer data sheets: Ilford, Kodak, and Tetenal all publish free, up-to-date PDF technical data sheets for their chemicals, covering times, dilutions, and capacities. These are a far more reliable reference than figures pulled from undated forum posts.