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Fixing the fixer problem.

Tim Gilbert chemistry


Let’s start by acknowledging that Fixers are seriously underappreciated. If not for the ability to “fix” our negatives and prints, “photography” would have remained nothing more than a laboratory curiosity.

The term “fix” is a bit odd. We’re not really fixing anything, we’re actually removing the unexposed, undeveloped silver ions from the emulsion. Which, if not removed, would eventually “develop” on their own into opaque solid silver and ruin the image.

So what's the problem that needs "fixing"?  Typically, fix either came in a slow acting but dusty powder or an awkward to ship, but fast acting, liquid. We thought we could do better.

In spite of its importance, there are really only a half dozen different fixer formulas in use today. In fact, you can break them down into two major categories¹: sodium thiosulfate vs ammonium thiosulfate. Actually, it’s the thiosulfate that does all work. (However, this is where a little magic involved, since the sodium or ammonium ions influence the behavior of the thiosulfate in ways that are beyond the scope of this document. Indeed, maybe even beyond current scientific theory.)

Once dissolved in water, the positively-charged sodium or ammonium ions will dissociate from the negatively-charged thiosulfate ions. Thiosulfate carries two negative charges and since positive and negative charges are attracted to one another, it reacts with the unexposed, undeveloped positively-charged silver ions in the emulsion. The result is a silver-thiosulfate complex that dissolves easily in water and is carried into the solution. Unfortunately, if left in the emulsion, the thiosulfate ions would also ruin the negative, which is why washing it out is so important.

Sodium based fixers were the mainstay of photography almost since the beginning. They traditionally come as a powder: easy to manufacture; easy to ship. However, many users find the dust generated by cutting open the bag and dumping it into a beaker to be annoying.

The effect of ammonium ions on the fixing process has been known about for almost a hundred years. They work twice as fast as sodium based fixers (hence the term rapid fixers), but weren’t commercially viable until a stable, inexpensive source Ammonium Thiosulfate became available. It is most commonly produced as a liquid, which is why rapid fixers come as a liquid concentrate. While liquids have their advantages, they’re expensive to ship. And kind of silly, since what you’re shipping is mostly water. Liquids are also a pain to package. Not only are empty bottles surprisingly expensive, shipping them (from our suppliers to us) is just about as expensive as shipping full bottles (we get hit with “dimensional weight” charges). You then have to deal with heat-sealing the caps and bagging everything.

So we created our wish list:

  •  Powder based so that we could package in the same pouch as our SP-Ultra 4LF. Just add water, this would help minimize the dust issue.
  • It also needed to use ammonium, since many experts believe that ammonium based fixers perform better with modern, high iodine films. And they fix faster and wash out faster.
  • A neutral to alkaline pH, which eliminates the need for an acid stop bath.

During our research we came across a hundred year old patent that describes a sodium thiosulfate fixer that contains a source of ammonium ions. Thus it has both sodium and ammonium ions present in solution. Our SP-FastFix, is a repackaged, updated fixer based on that patent. (Let’s face it, there really isn’t anything new in analog photography.) It offers numerous advantages:

  1. First, it’s powdered based. Just add 450 ml of water at 95°F/35°C, agitate gently and wait for the powder to dissolve.
  2. Fixing time stays more consistent than other fixers (see the Clearing Time Chart below.)
  3. Capacity is about 20% greater than rapid fixers (about 60 sheets of 4x5).
  4. It has an alkaline pH so an acid stop bath is not required (a plain water rinse is recommended.)
  5. Sp-FastFix works faster than standard sodium-only fixers but not as fast as a true rapid fixer. Its performance is about half way in between, which frankly, is fast enough for most purposes.
  6. Shelf life (unmixed) is at least two years; mixed is probably at least a year.
  7. It does not contain a hardening agent since most modern films don’t require one.

Fixing Time: to determine your fixing time, soak a small piece of exposed but undeveloped film in water for twenty to thirty seconds (wet film behaves differently than dry film) and then drop it into a beaker of fixer. It should turn clear in 25-5 seconds. The rule of thumb is to fix for four times (4x) the clearing time. If it takes longer than 50 seconds, the fixer should be replaced.

Fixer Capacity: three factors influence just how much film you can fix in a given amount of fixer:

  1. Type of film: some film has more silver halide than others and will not only take longer to fix but will exhaust the fixer sooner.
  2. Exposure: underexposed film will obviously have more silver halide left over after development and will exhaust the fixer sooner.
  3. Development time: just like and underexposed negative, an underdeveloped negative will also cause the fixer to work harder and die sooner.

Frankly, in the real world, when working with real (that is typical) negatives, we can ignore the second two factors. SP-FastFix is good for about 60 sheets of 4x5 film per 500 ml. Of course, you should test your fixer regularly.

Fixer Removal: be sure to wash your film for at least five minutes after fixing. We recommend the rinse/agitate/drain/repeat technique (

Here's the link: SP-FastFix

Your comments/questions are always welcome.

1: Yes, we’re oversimplifying the subject and standard disclaimers apply. This is intended as short introduction to the topic and not as a compressive dissertation on the fixing process. Keep that in mind before sending us a three page email pointing out everything we omitted.

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