Formulating Fixers

by Derek Watkins

A tip from a WWW reader on preserving developers

Most photographers know about the various types of developer and their characteristics - fine grain developer, high acutance, or compensating. But many don't think twice about the fixer they use or realise that there are different types of fixer, too, each of which has characteristics that make it more suitable for some applications than others.

What exactly does a fixer do, though? To answer that question we need to go back to the beginning of the processing cycle and look first at development.

When you develop a black and white film or print, the silver halides in the emulsion that were affected by light during exposure are reduced to black metallic silver. But the silver halides that werenÕt affected by light - the very deep shadow areas in a negative or the very bright highlights in a print - are still active when you've finished the development stage. If you turn on the white light in your darkroom, these areas will quickly fog to complete black.

To prevent this happening, you must remove the unexposed and unreduced silver halides from the emulsion. This is a two-stage task performed by the fixer and the final wash. The fixer converts the silver halides into a form thatÕs soluble in water, and the wash flushes the last remains out.

A basic fixer

The best known of all fixers is sodium thiosulphate, or hypo. You can make a perfectly adequate fixer by dissolving 200 grams of hypo crystals in a litre of water. It's preferable, though, to acidify the fixer slightly, especially if you don't use a stop bath between developing and fixing. If you don't the developer can continue working while the film or print is in the fixer and this can lead to staining. But if you add a weak acid to the fixer it will immediately neutralise the developer, which is alkaline, in the same way as an acid stop bath.

Unfortunately, if you add even a very weak acid like acetic acid to a fixer it will start to decompose and deposit sulphur. So to prevent this you must also add sodium sulphite.

A much simpler solution is to use a chemical which acts as both an acidifying agent and an anti-decomposition salt. Two such chemicals are potassium metabisulphite and sodium bisulphite.

You can make up simple acid fixer, then, like this: 

Sodium thiosulphate crystals                200 grams
Potassium metabisulphite
(or sodium bisulphite)                      20 grams
Water to                                    1000 ml
Hypo fixer has a 5 to ten minute fixing time for films and FB papers, 3 to 5 minutes for RC prints.

Hardening fixers

Because negatives and, to a lesser extent, prints are rather delicate, many photographers prefer to fix their films in a solution that also contains a hardening agent such as potassium alum. A typical fixer which contains both acidifying and hardening agents is: 
Potassium thiosulphate crystals             250 grams		
Sodium sulphite, anhydrous                  12 grams
Acetic acid, glacial                        15 ml
Boric acid                                  8 grams
Potassium alum                              15 grams
Water to                                    1000 ml
Personally, I don't like hardening fixers. They're much more difficult to wash out of the emulsion than simple acid fixers. Not only does this use more water, but you can never be quite sure you've washed all the fixer out.

You only find out when it's too late and your precious negatives have started to deteriorate. I prefer to use extra care when handling my negatives.

When you use either an acid fixer or an acid hardening fixer it's perfectly safe to switch on the white light as soon as the fixer has begun to work on the film or paper. This is because the acid in the fixer neutralises the developer almost immediately.

Leave the film or prints in the fixer for at least five minutes before washing, because the compound formed by the initial reaction between the silver halides and the fixer is virtually insoluble in water. But as the reaction continues, the compound changes and becomes soluble. On the other hand, though, don't leave the film or prints too long or the fixer could begin to bleach the delicate light areas of the image. Ten minutes is about right as long as the fixer is reasonably fresh.

Fixer capacity

The more films or prints you pass through a fixer, the more exhausted it naturally becomes until eventually it becomes completely inactive. This exhaustion is caused by the reaction between the hypo and silver salts in the material forming a complex compound of sodium silver thiosulphate.

The increasing amount of this compound causes the fixer to take longer and longer until soluble compounds are formed. At this stage staining and instability of the emulsion become not so much possible as likely.

The average life of a sodium thiosulphate fixer, whether acid or hardening, neither or both, is given in the following table. It's based on the number of films or sheets of paper you can safely fix in a litre of working solution. 

Films          Size                     Rolls or sheets 
                                        per litre 
35mm            20 exposure             60 
35mm            36 exposure 	        30 
120 roll                                30 
4 x 5" sheet                            120 

Papers
                4 x 6"                  75 
                7 x 5"                  50 
                6.5 x 8.5" (wholeplate) 40 
                10 x 8"/A4              30 
                12 x 15/12 x 16"        18 
                16 x 20"                15
It's quite easy to see when a fixer isn't fixing a film properly because the milky appearance of the emulsion doesnÕt clear, or clears very slowly. You can also compensate for exhaustion and temperature changes, as it only necessary to allow twice the clearing time as the total fixing time. When it reaches the point that the clearing time is the same as your original complete fixing time, it's time to renew the fixer.

But it's more difficult to know when a print fixer is approaching exhaustion. One way of making absolutely sure that your prints are properly fixed is to use the two-bath technique.

Two bath fixing

Instead of using just one fixer, the two-bath technique uses two, both dishes containing fixer of the same strength. You immerse the prints in the first dish for roughly half the fixing time and then transfer them to the second.

In this way the first fixing bath does all the hard work of converting the unexposed silver salts into soluble compounds and the second bath removes any remaining traces of silver to complete the job.

When you've fixed the number of prints indicated in the table, discard the first fixer, make the second fixer the first, and mix up a fresh second fixer. This way you'll get the maximum life out of the solutions and make sure your prints are properly fixed.

Rapid fixers

An alternative type of fixer uses ammonium thiosulphate as a fixing agent instead of sodium thiosulphate. The main advantage of this is to speed up the fixing process, because these fixers act in roughly a quarter of the time taken by a hypo fixer - films may be fully fixed in 2-3 minutes, fibre based papers in the same time, and RC papers in under a minute.

Further advantages are that the ammonium silver thiosulphate formed by the chemical reaction is much more soluble than sodium silver thiosulphate, and shorter immersion of fibre-based papers means the by-products have less chance of permeating the paper base itself. This means that you can reduce washing times considerably without the risk of staining later.

A typical rapid fixer using ammonium thiosulphate is: 

Ammonium thiosulphate                       200 grams
Sodium sulphite, anhydrous                  15 grams
Acetic acid, 28 per cent                    50 ml
Boric acid                                  8 grams
Water to                                    1000 ml
If you want to make it into a hardening fixer, simply add 25 grams of potassium alum.

Testing fixer

There's only one way to be quite certain that your fixer is still active and that's by testing it. There are two tests that you need to carry out. The first is for silver content and the second for acidity.

You can check the silver content by dropping a single spot of 10 per cent sodium sulphide solution on to a print which has been in the fixer for its full fixing time. If the spot turns brown the fixer has too high a silver content to be of any further use.

Litmus paper is the easiest way to test for acidity. If you dip a piece of blue litmus paper in the fixer it should immediately turn bright red. If it doesn't, the acidity of the fixer has been neutralised by alkali carried over from the developer.

If the silver content is sufficiently low to indicate that the fixer is still active but the acid content has been neutralised, you can restore the acidity of the fixer by adding 20 grams per litre of potassium metabisulphite.

The author's e-mail: [email protected]

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