Hardener sludge

Owned by fixer faults, washing faults and the stains they leave. Part XI owns the hardener’s chemistry and the window it has to live in.

What you see

In the bottle or the tray: a white, gelatinous precipitate that settles slowly, leaving the bath cloudy while it is stirred and clear above the sludge when it stands.

On the material: a white scum, most visible on a dried print or a dried negative held to the light at an angle, sitting on the surface rather than in the layer.

It must not be confused with the other milkiness an acid fixing bath produces. Kodak’s 1928 primer separates them by colour and settling behaviour: a pale yellow precipitate that settles very slowly is sulphur, from too much acid; a white gelatinous sludge is aluminium sulphite, from too little.

Likely causes

The primer names four, and they are all one condition arrived at four ways: not enough acid.

1. Too little acid put in when mixing. Its worked example is exact and is a mistake anyone could make: 28 per cent acetic acid used where the formula calls for glacial gives “less than one-third the required concentration of acid”.
2. Too little hardener in the bath, which changes the balance the other way.
3. Developer carried over, which neutralises the acid that is there. The primer singles out printing: “when fixing prints, a relatively large proportion of the developer is carried over to the fixing bath (unless a water or acid rinse bath has been used) which soon neutralizes the acid.”
4. Exhaustion. A worked bath “still contains alum and sulphite but no acid, and these combine to form a sludge of aluminium sulphite”.

For the scum on the material, the primer gives three: insufficient rinsing after development, too low an acid concentration, and insufficient agitation on first immersing in the bath.

What is happening: the chemistry and physics

An alum hardening fixer holds three things that only coexist within a window: thiosulfate, which does the fixing; sulphite, which protects the thiosulfate from the acid; and an aluminium salt, which hardens the gelatin. The acid is what keeps the aluminium and the sulphite apart.

Take the acid away — by mixing it short, by exhausting it, or by pouring alkaline developer into it — and the aluminium and the sulphite combine into aluminium sulphite, which is insoluble under those conditions and comes down as a white gelatinous solid.

⚗The trade the primer measured, and why there is no setting that avoids everything

Kodak’s own finding, in the primer’s words: “the hardening properties of an alum-acid fixing bath bear a relation to the tendency of the bath to precipitate aluminium sulphite. In other words, a bath containing an excess of acid (and which therefore may be used for a relatively long time before the aluminium sulphite precipitates), does not harden as well as a bath which precipitates when a much smaller volume of developer is added.”

So the hardening and the sludging life pull in opposite directions on the same variable. A bath set for good hardening sludges sooner; a bath set for a long sludging life hardens less. There is no setting that gives both, which is why the primer’s practical answer is a maintenance instruction rather than a formulation one: with a bath containing a minimum of acid, “it is advisable to add a further quantity of acid as soon as a slight precipitate appears; a satisfactory quantity being about one-half that originally present”.

Diagnostic questions

• Is the precipitate white and gelatinous, or pale yellow and slow-settling? White is aluminium sulphite and this entry; yellow is sulphur, which is a different fault, is caused by too much acid rather than too little, and can fade the image.
• Does the deposit sit on the surface of the material? Look at a grazing angle. Aluminium sulphite scum sits proud; a change in the emulsion does not.
• What acid was used in mixing, and at what strength? The primer’s worked example is a factor of three.
• Is there a rinse or a stop bath between developer and fixer? Without one, the acid is being neutralised every time a print goes in.
• How much has been through the bath? An exhausted bath sludges by definition.
• Was the material agitated on first immersion? One of the three named causes of scum on the material rather than sludge in the bath.
• Does the bath still harden? If it hardens well and sludges quickly, it is at the low-acid end of the trade above.

Corrective action

For the scum on the material, there is a published removal and it is the only one in this part of the atlas. Because aluminium sulphite is soluble in alkali, the 1928 primer instructs that the scum “may be removed by swabbing the film or print with a 10 % solution of sodium carbonate and then washing thoroughly”. Swab gently, on a support, and wash properly afterwards — a carbonate residue is its own problem.

For the bath: where a slight precipitate has just appeared and the bath is otherwise within its capacity, the primer’s answer is to add about half the original quantity of acid. Where the bath is exhausted, discard it; adding acid to a spent bath restores the clarity and not the fixing.

Prevention

The simplest prevention is not to use a hardening fixer at all, and it is the course’s default. ILFORD state that a fix hardener “is no longer generally recommended because modern camera films are sufficiently hardened at manufacture”, and their paper sheets recommend against one because it reduces washing efficiency. No alum, no aluminium sulphite. The case that still wants a hardener is a hand-coated emulsion that frills, which Part V owns.

Where a hardening bath is used deliberately: mix with the acid the formula names, at the strength it names; rinse or use a stop bath before the fixer, which is what removes the carry-over that neutralises it; agitate on first immersion; and keep a running total so the bath is discarded rather than nursed.

And note the historical alternative Kodak reached for: an alkali that does not evolve carbon dioxide on acidifying gives, in their own words, “less tendency to precipitate aluminium sulphite sludge from fixing baths containing alum”. Changing the developer’s alkali changes what arrives in the fixer.