Precipitate in a stock solution

Owned by chemistry in the bottle. The specific case of a concentrate that dropped out because it was stored too cold is stock crystallised in cold storage, owned by Parts II and III; this entry is the chemical cases.

What you see

Solid where there was none. Four appearances, and they mean four different things:

Appearance	What it is	What it means
White, appearing while you mix a developer, especially as the sulfite goes in	Metol, thrown out of solution	Usually nothing; it normally redissolves
White and gelatinous, in an alum fixing bath, settling on standing	Aluminium sulphite	The acid has gone — see hardener sludge
Pale yellow, settling very slowly, in an acid fixing bath	Sulphur	The bath is sulphurised and must be discarded
Fine, general, in a bath made with hard water	Calcium and magnesium salts	Water hardness, and a sequestering agent is the answer

And two appearances that are not faults at all: ILFORD state that when their powder developers are made up “it is normal for a few grains of powder to remain un-dissolved”, and that “discoloured or darker particles” in the white powders “will not affect the final development”.

Likely causes

1. Sulfite dissolved before the metol, in mixing a classic developer.
2. An alum fixing bath short of acid, whether mixed short, exhausted, or neutralised by carry-over.
3. An acid fixing bath with too much acid, too little sulphite, or run warm.
4. Hard water meeting carbonate or sulphite.
5. A bath that has been reused, which ILFORD name as a route to precipitates in its own right, along with suspended particles of emulsion from previous films.

What is happening: the chemistry and physics

The metol case is a salt effect, not an oxidation effect, and this is where the received wisdom is usually wrong. Kodak’s 1928 primer states the general rule that the preservative goes in first — and then states an exception for metol which explains itself: metol “is readily soluble in warm water (about 125 °F) (52 °C) and does not oxidize rapidly”, so oxidation is not the reason. The reason is that “Elon is a combination of an insoluble base with an acid which renders it soluble. When the acid portion is neutralized by a weak alkali such as sodium sulphite, the insoluble base is precipitated.”

The primer adds the asymmetry that makes the practice sensible — “when once the Elon is dissolved, it takes a fairly high concentration of sulphite to bring it out of solution again, though only a low concentration of sulphite is required to prevent the Elon from dissolving” — and, best of all, the reassurance: a precipitate formed this way “will usually redissolve on adding the carbonate and no harm has been done”.

The fixing-bath cases are a balance being lost. An acid hardening fixer holds thiosulfate, sulphite, aluminium and acid in a window. Lose the acid and the aluminium and sulphite combine; add too much acid, or run the bath above 29 °C, and the thiosulfate decomposes to sulphur.

The hard-water case is solubility. Calcium and magnesium form insoluble carbonates and sulphites, and a bath made with a hard supply contains both partners.

!One test tells you whether it matters: warm it and see

Stand the bottle in warm water — 20–25 °C is enough, and there is no reason to heat a photographic solution further — for an hour, and swirl it.

If the solid redissolves and the volume is unchanged, nothing has changed chemically and the bath is what you made. This is the ordinary outcome for a cold-storage crystallisation and for the metol case.

If it does not, something has reacted, and the composition is no longer the formula. Then the colour tells you which of the three chemical cases you have, from the table at the top of this entry — and only the metol case is recoverable.

And one case must not be warmed at all. A pale yellow sulphur precipitate means the bath has sulphurised; Kodak’s primer says the only remedy is to throw it away, and warming it accelerates the very decomposition that produced it.

Diagnostic questions

• What colour is the solid, and how fast does it settle? The table at the top of the entry.
• Which bath is it in? A developer and a fixer fail differently and the candidate list is short in each.
• Did it appear during mixing, or afterwards? During mixing points at the order; afterwards points at storage, exhaustion or contamination.
• Does it redissolve on warming to room temperature?
• How was the bath mixed — what order, at what temperature? Kodak’s Z-133 names improper solution mixing first among its nine causes of an out-of-control process.
• Is your water hard?
• Has the bath been reused, and how much has been through it?
• Is it just a few grains of undissolved powder? ILFORD say that is normal, and so are dark specks in the powder. See undissolved chemical.

Corrective action

Metol precipitated during mixing: carry on. Add the alkali as the formula directs and it will normally redissolve. If it does not, warm gently and stir.

Aluminium sulphite in a fixing bath: hardener sludge has the published answer — about half the original quantity of acid, where the bath is otherwise sound; discard where it is exhausted.

Sulphur in a fixing bath: discard. There is no other remedy, and the sulphur is apt to penetrate the gelatin and later fade the image.

Calcium salts: discard the bath, and make the next one with a sequestering agent or with softened water.

And in every case where you are not sure: a bath whose composition is in doubt is not a bath to put a film you care about through. Run a clip test or discard it.

Prevention

Dissolve in the order the formula gives, completely, one at a time, at the temperature it specifies — about 52 °C for metol per the 1928 primer, about 40 °C for ILFORD’s part A. Add the alkali only after the developing agent is fully dissolved.

Rinse or use a stop bath before the fixer, which is what keeps the acid in an alum bath.

Keep an acid fixing bath below 29 °C, and keep a running total so it is discarded rather than nursed.

Use a sequestering agent or softened water for mixing where the supply is hard — Kodak Limited sold Calgon for exactly this — and see calcium scum.

Mix only what you will use inside the keeping time, which is Kodak’s own instruction and prevents most of the storage cases outright.