Undissolved chemical

Owned by chemistry in the bottle. Not to be confused with a precipitate, which is something that was in solution and came out; this entry is something that never went in.

What you see

Solid at the bottom of a made-up bath, or caked on the bottom of the mixing vessel, or a bath that is still faintly cloudy after stirring has stopped.

And the first thing to establish is whether it is a fault at all, because two of the commonest sightings are not.

=Two things ILFORD say are normal, and a great deal of good chemistry gets poured away over them

From their own instructions for the powder film developers:

“NB it is normal for a few grains of powder to remain un-dissolved.” The formulation includes constituents that are only sparingly soluble, and a small residue after full stirring is expected.

“Sometimes the white powders may contain some discoloured or darker particles. This is perfectly normal and will not affect the final development.” A dark speck in a white powder is not contamination.

A few grains is normal. A layer is not, and neither is a cloudy bath that does not clear.

Likely causes

1. Water too cold. Solubility falls with temperature, and several photographic solids are slow at 15 °C and easy at 40 °C.
2. The next chemical added too soon, so that the solution was already loaded when the second went in.
3. Powder tipped in as a mass, which cakes and seals its own surface against the water.
4. Insufficient stirring, or stirring stopped too early.
5. Made up to the final volume too soon, which is the worst version, because the bath is then both incomplete and at the wrong concentration.

What is happening: the chemistry and physics

A solid dissolves at its surface, so the rate depends on how much surface is exposed and on how fast the dissolved material is carried away from it. Tip a mass of powder into still water and the outside dissolves, the solution immediately around it saturates, and the inside is sealed in a shell of saturated liquid. Stirring is not a courtesy; it is what removes the saturated layer and lets the next of the solid meet fresh water.

Temperature does the same job from the other side. Both makers specify it: Kodak’s primer gives about 52 °C for metol, and ILFORD ask for part A in warm water “at about 40 °C/104 °F”.

Order matters for the same reason. A solution already carrying a great deal of one solid has less room for the next, which is why every classic formula gives an order and says to follow it. Kodak’s primer states the general rule — dissolve each chemical completely before adding the next — and gives a specific consequence of breaking it that has nothing to do with solubility: “if the alkali is added before the crystals of the developing agent are dissolved, each crystal becomes oxidized at the surface and the resulting solution will give fog.”

!Why the concentration is wrong as well, which is the part people miss

If a bath is made up to its final volume while solid is still undissolved, two things are wrong at once.

The formula is incomplete: some of what should be in solution is at the bottom.

And everything else is over-concentrated relative to it, because the volume is right but the dissolved content is short. The bath is not “a weak version of the formula”; it is a different formula.

Kodak’s Z-133 makes the general point in its troubleshooting table, listing a liquid concentrate developer “over-concentrated or under-concentrated” among the causes of an abnormally light film, with the corrective action of mixing in the proper ratio. The same applies to a powder that did not all go in.

ILFORD add the corresponding rule for their packs: always make up to the volume stated, and never prepare a smaller quantity by using fractional parts of each powder — because two bags of different constituents cannot be divided by eye.

Diagnostic questions

• How much solid, and where? A few grains after full stirring is normal; a layer or a cake is not.
• Was it warm water, at the stated temperature? 40 °C for ILFORD’s part A; about 52 °C for metol.
• Was each constituent fully in before the next went in?
• Was it made up to the final volume before stirring stopped?
• Was the whole pack used for the stated volume? ILFORD forbid fractional packs and the reason is arithmetic rather than caution.
• Has the powder been open a long time? ILFORD say an unopened packet in cool dry conditions keeps indefinitely and that a stock should be prepared immediately once the packet is opened — an opened powder takes up water and cakes.
• Was the water freshly drawn from the mains? ILFORD advise letting it stand a few minutes first, because mains water is highly aerated; that is about oxidation rather than dissolving, but it is the same step.

Corrective action

Keep stirring, warm the vessel gently, and give it time. Most cases are recoverable at the mixing stage and are not faults at all once the solid is in.

If the bath has already been made to volume with solid still in it, the honest position is that you do not know its concentration. For work that matters, discard it and mix again; for a test strip, use it and label it as suspect.

Never filter out undissolved developer solids and use the liquid, on the reasoning that what is left is clear. What you have filtered out is part of the formula, and Kodak Limited’s own warning about cold-stored stocks makes the general point: the precipitate often contains the most important constituents.

Prevention

Dissolve in the order given, one at a time, completely.

Use the stated temperature, and let a stock cool to the working temperature before use rather than processing in it warm.

Add powder gradually while stirring rather than tipping it in — which is ILFORD’s own instruction for part B: “continue to stir while gradually adding the contents of Part B”.

Make up to about three-quarters of the volume first, dissolve everything, and only then top up. Both makers’ instructions have this shape and it is the single habit that prevents the concentration error.

Use the whole pack for the stated volume.

And weigh and mix outside the darkroom, per Kodak’s primer, taking care not to raise dust from the powders — which is a handling instruction rather than a mixing one, and belongs to Part II.