Calcium scum

Owned by fixer faults, washing faults and the stains they leave. Its close relatives are drying marks and water spots; this entry is the case where the water supply rather than the drying pattern is the cause.

What you see

A whitish, faintly cloudy film over a dried negative or print, general rather than localised, most visible when the material is tilted against a strong light. Kodak’s own description in the Z-133 troubleshooting table is exact: “Scum — very fine dissolved material that dries and makes surface cloudy”.

On a negative it lowers contrast in printing by scattering light. On a print it dulls the surface, most obviously in the blacks.

Unlike a drying mark it has no shape and no direction: it is everywhere the water was, which is everywhere.

Likely causes

1. Hard water in the final rinse. The principal cause and the one the manufacturers address directly.
2. Too much wetting agent, which is itself dissolved solid and adds to the deposit.
3. Calcium meeting carbonate or sulphite carried over from the baths, forming an insoluble salt in the wash rather than merely being left behind by it.
4. A dirty wetting-agent solution, which Kodak name in the same table row as a distinct cause with its own corrective action.
5. Dirt from the dryer or in the solutions, which produces the same cloudy appearance by a different route and must be eliminated before the water is blamed.

What is happening: the chemistry and physics

Hard water carries dissolved calcium and magnesium — in the geological areas where limestone is common, a great deal of it.

When the last of the water on a film evaporates, everything dissolved in it is left behind. There is no mechanism by which it could leave: evaporation removes the solvent and not the solute. A soft supply leaves almost nothing visible; a hard one leaves a film of calcium salts across the whole surface.

Two things make it worse than a simple deposit. Carried-over carbonate or sulphite from the baths gives the calcium something to precipitate with, so the deposit forms in the wash rather than only at the end of the drying. And any wetting agent in the rinse is itself a dissolved solid: at the right dilution its benefit — sheeting rather than beading — far outweighs what it leaves, and at too high a dilution it does not.

!The manufacturers' answer, and the part of it that is counter-intuitive

The instruction has been the same for eighty years and appears in both Kodak’s modern processing sheets and Kodak Limited’s 1944 formulary.

Modern, in one line in the Z-133 and T-MAX processing tables: “to reduce drying scum, mix KODAK PHOTO-FLO Solution with distilled water in areas that have hard water.”

1944, with a refinement that almost nobody passes on: “For hard water use a smaller amount of wetting agent. Where the water is very hard and tends to deposit a scum on drying, give the final rinse in distilled water containing this addition of wetting agent.”

Less wetting agent, not more. The instinct when a film dries badly is to increase the dose; in hard water that adds to the very deposit you are trying to remove. ILFORD say the same thing from the other direction: too little or too much can lead to uneven drying, and the right dilution depends on local water quality.

And note the economics, which are what make this a cheap fault to fix: a final rinse is a few hundred millilitres. The wash is tens of litres and the mixing water is litres. Buy pure water for the rinse first; it is the smallest volume in the process and the only one where the water is left on the film.

For the solutions rather than the rinse, the historical answer is a sequestering agent: Kodak Limited listed Calgon “for addition to developing solutions to prevent the precipitation of calcium salts when using hard water”. A sequestrant binds the calcium and magnesium into a soluble complex so that they remain in the bath but are no longer available to precipitate.

Diagnostic questions

• Is the cloudiness general, or does it have a shape? General is the water; shaped is drying marks or water spots.
• Does it sit on the surface? Look at a grazing angle, and touch a corner that will be trimmed with a wet swab. A deposit softens and moves.
• Is your supply hard? Look at the kettle, and ask your water company — hardness is published for every supply area.
• Was the final rinse made with tap water?
• What dilution of wetting agent, and measured how?
• Is the wetting-agent solution fresh? Kodak name a dirty one as a cause in its own right, and its corrective action is to dump and replace routinely.
• Could it be the dryer, or the fixer? Kodak’s scum row lists dirt from the dryer and a sulfurised fixer alongside the water. A sulfurised fixer is a serious separate fault and shows as a pale yellow precipitate in the bath.

Corrective action

Rewash and re-rinse in distilled water with the wetting agent correctly diluted, then dry properly. A surface deposit of soluble salts will usually go: the layer swells again and releases what is on and in it.

Where it does not, the deposit has combined with something in the layer and the course has no source stating that it can be removed. Do not escalate to abrasives on an original.

For a print, a re-wash carries its own risk if the print was marginally fixed, so check the wash tests first if the session was in any doubt.

Prevention

Final rinse in distilled or deionised water, with the wetting agent in it. This is the whole answer for most people and it costs the price of a few litres of deionised water from a hardware shop.

Use less wetting agent, not more, in hard water — Kodak Limited’s own refinement.

Keep the wetting-agent solution clean and make it fresh.

Where the baths themselves are affected, a sequestering agent is the published answer, and Part III owns the chemistry.

Establish whether your water is actually the problem, once. Mix one batch of final rinse with tap water and one with distilled, dry two strips from the same film side by side, and look at them. If you cannot tell them apart, your supply is fine and you have settled the question for your address.