Drying marks

Owned by the emulsion under stress. See also water spots, which is the discrete-drop version of the same physics.

What you see

Marks with a distinct edge on a dried film — often tear-shaped or elongated, running in the direction the water drained, sometimes with a paler centre and a darker rim. They are commonest on the lower part of a hanging film and around the perforations of 35 mm film, where water collects.

They print. That is the surprise, and it is the clue to what they are.

Likely causes

Kodak’s Z-133 lists five for Water marks — marks caused by excess water that causes differential drying, and each has its own corrective action:

1. Water collecting in the perforations — check that the wetting agent is diluted correctly.
2. Water running down from the clips — check that the wash and wetting-agent levels do not cover them.
3. Water splashed on the film — find and eliminate the source.
4. Inadequate drying — check that the drying temperature is adequate.
5. Film drying too quickly — reduce the drying temperature.

Note that causes 4 and 5 point in opposite directions. Both too slow and too fast produce marks, and the window is real.

And hard water makes all five worse, because there is more dissolved solid to leave behind.

What is happening: the chemistry and physics

They are not dirt sitting on the gelatin. That is the single most important sentence in this entry, because it explains why they will not simply wipe off and why they print.

As a film dries, water evaporates from a shrinking volume. Whatever is dissolved in that water — dissolved salts from the supply, residues from the baths, the wetting agent itself — becomes progressively more concentrated in what is left, and at the very end it is deposited into and onto the swollen gelatin at that spot. The gelatin there also dries under different conditions from the rest, so it shrinks differently.

The result is a local change in the layer: its density, its thickness and its refractive index. A change in the layer is in the light path, so it prints.

⚗What a wetting agent actually does, and why more is not better

A wetting agent lowers the surface tension of the rinse water so that the water drains as a continuous sheet rather than breaking into drops. No drops means no local concentration and no marks. Kodak Limited’s 1944 note gives the whole function in one line: it ensures uniform wetting when the material goes in and uniform draining when it comes out, which “prevents drying marks or ‘tear marks’” and greatly shortens the drying time.

ILFORD’s starting dilution is 5 mL per litre (1+200), with two cautions that matter more than the number: the dilution depends on your water, your equipment and your drying method — and “either too little or too much wetting agent can lead to uneven drying”.

Too much is a real fault. The wetting agent is itself a dissolved solid, and a strong solution leaves more behind than a weak one, so an over-dose replaces a water mark with a wetting-agent mark. Kodak Limited make the point from the other side: for hard water, use a smaller amount.

Diagnostic questions

• Do the marks follow the direction the film drained? Then it is drainage, and the clips and the hanging are the suspects.
• Are they concentrated around the perforations? Kodak’s first named cause, and it points at the wetting agent’s dilution.
• Are they at the top of a hanging film, under the clip? Water running down from a clip that was submerged.
• What dilution of wetting agent, measured how? ILFORD ask that it be measured and dispensed accurately, and a capful is not a measurement — though they helpfully record that their own 1 litre bottle cap holds about 20 mL brim full, which at 1+200 is enough for 4 litres.
• How hard is your water? If it is hard, the answer is distilled water for the final rinse, not more wetting agent.
• How was it dried, and at what temperature? ILFORD’s window is 30–40 °C in a cabinet, or room temperature in a clean still place. Both ends of Kodak’s list are temperature faults.
• Will a rewash remove it? See below — this is a test as well as a treatment.

Corrective action

Try rewashing once, and know what you are testing.

A deposit on the surface — soluble salts, dried wetting agent — can sometimes be removed by rewashing, re-rinsing in correctly diluted wetting agent and drying properly the second time: the layer swells again and releases what is water-soluble.

A change in the gelatin will not come back, because nothing has been added that can be taken away.

No manufacturer sheet or conservation guidance in this course’s corpus states which case a given mark is, and the course will not guess. So the practical rule is: rewash once, on the understanding that it may do nothing, and do not escalate to solvents or abrasives on an original. A failed rewash has cost you nothing; a solvent can cost the negative.

Prevention

Use a wetting agent in the final rinse, at a measured dilution. ILFORD’s 1+200 is the place to start and the sheet says plainly that it may need adjusting for your water and your drying method.

Use distilled or deionised water for the final rinse if your supply is hard. Both Kodak’s modern processing sheets and Kodak Limited’s 1944 formulary give the same instruction, eighty years apart. It is a few hundred millilitres, so it is the cheapest place in the whole process to spend on pure water.

Remove the excess before hanging. ILFORD ask for the excess rinse solution to be removed, by a clean squeegee or a clean chamois — with the warning that any grit caught in it will scratch the whole film. That is the trade, and clean is what makes it safe.

Keep the clips out of the solutions, per Kodak’s second corrective action, so that nothing runs down from them.

Dry in the window: 30–40 °C in a cabinet or room temperature in a still, clean, dust-free place. Faster is not better and Kodak names both ends.