Base-side scratches

Owned by the emulsion under stress. Read it with emulsion-side scratches: the two are one diagnosis with two answers.

What you see

A line that looks dark on a light box and prints white. Usually straight, usually running the length of the film, and frequently more than one at a constant spacing — the pattern that gets called tramlines.

Under a loupe it is sharp when the loupe is focused on the base surface and soft when focused on the emulsion. Under raking light on the base side it is a groove.

Likely causes

1. Grit in the cassette’s felt light trap. A single particle there scratches the base at every wind and every rewind, which is why the marks run the whole length and why there are often several.
2. The camera’s pressure plate or film path — worn, or carrying a particle.
3. Anything the film ran past in a processor: Kodak’s table names dirty rollers, misaligned rollers, worn bearings and mechanical problems.
4. Handling the dry film on a gritty bench or in a gritty sleeve.

What is happening: the chemistry and physics

No image material has been removed. The base carries no silver — it is clear acetate or polyester, 0.125 mm thick on ILFORD’s 35 mm film and 0.180 mm on their sheet film — so a groove in it takes nothing from the picture.

What the groove does instead is bend light. Its sloping walls act as a small prism or lens, deflecting the directional beam of a light box or a condenser enlarger away from the viewing axis. Less light reaches your eye along that line, so it reads dark by transmission; and less light reaches the paper under it, so it prints light.

!The refractive-index test, which is also the repair

Put a little clean water on a swab and wet the base side over the mark, then look at it wet on the light box.

A base-side scratch all but disappears. Filling the groove with a liquid of similar refractive index removes the air–plastic interfaces that were doing the bending, so the light goes straight through and the line stops being visible.

An emulsion-side scratch does not change, because it is a loss of silver rather than a distortion of the light path, and no fluid replaces silver.

That is the discriminating test, and it doubles as the principle behind every practical remedy: the trade’s answers — wet mounting, a scratch-filling fluid, or simply printing on a diffusion enlarger whose illumination is scattered rather than directional — all work by denying the groove a beam to bend.

And the diagnosis this fixes in the chain: the base is never soft, so a base-side scratch tells you nothing about the wet side. It points at the cassette, the camera, the film path or the handling — that is, almost always at something the film ran past repeatedly.

Diagnostic questions

• Which side, by the loupe or the wetting test? First, always.
• How many lines, and are they evenly spaced? Several parallel lines at fixed spacing is a felt trap or a roller with particles in it.
• Does the line run the entire length of the roll, including the leader and the unexposed end? Then it was made by something the whole film passed, which points at the cassette rather than at the camera back.
• Is it on every roll from the same cassette or the same camera? The confirming test: run a short roll through the suspect camera with a fresh cassette, and another through a different camera.
• Is the mark on the leader only? Then it was made during loading or rewinding rather than during transport.
• Did it appear after the film was dry? Look at the sleeve and the bench.

Corrective action

Print it wet, or print it diffuse. Both work on the same principle as the wetting test above, and both are legitimate and reversible.

Clean the base carefully — clean hands, a clean soft cloth, nothing abrasive, and nothing that will attack the base. This removes dirt, not the groove.

Do not attempt to polish out a scratch, and do not put anything permanent on the negative. The Library of Congress lists marking originals and repairing them with pressure-sensitive tape among examples of negligence; the same principle applies to a working negative you may want again.

Prevention

Suspect the cassette first. A single grain in the felt light trap will scratch every film you put through that cassette, and reloadable cassettes accumulate grit. Discard and replace rather than trying to clean.

Keep the camera back clean, and inspect the pressure plate and the film rails. A blower and a soft brush, not a solvent.

Rewind slowly, which reduces the force with which any particle is dragged.

Test a suspect camera deliberately: a short roll, wound on and rewound, developed and examined. Ten minutes settles what months of wondering will not.

Store dry negatives in clean sleeves and handle them by the edges, with clean hands or gloves.