Room light leak, with a direction

Owned between two Part XVI pages. Break/Fix — the room that fogs and the light that is not even owns the diagnosis, and its rule is this entry’s rule — a leak has a direction and the direction points at the hole — while making a room dark and proving it owns the leak test and the dated room certificate that the break/fix page reads as evidence. That page also adds the reason a room that was proved dark in July fails in October: rooms change, and the certificate has a date on it for that reason. This entry was written before either, from Part XIII’s step-wedge exposure series, the safelight test SOP and the manufacturers’ own darkroom publications.

What you see

Fog with a gradient. Heavier at one edge of the sheet than the other, or heavier on the sheets that were on top of the stack, or heavier on the side of the easel nearest the door.

That gradient is the entire diagnosis, and it is what separates this from the faults it resembles. Safelight fog is broadly uniform over a sheet held flat at a stated distance. Base fog and chemical fog are uniform by construction. A leak has a direction, because light travels in straight lines, and the direction points at the hole.

Likely causes

1. A gap around a door, which Kodak name as the large-leak example and treat with dark heavy cloth or weather stripping rather than tape.
2. A window, a ventilator or a service penetration. Kodak send readers to buy lighttight vents rather than to improvise them.
3. A seam that is not opaque. Kodak’s example is the one nobody expects: plywood that appears opaque may admit infrared, and pinholes between the darkroom and a lighted area admit visible light.
4. Something inside the room. An enlarger head, a lighted dial, a display. Kodak list these in the same sentence as the safelight itself.

What is happening: the chemistry and physics

Photographic fog is exposure that carried no information: every grain that receives it moves towards developability without recording anything. What a leak adds is exposure with a geometry — an intensity that falls off with distance from the aperture and with the cosine of the angle to it — so the fog it produces is strongest where the sheet faced the hole most directly and nearest to it.

Two consequences follow, and both are useful.

The gradient is a bearing. Fog heaviest along one edge means the aperture lies in that direction, at roughly the height the sheet was at. That is more information than any chemical fault gives you.

Time multiplies it. A leak that fogs nothing in thirty seconds fogs a great deal in the fifteen minutes a box of paper sits open on the bench, which is why Kodak’s remedy for the working stock is a lighttight dark drawer rather than a faster hand.

!The confirming test: five minutes in the dark, and then a coin

Kodak’s own check. Stay in the darkroom for five minutes with every light off. After five minutes, if you still cannot see a sheet of white paper placed against a dark background, the room passes. If there are leaks you will see them, because by then your eyes have adapted.

Five minutes is not arbitrary and shortening it defeats the test: dark adaptation is what turns an invisible leak into a visible one.

Then locate it with a sheet and a coin. Lay a sheet of paper where the material actually sits, put a coin on it, leave it for as long as the material is normally exposed to the room, and process it. A visible coin shape says the room fogs; the direction of the shading around it says where from. Repeat with the sheet turned ninety degrees, and the leak’s bearing is fixed by two readings.

Note the limit Kodak put on this class of test: a coin test checks only for fog, not for the added effects of room light and enlarger exposure together, and excessive fogging may show up only in the image area. For paper, run it through the enlarger first.

Diagnostic questions

• Is the fog even, or does it have a direction? One look at the sheet settles which family of fault you are in.
• What time of day was it? A leak that faces a window is a different fault in the morning and the evening, and a fault that comes and goes is almost always geometric.
• Was anybody in and out? A door opened for two seconds while paper is on the bench is a leak with legs.
• What is the construction? Kodak’s plywood-and-infrared example is worth taking seriously in a self-built darkroom.
• Does the paper fog in its box? If it does, this is not a room fault but a paper storage one.

Corrective action

Find it before you seal it. A darkroom sealed by guesswork is a darkroom that will be sealed again next month.

Small leaks with black tape; large ones with cloth or weather stripping, which is Kodak’s own division and a sensible one — tape over a door crack fails the first time the door is used.

Ventilate lighttight, not less. The temptation to close a vent is real and the answer is a lighttight vent, because the room still has to be breathable during mixing and processing.

Then re-run the five-minute test, because a leak fixed is a leak that must be shown to be fixed.

Prevention

A dark drawer for the working paper, built the way Kodak describe: a sliding lid in a groove around the top perimeter, the inside and the lid painted flat black, and a block of wood that pushes the lid closed as the drawer shuts. It removes the whole exposure of an open packet on a bench.

Return unused paper to its original packaging, which is also what its own keeping specification assumes.

Re-test after any building work, and after anything is moved that was covering something.