Why am I getting occasional cloud-like artifacts on my 4x5 film shots?

Question

Not always, just from time to time, I'm discovering these cloud-like artifacts on my 4x5 film shots. They are present on the film itself; it's not scanning artifacts. Artifacts can be present on different film types (first photo is EKTAR100 color negative, second is EKTACHROME E100 slide). These two shots were done with different cameras but with the same film holder.

The cloud-like artifact pattern is always different on every frame that has them.

First, color and B&W images are the same. Second, only one color channel with curves adjustments can make the artifacts more visible. On the B&W image, notice that the cloud-like artifacts have quite the same silhouette-like exposed part of the frame, but with an offset. The silhouette is a TOYO 4x5 film holder inner ledge for holding the film frame.

On the first photo, the artifact structure on the right side of the frame is more shallow than on the rest of the frame.

Any thoughts on the source of the color noise?

Answer 1

This is mottling.

I've seen it on my own film, which I processed myself, after the film sat in the film holder for too long. My hypothesis is that it's caused by condensation due to temperature changes; liquid water on the emulsion of the film can dissolve sensitizing dyes and redistribute them unevenly, making parts of the film less sensitive to light, and parts more, or changing spectral sensitivity locally.

The main way I'm aware of to avoid it is to load film holders shortly before you'll expose the film, and unload them (into a suitable light-tight holding container) as soon as practical after exposure (and then process them as soon as practical after that). Sadly, this can also happen to film still in the box, if it's not airtight and is subject to high humidity and/or multiple temperature cycles (for instance, being put into and taken out of a refrigerator multiple times).

This means the rule for storing film should be "Keep it cold until you're ready to use it, let it warm to room temperature before you open it, then store it at room temperature thereafter."