What causes this?

I just developed these two rolls. In the first pic the roll on the right has been drying for about 15 mins or so. It has curled. In the second pic, about 30-40 mins later, the first roll on the right has flattened, and the second roll on the left has curled. It's probably a stupid question, but what causes this?

are they both curled or flat when dry? Same kind of film on both rolls.

As the film dries the emulsion shrinks a bit -- it is gelatin, after all -- and this causes the film to curl with the emulsion side on the outside first, then inside. Some films curl more than others when they are dry, and in a dry climate (Utah) they tend to curl more.

Do you understand how a bimetallic strip in a thermometer works?
You have two dissimilar metals like copper and steel which expand and contract from heat at different rates. As the strip or coil of metal heats and cools, the expansion of the two metals causes it to curl in proportion to the temperature. That, in turn, moves a pointer or activates a switch, etc.

The same things happens with film and photographic paper. You have two materials which react to moisture in different ways. The emulsion absorbs a lot of water but the base absorbs very little. As the two layers expand and contract in proportion to their moisture content, they will cause the film to curl. This is what causes the effect you are seeing.

If you like to experiment, you can use a piece of film or photographic paper as a hygrometer. Tack a piece of film or paper to the wall by one corner then watch its response to the humidity in the air.

I actually have a piece of photo paper tacked to my darkroom wall which I use as a makeshift humidity indicator. I know just how much it should curl when the humidity is right. If it curls too much or too little, I know that I have to turn my dehumidifier on or off, depending on how the paper reacts.

This effect can also be a good indicator of when your film is dry enough to handle. Just watch the curl and, when it flattens out, your film should be almost dry.

Personally, I've found the Kodak film I've shot curls a little more. (Granted mine has never curled like that.)

However, I've been shooting Fuji Acros 100 lately. That stuff drys as flat as a pancake.

summicron1 said:

are they both curled or flat when dry? Same kind of film on both rolls.

Click to expand...

They are both TriX, both developed in the same way, and both hung in the same area to dry. This morning, they are both flat and dry.

Worker 11811 said:

Do you understand how a bimetallic strip in a thermometer works?
You have two dissimilar metals like copper and steel which expand and contract from heat at different rates. As the strip or coil of metal heats and cools, the expansion of the two metals causes it to curl in proportion to the temperature. That, in turn, moves a pointer or activates a switch, etc.

Click to expand...

Ah ha! Great explanation! Thanks!