Assuming that it is on fiber paper, then place it in a closed container that has a damp piece of paper on the bottom. It is important to have it raised so it doesn't touch the damp paper.
Seal it up and leave it at least overnight and it should be pretty flat by morning.
The reason the photo curls is due to the differential shrinkage between the substrate (the paper) and the image layer (the gelatin). The image layer shrinks more and causes the curl.
So, when you get it flat, then press it between a couple of acid free absorbent layers and let it dry for a day or two. In that condition it should remain flat.
Don't try flattening it directly without moisture, it will cause cracks in the emulsion.
Sent from David's iPad
I don't have much of my
darkroom tools left and have a question. I found an old black
and white photograph that I printed a long time ago
It is curled, no bends or folds, just curl.
What is the best way to decurl it?
Money can't buy happiness--- But somehow
it's more comfortable to cry in a Porsche than a Kia.