How do you flatten a curling photo?

I am not a paper conservator, but this is how I've flattened old, curled photographs: Allow the photograph to absorb moisture by exposing it to relatively high humidity (do not use water directly on the print). I only flatten photographs during humid periods in the summer. Once the photograph has absorbed some moisture (allow several days in humid air), it should be more flexible and less resistent to flattening. A dry, curled photograph will resist bending, and the surface will cack easily if forced. Slowly and gently flatten the photograph using your fingers and hands. Potential damage (creases, cracks, kinks) can occur, especially at the print's edges as you press them, so go slowly and carefully. Carefully flatten the photograph and place it under a heavy, flat object. You can use a book for small prints. Place a piece of acid free paper or mat board on both sides of the print. The time to flatten the print will depend; just check it over many days or even weeks or months for the print to relax and flatten.