Jasey Michelle's 365/366 Photo a Day Blog

Paper curl can be caused by uniform wetting but non-uniform drying. This can be used to advantage with some types of curl control. If a uniformly wetted paper sheet is exposed to a heat source like a light bulb that dries one side more rapidly than the other, curl will initially be toward the light because fibers on that side of the paper shrink and pull the paper. This places the fibers on the non-dried side under compression, however, and they are forced to slip and buckle slightly. When the second side of the paper dries, it shrinks by the amount of the first side plus the additional caused by the compressive stresses. Its ultimate length is shorter than the first side causing curl to form toward the unheated side. Often the axis of curl flips 90 degrees as well. A general rule is that paper will curl toward the side dried last.