What happens when paper gets wet?

Answer 1

When paper gets wet, the cellulose fibers absorb the water and swell. The wet paper becomes larger in all dimensions -- thicker as well as wider and longer -- than it was when dry. Papers that have a pronounced grain direction usually swell more across the grain than they do along the grain. If the paper is wetted unevenly or incompletely, it will warp or cockle as the wetter parts expand more than the drier parts. Water-soluble inks, stains, and dirt will also be dissolved in the water, and carried towards the edge of the wetted area, resulting in blurred writing and tide lines. If wet paper dries unevenly, or if it is not put under pressure during the last part of the drying process, the warping and cockling that happened when it was wet can become more or less permanent. If wet paper dries slowly, with insufficent air circulation, it can easily grow mold.