Lignins, which are the combined glues that hold plant cells together, are undesirable in a finished paper product. They age poorly, turn brown, become acidic over time, are waterproof, and resist the natural bonding of cellulose fibers to each other. If lignins are not removed and are left in contact with the surrounding cellulose fibers in paper, their acidity will break down the cellulose and the paper will become brittle.
Lignins comprise 20 to 30 percent of wood, but only 1 percent of cotton fibers. Because of the high concentration of lignins in wood, papers made from wood pulp discolor and eventually self-destruct. Although there are methods for the removal of most or all of the lignins, unless the residual chemicals used in these processes are also dealt with, embrittlement and acidification will only be postponed. For this reason, wood-pulp papers are generally avoided for permanent artwork. Because it is nearly lignin-free, paper made from 100 percent cotton is most desirable. The recently developed process for the removal of all lignins is being used at this time primarily to manufacture boards and storage containers used in archives, in conservation, and in museum-style framing.
Another major consideration in paper is its pH. The scientific symbol indicating the concentration of hydrogen ions in a liter of solution, pH