Paper is composed of cellulose fibers. Cellulose is a polymer of the sugar glucose and is used by plants to produce cell walls. Plant matter that has been processed to create a solution consisting of cellulose filaments suspended in water can be made into paper. A screen is passed through the solution so that the filaments can collect on it and thus form a layer. This layer of cellulose fibers is then pressed and dried to produce a usable sheet of paper. The source of the cellulose fibers, and the degree to which that source is refined, determine the nature and quality of the paper produced. The two most important factors that affect the quality of paper are the presence of impurities and an acidic pH. Finished papers may contain natural impurities, such as lignins that have not been removed during processing, unnatural impurities, such as residual chemicals, like sulfites, not washed out during final processing, or such chemicals as alum that have been added during final processing.

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