The fibrous material forming the basis of paper, the production of which is the first stage in the papermaking process. Pulp is produced either mechanically, chemically, or semichemically from fibrous raw material, primarily wood, cotton, or linen. (See pulping, mechanical pulping, chemical pulping, and semichemical pulping.) The goal of pulping is to liberate fibers of cellulose from other impurities in the raw material. The higher the cellulose content of a paper, the better the quality, so the best pulps are those that contain cellulose exclusively and little else. Cotton, being nearly 100% cellulose, produces the best pulp, as do linens, and other textile materials, but are more expensive than other fibrous sources. Wood, having a lower cellulose content, produces pulp that is not as good as cotton or linen pulps, but chemical treatment reduces the amount of impurities and enhances the quality of the pulp. The type of pulping method used also directly affects the quality of the paper that is produced. Mechanically-produced pulp (also called groundwood pulp) has a high lignin content, which reduces the permanence of paper, and is chiefly responsible for the yellowing of paper over time. Mechanical pulp is used for low-quality papers used in newspapers, catalogs, directories, and "pulp" magazines. Semichemical pulps have a somewhat reduced lignin content, and are higher in quality than mechanical pulps. Pulps produced chemically (especially utilizing the kraft process) are used for high-quality printing and writing papers. Wood pulp is brown in color (due primarily to the presence of residual lignin), and can often be used as is for brown wrapping paper or paper bags. More often, however, pulp must be bleached in order to produce printing and writing papers. Chemical pulping can only be done for so long before severe fiber degradation occurs, so bleaching is typically done to remove or alter the lignin without harming the cellulose fibers. (Mechanical pulps are also bleached, but not to the extent that chemical pulps are, as bleaching decreases pulp yield, defeating the purpose of the mechanical pulping process.)
It is at the end of the pulping process that the addition of non-organic fillers—a process called loading—occurs. The degree of refining of the pulp also dramatically affects the paper quality. (See Paper and Papermaking: Paper Properties.)