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RAW MATERIALS • Paper is made from fibers of cellulose. The most common source of cellulose is trees, but cellulose from cotton, hemp, and many other materials can be used. • Cellulose from trees contains lignin: "brown stuff" that must be bleached and rinsed out to make printing and writing papers. • Bleaching can leave traces of acid, which is what causes paper to turn yellow and crumble with age. "Acid free" papers contain no acid residue and last much longer. PULPING • Cellulose fibers are separated from wood chips by washing, cooking, and bleaching. The pulp stock (a.k.a. "furnish") that remains is 99 percent water. "WET END" • In the papermaking machine, the stock is sprayed onto a long, wide, moving screen called a "wire." Water drains through the wire, and the pulp fibers begin to bond together in a very thin mat on the top side of the wire. • The fiber mat is then squeezed between felt-covered press rollers to absorb more of the water. At this point the "paper" on top of the wire is still about 60 percent water. "DRY END" • The wet paper is heated and dried by passing through a series of steam-filled metal cylinders. Heating and drying the wet sheet seals the fibers together and turns them into finished paper. "CALENDERING" and COATING • The paper is run through a series of smooth-surfaced, chilled metal cylinders known as the "calender." The calender presses the drying paper smooth and uniform in thickness. • "Supercalendering" the paper through an additional set of rollers gives more smoothness and adds gloss. • Sometimes the paper is coated with a fine clay (kaolin) to fill in surface irregularities. This makes the paper brighter, more opaque, and easier to print on. RETURN TO FAST ACCESS MENU RETURN TO SESSION OUTLINE |