paper, thin, flat sheet or tissue made usually from plant fiber but also from rags and other fibrous materials. It is used principally for printing and writing on but has many other applications. The term also includes various types of paperboard, such as cardboard and wallboard.
The Formulation of Paper
A quarter to a third of most new paper is made from waste paper. The body of paper is made up of matted cellulose fibers—since c.1860 derived principally from wood. Rags, mostly cotton cuttings from textile and garment factories, are used to make fine stationery and for such purposes as cigarette paper. For other special papers, or where wood is not available, manufacturers may use pressed sugarcane, bamboo, manila rope, cereal straws, esparto grass, or other fibers.
Preparation from Wood Pulp
Most paper is made from wood pulp. Mechanical pulp, or groundwood, prepared by grinding the wood, is used to make newsprint, tissue, towel, and other inexpensive papers. For paper whose whiteness is important, a chemical pulp must be prepared. Lignin, which holds wood fibers together, turns yellow in sunlight and therefore must be removed by alternating treatments with acid and alkaline solutions. The wood pulp, boiled under pressure and treated to dissolve the lignin binder, is thus turned into cellulose fiber. The mixture is then washed and bleached; because the resulting pulp is more than 90% water, the water is usually treated before mixing.
Once the wood pulp has been treated, washed, bleached, screened, and beaten, it is blended to achieve the characteristics required for the intended use. The pulp, suspended in water, is poured over a wire screen in one of two machines that differ mainly in the form of the screen: a belt screen is used in the Fourdrinier machine and a cylindrical one in the cylinder machine. As water drains through the screen, a layer of fibers forms, which in the Fourdrinier is shaken to turn the fibers in different directions so that they mat. A wet felt belt pressed against the screen picks up the paper for feeding through sets of drying rollers. During this stage a rubber roller may be used to imprint a watermark. At the end of the process the paper is passed through a calender (stack of iron rollers), which presses the paper and smooths its surface. Fillers—chiefly clay or starch—are used to improve the printing, texture, and wet and dry strength of paper and to produce other special properties.
Treatment for Special Properties
Book paper is any kind of printing paper except newsprint; in order to prevent rapid deterioration of the paper through a reaction between the acids in the pulp mixture and the humidity in the air, modern book paper is further treated to make it acid-free. For the best reproduction of illustrations, especially halftones, book paper is coated with a layer of mineral pigment, usually clay, mixed with an adhesive. All writing papers are "sized" ; i.e., a water-resistant substance such as rosin is added to the pulp to prevent the spreading of writing ink. Hanging paper, or wallpaper, is soft and bulky; it is rosin-sized for water resistance and coated to take a printed design. Bag and wrapping papers are made of kraft paper, the product of the sulfate process, because of its strength.
The Introduction of Paper
Paper is believed to have been invented by Ts'ai Lun c.105 in China, where it reached an advanced state of development. Chinese paper was a mixture of bark and hemp. Papermaking spread to Japan c.610 and to Samarkand c.751, whence it was introduced by the Arabs into Egypt c.900 and by the Moors into Spain at Játiva c.1150. Mills were established in Italy c.1276; in France, c.1348; in Germany, 1390; and in England, 1495. European paper was usually made of flax and hemp. Primitive bark paper had been made in Mexico and Central America in pre-Columbian times. Paper was first produced in the American colonies in 1690 by William Rittenhouse at Germantown.
See J. P. Casey, Pulp and Paper (3d ed., 4 vol., 1983); J. R. Lavigne, Pulp and Paper Dictionary (1986); N. A. Basbanes, On Paper (2013).