I.

Introduction

Paper, thin sheets of compressed vegetable cellulose fibers. Paper is used for writing and printing, for wrapping and packaging, and for a variety of special purposes ranging from the filtration of precipitates from solutions to the manufacture of certain types of building materials. Paper is a necessity in modern civilization, and the development of machinery for its high-speed production has been largely responsible for the increase in literacy and the raising of educational levels of people throughout the world.

II.

Hand Papermaking

The basic process of making paper has not changed in more than 2,000 years. It involves two stages: the breaking up of raw material in water to form a suspension of individual fibers and the formation of felted sheets by spreading this suspension on a suitable porous surface, through which excess water can drain.

In making paper by hand, the raw material—straw, leaves, bark, rags, or other fibrous material—is placed in a vat or trough and is pounded with a heavy pestle or hammer to separate the fibers. During the first portion of this operation, the material is washed with running water to remove impurities, but after the fibers have been sufficiently broken up, they are kept in suspension, and the water in the vat is not changed. At this stage the liquid material, called half stuff, is ready for the actual process of papermaking. The chief tool of the papermaker is the mold, a reinforced sheet of metal mesh having either a square mesh pattern, called a wove pattern, or a pattern of more widely spaced longitudinal wires held together with smaller transverse wires, called a laid pattern. The mold pattern imprints itself on the finished sheet of paper, and thus handmade papers that are not given special finishes are identified as wove or laid papers, depending on the style of mold that is used in their making.

The mold is placed inside a removable wooden frame called a deckle, which forms a low rim around its edge. The papermaker dips the mold and deckle into a vat containing the half stuff; when the mold and deckle are removed from the vat, the surface of the mold is coated with a thin film of fiber-water mixture. The device is then shaken forward and backward and from side to side. This shaking has two effects; it distributes the mixture evenly on the surface of the mold and causes the individual fibers to interlock with those adjacent, giving strength to the sheet. While the device is being shaken, much of the water from the mixture drains out through the mold mesh. The device, with its formed sheet of wet paper, is then laid aside until the paper is sufficiently cohesive to permit the removal of the deckle.

After the deckle has been taken from the mold, the mold is turned over and the sheet of paper is laid smoothly on a sheet of woven woolen cloth, called a felt. Another felt is laid over the sheet of paper, and the process is repeated; the process of placing the paper between two felts is known as couching. When a number of sheets of paper have been interleaved with felts, the entire pile, called a post, is placed in a hydraulic press and subjected to a pressure of 100 or more tons, expelling most of the water remaining in the paper. The sheets of paper are then separated from the felts, stacked, and pressed. The process of pressing the stack of paper is repeated several times, and each time the stack is built up in a different order with the individual sheets in different positions relative to one another. This procedure is called exchanging, and its repetition improves the surface of the finished paper. The final stage in papermaking is drying. The paper is hung, in groups of four or five sheets, over ropes in a special drying room until its moisture has almost completely evaporated.

Papers that are to be used for writing or printing with ink require additional treatment following drying, because without such treatment the paper would absorb ink and yield fuzzy lines or impressions. The treatment consists of sizing the paper by dipping it into a solution of animal glue, drying the sized paper, and finally finishing the paper by pressing the sheets between sheets of metal or smooth cardboard. The amount of pressing determines the texture of the surface of the paper. Rough-textured papers are pressed lightly for a comparatively short period of time, and smooth-surfaced papers are pressed heavily for comparatively long periods.

III.

Machine Papermaking

Although the essential procedures of papermaking by machine are identical with those of hand papermaking, machine papermaking is considerably more complex. The first step in machine papermaking is the preparation of the raw material. The materials chiefly used in modern papermaking are cotton or linen rags and wood pulp. Today more than 95 percent of paper is made from wood cellulose. For the cheapest grades of paper, such as newsprint, groundwood (mechanically processed) pulp alone is used; for better grades, chemical wood (pulp in which undesirable materials are chemically removed), pulp, or a mixture of pulp and rag fiber is employed; and for the finest papers, such as the highest grades of writing papers, rag fiber alone is used.

Rags used in papermaking are first cleaned mechanically to remove dust and foreign matter. Following this cleaning, the rags are cooked in a large rotary boiler. This process involves boiling the rags with lime under steam pressure for a period of several hours. The lime combines with greases and other impurities in the rags to form nonsoluble soaps, which can be washed away in a later process, and at the same time reduces any colored dyes present to colorless compounds. The rags are then transferred to a machine called a beater, or Hollander, which is a long tub divided longitudinally so as to form a continuous channel around the tub. In one half of the tub, a horizontal cylinder carrying a series of knives revolves rapidly close to a curved bedplate, which is also provided with knives. The mixture of rags and water passes between the cylinder and the bedplate, and the rags are reduced to fibers. In the other half of the tub, a hollow washing cylinder covered with fine mesh screening is arranged so that it scoops water from the tub, leaving the rags and fibers behind. As the mixture of rags and water flows around the beater, the dirt is removed and the rags are gradually softened until they are finally resolved into individual fibers. The half stuff is then passed through one or more secondary beaters to break up the fibers still further. At this point are added coloring matter, sizing material such as rosin or glue, and fillers such as sulfate of lime or kaolin, which give added weight and body to the finished paper. In many American paper mills the second beater is of the type known as a Jordan engine. This machine consists of a stationary cone fitted with knives mounted outside a revolving cone also equipped with knives. The fiber material flows between these two sets of knives, and the cones can be adjusted relative to each other with great accuracy to regulate the fineness of the fibers.

The preparation of wood for papermaking is accomplished in two different ways. In various chemical-solvent processes, wood chips are treated with solvents that remove resinous material and lignin from the wood, leaving pure fibers of cellulose. The oldest of the chemical-solvent processes, the soda process, introduced in 1851, employs a solution of caustic soda (sodium hydroxide) as a solvent. The wood is cooked or “digested” in this solution under steam pressure. The fibers produced by this process do not have great strength but are used in mixtures with other wood fibers. The process most generally employed in the United States is the sulfate process, which is named for the solvent used, either sodium sulfate or magnesium sulfate.

In the groundwood process, blocks of wood are held against a rapidly revolving grindstone that shreds off short wood fibers from the block. The fibers produced by this process are used only in the production of cheap newsprint and for admixture with other types of wood fiber in the making of high-quality paper. To produce white paper from this pulp, paper mills have historically bleached the pulp with chemicals such as chlorine. Chlorine removes lignin, which gives paper an often undesired brown color. However, because bleaching paper with chlorine produces a carcinogen (cancer-causing compound) called dioxin, in 1998 the United States Environmental Protection Agency (EPA) published the pulp and paper industry Cluster Rule, which required U.S. paper companies to eliminate chlorine from the bleaching process by 2001. Instead, the mandate will require the companies to switch to safer compounds such as chlorine dioxide or sodium hydroxide.

Most paper today is made on Fourdrinier machines, which are patterned after the first successful papermaking machine, developed in 1803 by the British brothers Henry Fourdrinier and Sealy Fourdrinier. The heart of the Fourdrinier machine is an endless belt of wire mesh that moves horizontally. A flow of watery pulp is spread on the level belt, which passes over a number of rolls. A shallow wooden box beneath the belt catches much of the water that drains off during this stage. This water is remixed with the pulp to salvage the fiber contained in it. Spreading of the sheet of wet pulp on the wire belt is limited by rubber deckle straps moving at the sides of the belt. Air suction pumps beneath the belt hasten drying of the paper, and the belt itself is moved from side to side to aid the felting of the fibers. As the paper travels along the belt it passes under a turning cylinder called a dandy roll. The surface of this cylinder is covered with wire mesh or single wires to impart a wove or laid surface to the paper. In addition, the surface carries words or patterns worked in wire; these are impressed on the paper and appear as watermarks that identify the grade of paper and the maker. In handmade papers, the watermark patterns are fixed to the surface of the mold.

Near the far end of the machine, the belt passes through two felt-covered couching rolls. These rolls press still more water out of the web of paper and consolidate the fiber, giving the paper enough strength to continue through the machine without the support of the belt. The function of these rolls is the same as that of the felts used in couching handmade paper. From the couching rolls, the paper is carried on a belt of cloth through two sets of smooth metal press rolls. These rolls impart a smooth finish to the upper and lower surfaces of the paper.

After pressing, the paper is fully formed. It is then carried through a series of heated rolls, which complete the drying. The next step is calendering, pressing between smooth chilled rolls to produce the smooth finish known as machine finish. At the end of the Fourdrinier machine, the paper is slit by revolving cutters and wound on reels. The manufacture of the paper is completed by cutting into sheets, unless the paper is to be used on a continuous press that employs rolls of paper. Special papers are given additional treatment. Supercalendered paper is subjected to a further calendering process under great pressure between metal and paper-covered rolls. Coated paper, such as is used for fine halftone reproduction, is sized with clay or glue and calendered. Paper is also made on cylinder machines. Much of the tissue paper manufactured is made on Yankee machines, which have a single steam-heated cylinder for drying. Equipment used in pulp making and papermaking is constantly being improved and modernized. For example, the Inverform machine, which was invented in England in the 1940s, is a high-speed machine that produces a range of box board used by the food packaging industry.

IV.

Paper Sizes

Paper is usually sold by the ream in sheets of standard sizes. A ream of paper usually contains 480 sheets, but reams of drawing paper and handmade paper contain 472 sheets. Book paper and newsprint for flat-plate printing are sold in reams of 500 sheets and in perfect reams of 516 sheets. The most common book-paper size is octavo (112 by 168 cm/44 by 66 in). Newsprint for rotary-press printing comes in rolls of varying sizes; a typical roll of newsprint, as used by large metropolitan newspapers in the United States, is 168 cm (66 in) wide and 7,925 m (26,000 ft) long, and weighs about 725 kg (1,600 lb).

V.

Synthetic-Fiber Paper

In 1955 papers were prepared from nylon, Dacron, and Orlon fibers, and from blends of these fibers with wood pulp (see Plastics). Such papers are produced on conventional papermaking machinery and can be made with a wide range of appearances and characteristics, from crisp stock resembling ordinary paper to drapable, fabriclike materials. Because of their unique properties, synthetic-fiber papers have many applications for which ordinary paper is unsuitable, notably as electrical insulation, filtration material in air-conditioning equipment, electrical tapes for sound recording, shoe fabrics, and interlining in clothing.

VI.

History

According to tradition, paper was first made in ad 105 by Ts'ai Lun, a eunuch attached to the Eastern Han court of the Chinese emperor Ho Ti. The material used was probably the bark of the mulberry tree, and the paper was made on a mold of bamboo strips see Bamboo; Mulberry; Rice-Paper Tree. The earliest known paper still in existence was made from rags about ad 150. For approximately 500 years the art of papermaking was confined to China, but in 610 it was introduced into Japan, and into Central Asia about 750. Paper made its appearance in Egypt about 800 but was not manufactured there until 900 (see Papyrus).

The use of paper was introduced into Europe by the Moors, and the first papermaking mill was established in Spain about 1150. In succeeding centuries, the craft spread to most of the European countries. The introduction of movable type about the middle of the 15th century made book printing practical and greatly stimulated papermaking (see Printing). The first paper mill in England was established in 1495, and the first such mill in America in 1690.

The increasing use of paper in the 17th and 18th centuries created shortages of rags, which were the only satisfactory raw material known to European papermakers. As a result, many attempts were made to devise substitutes, but none was commercially satisfactory. At the same time, attempts were made to reduce the cost of paper by developing a machine to supplant the hand-molding process in paper manufacture. The first practical machine was made in 1798 by the French inventor Nicholas Louis Robert. Robert's machine was improved by the British stationers and brothers Henry Fourdrinier and Sealy Fourdrinier, who in 1803 produced the first of the machines that bear their name. The solution of the problem of making paper from cheap raw material was achieved by the introduction of the groundwood process of pulp making about 1840 and the first of the chemical pulp processes approximately ten years later.

See also Fiber; Paleography; Papyrus; Parchment and Vellum.

Contributed by: American Paper and Pulp Association