Paper

I

Introduction

Paper, material in the form of thin sheets, manufactured by the webbing of vegetable cellulose fibres. 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. In 20th-century civilization, paper is a basic material, 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 fibres 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 fibres. During the first part of this operation, the material is washed with running water to remove impurities, but when the fibres 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 mould, 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 mould 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 mould that is used in their making.

The mould is placed inside a removable wooden frame called a deckle, which forms a low rim around its edge. The papermaker dips the mould and deckle into a vat containing the half stuff; when the mould and deckle are removed from the vat, the surface of the mould is coated with a thin film of fibre-water mixture. The device is then shaken forwards and backwards and from side to side. This shaking has two effects; it distributes the mixture evenly on the surface of the mould and causes the individual fibres 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 mould 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.

When the deckle has been taken from the mould, the latter is turned over and the sheet of paper is laid smoothly on a sheet of woven woollen 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 tonnes, 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 require additional treatment following drying, because without such treatment the paper would absorb ink and yield fuzzy text and images. 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, 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 per cent of paper is made from wood cellulose. For the cheapest grades of paper, such as newsprint, groundwood pulp alone is used; for better grades, chemical wood pulp, or a mixture of pulp and rag fibre, is employed; and for the finest papers, such as the highest grades of writing papers, rag fibre 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 non-soluble soaps, which can be washed away in a later process, and at the same time reduces any coloured dyes present to colourless compounds. The rags are then transferred to a machine called a beater, or Hollander, which is a long tub divided longitudinally so that there is a continuous channel around it. 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 fibres. 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 fibres behind. As the mixture of rags and water flows around the beater, the dirt is removed and the rags are gradually macerated until they are finally resolved into individual fibres. The half stuff is then passed through one or more secondary beaters to break up the fibres still further. At this point colouring matter, sizing material such as rosin or glue, and fillers such as sulphate of lime or kaolin, which give added weight and body to the finished paper, are added. 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 likewise equipped. The fibre 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 fibres.

The preparation of wood for papermaking is accomplished in two different ways. In the groundwood process, blocks of wood are held against a rapidly revolving grindstone that shreds off fibres. The fibres produced by this process are short and are used only in the production of cheap newsprint and for admixture with other types of wood fibre in the making of high-quality paper. In various chemical-solvent processes, wood chips are treated with solvents that remove resinous material and lignin from the wood, leaving pure fibres of cellulose. The oldest of the chemical-solvent processes is the soda process, which was introduced in 1851 and employs a solution of caustic soda (sodium hydroxide) as a solvent. The wood is cooked or “digested” in this solution under steam pressure. The fibres produced by this process do not have great strength but are used in mixtures with other wood fibres. A common process employed today is the sulphate process, which is named after the solvent used, either sodium sulphate or magnesium sulphate.

Most paper today is made on Fourdrinier machines patterned after the first successful papermaking machine, which was developed in the early years of the 19th century. 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 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 fibre 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. Suction pumps beneath the belt hasten the drying of the paper, and the belt itself is moved from side to side to aid the felting of the fibres. As the paper travels along the belt it passes under a revolving 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 mould.

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 fibre, 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 surface 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 that used for fine half-tone 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.

IV

Paper Sizes

Paper is usually sold by the ream in sheets of standard sizes. A ream usually contains 480 sheets, except that 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 can be 168 cm (66 in) wide, 7,925 m (26,000 ft) long, and weigh about 725 kg (1,600 lb).