Paper

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. Paper can also be recycled, saving energy and resources.

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.

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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 nearly all paper is made in whole or in part from wood cellulose. For the cheapest grades of paper, such as newsprint, groundwood (mechanically processed) pulp is used, often along with recycled paper material. For better grades, chemical wood (pulp in which undesirable materials are chemically removed), pulp, or a mixture of pulp and rag fiber is employed. For the finest papers, such as the highest grades of writing papers, rag fiber alone may be used.

A

Paper Made from Rags

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.

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