The Constituents Of Paper
( Originally Published 1920 )
THE technique of paper-making varies greatly in accordance with each particular product. In fact, so wide is the range of paper products, that the different branches of paper-making severally require knowledge so special that an artisan in one branch might be as useless in another as if it were an entirely different industry. The coating of paper, for example, is an absolutely different trade from that of paper-making.
This remarkable diversification is entirely the development of a century, and principally the evolution of the past forty years consequent to the discovery of wood cellulose. Today the products of the paper-mill are no longer confined to the use of pen or press. We ride on car wheels made in part of paper; sit in paper-seated chairs; drink from paper cups; eat from paper plates; use paper napkins; wrap our food in parchment paper; sheath our buildings with paper without, and wall paper or wall board within; keep out the rain with roofing paper if we please. Our shoes, even, contain a paper part, said to be more durable than leather. Millions of packages, mailing-tubes and boxes are made of paper. It is even spun into a kind of yarn and woven into imitation cloth, while a surprising imitation silk necktie is produced from wood-pulp. In electrical engineering, paper as an insulator is almost indispensable.
All these paper commodities, and more, too numerous to mention, require special machinery and treatment. To give an exhaustive treatment of the subject would require volumes, but for the purpose of this book we are principally concerned with printing and writing papers.
Broadly speaking, there are five steps in the manufacture of paper:
1. The isolation of the paper-making fiber from the raw material.
2. The conversion of the fiber into pulp.
3. The beating and refining of the fiber, and the admixture of non-fibrous components.
4. The manufacture of the mixture into paper.
5. The finishing of the paper and its preparation for the market.
Cotton and linen rags, hemp, woods and plants each require their peculiar treatments. Cotton and linen, being the original paper-making fibers, will be considered first.
Rag papers may be made from all sorts and conditions of rags, so the fineness of the finished product depends upon the newness and quality of the rags. New white cuttings from textile factories are the best, as their strength is unimpaired by previous use, and they may be prepared for manufacture with a minimum use of chemicals.
From this high standard, rags are graded down in accordance with their color, cleanliness and condition. The first sortings are made by stock-dealers, and the paper-maker orders whatever grades are suitable to his purpose. After their receipt at the mill, the bales of rags are opened, dusted by machine and distributed to girls, who sort them, open up the seams so as to release hidden dirt, remove bottons and other foreign material.
In the making of the highest grades, the new white rags are cut by hand into small pieces of uniform size, but ordinarily they are fed into a mechanical rag cutter. After this they are passed through a dusting machine to rid them as far as possible from dirt and foreign matter, which might otherwise appear as specks in the paper.
BOILING.—DYES and greasy matters are associated with the fibers, and in order to obtain the pure cellulose fiber the rags are cooked, under steam pressure, in rotary boilers with alkali. This saponifies and dissolves the non-cellulose compounds, and the soda in combination with these soluble materials is subsequently washed out. The amount of steam pressure, the quantity of chemicals, and the duration of the cooking, are subject to variation under different conditions. At the conclusion of the process the manholes in the boilers are opened, and the contents are deposited on the floor, later to be transferred to the washer room.
WASHING.-A washing engine consists of an oval tub about four feet high. It is divided longitudinally by a partition or "mid-feather," with a passage left at either end for the circulation of the stock. On one side is located a large roll, having a continuous parallel series of knives horizontally inserted in its surface. The floor of the engine slopes gently to a point under the roll, where a bed plate is set. Behind the roll is a raised partition or dam, over which the stock is thrown as it passes between the beater roll and the bed plate. This is known as the "back-fall," and assists in the circulation. The roll may be raised or lowered over the bed plate, and by this means the breaking of the stock is regulated.
Affixed to the tub are, one or more washing cylinders, so arranged that they may be lowered into the stock. These are constructed in such a way that during the process of washing the water passes through their wire-covered surfaces and is drained into the hollow axle of the roll by an interior arrangement, called buckets. The axle, being open at one end, permits the wash water to escape.
At first the engine is partly filled with water, then the rags are gradually thrown in until the tub is full. The revolving roll keeps the mass in circulation, while the rags are broken and shredded as they pass beneath it. A continuous stream of fresh water runs into the tub, and in running out through the revolving washer drums carries off the dirt, but the fibers themselves can not pass through the wire coverings, so remain until cleansed. Necessarily the water used must be free from sediment or mineral impurities, such as iron, otherwise it would fill the stock with specks. Therefore, a filter plant is usually maintained.
BLEACHING.—After the washing has been completed the drums are raised clear of the stock and bleaching liquor is introduced. This is an important step, and if not carefully managed may impair the stock. For instance, if bleaching is carried on at too high a temperature, the white color obtained will not be permanent, and discoloration will occur after the paper is made. Much of the paper, which at first displays a brilliant white color, will afterward take on a yellowish tinge, especially if it is exposed to light. A comparison between the century-old hand-made papers and modern "fine writings," makes the old papers appear a "natural" shade, but place both for a few hours in the sunlight and often the mod-ern paper will fade, whereas the old sun-bleached papers remain unaltered. The high artificial bleaching does not insure permanent results.
After the bleach liquor has been thoroughly mixed in, the stock is discharged into drainers and allowed to stand for a week or more, until no traces of chlorine remain. In this state the pulp is known as "half-stock."
The treatment of hemp is so similiar to that of rags that a description here of the process is superfluous.
Wood-pulps are of two classes, mechanical and chemical. In the lay mind there often appears to be some confusion between the two, leading to an unreasonable prejudice against papers made from either class. The fact is so generally known that news-print, one of the cheapest grades of paper, is made from wood, that the partially informed person is prone to think that all wood papers are of low quality, whereas paper of permanence and excellent quality may be made from the high grades of wood cellulose chemically prepared.
GROUND WOOD.—The mechanical, or ground wood, as its name implies, is made by grinding logs from which the bark has been removed. The logs are shipped, or floated from the lumber camps to the mills, where they are cut to convenient length and the bark is removed. Next they are taken to the grinders. One type of grinder consists of a vertical grindstone encased in an iron jacket. There are three pockets over its circumference into which the logs are placed. They are held by hydraulic pressure against the revolving stone, over which flows a stream of water, and are rapidly reduced to fibers. These fibers are carried by the flowing water into a chamber below the grinders, passing through a screen which catches the coarser bits, the fibers of suitable size thus being separated from the rest. This pulp is still not sufficiently fine or uniform, so it is pumped into screens and forced through the finely perforated plates. The fibers are carried through with a large quantity of water, and are formed into thick sheets by means of a so-called "wet machine."
WET MACHINE.—The wet machine consists of a vat, in which a partially submerged hollow drum rotates. The surface is covered by a wire cloth, and the hollow axle of the drum acts as a drain for the fiber-laden water, which, in passing through the drum, deposits a film of fibers upon the revolving surface. This soft pulp film, continuously forming, is removed from the top of the drum by an endless felt running tangent to it, and held in close contact with it by a couch roll, the pressure of which causes the web of pulp to adhere to the felt.
The felt passes between two squeeze rolls, and the pulp adhering to the upper roll is wound up until a certain number of layers have accumulated, when it is cut across by a knife and removed as a thick sheet.
The sheets, folded to a convenient size, separated by alternate pieces of sacking, are put in a hydraulic press and squeezed to remove the water. The pulp is taken from the press about fifty per cent moist; the sheets are separated from the sacking and are now ready for use or for shipment. It is also quite customary to ship the pulp without having pressed it. In this case it contains about 70% water, due allowance for which is made in billing.
This pulp contains practically all the constituents of the original wood, has little strength, inferior felting properties, and is not of permanent character. Its utility results largely from its cheapness. When made into paper with a suitable admixture of sulphite pulp, for strength's sake, it proves to be admirably adapted for the fast-running newspaper presses, as ink dries upon it almost instantly.
It is also used in the making of boxboards, cheap card-boards, pie plates, wall papers, etc. It should, however, be strictly excluded from all papers of more than ephemeral purposes, because of its lack of permanence. The appearance of a paper containing much ground wood is inferior, as the color is poor and small shives of wood may be discerned on the surface. An easy and reliable way to ascertain the presence of ground wood is to moisten the paper with a drop of strong nitric acid, which develops a dark-brown stain if ground wood is present. Another good test is phloroglucine, which turns ground wood to a bright car-mine shade. The quantity of ground wood is roughly indicated by the intensity of the stain.
BLEACHED GROUND WOOD
A quality of pulp intermediate between chemically produced wood cellulose and ground wood is obtained by bleaching an especially finely ground quality of pulp wood. This product is excellent as a filler for medium grades of paper, as it is opaque—fine, and of fair color. Nevertheless, it is open to the same criticism as other ground wood as to permanence, though in a less degree.