(2) the methods by which the prepared pulp or fibres are converted into paper ready for the market; this is paper-maliing proper, and includes the operations of beating, sizing, colouring, making the sheet or web, surfacing, cutting, &c.
Rags arrive at the mill from the rag merchants, either roughly sorted into grades or mixed in quality and material, and the first process is to free them from sand, dust and other impurities. To effect this they are usually passed in bulk through an ordinary revolving duster. They are then sorted Rags. into grades, and cut to a workable size about four inches square. For the best work, hand -cutting, done by women, is still preferred, but it is expensive and good machines have now been designed for this purpose. After further thrashing and dusting, the rags are ready for boiling, the object of which is not only to get rid of the dirt still remaining in them and to remove some of the colouring matter, but also to decompose a particular glutinous substance which would impair the flexibility of the fibres and render chem too harsh and stiff for readily making into paper. Various forms of vessels are used for boiling, but usually they are made to revolve by means of suitable gearing, and are either cylindrical or spherical (fig. 1).
Fig. 1.—Revolving Spherical Rag Boiler.
In these the rags are boiled with an alkaline solution under a low steam pressure for six to twelve hours. The next step is that of washing and "breaking in," which takes place in an engine called the "breaker." This (fig. 2) is an oblong shallow vessel or trough with rounded ends and dished bottom, usually about 13 ft. long by 6 ft. wide, by about 2 ft. 6 in. in depth, but the size varies greatly. It is partly divided along the centre by a partition or "mid-feather," and furnished with a heavy cast-iron roll fitted round its circumference with knives or bars of steel in bunches or clumps. Underneath the roll and fixed in the bottom of the trough is the "plate," consisting of a number of parallel steel bars bedded in a wooden frame. The roll can be raised or lowered on the plate so as to increase or diminish, as desired, the cutting action of the bars and plate on the material. The duty of the roll is to cut and tease out the rags, and also to act as a lifter to cause the stuff to circulate round the trough. The breaker is half filled with water and packed with the boiled rags; an ample supply of clean water is run into the engine for washing the rags, the dirty water being withdrawn by the "drum-washer," a hollow cylinder fitted with buckets and covered with fine wire-cloth. During the washing process the roll is gradually lowered on the plate to tease out the rags into their original fibres; this operation takes from two to four hours. As soon as all signs of the textile nature of the material are destroyed, the washing water is turned off, the drum-washer lifted, and a solution of chlorine or bleach is run in to bring the pulp up to the degree of whiteness desired, after which the rag "half-stuff," as it is now called, is emptied into steeps or drainers, where it is stored ready for use.
Fig. 2.—Rag-breaking Engine.
In treating esparto (the use of which for paper-making is almost confined to Great Britain) the object is to free it from all encrusting and intercellular matter. To effect this it is digested with a strong solution of caustic soda under a high temperature, in boilers which are almost invariably stationary. Esparto. The most usual form is that known as Sinclair's patent (fig. 3). This boiler is constructed of wrought-iron or steel plates, and holds from 2 to 3 tons of grass. It is charged through the opening at the top A, and the boiled material taken out from a door B at the side; the grass rests on a false bottom of perforated plates C, through which the liquor drains, and by means of two "vomiting" pipes, D, D, at the sides of the boiler, connecting the space at the bottom with a similar space at the top, a continuous circulation of steam and liquor is maintained through the grass. The steam pressure is kept up to 30 to 40 ℔ per sq. in. for three or four hours; then the strong liquor or lye, which contains all the resinous and inter cellular matters dissolved by the action of the caustic soda, is run off and stored in tanks for subsequent recovery of the soda, while the grass is taken to the "potcher" or washing engine. In construction and working this is similar to the breaking engine used for rags; in it the grass is reduced to pulp, and washed for about twenty minutes to free it from the traces of soda liquor remaining after the partial washing in the boiler. As soon as the wash water is running clear it is shut off, and the necessary quantity of a solution of bleaching powder or chlorine (averaging about 6 to 8 % on the raw material) is run into the potcher, and the contents are heated by steam to a temperature of about 90° F. After about four to six hours the bleaching is complete, the drum-washer is let down, fresh water run into the potcher, and the grass washed to free it from all traces of chlorine, an operation generally assisted by the use of a little antichlor or hyposulphite of soda. The esparto, as shipped in bales from the Spanish or African fields, is mixed with roots, weeds and other impurities; and as most of these do not boil or bleach as rapidly as the esparto they would, if not taken out of the pulp, show up in the finished paper as specks and spots. To get rid of them the esparto pulp when washed and bleached is run from the potcher into storage chests, from which it is pumped over a long, narrow serpentine settling table or "sand-table," made of wood and fitted with divisions, or "weirs," behind which the heavy impurities or weeds fall to the bottom and are caught. The pulp is next passed over what is known as a "presse-pâte" (fig. 4) or "half-stuff" machine, very similar to the wet end of a paper machine, consisting of strainers fitted with coarse-cut strainer plates, a short wire and a pair of couch and press rolls. The pulp is drawn by suction through the strainers, which keep back the finer impurities that have passed the sand-table, and then flows on to the wire-cloth in the form of a thick web of pulp. After passing through the couch and press rolls, the pulp leaves the machine with about 70% of moisture, and is ready for the beating engine, the first operation of paper-making proper. This is the usual process, though various modifications are introduced in different mills and for different purposes.
Most kinds of straw can be utilized for making into paper, the varieties generally used being rye, oat, wheat and barley; of these, the two former are the most important, as they give straw the largest yield in fibre. Germany and France are the two principal users of straw, which closely resembles esparto inStraw. its chemical constitution, and is reduced to a pulp by a somewhat similar process.
Scandinavia, Germany, the United States and Canada are the countries which mainly use wood as a material for paper-making,
