Page:Paper and Its Uses.djvu/23

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11
REDUCTION TO PULP

usually a weak solution of bleaching powder (chloride of lime) is let into the engine and thoroughly mixed with the pulp. When the bleach is thoroughly incorporated the half-stuff is let down into large tanks, made of stoneware or cement, having perforated bottoms, and there the bleach completes its task, and the pulp is allowed to drain.

Next comes the beating, at which stage the blending of different fibres may take place. The object of beating is to reduce the bleached pulp to fibres, and to reduce the length of the fibre in accordance with the requirements of the paper to be made. The rags are chosen according to the class of paper desired—softer rags for soft papers, and, of course, stronger rags for strong papers. For blottings, filter papers, and lithographic papers, soft rags, sharp beater knives, quick beating are adopted. For dense, hard papers, such as ledger, type-writing, bank, imitation parchment papers, duller knives, slow beating, with a gradual lowering of the beater roll is the order. The normal time for beating the pulp for an ordinary rag paper may be taken as eight hours.

To take the next material, esparto, and to follow it in the same way. Esparto arrives in bales, fastened either with ropes of esparto or with iron bands. Esparto travels through the mill in the same way as rags, that is, from the ground floor, where it is unpacked and dusted, upwards by means of a series of claws, along a travelling band where pickers remove foreign substances. In its travel broken fibres and dirt escape, and the grass arrives at the mouth of an upright cylindrical boiler, stationary, and so arranged that the boiling liquor is vomited over the mass of esparto. The boiler is filled, and a fairly strong solution of caustic soda is run in, the manhole is fastened down, and steam under pressure introduced. After several hours boiling the