When the boiling is judged complete the rags are discharged from the boiler, varying in colour from light brown to black, into a "breaker," which is an oval shaped vessel divided down the centre by a "midfeather" or partition which is placed along the major axis of the vessel, without extending to its ends. In the bottom, at one point of the channel thus formed, is a bed plate with fixed steel or bronze knives against which play similar knives on the periphery of a heavy cast-iron roller, suitably mounted on bearings and rotating in the channel, like a paddle wheel. Its motion circulates the mixture of rags and water round and round the circuit of the channel, disintegrating the rags all the while: the water is continuously renewed, and the rags thus washed, the quantity of water thus consumed being very large. Eventu ally clean rag pulp is obtained, which is then bleached up to the white colour required, by a chloride of lime (bleaching powder) solution, which is previously freed from turbidity by settlement and decantation : the bleaching is commonly done in the breaker, or after partial bleaching in the breaker the half bleached pulp may be tipped out into a tank and the bleaching process left to continue until complete. The material is then known as half stuff, and, after thorough washing, it is ready for making up into paper. For this purpose it is passed through the beater and thence to the wet end of the paper making machine.
(2) "Chemical" and "Mechanical" Pulp.—The principles of this procedure are followed for the manufacture of half stuff from compound celluloses, e.g., wood and esparto, after, however, such previous chemical action as is necessary to remove the in crustants. Wood cellulose, thus prepared as a pulp free from incrustants, is called "chemical wood" to distinguish it from "mechanical wood." Mechanical wood is prepared as a pulp by grinding wood wet against a grindstone, without removal of in crustants, and it is therefore a material of very inferior grade, suitable only for the manufacture of low grade products. Chemi cal wood pulp and mechanical wood pulp are prepared in mills ad joining the forests of the wood growing countries, and thence exported to papermakers all over the world, in air dry condition: the papermaker uses this pulp, plus water, in much the same way, mutatis mutandis, as he uses the rag half stuff, the preparation of which has been described above. Wood pulp is prepared in the roll, and then cut into sheets and baled for shipping. Chemical pulp is sold as containing io% moisture; mechanical as contain ing so%.
Esparto grass is derived from the littoral lands of the Medi terranean, whence it is imported almost entirely into Great Britain, and mainly Scotland, in fairly constant amount, of value about L750,000 per annum. It is shipped in bales which contain roots, weeds and other impurities. It is cleaned and boiled with 15-18% caustic soda at 4-5% strength and at 4o-5o lb. steam pressure : the boiler used is almost invariably stationary and of the type known as Sinclair's patent esparto boiler which com monly takes between 21 or 3 tons of grass. The boiler is
charged through an opening at the top and when the cooking has been judged completed, which may be in about three hours, the boiled material is taken from a false bottom through a door at the side : the liquor drains from the false bottom and is removed through vomit pipes. To get rid of the roots, weeds and other impurities, the pulp, after washing and bleaching, is pumped from storage chests over a settling table or "sand table," made of wood and fitted with divisions or "weirs" behind which the heavy impurities collect, fall to the bottom and are removed. The yield of cellulose is about 40-50% on the dry weight of the grass, and the cellulose is of not very permanent quality: its fibre is short and therefore incapable of giving strength to paper made from it, but it gives a certain soft bulkiness which is esteemed for some purposes, e.g., magazine papers, and it is much used in all sorts of combinations with other fibres, especially bisulphite wood pulp. Straw is treated in much the same sort of way but it requires higher boiling temperatures, gives a lower yield and consumes more bleach : it is used to a small extent as an ingredient for papers, to which it gives a hard, brittle quality ; but it is used much more for making straw boards, for which purpose it is boiled with lime which is cheaper than caustic soda.
There are three processes in use for the chemical removal of incrustants from wood, with formation of "chemical wood pulp," i.e., wood cellulose prepared from wood by removal of incrustants by chemical action. These are the so-called soda, sulphate and bisulphite or sulphite processes. The woods em ployed in Europe are mainly those of the Scotch fir (Pinus syl vestris), the Norway spruce (Picea excelsa), the poplar (Populus alba), the aspen (Populus tremula) and, in America, the black spruce (Picea mariana), the red spruce (Picea rubra), the hem lock (Tsuga canadensis), the balsam fir (Abies balsamea), the poplar (Populus grandidentata) and the aspen (Populus tremu loides). The wood is cut up into logs, de-barked and then con verted into chips, 1 or inch in size, by revolving cutters : after wards the chips are bruised by passing between rollers to enable the boiling solution, later on, to penetrate them the more easily. In the soda process the wood chips are boiled in much the same way as esparto is boiled : and with about the same proportions of caustic soda. The boiler is sometimes a rotating boiler, how ever, and the time of boiling longer (5-8 hours) and the steam pressure higher (9o-15o lb.). The soda is recovered and used again, in this as in the esparto industry, by evaporating down the liquor to dryness and igniting the residue obtained, which is then recausticized by treatment with lime and used over again.