Boiling.—The slices of wood, which are about in. thick, are then taken to the boilers. These are vertical cylinders, about 5 ft. in diameter and 16 ft. high, divided into compartments by means of perforated diaphragms. The boilers are heated either directly by means of furnaces underneath, or by steam circulating round an outer jacket. The boilers are partially filled with a solution of caustic soda at about 17° Tw., and the heat is kept up for 6 hours, the pressure being equal to about 70 lb. a sq. in. At the end of this time, the contents of the boiler are ejected with consider able violence into a large iron chamber placed underneath (see Bamboo and Cane, p. 1485). The pulp is then allowed to pass into large iron drainers, mounted on wheels for convenience of loco motion. In the centre of the tramway along which the drainers pass, are placed sewers to receive the liquor. When as much as possible of the liquor has drained away, hot water is poured upon the pulp. A small quantity only is used, so as to keep the liquor sufficiently concentrated to pay for evaporation. The pulp is then taken to washing-engines, similar in principle to those described under Esparto, p. 1487. The washed fibre is next freed from unboiled portions, sand, &c., by being passed over sand-tables and through screens (see Paper-machine, p. 1494), and is then passed over the wet end of a machine, as in the case of straw. It is bleached in the ordinary way, and is then made into a very thick coarse web on a cylinder paper-macbine; in this form, it is sold to the paper-manufacturers. The subsequent treatment of it in the paper-mill calls for no special remarks. Wood-pulp so prepared may be used alone, or mixed with rags or other material, for almost all classes of paper.
The fibres prepared from poplar are very white, but are somewhat deficient in strength; those from other woods, such as members of the family of Coniferco (pines), are much longer and stronger, but owing to the fact that, in the raw state, they usually contain a very large quantity of resinous matter, these woods are much more difficult to digest, requiring a stronger solution of caustic soda, and a considerably higher pressure. The process just described is that actually in use at a mill in America ; at other places, a similar method of treatment is adopted, or if any material difference exists, the details are kept secret. Many other reagents have been suggested as substitutes for caustic soda. Among these, may be mentioned sulphide of sodium, chlorine, and a mixture of hydrochloric and nitric acids (aqua regia). But, as far as is known, they have all been abandoned, on account of the expense and extra trouble involved in their use. Many attempts have been made to utilize the fibre in sawdust, by treating it with one or other of the before-mentioned chemicals, but hitherto without much success. Works have also been erected in this country with a view of obtaining pulp from other waste forms of wood, such as small chips and shavings ; but, principally owing to the nature of the material, and the consequently severe treatment necessary for their disintegration, they too have been given up.
The so-called "mechanical" wood-pulp is obtained by disintegrating the wood entirely by means of machinery, without the use of chemicals. As long as 100 years ago, it was proposed to utilize sawdust and shavings, by stamping them into a pulp ; but owiug to the want of suitable machinery, the attempt was unsuccessful. The next idea was to disintegrate blocks of wood, by grinding with rapidly-revolving cylinders of stone. This, after many years of labour, has been brought to a state in which it is a commercial success. The details of the process are as follows : The wood is first out up into blocks, the size of which is determined by the width of the stones used for grinding ; any knots present are cut out with an axe. The stones are made of sand stone, and are covered over three quadrants with an iron casing, the remaining quadrant being exposed. The surfaces of the stones are made rough by the pressure of a steel roll studded with points, and which is pressed against it while revolving. In addition to this, channels, about I in, deep are cut into the stone at distances of 2-3 in. They are made in two sets, crossing each other iu the centre of the stone, and serve to carry off the pulp to the sides of the stone, in addition to giving increased grinding-surface. The pressure of the blocks of wood against the stones is steadily maintained by screws worked by suitable gearing ; this is necessary in order to obtain a pulp of uniform character. A stream of water is kept constantly playing on the stone ; by this means, the pulp as formed can be conveniently carried away. It is first passed through a rake, which retains small pieces of wood that have escaped grinding. The stream of pulp then passes through the sorters. These are cylinders about 3 ft. long and 2 ft. in diameter, covered with a coarse wire-cloth (No. 18). The fibres that are retained by this wire fall into the refiners, which consist of a couple of horizontal cylinders of sandstone, the upper one only of which revolves. Here they are further disintegrated, and are again passed through the wire-cloth ; this is repeated until all the fibres have passed through. The pulp, after passing through the first sorter, may be conducted through a series of gradually increasing fineness, and, by this means, be separated into different qualities. Though pulp so prepared cannot compete with chemically-prepared stuff, as the fibres are extremely short, and have comparatively little felting-power, it may be used with advantage as a sort of filling-material. It is said to be used entirely for some low-quality newspapers.