The wood to be pulped, mostly consisting of poles from 4 to 8 in. in diameter, has first of all to be (1) cleaned and barked, (2) freed from knots and unsound parts, (3) cut into convenient lengths, and (4) split if necessary, before it is ground into pulp.

The Cleaning and Barking consist in freeing the wood from earth, sand, and other impurities which collect on the ends of the logs, and in removing the bark and cambium, which is usually done by hand in small pulp-mills, otherwise by steaming, or with special machines (see below). The branch-knots and unsound parts are then bored out by means of a rotating auger, or punched out with a chisel in small establishments, and cross-cut into sections of 10 to 20 (usually 14 to 16) in. long, thick billets being split to make the grinding easier.

These billets are then ground down by being pressed, generally lengthways, against a grinding-stone made of hard, fine, and even-grained sandstone, which rotates quickly so as to loosen the fibres at a uniform rate, and resolve them into a loose, pulpy condition. The grinding is usually done under a continuous stream of water to facilitate the separation of the fibres, which have then to be strained, dried, and pressed in moulds.

The grindstones are 40 to 60 in. in diameter, and about 18 or 20 in. thick; and they may be made to rotate either vertically or horizontally. Sometimes they are made of several segments, in place of one solid block, so as to allow of hollow portions being replaced if the stone should get worn out through chance difference in grain or hardness.

About five to eight small billets of the wood to be pulped are put into a box and then steadily pressed under or against the quick-rotating grindstone, and kept wet with the continuous flow of a small stream of water (wet process). The separation of the fibres may take place either longitudinally (as is usual), or transversely; and the force required is calculated at 4 horse-power for every cwt., or 80 horse-power per ton, of dry pulp produced per diem.

The water containing the pulpy fibres is first strained to remove splinters and coarse, incompletely ground pieces, and then passed on to the sieves. These strainers are made of perforated copper, and are set at a slight angle and kept in quick, jerky, side-to-side motion by means of a crank; and there are usually two or three of such strainers placed one above the other, with holes of different sizes. The water containing the pulp in suspension is run on to the upper strainer, then percolates to those with smaller holes, and is run off by a funnel to the pulpmachine, where the pulp is pressed into sheets. The coarser fibres thrown out from the lower edge of the strainers are collected in a gutter leading to a vat, from which they are pumped up to a churn, where they are further reduced by friction, and then once more passed through the strainers. Besides such strainers, however, cylindrical and flat rotating sieves are also in use, where the sorting of the pulp takes place by centrifugal force.

In the pulp-machine the stuff coming from the strainers is run through fine.meshed sieves, the water being drained off and the fibres left behind. But such pulp still contains 80 to 90 per cent of water, and has to be reduced by pressure to at least about half its weight before it can be transported for use elsewhere; while for more thorough drying, it has also to be heated. The pulp has a dullish indefinite colour, is unbleachable, and has a short, stiff fibre, so that it can only be used for making pasteboard, cardboard, and coarse kinds of paper; and unless thus utilised at once, or else pressed into sheets in the machine, the pulp soon changes colour and gets damaged. Pulp thus prepared is known commercially as white pulp.

The grinding of the pulp is greatly facilitated by steaming the wood first of

all under a pressure of 4 to 5 atmospheres (56 to 70 lb. per square inch), when the disintegration takes place more easily, and in longer and softer, and more flexible and felty fibres. But steaming darkens the colour of the wood, and this process is therefore confined to the preparation of brown pulp. Grinding is even still easier, however, if the wood is alternately steamed and then steeped in boiling water under steam-pressure.-A common method in America is the dry process, when little or no water is allowed to flow on to the wood during the process of grinding. A great amount of heat is thus generated by friction, but considerably less force is required, 2 to 3 horse-power being sufficient for each cwt. of dry pulp made every 24 hours, or 50 to 60 horse-power per ton (as compared with 4 horse-power per cwt., or 80 horse-power per ton for the wet process).

The preparation of wood-pulp is now confined almost entirely to out-of-theway places, where there is good water-power, and where the quantity of suitable wood is either too small, or else transport is too dear, to make it worth while to convey it to the nearest cellulose-mill.

2. Cellulose is made from wood which is first cut into thin slices and small chips by machinery, then ground down between grooved rollers, and afterwards macerated or disintegrated by being boiled under high pressure in a solution either (1) of soda (the alkali process), or (2) of calcium sulphite (the acid process, the usual method), or else (3) treated by an electrochemical process. The raw cellulose thus obtained is washed, then bleached with chloride of lime, and finally pressed between heated rollers and dried. Many paper-mills prefer, however, to get the raw cellulose and bleach it for themselves.

Good cellulose should be free from all such impurities as woody knots, bits of bark, earth or sand, dirt from the machinery, &c. Soda-cellulose is chemically purer than sulphite-cellulose, partly because the incrusting substances are less thoroughly dissolved by the latter process, and partly because the chemicals cannot be washed out of the raw pulp so thoroughly. From neither kind of cellulose, however, can such tough, strong paper be made as from rags, although soda-cellulose is the tougher and more like cotton rags, while sulphite-cellulose is weaker and more like linen or hempen rags.

Although Aspen, Poplar, and Lime give the whitest cellulose, much sought after for special purposes, yet the wood of coniferous trees, taken generally as a class, yield a better kind of fibre-longer, more flexible and felty, paler in colour, and more easily bleached-than broad-leaved trees, among which the softwoods are far preferable to most hardwoods. The kinds of wood found most suitable for preparing cellulose on a large scale are Spruce, which is chiefly used, then Scots Pine, and then Silver Fir; and to a much less extent also Larch, Aspen, Poplar, and Birch (see footnote to p. 428).

Resin hinders maceration. But if the wood-chips be treated with a mixture of alcohol and benzine, the resin is dissolved (Müller and Meyer's patent). Paper made from such de-resinised chips is said neither to turn yellow nor to become brittle. Birch yields a pinkish, and Maple and Sycamore a purplish pulp, of low grade and value. The other kinds of wood are practically unsuitable. As small poles and "lop and top" can be used, this often enables thinnings to be disposed of with advantage, unless the cost of transport to the mill is too great.

The various stages in the process of preparing the cellulose are (a) the cleaning and barking; (b) freeing from knots and unsound parts; (c) cutting into thin chips; (d) macerating in the alkali or the acid lye; and (e) washing, bleaching, and drying the cellulose.

The cleaning of the ends of the logs, the stripping of the bark and cambium, and the augering or cutting out of branch-knots and unsound places that cannot be macerated and bleached, take place in the same way as in a wood-pulp mill; but it must be done even more carefully here, as dirt of any sort cannot afterwards be removed from the cellulose. Cleaning can best be done by hand, though this is of course the dearest method, and in all large mills machinery is used to as large an extent as possible.

The wood is first cross-cut in lengths of 2 to 3 ft., and any unsound parts shown at the sawn surface are cut out. The bark and cambium are then removed either by being pressed against a revolving plate set with four plane-like knives, or by

violent friction inside a revolving drum fitted with teeth-like ridges, from between which the pieces of bark are carried away by water; but as the sections of wood are seldom quite round, such barking-machines waste a good deal of the wood. It is therefore better to steam it and strip off the bark by hand, as it comes away easily and there is no loss of wood.

The branch-knots are bored out by pressing the wood against a rotating auger, or else they are, together with unsound pieces, cut out by hand after the wood has been cut into chips by the wood-chopping machine. There are two kinds of chippers or machines for cutting the billets of wood into thin chips or slicing it into small pieces. In the one the cutting chisels work vertically up and down; and in the other, the usual kind, they rotate. The chipper cuts or shaves off chips of wood at an angle of 45° to 60° to the axis of the billet; and the thinner the chips, the easier they can be acted on by the lye during the process of boiling and maceration.

Ransome's Pulp-Wood Chipping Machine consists of a large disc, on the face of which are fixed a number of steel cutters, so arranged as to act successively on the piece of wood to be cut. Holes slotted through the disc contain the chisels, placed with their bevelled edge towards the wood, so that they cut with the short side of their cutting angle in about the same plane as the direction of the cut. These chisels thus operate across the grain of the wood, and chop or break it into small pieces, which are so bruised and loosened in fibre

as to be easily ground to a fine powder by the grinding machine. The disc and its spindle are tilted so as to strike the wood at an angle of 45° to 60°. The disc is fed with wood by an automatic roller worked by ratchet and pawl gearing.

Fig. [281 shows a Wood-Chipping Machine with 7 ft. Disc capable of chipping either a log or a scantling (not exceeding 14 in. x 8 in.) into small pieces across the grain. It consists of a massive cast-iron disc mounted on a steel shaft, set at such an angle that the cutters strike the wood at an angle of 60°. Ten cutters are arranged in echelon on the face of one side of the disc, and project through it just sufficiently to cut chips of the desired thickness. The disc makes 240 revolutions per minute, and as the cutters are so spaced that they cut only during a period of somewhat less than half a revolution of the disc, the remainder of the revolution enables the disc to recover any momentum lost while the cutting is taking place. The chips are projected through the back of the disc, and are carried away either by a pneumatic exhaust apparatus or a conveyor.

The timber is fed up to the cutters along a horizontal table by a positive variable feed consisting of a flat-link chain running in the bed of the table, and into which suitable steel dogs can be inserted at any desired point to suit scantlings of different lengths. The dogs are so constructed that they fall out of the chain as it turns round the chain tumbler, and before the dogs come within reach of the cutters. The rate of feed can be varied to suit the hardness of the timber being cut, and the gear is controlled by levers worked by a hand-lever placed near the operator.

From the chipper the wood-chips go into a grinding-mill, in which they are ground down or minced between grooved rollers into small pieces about threequarters of an inch long and one-fifth to one-third of an inch thick, and then passed on to a drum-sieve, where they are freed both from fine dust and from coarse bits of branches, unground chips, &c., and thus made ready for maceration. The loss of wood in these preliminary processes of cleaning and preparing amounts to from 5 or 6 up to 10 or 15 per cent for barked poles and stems with few branches, according to their quality; while, for wood of branching growth and with the bark on, it varies from 15 to 20 per cent for large poles, and up to 30 per cent for small poles below about 3 in. diameter.

(1) The Alkali or Soda process of macerating the wood is the oldest method of treating the chips; but as it is the more expensive, and produces only a smaller quantity of cellulose, it is now less generally in use than the acid process. It is still in use, however, and likely to remain so, as its product makes a good class of paper, fine, soft, and pliable, which takes ink well. There are two varieties of this method, in one of which caustic soda (NaHO) is used in producing maceration, while in the other (Dahl's process) this is effected by means of sulphate of soda (Na„SO4). The latter is less caustic in its action, though sufficiently so to dissolve the lignin; it therefore causes less waste of cellulose. It is also much cheaper and easier to prepare and renew the lye; but it has the great disadvantage of causing an abominable stench, both in preparing the lye and while boiling the chips, so that such factories must form a public nuisance to any residents round about where they are in operation. Only the caustic soda process need therefore here be considered in detail.

When Caustic Soda or Hydrated Sodium (Sodium hydroxide, NaHO) is used in a solution of 8 to 10 per cent strength (generally about 10 per cent) to boil the wood-chips at a temperature of 320°-365° Fahr. (thereby generating an extra pressure of 5-10 atmospheres, or about 70 to 140 lb. per square inch), the ligneous and other incrusting substances become entirely dissolved from the woody fibres, and a very pure cellulose remains after the lye is run off. The process is therefore effective; but its drawbacks are (1) that caustic soda is dear, (2) that it can only be partially re-obtained from the lye after use, and (3) that it eats into part of the cellulose and thus causes wastage.

The caustic lye is prepared by causticising a solution of soda with quicklime in iron vessels provided with a stirring apparatus and an intermittent steam blast-pipe. Carbonate of soda and quicklime are mixed in the proportion of 2 to 1.1 (although this proportion of quicklime is about 2 per cent more than is needed theoretically), and the transformation which takes place in boiling this by means of jets or blasts of steam and continuous stirring is that NaCO3 + CaO + H2O = 2 NaHO + CaCO3, and when the steam is shut off the carbonate of lime (CaCO3) is allowed to settle at the bottom, and the hydrated sodium or caustic soda lye is drawn off and filtered to purify and clear it. The deposit is also washed when sufficient in bulk, to get the caustic soda contained in it.

For every cwt. of dry cellulose produced, from 150 to 160 gallons of lye are needed, or about 13 gallons for each lb. in weight.

The lye becomes discoloured and dark-brown from the decomposed lignin, &c., during the boiling and maceration of the wood-chips, but most of the soda remaining in it can be recovered and the lye brought up again to the proper strength by adding new soda. The operation of recovery is, however, tedious and expensive. The discoloured lye has to be heated till all the water is evaporated as steam, and the dry residuum is then calcined in a furnace, when the organic matter is consumed and carbonate of soda is left-the soda being burned as white as possible, but without quite reaching its melting-point. If fine particles of carbon then still remain in the soda they cannot afterwards be removed by filtering; and if they subsequently get in among the cellulose during boiling, they produce a greenish-blue discolouration that cannot be removed in bleaching. The calcining occasions a loss of about 12-15 per cent of the soda, and as the soda becomes more and more impure silicates are formed, while the wear and tear of the sole-plates of the furnace make frequent repairs necessary.

The wood-chips are macerated in upright cylinders of about 350 cub. ft. capacity, made of strong sheet-iron, packed with asbestos, and cased in wood to keep in the heat as much as possible; and all the piping and other parts coming in contact with the caustic lye must also be of iron. Horizontal boilers placed directly over a furnace were formerly used, but upright boilers are now generally preferred. Inside the boilers there are perforated sheet-iron cases, with holes large enough to allow the lye to circulate freely, but small enough to prevent the wood-chips being washed out. The wood-chips are packed into these perforated cases from an opening on the top of each boiler, and the raw cellulose is taken out from a side-hole below. The boilers are of course fitted with piping and taps for bringing in and leading off steam, and for running in fresh lye and carrying off what has been used. The boiler is filled from below with the caustic solution, so as to drive the air upwards from between the chips, and thus get rid of it before the boiler gets filled and hermetically sealed, and the boiling and maceration begin. In large pulp-works two or more boilers are sometimes connected together with piping, so that the caustic solution circulates from one to the other; this utilises the lye more thoroughly, and produces a fine quality of cellulose. The strength of the lye used, the time allowed for boiling, and the temperature at which, and consequently the atmospheric pressure under which, the chips are boiled, depend on the kind of wood being treated. The caustic lye usually varies from 8 to 12 per cent, the time of boiling from 6 to 24 hours, and the pressure from 5 to 10 atmospheres (70 to 140 lb. per square inch); but accurate details as to temperature (i.e., pressure) and duration of boiling are generally trade secrets. Conifers are easier to treat than the wood of broad-leaved trees; and among Conifers, Spruce-wood is most easily pulped, while Scots Pine and Larch can both be macerated much more easily than Silver Fir.

In the above soda process, where as much as possible of the caustic soda remaining in the lye is recovered, the raw cellulose is washed for this purpose before being bleached. It is therefore moved direct from the boiler to an iron or cement cistern with a perforated bottom, which forms one of a set of four to eight similar cisterns, ranged at different levels