CHAPTER V

WOOD PULP AND WOOD PULP PAPERS

THE MANUFACTURE OF MECHANICAL WOOD PULP.

Wood is converted into pulp suitable for the manufacture of paper by methods which produce two distinct varieties. The first is _mechanical wood pulp_, so called because it is made by a purely mechanical process. The second is termed _chemical wood pulp_ from the fact that the material is submitted to chemical treatment.

_Ground Wood and Cellulose._--The two varieties of pulp are sometimes distinguished by the use of the terms ground wood and cellulose. In the former case the description implies a product consisting of pulp obtained by grinding wood into a fibrous condition, while in the second the word suggests a purified chemical product freed from the resinous and non-fibrous constituents found in wood. This is, in fact, the essential difference, for mechanical wood pulp consists of fibres which have been torn away from wood by means of a grindstone; it differs but slightly in chemical composition from the original raw material and contains most of the complex substances natural to wood. Chemical wood pulp, on the other hand, consists of fibre isolated from wood in such a manner that the complex non-fibrous substances are more or less entirely removed. The difference between these two pulps is shown in the following approximate analysis of spruce wood, and of the pulp derived from it. The composition of the mechanical pulp is practically identical with that of the wood itself.

COMPOSITION OF SPRUCE WOOD, AND OF CHEMICAL WOOD PULP (SPRUCE).

----------------+---------+---------- -- | Wood | Chemical |(Spruce).|Wood Pulp. ----------------+---------+---------- Cellulose | 53·0 | 88·0 Resin | 1·5 | 0·5 Aqueous Extract | 2·5 | 0·5 Water | 12·0 | 8·0 Lignin | 30·5 | 2·5 Ash | 0·5 | 0·5 +---------+---------- | 100·0 | 100·0 ----------------+---------+----------

The use of mechanical wood pulp is generally confined to the manufacture of news, common printings and packing papers, cardboards, and boxboards. It possesses very little strength, quickly discolours when exposed to light and air, and gradually loses its fibrous character. The chemical wood pulp is a strong fibre, from which high-class papers can be manufactured, the colour and strength of which leave little to be desired.

_Species of Wood._--The woods most commonly used for the manufacture of wood pulp belong to the order Coniferæ, or cone-bearing trees. In Europe the spruce and silver fir are the chief species, while in America spruce, balsam, pine, and fir are employed. The harder woods, such as hemlock, beech, larch and others, are not converted into pulp by the mechanical process.

_Timber Operations._--The trees are cut down in the early part of winter by gangs of men specially trained to the work. The organisation of a lumber camp when the operations are of an extensive character is very complete and carefully arranged, every detail being attended to in order to get out the wood as cheaply and expeditiously as possible. The branches and small tops are removed from the trees when they are fallen, and the trunks cut into logs of 12, 14, or 16 feet in length, and afterwards piled up on the banks of the nearest river, or on the ice, ready for the breaking up of the winter.

As soon as the ice breaks up and the rivers become navigable the logs are floated down to their destination, in some cases hundreds of miles from the scene of operations. Where rivers are not available the timber is brought out by horses or bullocks, or by means of a light railway.

_Log Cutting._--As the timber arrives at the mill it is carefully measured, both as to its diameter and length, in order that a record may be kept of the quantity used. Some of the logs are piled up in the storeyard for use in the winter, and the remainder converted into pulp day by day. The logs are first cut into short pieces about 2 feet long by means of a powerful circular saw, the arrangements for this work being devised so as to keep down the cost of labour as much as possible. All waste pieces are thrown aside to be utilised as fuel.

_Barking._--The bark on the logs is removed in one or two ways. Much of it is knocked off during the transfer from the forest to the mill, but even then the wood requires to be cleaned. In Norway and Sweden the wood is treated in a _tumbler_ or a _barker_, while in America and Canada the use of the tumbler is practically unknown.

The barker consists of a heavy iron disc fitted with knives, usually three in number, which project from the surface of the disc about half or three-quarters of an inch. The barker rotates in a vertical position, and the short pieces of wood are brought one by one into contact with the disc in such a manner that the bark is shaved off by the knives. The machine is provided with conveniences for pressing the wood against the disc and for turning the logs as they are barked.

The machine is encased in a strong cast-iron cover, and all the bark shaved off is carried away by the strong current of air set up by the rapid motion of the disc, and subsequently burnt.

The tumbler system is quite different. In this case the short pieces are thrown into a large circular drum with hot water, and the bark taken off by the friction of the pieces as the drum rotates. The loss of material is of course less in this process, but the wood is not cleaned quite so effectively.

The wood at this stage can be used either for the manufacture of mechanical or chemical pulp. As a general rule the pieces are taken indiscriminately for either process, but sometimes the wood is sorted out, the clean stuff free from knots and blemishes being reserved for high quality chemical pulp.

_Grinding._--The main feature of the grinding process is the attrition of the wood when held against the surface of a rapidly revolving grindstone, the fibres as they are rubbed off being instantly carried away from the stone by a current of water. A complete description of the machines used and the modifications of the process practised by manufacturers is impossible in this book, but the following points will be sufficient.

The machine consists of a large grindstone about 54 inches in diameter, and 27 inches thick. It rotates in a vertical or in a horizontal position at a high speed. The stone revolves inside a casing which is provided with a number of _pockets_, so called, into which the pieces of wood are thrown at regular intervals, as fast as the wood is ground by the friction of the stone.

A continual stream of water playing upon the surface of the stone washes away the pulp into a tank or pit below the machine.

The quality of the pulp may be varied by the conditions under which it is made. By limiting the proportion of water so that the wood remains in contact with the stone for a longer time the temperature of the mass in the pockets rises. Such _hot ground pulp_, as it is termed, is tough and strong.

When the fibres are washed away from the stone as fast as they are produced the temperature does not rise, and _cold ground_ pulp is made, which is not characterised by the somewhat leathery feel of the pulp made at the higher temperature.

The surface of the stone plays an important part also. If the stone is smooth the wood is rubbed away slowly, but if the surface has been roughened and grooved by means of a special tool the fibres are torn away quickly. In the first case the pulp comes from the stone in a finely-ground state and in a uniform condition, while in the second the pulp is coarse and chippy.

The output of the machine is, however, much increased by the use of sharp stones and by the application of considerable pressure to the blocks of wood.

_Screening._--The mixture of water and pulp leaving the grinder falls into a tank below the stone, all large chips being retained by means of a perforated plate. The finer pulp, still too coarse for use, is then pumped to the screens, which serve to remove all chippy and coarse fibres and produce a uniform material. The _shaking sieve_ consists of a shallow tray, the bottom of which is a brass plate or series of plates perforated with small holes or slits. The pulp flows on to the tray, which is kept in a state of violent agitation, the fine pulp passing through the holes and the coarser pieces working down to the lower edge of the tray into a trough which carries them away. The _flat screen_ is somewhat different in construction, but the principle of separation is the same. It consists of brass perforated plates forming the bottom of a shallow cast-iron tray, continually agitated by means of cams fixed to the under surface of the trays.

The _centrifugal screen_ is a cage made of finely perforated brass sheeting which revolves at a very high rate of speed inside a circular cast-iron vessel. The pulp flows into the interior of the cage, the fine fibres being forced through the screen by the centrifugal action of the machine, and the coarse material is retained.

_Wet Pressing._--The pulp leaving the screens is mixed with such a large quantity of water that it is necessary to concentrate it. This is effected by means of the wet press machine (Fig. 41). The pulp and water are pumped into a wooden box in which revolves a large hollow drum, the surface of this drum consisting of a fine wire cloth of about 60 or 70 mesh. The drum is not entirely immersed in the mixture, so that as it rotates the pulp forms a skin or thin sheet on the surface, and the water passes away through the wire into the interior of the hollow drum. The drum carries the thin sheet out of the box and above the level of the mixture until it comes into contact with an endless blanket or felt, which is pressed against that part of the drum not immersed in the liquid.

By this means the thin sheet is transferred to the felt and carried between squeezing rolls to the finishing rolls. The felt, carrying on its upper surface the thin sheet of pulp, passes between two rolls, usually 16 to 20 inches in diameter, the upper being made of wood and the lower one of cast iron. The pulp adheres to the upper drum and the felt passes round the lower drum back to the box containing the mixture of pulp and water; the thin sheet is continuously wound on the upper roll until a certain thickness is reached.

When this occurs the attendant removes the thick sheet by a dexterous movement of a sharp stick across the face of the roll. The wet pulp at this stage consists of 30 per cent. air-dry pulp and 70 per cent. of water.

_Hydraulic Pressing._--The sheets taken from the wet press machine are folded into a convenient shape and piled up, coarse pieces of sacking being placed between the sheets. At stated intervals the piles are submitted to pressure in hydraulic presses in order to remove further quantities of water, which slowly drains away through the sacking. In this way a mass of pulp in the form of thick folded sheets containing 50 per cent. of dry wood pulp is produced.

The pieces of sacking are taken out and the sheets put up in bales of any required weight, usually 2 cwt. or 4 cwt.

THE MANUFACTURE OF CHEMICAL WOOD PULP.

Most vegetable fibres are converted into pulp by alkaline processes, that is by digesting the raw material with caustic soda and similar alkaline substances. Wood may be treated in two ways, one of which is the ordinary soda process, and the other an acid treatment requiring the use of sulphurous acid.

_Preparation of the Wood._--The logs of wood are cut up and barked exactly as in the case of mechanical pulp. The short two-foot pieces are then cut up into small flakes about one inch square and half an inch thick by means of a machine known as a _chipper_. This is similar in construction to a barker, consisting of a heavy iron disc rotating at a high speed inside a stout cover. The disc revolves in a vertical position, and three projecting knives slice up the logs into flakes. For this purpose the disc is provided with three slots which radiate from the centre towards the circumference for about 12 inches. The knives can be adjusted so that they stand up through the slots and above the surface of the disc to any required distance.

In order to ensure uniformity in the size of the chips, the practice is frequently adopted of sifting the wood leaving the chipper. The sieve is a large skeleton drum, the outer surface of which is made of a coarse wire cloth capable of passing all pieces of the size mentioned. Larger chips and pieces are retained in the drum as it revolves in a horizontal position and only fall out on reaching the extreme end of the machine.

_The Digesters._--The object of boiling the wood under pressure with chemicals is to dissociate the valuable fibrous portion of the plant from the resinous and non-fibrous portion. In this process the wood loses half its weight, the yield of pulp being about 50 per cent., and the remainder is dissolved out by the chemical solution. The conditions of treatment are extremely varied in character, the quality of the pulp produced varying in proportion.

The digesters are either spherical, cylindrical, or egg-shaped, being constructed to revolve at a slow rate of speed, or fixed permanently in an upright position. Spherical boilers are usually 9 or 10 feet in diameter, the cylindrical digesters being 40 or 50 feet high and 12 or 15 feet diameter, the larger ones being capable of taking 20 tons of wood for each operation.

For the alkaline process the interior of the digester does not require any special treatment, but with the acid process the internal portion of the boiler is carefully lined with a thick layer of acid-resisting brick and cement.

The contents of the digester are heated by means of high-pressure steam, which is blown direct into the mass or passed through a coil lying at the bottom of the vessel. In the former case the steam is condensed by the liquor, the volume of which is consequently increased, while in the latter case the condensed steam is drawn off continuously from the pipes. Each system has its own particular advantages.

_Different Kinds of Chemical Wood Pulp._--According to the method of treatment so the quality of the pulp varies. The chemicals used, the system of boiling, the temperature of digestion, the strength of the solutions, the duration of the cooking period, and, last but not least, the species of wood, are all determining factors in the value of the ultimate product.

_Soda Pulp._--This is prepared by digesting wood with caustic soda in revolving boilers for eight or ten hours at a pressure of 60 to 80 lbs.

_Sulphate Pulp._--Prepared by digesting the wood with a mixture of caustic soda, sulphide of soda, and sulphate of soda.

_Sulphite Pulp._--The process most generally adopted for the manufacture of wood pulp is the treatment of the material in brick-lined digesters with bisulphite of lime for eight to nine hours at a pressure of 80 lbs.

_Mitscherlich Pulp._--This is sulphite pulp prepared by digesting the wood at a much lower temperature and for a longer period than the ordinary sulphite. The steam is not blown direct into the mass of wood, and the pressure seldom exceeds 45 or 50 lbs., the time of boiling occupying 45 to 50 hours. So called from the name of the inventor.

_Sulphite Wood Pulp._--This name is given to pulp prepared by digesting wood with solutions containing sulphurous acid, or salts of sulphurous acid. The acid is produced by burning sulphur or certain ores containing sulphur, such as copper or iron pyrites, in special ovens. The most modern form of oven consists of a cylindrical cast-iron drum revolving slowly in a horizontal position on suitable bearings. The sulphur is thrown at intervals, or fed automatically, into the oven, the amount of air being carefully regulated to avoid the formation of sulphuric acid in the later stages of preparation. The sulphur is also burnt in stationary ovens which consist of flat shallow closed trays.

The hot sulphurous acid gas passes through pipes and is cooled, after which it is brought into contact with water and lime for the production of the bisulphite of lime. This is accomplished by one of two methods as follows.

_Tower System._--The cool gas is drawn into high towers usually built of wood, 7 or 8 feet diameter, which are filled with masses of limestone. From tanks at the top of each tower a carefully regulated quantity of water flows down upon the limestone and absorbs the ascending column of gas, this being drawn into the tower from the bottom. The limestone is simultaneously dissolved, and the liquid which flows out from the pipes at the bottom of the tower consists of lime dissolved in sulphurous acid, together with a certain proportion of free sulphurous acid. This is generally known as a solution of bisulphite of lime.

_Tank System._--The somewhat costly tower system has in many cases been superseded by the use of a number of huge wooden vats, 10 to 12 feet diameter and 8 to 10 feet high. These tanks are filled with water and a known quantity of slaked lime. The gas is forced into the tanks by pressure or drawn through by suction, and the conversion of the milk of lime into bisulphite of lime proceeds automatically. In order to ensure complete absorption the gas passes through the tanks in series, so that the spent gases leaving the vats do not contain any appreciable amount of sulphurous acid.

In order to obtain pulp of uniform quality it is necessary that the liquor should be of constant composition. The formula differs in the various mills according to the conditions which are found most suitable.

_Sulphite Digesters._--The almost universal form of boiler employed in cooking wood by the sulphite process is a tall cylindrical vessel of about 50 feet in height, and 14 to 15 feet internal diameter, lined with acid-resisting brick.

This form of digester is capable of holding 20 tons of wood at one charge, yielding 10 tons of finished pulp.

The chipped wood is discharged into the digesters from huge bins erected just above the openings to the digesters, so that the latter can be filled without any delay and the requisite quantity of sulphite liquor added.

The manhole or cover is at once put on, securely fastened, and steam turned on gradually until the pressure reaches 70 or 80 lbs., at which pressure the cooking is steadily maintained. The progress of the operation is watched and samples of the liquor drawn from the boiler at intervals to be tested, so that the boiling may be stopped when the results of the testing show the wood is sufficiently cooked.

There is no special difficulty in this operation, provided the necessary conditions are observed. It is important that the wood should be dry, and that the proportion of sulphite liquor per ton of dry wood should be constant. If the wood happens to be wet, due allowance must be made for the excess water and a somewhat stronger liquor used in order to compensate for this. Other precautions of a similar character are observed in order to minimise the danger of an insufficiently cooked pulp.

_Washing._--When the pulp has been boiled, a process which generally occupies seven or eight hours, the steam is shut off and the contents of the boiler blown out into large vats known as blow-out tanks, the pressure of steam remaining in the digester being sufficient to empty the softened pulp in a few minutes. Much of the spent sulphite liquor, now containing the dissolved resinous and non-fibrous portions of the original wood, drains away from the mass in the tank, and then copious supplies of clean water are added in order to wash out the residual liquors which it is essential to remove.

Numerous other devices are employed to ensure the complete washing of the boiled pulp.

_Screening._--The production of a high-class pulp necessitates proper screening to eliminate coarse pieces of unboiled wood and the knots, the latter not being softened completely. The methods adopted vary according to requirements.

For uniform clean pulp that can be bleached easily the material from the blow-out tanks is, after washing, mixed with large quantities of water and run through sand traps, which consist of long shallow wide boxes provided with slanting baffle-boards to retain knots and large pieces of unsoftened wood, the pulp thus partially screened being subsequently treated in the proper screening apparatus.

Sometimes the washed pulp is sent direct to the screens and the well-boiled fibres sorted out by a system of graded screens, which separate the completely isolated fibres from the bulk and retain the larger pieces, these being broken down in a suitable engine and put back on the screens.

The machinery employed for screening chemical pulp is identical with that used for the treatment of mechanical wood pulp.

_Finishing._--The ordinary sulphite pulp is worked up into the form of dry sheets for the market and not sent out in a wet state as the mechanical wood. There are several practical disadvantages in preparing the latter in a dry condition which do not, however, occur with chemical pulp.

Hence the pulp after being screened is not pressed but submitted to a different process. From the screens the mixture of pulp and water, the latter being present in large quantity, is pumped into a concentrator, or slusher, as it is termed, by means of which some of the water is taken out.

The slusher consists of a wooden box divided into two compartments by a vertical partition. In the larger compartment a hollow drum covered with a fine wire cloth revolves, the construction and purpose of which are precisely the same as that of the wet press machine used for mechanical pulp.

As the drum revolves the pulp adheres to the outer surface, while the water passes through the wire cloth. The drum is not completely immersed in the mixture, so that the skin of pulp is brought out of the water by the rotation of the drum. When this takes place the contact of a wooden or felt covered roll which revolves on the top of the drum causes the pulp to be transferred from the drum to the roll. The wet pulp is continuously scraped off by an iron bar or _doctor_, as it is called, resting on the surface of the roll, and it finally drops into the second compartment of the slusher in a more concentrated form ready for the drying machine.

_Drying._--The mass of wet pulp from the slusher is conveyed into a circular reservoir or _stuff chest_, which serves to supply the machine used for converting the pulp into dry sheets.

The machine is to all intents and purposes a Fourdrinier paper machine, and the process is similar to that used for the manufacture of paper. The pulp flows in a continuous stream on to a horizontal endless wire, which carries it forward as a thin layer; the water drains through the meshes of the wire, further quantities being removed by _suction boxes_, which draw away the water by virtue of the vacuum produced by special pumps. The wet sheet then passes between the _couch rolls_ which compress the pulp, squeezing out more water, and then through _press rolls_, which finally give a firm adherent sheet of pulp containing 70 per cent. of water. The sheet is dried by passing over a number of steam heated cylinders, which cause all the moisture to evaporate from the pulp. At the end of the machine the dry pulp is cut up into sheets of any convenient size, and packed up in bales of two or four cwts.

_Mitscherlich Sulphite Pulp._--This term is applied to sulphite wood prepared by submitting the chipped wood to a comparatively low pressure for a long period. The wood is placed in the stationary upright form of digester with the requisite amount of liquor, and the heating produced by the passage of steam through a leaden coil lying at the bottom of the digester, so that the steam does not condense in the liquor but in the coil, from which it is drawn off. The pressure seldom exceeds 45 lbs. but the duration of the cooking is thirty-six to forty-eight hours. The boiler is not emptied under pressure, but the pulp is discharged from the digester after the pressure has been lowered, and the manhole taken off. The contents are usually shovelled out by the workmen.

The pulp is carefully washed, screened and made up into wet sheets on the ordinary wet press machine. This pulp is never dried on the Fourdrinier like the common sulphite, as its special qualities can only be preserved by the treatment described. This pulp is particularly suitable for parchment papers, grease proofs and transparent papers.

_Soda Wood Pulp._--The chipped wood is boiled in stationary or revolving digesters for eight or nine hours at a pressure of 70 or 80 lbs. A solution of caustic soda is employed, about 16 to 20 per cent. of the weight of the wood being added to the contents of the digester. Live steam is blown direct into the mass, and after the operation the spent liquor is carefully kept for subsequent treatment. The pulp is washed in such a manner that the amount of water actually used is kept down to the smallest possible volume consistent with a complete removal of soluble matters. This is done in order that the spent liquors may be treated for the recovery of the soda.

_Recovery of Spent Liquors._--When wood is cooked by the soda and sulphate processes the solutions containing the dissolved organic matter from the wood can be evaporated, and the original chemical recovered. In the case of soda pulp the method of treatment is as follows: the spent liquors and the washings are evaporated by means of a multiple effect vacuum apparatus to a thick syrup. The concentrated liquor produced is then burnt in special furnaces, all the organic matter being consumed, leaving a black mass which consists mainly of carbonate of soda. The mass is washed with water to remove the carbonate which is afterwards converted into caustic soda by being boiled with lime.

The spent liquors from the sulphite process have no value, for they cannot be recovered by this method. At present the whole of the sulphur used and the organic matter dissolved from the wood is lost. This means the loss of about 250 to 350 lbs. of sulphur and nearly 50 per cent. of the weight of wood for every ton of pulp produced.

WOOD PULP; MICROSCOPIC FEATURES.

Mechanical and chemical pulps are readily distinguished under the microscope. The former consists of fibres of irregular shape and size, mixed with a large proportion of structureless particles, all bearing evidence of having been torn apart and separated by mechanical methods. The chemical pulp, on the other hand, consists of fibres isolated by a process which preserves them in perfect condition and form. The pulp from the various woods can be differentiated by minute details in fibre structure, some of the woods being determined from the presence of characteristic cells.

The use of aniline sulphate can also be resorted to, and for microscopic work the most useful reagent is a mixture of zinc chloride and iodine. This produces an intense yellow colour with mechanical pulp and a bluish colour with sulphite and other chemical wood pulps.

THE DAILY NEWSPAPER.

The newspapers of the present day are made almost exclusively of wood pulp. The use of the latter material for paper-making has steadily increased from the date of its introduction about A.D. 1870, when wood pulp was imported into England in considerable quantities.

News and cheap printings consist of mechanical and chemical wood pulps mixed in varying proportions determined chiefly by the price paid for the finished paper. In some cases the proportion of mechanical wood pulp is as much as 85 per cent., though the average composition of a cheap wood paper is represented by the following proportions: Mechanical pulp, 70 per cent.; sulphite pulp, 20 per cent.; loading, 10 per cent.

Some idea of the enormous quantity of material used for the daily press may be judged from one or two examples. A certain popular weekly newspaper having a circulation of one and a quarter million copies per week requires every week 137 tons of paper produced from 170 tons of wood. A popular halfpenny newspaper boasting a circulation of about one-half million copies per day consumes 185 tons of paper manufactured from 230 tons of wood, every week.

It is easy also from these facts to estimate the amount of timber which must be cut down to supply the demand for newspapers and cheap printings.

The manufacture of news calls for considerable skill and able management, owing to the keen competition amongst the paper mills devoted to this class of paper. The process as carried on in England is as follows:--

The mechanical pulp, reaching the mill in the form of thick sheets suitably packed up into bales, is first broken up again into moist pulp. Various machines are used for this, such as Wurster's kneading engine, Cornett's breaker, or some similar contrivance. An old potcher, such as is used for the breaking and washing of rags, makes a good pulp disintegrator. The broken pulp is discharged into beating engines in any suitable or convenient manner and the right proportion of chemical wood pulp added in the form of dry sheets. The beating process only occupies thirty to forty minutes in the case of the common news, a marked contrast to the eight or nine hours required by rags. China clay is added to the contents of the beater, ten to twelve per cent. being the general practice. This is followed by a measured quantity of rosin size, and after thorough incorporation the size is precipitated upon the fibres by means of alum.

In the commoner qualities of these papers the materials are added in the dry state, but for finer grades of newspaper the china clay is mixed with water, and carefully drained through a fine sieve before use. The alum cake is also dissolved and treated in a similar manner in order to keep out dirt and coarse particles likely to produce holes in the paper.

The paper machine used for the manufacture of cheap printings is constructed to produce as much as 100 to 180 tons of finished paper per week, every detail being arranged for a large output at a very high speed. In the modern machine it is possible to produce paper at the rate of 450 to 550 feet per minute, the width of the sheet being from 120 to 160 inches.

Careful attention is paid to economy of every kind with regard to the power required for driving the machine, the amount of steam consumed in drying the paper, recovery of excess of fibre and china clay which escapes from the machine wire, and similar details of a mechanical order.

The beaten pulp, after being sized and coloured, is discharged into huge circular brick tanks, or stuff chests, two of which are found with each paper machine. The supply of pulp and water for the machine is taken from one stuff chest while the second is being filled up from the beating engines, in order to secure a mixture of constant composition.

The pulp is pumped from the stuff chest into a small regulating box placed above the machine wire, and this box is kept full of beaten pulp so that the supply of pulp and water to the machine is perfectly constant. The pulp, diluted with the proper quantity of _back-water_, is carefully strained through rotary screens and allowed to flow through a distributing box on to the machine wire, where it rapidly forms a sheet of paper.

The excess of water, together with a certain proportion of fine fibre and china clay, falls through the wire, and is caught below in a shallow box, called the save-all. This _back-water_, as it is called, is used over again for diluting the beaten pulp to the right consistency, as already described.

The whole of the water obtained in this way is not all utilised in the regulating box, and any surplus is pumped up continually into large store tanks and used in the beating engines for breaking down the dry pulp.

In many cases, where a large quantity of water is used on the machine, special methods have to be adopted for the recovery of all the fibre and clay, which would otherwise be lost, and there are many ingenious systems in use whereby this saving is effected.

The most usual practice is to allow the excess of water, which contains from 8 to 15 lbs. of suspended matter per thousand gallons, to flow through a series of brick tanks at a slow rate of speed. The clay and fibre settle to the bottom of the tanks, and the water passes away from the last tank almost clear and free from fibre and loading.

The drying of the moist paper leaving the press rolls of the machine is effected in the usual manner by means of drying cylinders. On account of the great increase of speed at which the paper is produced, the number of drying cylinders has also been increased, and at the present time a machine of this description is provided with 28 or 32 cylinders, the object being to dry the paper economically.

MECHANICAL WOOD PULP IN PAPER.

The presence of mechanical wood pulp in paper is detected by means of several reagents, which produce a definite colour when applied to a sheet of paper containing mechanical wood. The depth of colour obtained indicates approximately the percentage present, but considerable practice and experience is necessary to interpret the colour exactly. A more reliable method of estimating the percentage of mechanical wood in a paper is by microscopic examination.

The reagents which can be used are--

(1) _Nitric Acid._--This produces a brown stain on the paper, but it is not a desirable reagent for ordinary office purposes.

(2) _Aniline Sulphate._--A solution of this is prepared by dissolving 5 parts of aniline sulphate in 100 parts of distilled water. When applied to the surface of news a yellow coloration is produced, more or less intense according to the amount of mechanical wood present. It can only be used with white papers, or papers very slightly toned.

(3) _Phloroglucine._--This sensitive reagent, which gives a rose-pink colour when brushed on to the surface of the paper, is prepared by dissolving 4 grammes of phloroglucine in 100 c.c. of rectified spirits, and adding to the mixture 50 c.c. of pure concentrated hydrochloric acid.

There are several other aniline compounds which give colour reactions of a similar character, but they are not often used. The phloroglucine reagent fails as a test for mechanical wood in papers which have been dyed with certain aniline colours, for example, metanil yellow. Paper which has been coloured with this dye will, when moistened with the phloroglucine reagent, give an intense pink colour, even if no mechanical wood is present. This is due to the fact that the dye itself is acted upon by the hydrochloric acid in the test reagent. The same colour is produced on the paper with hydrochloric acid _per se_.

There is little difficulty in distinguishing between the colour arising from the presence of such a dye, because the effect is instantaneous, whereas the coloration due to mechanical wood develops gradually. Moreover, the reaction due to the presence of metanil yellow gives a perfectly even coloured surface, whereas with mechanical wood pulp the fibres appear to be more deeply stained than the body of the paper.

_Output of a Paper Machine._--The quantity of paper which can be produced on the paper machine is readily calculated from the following data:--

Speed of machine in feet per minute _F_ Nett deckle width in inches _D_ Width of sheet of paper in inches _W_ Length of sheet of paper in inches _L_ Number of sheets in ream _S_ Weight of paper per ream _R_

The general formula for the output of paper per hour is

720 × _F_ × _D_ × _R_ Output in lbs. per hour = -----------------------. _S_ × _L_ × _W_

When the number of sheets in the ream is 480, this formula simplifies to

1½ × _R_ × _F_ × _D_ Output in lbs. per hour = --------------------. _L_ × _W_

The term "nett deckle width" applies to the width of the trimmed finished paper at the end of the machine. The formula takes no account of the allowance required for trimming edges. In most cases the deckle width of the machine is arranged so that the paper is cut into strips of equal width when leaving the calenders, _e.g._, a deckle of 80 inches will give 4 sheets, each 20 inches wide.

The method by which the general formula is obtained may be explained by an example.

What is the output of a machine having a speed of 100 feet per minute, with an 80-inch deckle, producing a sheet of paper 20 inches by 30 inches, weighing 30 lbs. per ream of 480 sheets?

The machine produces every minute a sheet of paper 100 feet long and 80 inches wide.

Hence output per minute in square inches

= 12 × 100 × 80. Output per hour in square inches = 60 × 12 × 100 × 80.

Now each (20 × 30 × 480) square inches is area of one ream.

Output of paper per hour in reams

60 × 12 × 100 × 80 = ------------------. 480 × 30 × 20

Output of paper per hour in lbs.

720 × 100 × 80 × 30 = ------------------- 480 × 30 × 20

= 600 lbs.

The general formula may be applied for the purpose of calculating the speed at which the machine must be driven.

_Example._--A machine with 75-inch deckle is required to produce 6 cwts. per hour of a paper 25 inches by 18 inches (500 sheets), weighing 19 lbs. to the ream. At what speed is the machine to be driven?

Output in lbs. per hour

720 × _F_ × _D_ × _R_ = --------------------- _S_ × _L_ × _W_

720 × F × 75 × 19 672 = ----------------- 500 × 18 × 25

_F_ = 148 feet per minute.