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.
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 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.
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
| Output in lbs. per hour = | 720 × F × D × R | . |
| S × L × W |
When the number of sheets in the ream is 480, this formula simplifies to
| Output in lbs. per hour = | 1½ × R × F × D | . |
| 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 | |
| 672 = | 720 × F × 75 × 19 |
| 500 × 18 × 25 | |
| F = | 148 feet per minute. |
Common Browns.—The raw material used in the manufacture of common brown papers is chiefly jute and waste fibres of every description, such as waste cuttings from boxboard factories, old papers, wood pulp refuse, and other substances of a like nature. The jute, in the form of sacking or old gunny bags, and the hemp refuse, in the shape of old rope and string, are subjected to a slight chemical treatment just sufficient to isolate the fibres to a condition in which it is possible to work them up into paper. The bagging and string are cut up in a rag chopper and boiled in revolving boilers with lime or caustic soda for several hours at a pressure of 20-30 lbs., the lime being used when it is desired to manufacture a harsh paper, and the caustic soda being employed for the production of paper having a softer feel. The pulp is not always washed very completely after the process of digestion, as is the case with white papers, and it is often possible to extract from brown papers of this class a considerable proportion of the alkaline matter which has not been thoroughly removed from the boiled pulp. The presence of this alkaline residue does not affect the quality of ordinary brown paper, but is frequently a serious defect in the case of middles or straw boards, which are afterwards utilised for boxes and covered with coloured papers. The colour of the paper pasted on to such incompletely washed boards is frequently spoilt by the action of the alkali when moistened with the paste used, many aniline dyes being susceptible to the small proportion of alkali present.
The stronger materials, such as jute or old rope and string, are either used by themselves or blended with inferior raw material according to the quality of the paper being made. The jute and hemp fibres are generally beaten by themselves in the engine before the other materials are added. The pulp is mixed with the required amount of loading, while the sizing and colouring operations are carried out in the usual way.
The common brown papers are known by a variety of trade names which at one time indicated the nature of the fibrous constituent, but at the present day the name is no guide or indication of the material used for the manufacture of the paper. The common heavy brown used for wrapping sugar and sundry groceries made in heavy grey and blue shades is a coarse paper made from cheap materials and containing a large proportion of mineral matter. It is usually supplied under the trade name of royal.
A somewhat lighter and stronger wrapping paper of a white or buff colour, used for wrapping groceries, tea, and cotton goods, is that known as casings, a name probably derived from the application of this paper originally to the lining of cases.
Manila papers so called were originally made from rope, but the term is now applied to papers which may be made entirely of wood pulp.
Rope browns are common papers made of fairly strong material of a miscellaneous character, this name having been derived from the fact that rope and similar fibre were at one time used exclusively.
Wood Pulp Wrappers.—Most of the papers of the present day are made from wood pulp, this material giving a thin, light, tough paper, which is pleasant to handle and forms a great contrast to the dense, opaque, heavily loaded, and inartistic specimens produced some years ago. Paper of this kind, though apparently more expensive than common browns, is really more economical in use. The paper is not only stronger, but it is possible to obtain a larger number of sheets for a given weight. The great advantage in the improvement of brown papers dates from the introduction of the now well-known kraft papers, which are of comparatively recent origin.
Kraft Paper.—The term Kraft, meaning “strength,” is applied to a remarkably strong cellulose paper prepared from spruce and other coniferous woods by the soda treatment, the special feature of the process being an incomplete digestion of the wood.
The wood previously chipped into pieces 1 inch to 1½ inches in length, is boiled with caustic soda, the digestion being stopped before the wood pulp has been quite softened, and while the pulp is still too hard to be broken up into isolated fibres by simple agitation in water. The pulp after thorough washing is disintegrated by means of an edge-runner, or some form of breaking engine, the first mentioned probably giving the most satisfactory results, and converted into paper by the usual methods.
The wood can also be reduced by the sulphate process, in which case the chipped wood is boiled in a liquor to which about 25 per cent. of spent lye from a previous cooking is added.
The best results are obtained by attention to the cooking process to ensure an under-cooked pulp, by careful isolation of the fibres in a kollergang, or edge-runner, which machine is capable of separating the fibres without shortening them, and by proper manipulation on the paper machine.
The paper produced under favourable conditions in this direction is wonderfully tough and strong and may be quoted as the most recent example of the fact that the latent possibilities of wood pulp have by no means been exhausted or even thoroughly investigated.
Imitation Kraft Paper.—If wood is boiled in water at high temperatures the fibre is softened and much of the resinous matter is removed. Such wood, if ground in the same way and by the same methods as ordinary mechanical wood pulp, is readily disintegrated, and a long-fibred pulp may be obtained. The process of boiling short 2 feet logs of wood in a digester under a pressure of 20-50 lbs. has long been known. The wood after boiling is partly washed and then worked up into pulp by the usual mechanical process. The wood is easily ground and yields pulp containing long fibres which in their physical properties closely resemble those of pure wood cellulose, but the original constituents of the wood are present almost unchanged, just as in mechanical pulp. The product obtained by grinding is a very tough flexible material of a brownish yellow colour, and the paper is known as Nature brown. It is chiefly used for the preparation of tough packing papers, for the covers of cheap pocket-books, and other miscellaneous purposes. When this brown mechanical wood pulp paper is glazed on both sides it is then known as ochre glazed, the word ochre referring to the colour. When made up into light weight papers it is sold as imitation kraft paper.
A great variety of wrapping papers are now made from wood pulp, such as sealings, sulphite browns, manilas, sulphite caps, but the distinctions between these papers relate chiefly to the amount of finish, the colour and size of the sheet. The methods of manufacture only differ in small details as indicated by these distinctions.
Fine Wrappings.—The papers used for packing small goods such as silver ware and other delicate articles are generally tissues, the better qualities of which are made from rag, and the cheaper qualities from wood pulp. These papers are known as tissue, crêpe, crinkled tissue, manila tissue, and by a variety of trade terms.
Many of the fine wrappings of the tissue class and the somewhat heavier papers known as M. G. Caps are manufactured on the single cylinder machine, which produces a paper having a highly polished surface on one side and a rough unglazed surface on the other side.
In the single cylinder machine the beaten pulp passes from the stuff-chest on to the wire of the ordinary Fourdrinier machine and through the press rolls, but instead of being dried over a number of cylinders the paper is led over one single cylinder of very large diameter which is heated internally with steam. The paper is usually pressed against the surface of the cylinder by means of a heavy felt, which is, however, sometimes omitted. The side of the paper coming into contact with the cylinder becomes highly polished, the surface in contact with the felt remaining in an unfinished rough condition. This paper is said to be machine glazed and is known as an M. G. paper.
Boards.—Cards, millboards, middles, boxboards, carriage panels, and similar paper products are manufactured either on a single board machine, by means of which single sheets of any required thickness can be obtained, or on a continuous board machine, which is capable of producing cards and plain or duplex boards of moderate thickness.
The raw material used consists, as in the case of browns and wrappers, of every conceivable fibrous substance mixed with mineral matter and then suitably coloured. The preliminary processes for the treatment of the pulp are exactly the same as those employed in the case of brown papers up to the point at which the beating has been effected.
The beaten pulp, diluted with large quantities of water, is pumped continuously into a large wooden vat of rectangular shape. Inside this vat revolves slowly a hollow cylindrical drum, the circumference of which is covered with wire gauze of fine mesh. The drum is not completely immersed in the mixture of pulp and water, so that as it revolves the water passes through the wire, while the pulp adheres to the surface. The water flows regularly into the interior of the drum and runs away through pipes fitted at each side of the vat near the axis of the drum, and the pulp is brought up out of the water until it comes into contact with a travelling felt. The thin moist sheet of pulp adheres to this felt, passes through squeezing rolls which remove part of the water, and is finally carried between two wooden or iron rollers of large diameter. The pulp adheres to, and is wound up on the upper roller, the felt being carried back by the lower roller to the vat. When the sheet on the upper roller has attained the desired thickness, it is immediately cut off and transferred to a pile of similar sheets, a piece of coarse sacking or canvas being interposed between every wet board. The dimensions of the full-sized board are determined by the diameter of the upper roller and its length. A roll 74 inches wide and 14 inches diameter will give a board 74 inches by 44 inches.
As soon as a sufficient number of wet boards has been obtained they are submitted to pressure in order to remove the excess of water and at the same time compress the material into dense heavy boards. The pieces of sacking are then taken out and the boards dried by exposure to air at the ordinary temperature or in a heated chamber.
The dried boards are finished off by glazing rolls. These rolls compress the boards still further and impart a polished surface. The amount of “finish” may be varied by the pressure, number of rollings, temperature of the rolls, and by damping the surface of the dry boards just before they are glazed. The boards are cut to standard sizes before or after glazing.
Duplex Boards.—If the single board machine is fitted with two vats instead of one, it is possible to manufacture a board with different coloured surfaces. A board coloured red on one side and white on the other is manufactured by having one vat full of pulp coloured red and the second vat full of white pulp. The thin moist sheets from the two vats are brought together and passed through the glazing rolls, which cause the moist sheets to adhere closely to one another, the double sheet of pulp so formed being wound up on the rollers at the end of the machine. The board is then dried, glazed, and finished in the usual way.
The same principle is occasionally adopted on the Fourdrinier machine for duplex wrappers. Thus a common brown pulp is worked up in conjunction with a dyed pulp to produce a brown paper having one surface of good paper suitably coloured. The brown pulp flows on to the wire of the paper machine, and after it has been deprived of part of the water at the suction boxes, a thin stream of coloured pulp, diluted to a proper consistency, flows from a shallow trough, placed across and above the wire, on to the wet brown web of paper in such a manner as to completely cover it as a thin even sheet of coloured pulp. The adhesion of the latter to the surface of the brown paper is practically perfect, and the weight of the couch and press rolls ensures uniform felting of the fibres.
Middles.—This term is applied to a thin or thick cardboard made of common material, the colour and appearance of which is of little importance for inferior goods. Boards of this kind are covered subsequently with papers of all colours and qualities, and the origin of the word “middle” is easily seen. The manufacture of a board consisting of two outside papers of good material and a middle produced from common stuff is effected by the continuous boxboard machine, unless the board is too thick to be passed over drying cylinders, calendered, and reeled, in which case the boards are produced on an ordinary wet machine and the paper pasted on the surface of the dry board.
The term is, however, now also applied to a common paper made of mechanical wood pulp with perhaps a little chemical pulp, used for tram tickets, cheap advertising circulars, common calendar cards, and similar purposes, to which no outer surface of a special character is added.
This machine differs from the single board machine in that the finished board can be produced from the pulp at one operation. It is used principally for cards and boards of moderate thickness which can be wound up in the form of a reel at the end of the machine.
The mixture of pulp and water is pumped into two or more vats and formed into a number of thin sheets, which are all brought together between squeezing rolls and passed through heavy press rolls which compress the several layers into a compact mass. The thick sheet obtained is dried over steam-heated cylinders which are placed at the end of the press rolls, and calendered. The whole process, indeed, resembles that of ordinary paper-making, the main difference being the method of producing the wet sheet or card.
Some machines are constructed with six or seven vats and forty to fifty drying cylinders, and are capable of turning out a large quantity of finished material.
The board can be made of uniform quality and texture throughout, or be finished off with high-grade paper on one or both sides. In the latter case the constituents of the “middle” part are waste papers and raw material of inferior quality, the outer surface of wood pulp, white or coloured according to circumstances. The variety of papers and boards which can be produced is due to the fact that the several vats of pulp are independent of one another and can be filled with any kind of paper stock. The combined sheets forming the ultimate board are dried on the ordinary cylinders, calendered, and reeled up at the end of the machine.
There are many varieties of paper products obtained by submitting finished paper to a number of special processes. Of these only a few of the more important will be described.
These products can be divided approximately into three classes:—
(1) Papers coated on one side or both sides with various substances, such as “art,” photographic papers, etc.
(2) Papers impregnated with chemicals, such as blue print, medicated, and cheque papers.
(3) Paper pulp converted into modified products by chemical treatment, such as vulcanised board, viscoid, etc.
Of the first class, the coated papers used for art and chromo illustrations are the most important.
Of the second class, the blue prints and papers impregnated with chemicals, chiefly employed for the production of engineers' drawings, may be regarded as typical.
In the third class, vegetable parchment and vulcanised board are the most familiar.
Parchment Paper.—This is produced by the action of sulphuric acid upon ordinary paper, the most suitable for this purpose being made from unsized cotton rag, free from such additions as mechanical wood pulp. The presence of the latter substance should be avoided, as it is liable to char or burn, so that in the finished product it shows itself in the form of small holes. The process depends upon the power of sulphuric acid to change the surface of the paper into a gelatinous mass, which has been shown to consist of a substance called amyloid.
The best parchment is made from pure cellulose such as rag or chemical wood pulp. The quality of the parchment depends upon attention to the strength of the acid, the temperature of the acid bath, the period of immersion, the complete removal of the acid, and the careful drying of the wet parchment.