R. W. Sindall

Fig. 6.—A Rag Sorting House.

The word rag is used to designate a very wide range of raw material suitable for conversion into paper. In the case of high-class hand-made writing papers only the best qualities are employed, such as new linen and cotton cuttings from factories, or well-sorted rags of domestic origin. The usual classification adopted by merchants who supply the paper mills is somewhat as follows:—

New white linen cuttings (from textile factories).
New white cotton cuttings (from textile factories).
Fine whites (domestic rags).
Outshots (a quality between fines and seconds).
Seconds (a grade inferior to fines).
Thirds (inferior and dirty well-worn rags).
Coloured prints (of all grades and colours).
Fustians and canvas.
Manila and hemp rope.
Baggy, gunny, and jute.

The total amount of rag used in England for paper-making is not known. The only figures available refer to rags imported; and these cannot be regarded as a measure of consumption, which could only be arrived at by first ascertaining the quantity of home rags used. The imports of rag at stated periods are given in the appended table:—

Rags Imported into England.

Sorting and Cutting.—All rags on arrival at the mill are carefully sorted. This process is conducted entirely by women, who sort and cut up the rags at special tables provided with cutting knives curved in shape similar to a scythe. These are fixed at an angle in the centre of the table, with the back towards and in front of each work-woman. The top of the table is made of thick coarse wire so that some of the dirt and foreign impurities may fall through. All buttons, hooks and eyes, pins, leather, pieces of rubber, and other articles are carefully removed, while seams and hems are also opened out. The rags are cut into slips 3-5 inches long and then recut crosswise, and thrown into suitable baskets or receptacles standing round the table, by which means the sorting operation is effectually carried out. The care and attention given to the sorting is an important item in the manufacture of papers of uniform quality, and in the best mills the sorting is carried out to such an extent that twenty or twenty-five grades are obtained.

Dusting.—The rags are next passed through a machine which removes dirt. This is a hollow cylindrical or conical drum having an external covering of coarse wire cloth, which rotates inside a wooden box. The shaft is provided with projecting spikes, so that the rags are violently agitated in their passage through the machine. The dirt and other impurities fall through the wire on to the floor of the room, while the clean rags are discharged from the lower end of the drum. The loss in weight varies according to the condition of the rags. With good materials the loss may only be 1-2 per cent., while with dirty common rags the loss during cleaning and dusting may amount to 10 per cent.

Boiling.—The further purification of the rags is effected by a chemical treatment, viz., boiling at a high temperature with alkaline substances, which process removes fatty, glutinous, and starchy matter from the material.

For this purpose a spherical digester is used, generally 7-9 feet diameter, and capable of holding 2-2½ tons of rag. The boiler or digester is filled with dusted rags, and the requisite amount of alkaline solution added. The manhole is then closed, and steam admitted through the hollow trunnions until the pressure reaches 20 or 30 lbs., at which pressure the boiling is continued for three to six hours according to requirements, the digester rotating slowly the whole time in order that the rags may be evenly and thoroughly boiled.

The liquor employed for boiling is a solution of caustic soda, carbonate of soda, or milk of lime. In the case of caustic soda the amount required varies from 5 to 10 per cent. of the weight of rag. Caustic soda is preferable to lime, because it acts upon the grease and other fatty matters, forming a soluble compound which is freely removed in the subsequent process of washing. Many paper-makers, however, use milk of lime, carefully strained through fine cloth, almost exclusively. Considerable experience and skill are necessary in this operation in order to avoid injury to the fibre not only as regards its strength, but also its colour.

Washing.—When the rags have been sufficiently boiled, the steam is turned off and the pressure allowed to fall. This can be effected quickly by blowing off from a valve fixed at the bottom of the boiler opposite to the manhole. The cover is removed from the boiler and the boiler slowly rotated in order that the contents may be discharged into a tank placed below. The “black liquor,” as it is called, is then drained away from the rags, which are immediately subjected to a preliminary washing. The process of washing must be carried out in a thorough manner in order to remove all soluble compounds, which if left would cause an unnecessary waste of bleach in the subsequent stages of purification. There are many schemes employed for washing, most of them being devised with the idea of using a minimum quantity of water.

The most general practice, in the absence of special machinery, is the preliminary treatment in the tank below the digester, followed by a more complete washing process in a machine known as a breaking engine.

This apparatus is a shallow oval-shaped vessel with circular ends, divided lengthwise by a partition called a mid-feather, which, however, does not extend the full length of the apparatus. In one of the two channels into which the vessel is thus divided a heavy roll is fitted, which is provided with a number of steel knives. On the floor of this channel there is fixed a “bed-plate,” also provided with projecting knives which are parallel with the knives in the roll. The distance between the knives in the roll and those in the “bed-plate” may be altered as required by means of an adjusting screw. In the other channel of the breaking engine there is fitted a “drum-washer,” which serves for the removal of the dirty water from the machine. This drum is divided into sections by means of partitions which reach from the centre to the circumference. The surface of the “drum-washer” consists of a fine brass wire cloth supported by a coarser material placed underneath.

The breaking engine is half filled with clean water, and the rags are thrown into the engine until it is suitably filled. The rotation of the heavy roll causes the mixture of rags and water to circulate round the vessel, the floor of which is so constructed that the pulp is drawn between the roll and “bed-plate” and discharged over the “backfall,” which is that portion of the sloping floor behind the “bed-plate.”

The “drum-washer” rotates with its surface in contact with the mixture in the engine, so that the dirty water passes through the wire cloth and is caught in the curved sections or buckets inside the drum and discharged into a trough adjacent to the centre, and thereby conveyed away from the engine. Clean water is allowed to run into the vessel at one end while the dirty water is discharged by means of the “drum-washer.” At the same time the rags are broken up by means of the knives on the roll, so that when the rags are sufficiently washed, a process which usually occupies four hours, they are also partially disintegrated.

Bleaching.—The clean disintegrated rag is next bleached by means of ordinary bleaching powder solution. Bleaching powder is a substance prepared by the action of chlorine gas on dry slaked lime, resulting in the formation of a compound which has the property of bleaching or “whitening” vegetable matters. The clear solution obtained by treating the powder with water is utilised by the paper-maker for bleaching the rag pulp.

Various methods are used for this purpose. Sometimes the requisite volume of clear bleach liquor is added to the pulp in the breaker, and the material kept in constant circulation until the operation has been completed. In other cases the broken pulp is transferred to a “potcher,” which is a vessel similar in shape to the breaker, but merely provided with paddles for keeping the pulp in circulation, and bleached by the addition of chloride of lime solution.

Another method frequently adopted is to discharge the pulp from the breaker, immediately after the addition of the bleach, into brick or cement tanks, allowing the bleaching action to proceed spontaneously without prolonged agitation.

In some instances the process is hastened by adding dilute sulphuric acid to the pulp after the bleach liquor has been run in, or by heating the mixture with steam. For high-class papers such devices as this are seldom resorted to, as experience shows that the colour of pulp bleached by drastic methods does not maintain a high standard.

The pulp is then thoroughly washed in order to remove every trace of residual bleach, and also the soluble compounds which have been formed during the operation. Very large quantities of water, clear and free from suspended dirt, are necessary. In some mills any excess of bleach is neutralised by the use of an “antichlor” such as sodium hyposulphite, or sodium sulphite, but the best results are undoubtedly obtained when the quantity of chemicals used is kept at a minimum.

If the pulp is bleached in a breaker or potcher, the washing is effected by the aid of the drum-washer. With pulp treated in steeping tanks, fresh water is allowed to percolate or drain slowly through the mass.

Electrolytic Bleaching.

The substitution of a sodium hypochlorite solution for the ordinary calcium hypochlorite solution obtained from common bleaching powder has been the aim of specialists for many years. As early as 1851 a patent was taken out by Charles Watt for decomposing chlorides of the alkali metals and the formation of hypochlorites. It was not until 1886 that a practical method was devised for producing an electrolysed solution of salt, but in that year Hermite introduced a continuous process in which an electrolysed solution having a strength of three grammes chlorine per litre was passed continuously into the potcher.

Many patents for the electrolysis of salt have been taken out during the last twenty years, of which the Bird-Hargreave process is in operation in England, the Rhodin process in America, the Siemens and Halske in Norway, and the Oettel and Haas apparatus in Germany. The figures relating to the latter apparatus may be mentioned as typical of the present condition of electrolytic bleaching. The apparatus consists of a narrow rectangular trough divided into a number of chambers through which a solution of brine flows at a constant and steady rate. The electric current is passed through the solution by suitable electrodes, the temperature being kept down by means of a cooling coil. The cost of producing the bleach liquor as given by the inventors of the apparatus from the results of actual working are shown in the following table:—

Table giving Analysis of Cost for Producing Bleach Liquor.

The above costs have been estimated on prices as follows:—

After 12 hours the 166 gallons (750 litres) are converted into electrolytic bleach liquor containing 26¾ lbs. of active chlorine (12·15 kilos.).

Beating.—Although the rags are reduced by the breaking engine to a condition of fibrous lint, called “half-stuff,” they are not fit for conversion into paper. They have to be beaten in special machinery until a complete separation of the single fibres has been effected, and this process is rightly regarded by many paper-makers as the most important stage of manufacture.

The beating engine is similar in construction to the breaking engine, but there are certain essential differences in arrangement and manipulation. There is usually no drum-washer; the roll contains a large number of knives which are fixed in clumps or sets of three round the circumference; the lowering of the roll upon the bed-plate is carefully watched and controlled, and the desired effects are only obtained by strict attention to the condition of the pulp during the whole process.

The beater is first partially filled with water, and the drained half-stuff added gradually until the “furnish,” a convenient term applied to the contents of the engine, has the proper consistency, which varies according to the nature and quality of paper required.

The mass is circulated steadily round the engine by the action of the beater roll, which is lowered from time to time until the distance between the knives on the roll and those on the bed-plate has been set to the desired adjustment. This lowering of the roll and its proper adjustment call for the greatest care.

Influence of the Beating.—The importance of this operation can easily be judged from one or two specific examples. In the case of rag papers the two extremes of variation are represented by the ordinary blotting paper on the one hand and a hard strong writing paper known as a loan on the other. Now the great difference in these papers may be traced to the careful selection of the rag and the treatment in the beater as the two primary causes of the final results.

For blotting papers it is essential that the rags should be old and tender. In the beating operation subsequent to the usual boiling and bleaching processes the half-stuff is beaten quickly with sharp knives, the roll being lowered soon after the engine is filled, so that the beating is finished in about one to one and a half hours.

For the strong writing paper new strong rags are selected. In the beating process the knives used are dull, the roll is lowered slowly and cautiously, and the beating goes on for eight to ten hours.

The effect of such difference in treatment is easily seen by examination of the fibres of the papers under the microscope. In the first case the fibres appear short with clean cut ends, the shape little distorted, the structure well defined, bearing a strong resemblance to the unbeaten material. In the case of the well-beaten paper the ends of the individual fibres appear to be drawn or frayed out, the fibres do not possess the sharp well-defined outline characteristic of blotting paper; they are partly split up into fibrillÆ which lie together in a confused mass.

In the blotting paper these effects are produced because the knives being sharp cut up the material quickly, and in the writing paper because the dull “tackle” tends to draw out the fibres and tear them up lengthwise.

The practical result is a spongy, soft, and bulky blotting and a hard, strong, heavy writing paper. Of course the great difference between a blotting and a writing paper is not all due to this one operation, but is obtained by a series of operations, of which one of the most important is, however, the beating.

Colouring the Paper.—The pulp is brought to any desired tint by the addition of mineral pigments or aniline dyes to the contents of the engine. The latter soluble dyes, however, are seldom used for high-class rag papers. Prussian blue, ultramarine, and smalts are chiefly used for this purpose, giving toned blue, azure, and blue laid papers.

Making the Paper.—The beaten pulp, when duly prepared, is run from the engine into store tanks known as stuff chests, ready for the actual manufacture. The pulp properly diluted with water is strained through special screens to remove any insufficiently beaten material and any impurities present, after which it is run off into the vat, a square-shaped vessel built of wood or stone.

The apparatus used in forming the sheets is called a hand mould. The mould is a rectangular frame of mahogany upon which is stretched tightly a fine wire cloth, the surface of the latter being kept flat by a coarser wire cloth fixed underneath, supplemented by wedge-shaped pieces of wood. A second frame called the deckle fits on to the mould in such a manner as to form a shallow tray, the bottom of which is the fine wire cloth.

The vatman takes up the mould with both hands and dips it into the vat full of pulp in a slanting position, drawing it through the stuff towards him in a peculiar manner and lifting it out from the vat with a definite quantity of the mixture in the frame. As the water drains away from the pulp, through the wire cloth, he imparts a shaking motion to the mould in order to cause the fibres to “felt” properly, this felting or interlacing of the fibres being an essential feature in the manufacture of a good sheet of paper. When the water has drained away sufficiently from the pulp, the vatman removes the deckle from the mould and passes the latter over to the coucher, who takes the mould, reverses it, and presses the contents, which may now be described as a wet sheet of paper, down on to a damp piece of felt, by which means the paper is transferred to the felt. He returns the mould to the vatman, who meanwhile has made another sheet with a duplicate mould, and then, having laid a second felt upon the wet sheet of paper, he proceeds to transfer the next sheet of paper to the second felt. This process is continued until a pile is formed consisting of wet sheets of paper alternated with pieces of felt.

The pile is at once submitted to great pressure in the hydraulic press, and the excess water slowly forced out, while at the same time the sheets are compressed and thus “closed up,” as it is termed. When all the excess water has been removed as far as possible, the pile is taken away and the sheets of damp paper taken out, the felts being placed in one pile ready for further use, and the sheets of paper in a second ready for the next process.

The papers are put back into the press without felts between the sheets and left for some time. In most cases the sheets are turned round or mixed in with the sheets of another pile, before pressing. In this way any unevenness or irregularity in the sheets is counteracted and a more uniform result obtained.

When these changes are repeated several times the paper acquires an even texture and becomes firm and hard.

Drying the Paper.—The sheets are hung up in the loft, as the drying room is called, upon poles or ropes. The moisture gradually evaporates, and the paper is thus dried by exposure to air. In winter it is necessary to warm the air in the loft, as the air is then saturated with moisture. In lofts of limited capacity the air is heated in order to hasten the process, but the best paper is allowed to dry naturally, as by this means the shrinkage is gradual and a maximum strength is attained.

Sizing the Paper.—The dried paper as it leaves the loft is termed Waterleaf because, being unsized, it readily absorbs water, and therefore before it can be used it must be sized. For this purpose it is dipped into a solution of gelatine, an operation described as tub-sizing or animal-sizing, the former term being used on account of the tub in which the size is kept, and the latter on account of the fact that the gelatine is made from animal matter such as hides, cartilage, hoofs, and other refuse.

Animal Size.—This is prepared from hide pieces, skins, and the like by a simple process, which, however, requires a good deal of care in order to obtain the best results. The material is first thoroughly washed in plenty of clean water, and then heated with a definite quantity of water in a steam jacketed copper pan. The pieces slowly dissolve until a solution of gelatine is produced, and after the dirt and impurities have settled to the bottom of the pan the clear liquid is drawn off into store vessels. There are many details of a technical character to be attended to in the manufacture of good gelatine, and as the process is expensive, considerable attention is demanded at this stage in the completion of a sheet of paper.

The dry sheets of paper are sized by the simple expedient of dipping, or by the passage of the paper through a long trough. In the first case the workman takes up a number of sheets and dips the bunch into a vat of size at the proper temperature, about 100° Fahrenheit. He then allows the surplus size to drain off, and the sheets are submitted to a slight pressure in order to remove the excess of gelatine that will not drain off.

In the second case a different method is adopted in that the sheets of paper are carried by travelling felts through a bath of heated size, the excess gelatine being removed by the action of rubber or wooden rollers through which the papers are passed before leaving the apparatus. The papers are quickly and evenly sized by this method, which is now most generally used.

Glazing.—When the sheets of paper are quite dry they are ready for glazing, a process which turns the dull rough surface of the sized sheet into a highly polished smooth surface fit for use. The sheets are placed singly between copper or zinc plates, and a pile of these passed several times through heavy iron rollers, great pressure being applied to the latter during the operation.

The amount of polish imparted by this plate-glazing process, as it is termed, can be varied considerably. With a light pressure and few rollings, the sheet of paper can be turned out having a fairly smooth surface, and without a conspicuously shiny appearance. By employing a great pressure and repeated rolling a much higher surface is attainable. If the plates are hot a still higher finish is possible. Machine-made rag papers are glazed usually by means of the supercalender, which is a stack of alternate steel and paper rolls placed one above the other in a vertical position. The reel of paper passes between these rolls and becomes highly surfaced.

This operation effects many changes in the paper, besides imparting a good finish. The thickness of the sheet is reduced by about 40 per cent., the fibres being compressed much closer together. The tensile strength of the paper is also materially increased, and in every way the paper is improved. Moderation is essential in this as in everything, because excess of glazing weakens a paper, rendering it brittle and liable to crack when folded.

Laid and Wove Papers.—When certain papers are held up to the light and carefully examined it will be noticed that they appear to contain delicate transparent lines running parallel with one another at equal distances of about an inch, and that these are intersected by similar transparent lines running at right angles, which are much closer together. Such papers are known as Laid Papers, and the peculiar formation of the transparent lines is due to the construction of the mould used in the making. The wire surface of this mould consists of a number of somewhat stout wires placed about one inch apart, interwoven with finer wires running across and at right angles, which are threaded much closer together. When the mould is dipped into the vat and withdrawn, the water drains away from the under surface of the wire, and the moist pulp settles down on the upper surface; but since the coarser wires project a little from the finer threads, the paper is slightly thinner along those wires, though to an almost infinitesimal extent, with the result that on drying the sheet appears to contain transparent lines.

Wove papers are so called from the nature of the mould used. The surface of the mould in this case consists of fine wires equally distributed, being woven in such a manner that the wires are equidistant from one another, as in ordinary wire gauze. A wove paper, on being examined in the light, simply shows a number of small diamond-shaped spaces, which in the majority of instances are difficult to detect.

The Watermark.—The transparent device observed in many papers when held up to the light is known as the watermark, a term probably derived from the conditions existing at the time the sheet of paper is made on the mould. The effect is produced by means of a raised design sewn or soldered to the surface of the mould, the design being fashioned out of fine wire.

When a mould thus fitted with the design is dipped into a vat of pulp and lifted out, the water falls through the wire, and the pulp sinks down on to the surface of the mould, forming a replica, so to speak, of the design, which is easily seen when the dry paper is held up to the light, because the paper is thinner just at those points where the wires forming the design come into contact with the wet pulp.

Some of the watermarks are very elaborate and interesting. A familiar illustration of a beautiful design of this description is to be found in the Bank of England notes. As a general rule the ordinary watermark consists of a mere trade term such as “Vellum,” “Zenobia,” or of the name of the manufacturer, such as “J. Whatman,” “R. Batchelor,” and so on. In the earlier days of paper-making many highly interesting designs were used, and some of these are still extant. In fact many of the names by which certain standard sizes of paper are known owe their origin to the watermarks employed.

The earliest known watermark bears the date A.D. 1301, being in the form of a globe and cross, as shown. Of equal interest are those designs from which certain papers are called foolscap, crown, pott, post, royal, columbier, and so on. The watermarks are now little used, but the terms are still retained, as indicating the size of the sheet.

Microscopic Features of Cotton and Linen Fibres.

The cotton fibre is about 30 mm. long, with an average diameter of ·025 mm. of tube-like shape, and having a prominent central canal. There are no cross markings on the cell walls, and the ends of the fibre are rounded off into a somewhat blunt point. It exhibits a marked tendency to twist itself, especially if dry, and this peculiarity is readily observed with the raw material.

The process of paper-making alters the characteristic structure of the fibre very greatly. The ends of the fibre are seldom to be seen; the curious twist is less prominent, and the fibres are torn and destroyed. The effect of the beating process, for example, on cotton is easily to be noticed by comparing the fibres of a blotting paper under the microscope with the fibres of a bank or loan paper.

The distortions produced by prolonged beating renders the determination of the exact percentage of cotton in a rag paper rather difficult, but the features to be looked for are the absence of pores, cross markings, the existence of a central canal, striations produced in many cases on the cell walls parallel to the length of the fibre. The structural features are more readily observed when the fibres are stained with a suitable reagent. (See page 71.)

The linen fibre has an average length of 27 mm. with a diameter of ·02 mm. The raw flax is very different from raw cotton and is easily distinguished. The fibre is slender in shape, having thickened knots at regular intervals throughout its length, the general appearance of which may be compared to a stick of bamboo. The central canal of the fibre is extremely narrow, running like a small thread through the length of the fibre. The cell walls are further marked by numerous pores, which appear as small dark lines running from side to side, but not meeting in the centre.

In the treatment necessary for making paper these characteristics are largely destroyed, and while it is quite easy to ascertain that a paper is of linen, or of cotton, or that a paper is mainly cotton with a small percentage of linen, yet there are conditions under which it is difficult to determine the exact percentage of cotton or linen in a rag paper. If, for example, a paper contains nearly equal quantities of cotton and linen, the exact proportions cannot be determined closer than 10 per cent., especially in well-beaten papers.

Reagent for Staining Fibres.

Preparation.—Dissolve 2·1 grams potassium iodide and 0·1 grams iodine in 5 c.c. of water. Mix this solution with a solution containing 20 grams of dry zinc chloride in 10 c.c. of water. Allow the mixture to stand; pour off the clear liquid into suitable bottles.

Coloration Produced.

Cotton, linen, hemp.—Wine red.

Esparto, straw and wood cellulose.—Bluish violet.

Mechanical wood, unbleached jute.—Yellow.

Manila hemp.—Blue, bluish grey to yellow.