It is almost impossible to utilise the whole of the back-water passing through the wire-cloth in the way described. In some mills a portion of this water is made to pass through a “pulp-saver,” such as is shown in Figs. 51 and 52. It consists of a conical drum A, the circumference of which is covered with wire-cloth, and it is caused to revolve slowly by suitable gearing. The water enters by the pipe B, and passes through the meshes of the wire-cloth, the pulp gradually finding its way to the wider end, where it is discharged into the box C. It can then be returned to the beaters.

This pulp-saver can also be used for recovering the fibre from other waste water, such as the wash water from the washing and beating-engines; or it can be used for freeing bleached pulp from water in cases where drainage or hydraulic pressure is not resorted to.

If any pattern or name is required on the paper, it is produced by means of a light skeleton roll, called a “dandy-roll,” covered with raised wires in the form of the desired pattern, {156} placed between the suction-boxes, and pressing lightly on the still moist paper. The paper is thinned where the wire pattern presses, and thus a mark (water-mark) is produced. The other side of the paper has a mark corresponding to the wire-cloth; by using a dandy-roll covered with wire-cloth, the two sides can be obtained alike, such paper going by the name of “wove.”

Paper in which a series of parallel lines are produced is called a “laid” paper.

Some water-marks are produced by means of a dandy-roll in which the pattern is formed by depressions in the surface. The paper is thus thicker where the pattern is formed than in the ground-work. De la Rue (Patent No. 8348, 1884) has patented the use of dandy-rolls so formed as to produce upon the paper the effects both of thickening and thinning.

Imitation water-marks can be produced on the finished paper by subjecting it to pressure in contact with plates on which the design has been produced in relief. In this way very beautiful results can be obtained. (See Patent No. 13,455, 1884.)

It sometimes happens that the wire-cloth slips slightly to one side. This can be obviated by the machine-man shifting, by means of screws, one of the rolls provided for the purpose with a movable journal, until its axis is at a slight angle to that of the other rolls. An automatic apparatus has been invented for this purpose. Two brass plates are fixed, one on each side of the wire-cloth, to a long rod, connected by suitable machinery with the screws working the movable journal, so that, as the wire presses against one or the other of these plates, the roll is shifted so as to correct this.

The paper, which, even after passing the suction-boxes, is still very wet, passes with the wire-cloth between the couch-rolls G G′. These are hollow copper or brass cylinders, covered with a tightly-fitting endless jacket of felt. They may also be made of wood (sycamore or mahogany) or of iron or iron and brass combined. The pressure of the upper couch-roll upon the lower can be regulated by means of {157} screws or levers. They serve to press out water from the paper, and to detach the paper from the wire-cloth. By dexterous manipulation on the part of the machine-men, the paper is transferred to the endless felt, travelling over the rolls k in the direction of the arrows. It is known as the “wet felt,” from the condition of the paper at this stage. In its passage along this felt, the paper passes between two iron rolls K, called the first press-rolls, with the object of having the water squeezed or pressed out of it. These rolls are sometimes covered with a thin brass case, and the top one is provided with an arrangement called the “doctor,” in order to keep it clean, and free from pieces of paper that may have stuck to it. The lower press roll is sometimes covered with an indiarubber jacket. The “doctor” is a kind of knife placed along the whole length of the roll, and pressing against it at every point. The pressure on the rolls can be regulated by means of levers, or, as in the illustration, by the screw k′. It will be readily seen that the under side of the paper that has been next to the felt will, in its still moist condition, have taken to some extent an impression from the felt, while the upper side will have been made comparatively smooth by the pressure against the top roll of the 1st press. In order to make both sides of the paper as nearly as possible alike, it is passed through another set of rolls L, called the 2nd press. This time it is reversed, and enters at the back of the rolls; thus the other side of the paper is next the metal, being taken through by the felt (called the “2nd press felt”) travelling on the small rolls l, the paper, after leaving the wet felt, and before being taken on to the 2nd press felt, travels over the rolls l′. The 2nd press felt is necessary, because the paper is too tender to withstand, unsupported, the pressure of the rolls.

FIG. 53.

The paper, after passing the 2nd press rolls, travels over the drying-cylinders M, the number of which varies somewhat. In the machine shown, there are in all eight cylinders. Sometimes as many as twelve are employed. Between the {158} 2nd press rolls and the cylinders, a passage S allows easy access to the other side of the machine. The paper generally passes alone over the first two, which are only slightly heated; afterwards it is led over the others by means of felts, as shown. The arrangements shown at R are for the purpose of stretching the felts. The cylinders are heated by means of steam, and are generally divided into two sets, between which is a pair of chilled-iron, highly-polished rolls N, called “smoothers,” the function of which is sufficiently explained by their name. They are also heated by means of steam. The cylinders are usually made of slightly decreasing diameter, in order to allow for the shrinking of the paper on drying. Messrs. G. & W. Bertram introduce into some of their machines one or two small drying cylinders, over which the 2nd press felt travels, the object being to drive off some of the moisture absorbed from the paper. This contrivance is said to give excellent results, and to ensure a considerable saving in felts. The arrangement is shown in Fig. 53. It is especially useful in machines running at a high speed, or in those having limited drying power. After leaving the cylinders, the paper should be quite dry; it is then led through the calenders, of which there are in some machines {159} as many as three sets, though only one is shown. These are similar to the smoothing-rolls, just described. Pressure is applied by the screws O′, or by levers and weights. The friction of the hot calenders on the dry paper develops a large quantity of electricity, which occasionally discharges itself in bright sparks.

It is the practice in some mills to cool the paper before passing it through the calenders. This is effected by passing it over a copper cylinder, through which a stream of cold water runs.

The degree of smoothness or “finish” that can be given to the paper by the calenders, depends to a large extent upon the degree of moisture which it contains. As it leaves the last cylinder it is perfectly dry, that is to say, it contains only that amount of water which cellulose, from whatever source, always carries. This amount varies slightly with the nature of the cellulose, and with the plant from which it has been isolated, and also with the state of the atmosphere, it being greater on damp days. It would be impossible so to regulate the drying action of the cylinders that the requisite amount of water should always be left in the paper; it is therefore better to dry it as thoroughly as possible, and then to add the water, by artificial means, just before it passes through the calenders. This method, moreover, has the advantage of damping only the surface of the paper.

Fig. 54 shows a damping arrangement as manufactured by Messrs. James Bertram & Son. The paper on leaving the last drying cylinder passes on to two copper cylinders c, filled with cold water. Steam passes through the pipe a, and issues through a number of fine holes in a pipe running at right angles to the direction in which the paper is travelling, and near to it. The steam condenses on the paper and on the surface of the cylinders, from which the paper absorbs it. The supply can be regulated by the cocks shown in the drawing. The troughs d carry off any excess of condensed water.

Amongst other methods proposed is one by Annandale; it {160} consists in breaking up jets of water into a very fine spray by means of a blast of air.

The finished paper, after passing through the calenders, is wound on the reels P. The gearing by which the whole machine is driven is shown in Plate II.

FIG. 54.

It sometimes happens that, owing to the increased tension due to the contraction of the paper in drying, the paper breaks. It is therefore necessary to alter the speed of some of the cylinders to compensate for this. A rough expedient which is largely adopted, consists in attaching, by means of a mixture of rosin and tallow, a piece of felt to one or other of the pulleys (Plate II.), and thus altering its speed.

The Fourdrinier Paper Machine. Plate II.

Tub-sizing.

—The fore­going de­scrip­tion is of a ma­chine for the man­u­fac­ture of en­gine-sized papers; some slight mod­i­fi­ca­tions are neces­sary in the case of papers that are tub-sized on the machine. In making the cheaper qualities of tub-sized papers, the paper, after being partially dried over a few cylinders, is passed through a ves­sel con­tain­ing a {161} solution of gelatine (see p. 162). It then goes between a pair of rollers, which press out the excess of size, and then again over drying-cylinders. The vessel or trough containing the size is filled to overflowing from a tank placed at a higher level; the overflow passes into a lower vessel, from which it is pumped up to the top tank. The size is kept warm by means of a coil through which steam passes placed in one or other of the vessels containing it. The paper passes between a pair of rollers placed in a trough and dipping into the size. The size may be made to flow through a pipe pierced with a number of holes, the stream running directly on to the web of paper. In the other, and perhaps most general way, at least for the better qualities, the paper is wound off immediately after leaving the last drying-cylinder A (Fig. 55), and sized at some future time, or it may be passed directly into the vat B containing the size. After passing between the squeezing-rolls. C, it is generally wound off as at D; and after having stood some time {162} to allow the size to be evenly absorbed by the paper, it is wound off from F, and passed over the cylinders H, of which there may be a very great number, some machines having over 300 of them. These cylinders are made of light spars of wood; inside them, and revolving rapidly in an opposite direction, are the fans G. The paper, in travelling over these drums, is only slowly dried, and is supposed by this means to be more perfectly sized, and increased in strength. It is wound on to reels again at I. Only the first and last two drums are shown. This method was devised to imitate, as far as possible, the sizing process of hand-made paper. Even now, paper that has been made on the machine is sized by hand, after having been cut into sheets, much in the same way as hand-made.
FIG. 55.

Preparation of Size.

—Very pure gelatine can now be obtained in the form of light-coloured cakes. It is, however, more economical for the paper-maker to prepare his own: this is therefore done in the majority of cases. A great many animal substances, such as clippings of hides, horns, bones, &c., yield gelatine when heated with water. Any of these substances may be used, the first being the chief raw material. They are first softened by soaking in cold water for some days: they are then cleansed by washing in a stream of water. The next operation is that of heating with water. This is usually done in iron or copper vessels provided with a false bottom or outer jacket, into which steam may be passed. The hide cuttings are covered with water, and the mass gradually raised to a temperature of about 85° (185° F.). In from 10 to 15 hours nearly the whole of the cuttings will have passed into solution as gelatine. This solution is then drawn off and any insoluble substance suspended in it is removed by subsidence or filtration. The hides contain a certain quantity of fat, which collects on the surface of the solution. The residue in the boiler is again heated with water and the solution added to the bulk. It is of the greatest importance that too high a temperature be avoided, as gelatine is liable, when strongly heated with water, to lose its power of gelatinising. {163}

To the filtered solution of gelatine, which should be, if properly prepared, of a pale colour, a quantity of alum solution is added. The effect of alum upon gelatine is very remarkable. If added slowly it will be found gradually to render it stiff until at a certain point the mass will become almost solid; a further addition renders it fluid again. It is in this state ready to be used for sizing purposes. The amount of alum necessary to produce this effect is about 20 per cent. of the weight of the raw material. The alum is also useful in preventing the decomposition of the gelatine, but its chief characteristic is to render the gelatine a more efficient sizing material. There is no doubt that, besides its action upon the gelatine itself, it has a considerable effect upon cellulose. It is of the greatest importance that the alum or the sulphate of alumina, which can be substituted, should not contain any free acid. This is especially necessary in the case of papers made from rags, in the bleaching of which an acid has been used; one effect of which is to remove all basic substances derived from the boiling or bleaching processes. In the case of esparto and similar fibres, a considerable quantity of such substances are present in the pulp, the result being that the free acid of alum is to a large extent neutralised, and its injurious effects prevented.

The effect of the free acid is seen in the weakening of the paper and the destruction of metal surfaces with which it comes in contact. The effect of acid upon cellulose will be found more fully described in p. 12.

Many paper-makers add to the gelatine a certain quantity of soap, the effect of which is to render the paper capable of taking a high finish. The soaps employed should be white and firm, and should be free from rosin. Some soap-makers prepare a special soap for paper-making. They are usually made from tallow, or a mixture of this with a small quantity of coconut oil.

If a solution of soap be added to one of alum, a double decomposition immediately occurs, the fatty acid being thrown down in combination with the alumina, and the soda {164} combining with the sulphuric acid. If, however, the soap solution be previously mixed with a solution of gelatine, no precipitation takes place, the mixture forming an emulsion having somewhat the appearance of thin milk.

Various attempts have from time to time been made to size paper in the engine with gelatine, by precipitating it in the pulp after the manner of rosin sizing, but as yet no successful method has been devised.

Single-cylinder Machines.

—Modified forms of the original Fourdrinier machine have been introduced to meet various requirements. One suitable for the manufacture of very thin paper, or of papers one side only of which is required to be glazed, called a single-cylinder or Yankee machine, is shown in Fig. 56.
FIG. 56.

It resembles the Fourdrinier machine as far as the couch-rolls A and B. The paper is taken off the wire-cloth on to an endless felt running round the upper couch-roll A, and travelling in the direction of the arrows. It is taken from the felt on to the large drying-cylinder C, of about 10 feet diameter, heated with steam. It is carefully turned and polished so as to impart a high gloss to the surface of the paper with which it is in contact. Calender-rolls are sometimes supplied in addition.

The arrangement shown at D is for the purpose of washing the felt. This is necessary to cool and open it out after passing between the cold press-roll E and the hot cylinder. {165}

The paper, after passing over the greater part of the surface of the cylinder, is sufficiently dried, and it is then wound off at F.

FIG. 57.

A machine of a very dif­fer­ent con­struc­tion from the or­di­nary form is shown in Fig. 57. The pulp, after passing through the strainer A, enters the vat B, in the centre of which a large drum or cylinder C revolves. This cylinder is covered with fine wire-cloth, and on it the paper is made. As it revolves, the fibres attach themselves to the wire, and the water passes through the meshes, being assisted by means of a pump. The sheet of paper thus formed is taken on to the endless felt passing round the couch-roll D, and travels along with it to the large drying-cylinder E, heated by steam. It leaves the felt at F, and is then taken on to the cylinder, after travelling round which it is sufficiently dried, and is then wound off as at G. The felt on its return journey passes through the washer H, where it is cleaned and freed from adhering particles by the scraper I. It is squeezed free from excess of {166} water by the rolls K. Paper made on such a machine is weaker than that made in the ordinary way, because it has not been found possible to give a shaking motion to the cylinder, and thus the fibres are only imperfectly felted.

A modification of this machine is used for making millboards, the difference being that it has no drying-cylinder. The felt carrying the paper passes between a pair of press-rolls, which squeeze out the water. The sheet of paper is then allowed to wind round the top press-roll until of the required thickness. When this happens, it is cut off the roll by a knife. The thick sheets so produced are dried either in the open air or in a room heated for the purpose. (See p. 108, Fig. 31.)