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Stock Preparation

( Originally Published 1920 )



WITH the exception of papers made chiefly from groundwood, such as newspapers, toilet tissues, and the cheaper grades of printing papers used in the so-called "pulp" magazines, almost all papers are made from fibers which have been given some sort of a mechanical treatment. This is termed "beating" or "refining," and is carried out in various types of machines. These are designed to cut or shorten the fibers and to develop their ability to cohere when formed into a sheet, and thus to produce the strength and even formation required in most papers.

Beating has been practiced from the earliest period at which paper was made, and in all probability the name originated from the crude methods employed at that time, or even when papyrus was used. The method of preparing sheets of papyrus involved pounding the moistened strips to make them adhere, and later methods of making paper from bark included wetting it, placing it on stones and pounding it with a club or mallet. The original Chinese method of preparing fibers for papermaking was to place the bark or cloth in a stone mortar with a little water and pound it with hand-operated pestles or mallets until the fibers were sufficiently separated. All of these operations are distinctly "beating," and included little of the cutting action which is a function of many of the modern beaters and refiners.

In the Chinese method of beating the mortars were frequently set in the ground and the pestles or hammers were raised by hand, or later by workmen treading on the ends of levers or tilt-bars.

The hammers probably had a weight of 60 to 70 pounds and dropped on the stock from a height of 8 to 12 inches. This laborious method of beating was somewhat improved about 1151 when a stamp mill operated by water power was invented in Spain. Mills of this stamp type consisted of rows of great wooden hammers operating up and down in troughs known as "vat holes," which were made from stone or from logs of oak, with their cavities lined with lead or iron. Later, probably toward the seventeenth century, the stampers were somewhat modified according to the kind of work they had to do. For the first treatment of rags the hammers were faced with iron spikes or teeth, which aided in fraying out the cloth. While this treatment was being given, a stream of water was run through the trough to carry the loosened dirt away through holes in the sides of the containers. Loss of fiber was prevented by screens of horsehair over the holes. When the rags were partly broken down and washed, they were bailed out into another stamper where the beating and washing was continued with more lightly shod stampers. The final treatment was in a third set of stampers, which were usually not metal faced, and which operated without the stream of water, because, at this stage of the beating, water would have removed too much fiber.

The number of such stamp mills in a plant naturally varied with the amount of paper being made, and as there is little information regarding the output of any of the early mills the capacity of a stamp mill can only be guessed. A good German mill early in the seventeenth century was said to have as many as twenty-five troughs, each with five stampers. Such stamp mills as these were used in the early American paper mills, notably the Willcox mill previously mentioned. It is not known when they went out of use, but it was probably not far from 1780 in America and somewhat later in Europe.

It is obvious that this manner of beating must have caused little shortening of the fibers, and that its chief function was to fray out their ends and split them lengthwise into long, slender fibrils. It is claimed that beating of this sort, together with the absence of alum and other chemicals in the paper, was responsible for the high strength and durability of papers of this period. This may be true, but the opinion is necessarily based on papers which have survived, and cannot be influenced by those papers which were so poor as to have perished. That there were such papers is proved by a statement of the Dutch archives in 1670 saying that papers (presumably of Dutch origin) cracked when folded, and after a few days could be rubbed to fragments between the hands. It seems that such a state of affairs must have been caused by the preliminary treatment of the rags, rather than by the beating operation itself.

It is generally considered that the essential features of modern beaters were first employed about 1680, when some Dutchman whose identity is unknown developed a beater consisting of a tub with rounded ends in which revolved a solid wooden roll, on the surface of which were fixed about thirty iron knives. This roll revolved directly over a bedplate made of stone or metal, which was fixed in the bottom of the tub. As the roll turned, the fibrous material was dragged between the bedplate and the knives and lifted over a backfall, down which it slid to complete its course around the tub. Beaters of this type are in common use today and are known as Hollanders because of the country of their origin.

Whether the date of 1680 is correct seems to be questioned by A. Blum in his book "On the Origin of Paper." In this he states that the Fabriano mill in Italy produced in 1268—1276 a rag paper from flax or hemp in which the pulp was ground more thoroughly by metal beaters than stock produced by stamp mills. In this paper the fibers were short and it was sized with gelatin and water-marked. This distinguished it from paper made in the Arabic manner, which was of long fibers beaten with pestles, and sized with starch. This question is merely of academic interest, but a bit of evidence tending to support Blum was supplied by a European paper made in 1486 in which the linen fibers were much shorter than most rag stocks treated in modern beaters, and many showed ends cut or broken sharply across.

Beaters of the Hollander type were in use in Germany as early as 1710, and in 1725 one such beater was said to prepare as much stock in a day as eight stamper mills could make in eight days. The change from stampers to Hollanders was gradual, and for some time both types were used in many mills, the stampers for breaking down the rags and the Hollanders for finishing the beating. Hollanders were used in America at least as early as 1775. All but one of the English mills had discarded stampers by 1810, but until 1861 the French government insisted that all paper for the stamp office be made from fibers beaten in stamp mills.

The principle on which the Hollander operates is still that of many modern beaters and the chief changes have been made to improve the efficiency and control of the beating and the capacity of the beater tubs. Literature of about 1885 mentions rag washers as having capacities of about 150 pounds of rags, and as late as 1888 trade journals mentioned the installation of new beaters of 600 pounds capacity. Today a beater of 1,000 pounds is relatively small and some will hold from 3,000 to 4,000 pounds. There are many different kinds of beaters of this general type, varying in size and shape of tub, weight of roll, number, thickness and spacing of roll knives, and position and type of knife filling the bedplate. These changes were all designed to increase the speed of beating and improve the quality of the beaten fiber in some way, but none has proved so outstanding that it has replaced all the others. Like much of the other machinery in a paper mill the cost of replacement is so high that once a beater is installed it is used more or less regardless of its efficiency. This is largely due to the fact that the efficiency of a beater may be calculated mathematically according to a number of theories, but can be proved only by practical operation. It is, therefore, necessary to install and use a beater in order to be certain that it is satisfactory.

In using a Hollander the sequence of operations includes filling, or "furnishing," beating and dumping. Furnishing includes adding water and fibers until the desired quantities are present. The fiber may be in slush form, or in lap form containing 30 to 50 per cent of fiber; or in dry sheets or roll fiber, each of which is usually about 90 per cent dry fiber. Sometimes wet waste, or "broke," from the paper machine is added, or dry broke or waste from the sorting room which has been put through some form of kneader to reduce it to a semi-pulped condition. As will be discussed later, fillers, coloring matters and sizing agents are customarily added to the pulp in the beater.

After the furnishing has been completed the beating is begun by lowering the roll toward the bedplate. This is done by turning a wheel which operates through a worm and gear to move one end of a lever on which the bearing of the roll shaft rests. A shaft attached to the hand wheel extends across the beater tub to a similar device on the other side, so that both ends of the roll are raised or lowered an equal amount. Adjustment in this way is very sensitive, and requires great skill and judgment for the best work. The setting of the roll may be judged by placing one end of a rod against the end of the bedplate and the other end of the rod in the ear; then the vibrations will tell the experienced man how hard his roll is set. Sometimes the degree of beating is specified as so many turns of the hand wheel, starting from the position when the roll just touches the bedplate. This requires almost as keen judgment in determining the starting point as in beating entirely by sound. There are also mechanical devices which weigh and record the pressure exerted by the roll on the stock, or which show the linear distance of the roll from the bedplate, but much the most common method of operating is still that of setting the roll by hand.

The amount of beating, as well as its severity, depends on the kind of paper being made and the fiber used. For example, papers as different as blotting and hard rattly bond can be made from the same rag stock by proper regulation of the beating. When rag stock was the fiber in common use the beaterman judged the degree of beating by taking a little of the beater contents in his hand and squeezing it. This same method was also employed in judging the length of the fiber delivered by the jordans, and experienced men became remarkably proficient in regulating their beating in this way. Since the use of chemical woodpulp has become so general the necessary shortening of the fibers is much less, and the art of beating by hand-testing has almost completely disappeared. It was probably never as skilfully carried out as the beatermen thought, for recording devices have proved that hand setting of the roll could not be done the same by different individuals, and not even twice alike by the same person.

When the beating in the Hollander is considered finished its contents are dumped through a large valve in the bottom of the tub into a tank or chest fitted with an agitator, and often known as the jordan chest because the stock next passes to the jordan. Dumping a beater requires care and system. Some water is necessary to enable the stock to flow out readily, but the amount used should not vary too much because the consistency of the stock going to the jordans should be practically constant in order to enable it to do uniform work. From the jordan chest the stock passes through the jordan to another chest which supplies the paper machine, and hence is called the machine chest. The placing of a second jordan (or other refiner) between the machine chest and a paper machine makes it possible to change the character of a paper more quickly than is possible if stock is accumulated in the machine chest before the effect of beating can be tested on the paper machine.

Beaters of the Hollander type require much floor space and their power consumption is high—much the greater part of the power being used to circulate the stock, rather than in useful beating. Some beaters can handle stock at 8 per cent concentration, but most are operated at 5 to 6 per cent. The control of the beating action is not too good and the action of the roll can change with the wearing away of the knives of the roll and of the bedplate. These shortcomings, and the desire for continuous, rather than batch operations, led to the development of the Kingsland refiner in 1856, the jordan in 1858, and since then a considerable number of others, which work on similar principles, but with different mechanical devices. Most of these devices consist in principle of two discs, one or both of which revolve, and between which the fibers pass from the center of the discs to the periphery. The inner faces of the discs are lined with bars or grinding surfaces of various kinds according to the character of the stock and the purpose for which it is to be used. The distance between the discs influences the capacity of the refiner, as well as the degree of beating of the stock, and it can be controlled accurately by proper mechanical devices.

The jordan, which has been widely used for a long time, consists of a conical plug, covered with longitudinal knives, which fits into a conical shell similarly lined with knives. A means of moving the plug in or out with respect to the shell enables the distance between the knives of the plug and those of the shell to be accurately adjusted so that the treatment of the stock can be varied from a light brushing action to one of sharp cutting. This permits considerable variation in the quality of paper made. The stock enters at an opening in the small end of the shell, passes spirally around the rapidly revolving plug, and is discharged at the large end by the centrifugal action of the plug. Originally the jordan supplemented the beater and was supposed to give the finishing touches to the stock and to even out inequalities in the stock from different beaters. Its function was much more important when beating long fibers, such as cotton, linen and hemp, than it is on chemical woodpulps. Today some mills have completely eliminated beaters and depend entirely on jordans or other refiners for their stock preparation (fiber treatment). This is true of such widely different papers as kraft wrappings and glassine paper. Other mills, notably those making newsprint, have done away with beating entirely and meter both the groundwood and sulfite directly to the paper machine.

Jordans and refiners require that the stock be supplied to them in slush form and preferably at a constant fiber concentration. The jordan can handle stock of 2 to 4 per cent concentration, but 3 to 3.5 per cent is the usual practice. Hollanders and other beaters of similar construction can be partially filled with water and then dry sheet or roll fiber can be fed in until the desired concentration is reached, though this is not considered to be the best practice.

In spite of all the developments which have been made in beaters and refiners, and all the study which has been given to the subject, there is still no sound, basic knowledge which will enable a mill to select the best equipment for any given type of paper. Probably there is no "best beater," at least so far as its effect on the paper is concerned, and an equally good sheet can be made from stock prepared in any good beater if it is properly handled. There is still much argument as to whether tub beaters are necessary and whether all the work cannot be done better in jordans or other refiners. This unfortunate and confusing state of knowledge is due to the impossibility of assembling and comparing large scale units of the different beaters which can then be used in preparing different fibers for a variety of types of paper.

The changes which take place in fibers during beating profoundly affect the character of the paper which can be made from them. Some of the changes can be seen when the fibers are examined under a microscope, but in many instances there is a marked change in the papermaking properties with no commensurate visible change. In general it may be said that the outer surface of a fiber consists of a network of fibrils, while the inner, secondary wall has parallel bundles of fibrils of the same general dimensions. Beating tends to remove that portion of the outer surface which remains after the processes of cooking, bleaching and purification. As beating proceeds further, the inner wall of the fiber starts to swell and disintegrate. Beating for most grades of paper involves only the first of the process, but for glassine papers it goes nearly to completion.

Fibers vary greatly among themselves in the way they respond to beating treatments. Hemp, linen, and to a less extent cotton, tend to split lengthwise into fibrils which are considerably longer and more slender than those from wood fibers. These fibers, therefore, are of special value in making such thin and opaque products as the Oxford India papers so largely used in printing Bibles and Prayer Books. Other fibers, such as sulfite and sulfate from coniferous woods, split very much less than linen, and hence are not suitable for very thin papers. The fibers from deciduous woods are naturally short and do not show visible effects of beating nearly as much as the long fibers, though there is a distinct change in the strength of the paper as the beating of such fibers proceeds.

Besides the type of fiber there are numerous other factors which may greatly influence beating. Degradation of the fibers may be caused by too severe treatment in the cooking process; too much bleaching, or bleaching at the wrong degree of acidity or alkalinity; contact of the fibers with acid, as in the drainer bleaching of rag stock which was acidified to hasten the bleaching action; or too high a temperature during bleaching. All of these may so seriously affect the fiber and its beating that the papers made from it are of very inferior quality. On the other hand, it is possible to have a fiber so highly purified that it has no papermaking value, as proved by a fiber of 99 per cent alpha-cellulose content, which could not be so beaten as to produce sheets which were strong enough to handle.

Besides these influences which cause variations, there are the different types of beaters and refiners with their different kinds of bar fillings, different speeds, and possible degrees of adjustment. Most of these have been designed to perform certain features of beating, such as cutting, or "hydration," but their action is so obscured by other factors of beating and papermaking that it is difficult to isolate the action of the beating device itself.

The term "hydration" is only one of those used to describe those properties of pulp caused by beating. "Freeness," "slowness," "wetness" or "greasiness" are also applied as descriptive of the rate at which water drains from the stock, or the way it feels when grasped in the hand and squeezed. Although "hydration" is very generally employed to describe this effect of beating its selection is particularly unfortunate because it seems to imply an actual chemical combination of water with the fiber substance. For many years there was active discussion of this theory, but at present it seems to be generally assumed that the effect is due to a softening and swelling of the fiber caused by penetration of the water between the fibrils.

Whatever its cause, the effect of pronounced "wetness" or "hydration" in a beaten stock is to reduce the rate at which it will drain on the paper machine wire and hence reduce the speed at which the machine can be run, as well as its output of paper. It also causes a tendency to slide on the cylinder machine molds and to "crush" under the dandy roll of the fourdrinier machine, both of which cause defective paper. In the paper itself beating induces greater shrinkage, density, hardness, and translucency. Such effects are noticeable to an extreme degree in papers of the glassine type.

Almost without exception, paper consists of a heterogeneous mixture of fibers ranging from long to very short. The proportions of long and short depend on the kind of fiber used and the degree of beating, which is merely another way of saying the type of paper. This variation in length is due to the fact that not all of the fibers receive the same amount of cutting in the beaters and jordans. In fact this could hardly be otherwise considering the enormous number of fibers in a beater and the chances that many will completely escape passing between two knives in such a way as to be cut. Probably the effect of variations in length is beneficial in most papers because it helps to average out the various strength factors, and thus make a more generally acceptable sheet.

As the degree of beating increases, the bursting, tensile and folding strengths of the paper increase. Tearing strength rises rather quickly to a maximum and then drops off with increased beating. At the same time shrinkage of the sheet on drying increases and also its susceptibility to change in dimensions with varying humidities in the surrounding atmosphere. In many cases the demand of the customer is for properties which are not produced by the same type of beating, as, for example, high bursting and tearing strength in the same sheet. This, together with the necessity for operating paper machines at high speed in order to make enough product to produce a profit, make the problem of beating a very complicated one which is not simplified by the fact that our knowledge of beating consists largely of a collection of diverse personal opinions.

The beating of the fibers, which has just been described, is only a part of the preparation of the stock for use on the paper machine. There are a number of other factors to consider, such as the addition of fillers, sizing agents, coloring matters and wet strength agents, and these materials are usually, though not always, added to the fibers in the beater.

Fillers, or loading materials, were originally considered as adulterants used chiefly to cheapen the paper. It was not long, how-ever, before they were recognized as serving perfectly legitimate purposes by increasing the opacity of the paper, aiding in obtaining a good finish on calendering, and improving printing qualities by reducing "show-through" and "strike-through" of the ink. To-day fillers are used in the great majority of printing papers, though there are some bulky papers and specialties in which they are not employed. Fillers increase the weight more than the bulk of a paper; they also affect adversely the strength of the paper and make it much more difficult to size with rosin. These facts must be remembered when setting specifications for a paper.

The most generally used filler is clay, which is a natural white mineral, formed by the weathering of certain types of rock. It is prepared for use by washing, separating sand and mica, and drying for shipment. Formerly much of the clay came from England, but the greater part now used in this country is of American origin. The important properties for a filler clay are good whiteness and freedom from grit and mica. Particle size is not nearly as important as in a coating clay and little attention is paid to this property in selecting a filler. Uniformity of color is also not as important as in coating clays because the filler is dispersed among the fibers and its effect is much less marked than when in a surface coating. This does not mean that the properties of a filler clay are unimportant and need not be controlled, but merely that they are less critical than for coating work.

In the early days clay was added dry to the stock in the beater, being measured by weight or more roughly as so many buckets full. Modern practice is to prepare a clay slip by mixing with water, adjust its dry clay content to a definite and constant value, and measure this to the beater by volume. In either case it is customary to add the clay fairly early in the beating process on the theory that if beaten into the fibers as they are cut and fibrillated its retention would be greater as the sheet is formed on the paper machine wire. It is very doubtful if this point is of any importance. No mat-ter how the stock is prepared a certain amount of the clay or other filler passes through the wire with the water, but much of this is later recovered in the white water save-ails and used over again.

The filler next in importance to clay is precipitated chalk, which is calcium carbonate. The natural ground mineral is seldom used and the manufactured product is made in several different ways and in very different degrees of fineness. Chalk is much whiter than clay and is of more help in increasing the opacity of the sheet. It is an alkaline material and its presence makes it extremely difficult to size the paper with rosin and alum. Because its alkalinity overcomes the acidity of the alum used in the beater contents, the permanence of paper filled with chalk is far greater than of that filled with clay and made acid with alum. Chalk was first extensively used in cigarette papers, which are often very heavily loaded with it, but its use in printing papers is now quite general.

Calcium sulfate in the form of ground gypsum, or more often artificially prepared as "pearl hardening," "crown filler," or material sold under some other trade name, is sometimes employed as a filler. Its crystalline form and transparency make it of doubtful value in improving the opacity and color of the paper, and its moderate solubility in water makes it of questionable economic value.

Talc is a natural mineral occurring in several forms, some of which find use as fillers. The soft, foliated kind is not usually of good enough color for white papers, but it has the interesting property of reducing the strength of the paper far less than an equal amount of clay. The fibrous talcs such as "asbestine," "talclay," etc., are of much better color than the foliated kinds, but have the disadvantages of being rather too abrasive, and of containing enough calcium carbonate to interfere with rosin sizing.

Titanium dioxide is one of the newer and more expensive fillers. It is used primarily for its very high brightness and its unusual opacifying power, and because so little of it is needed to cause appreciable improvement it can be used in bulky papers without seriously affecting their bulk-to-weight ratio.

Sizing is a term which the papermaker employs very loosely to cover the use of numerous materials in the beater, others on press rolls or calender stacks, and even in connection with the ratio of adhesive to pigment in coating mixtures. Only those substances used in the beater will be mentioned here.

The principles of rosin sizing were first worked out about 1806 and since then an enormous amount of research has been done on the subject, without appreciably changing the basic operations. Rosin size is prepared by boiling rosin with soda ash or caustic soda in enough water to give about 60 per cent solids in the finished size. The proportions of rosin and soda are generally such as to leave about 30 per cent of the rosin uncombined, but "dissolved" in the rosin soap formed from the rest of the rosin. When this size is diluted, mixed with the beaten fibers and precipitated with alum, the rosin adheres to the fibers, and when the paper is dried on the steam drums of the paper machine it develops sufficient resistance to the penetration of water so that it can be written on with pen and ink. There isn't a papermaker living who doesn't wish that rosin sizing was as simple as that, but unfortunately it is an extremely complicated operation and is affected by so many variable factors that at times it seems completely uncontrollable. Just a few of these factors are: (1) proportion of free and combined rosin in the size, (2) the amount used, (3) the degree of beating given the fibers, (4) the amount and kind of filler used in the paper, (5) the ratio of rosin to alum, (6) the nature of the water used, and (7) the temperature of the beater furnish. When these are combined with the numerous variable factors involved in running the paper machine and drying the paper, it can be understood how difficult it is to obtain reliable information about any of the individual variables.

All this trouble is taken to develop only the one property of resistance to the penetration of water. Rosin sizing has at best a slightly harmful effect on the strength of the paper, and it reduces its permanence over long periods of storage. The action of alum, used to precipitate the size, also causes the flocculation of finely suspended dirt in the water and thus is harmful to the color of the paper. Rosin sizing is necessary in some kinds of paper, but for many printing papers it is quite needless, and it is desirable to omit it wherever the use of the paper will permit.

Sizes containing various proportions of wax with the rosin are often used, especially for papers employed for drinking cups or for holding wet materials. Such sizes are used in much the same way as those containing rosin only, but sometimes are more difficult to prepare in dilute form.

Alum, which is the papermaker's name for aluminum sulfate, is the universal precipitant for both rosin and wax sizes. It has an acid reaction and if used too freely it may make the paper so acid that it will become brittle on aging, especially in hot climates. For this reason, as well as for economy's sake, as little alum as will give the desired result should be used. This minimum amount is always more than that which is needed just to precipitate the rosin, other-wise good sizing is not obtained. Moreover alum is one of the most generally used cure-alls in the mill and when the sizing is poor, or the stock sticks on the press rolls, or any one of a dozen things takes place, the first attempt at correction is often to add more alum. It is no wonder that alum consumption is very frequently too high.

Two materials which are often called sizing agents are sodium silicate and starch. Both serve chiefly to reduce the fuzz on the surface of the paper and to increase its strength; they do not aid in developing water resistance. Silicate is precipitated by alum, but except under certain very specific conditions starch is not. Starch is added to the beater in the dry condition or as a solution of some chemically treated starch. Probably the retention is greater where dry starch is used, but its effect may be no greater than a smaller amount of cooked starch, since the raw starch must be cooked by the heat and moisture on the drying cylinders before it can increase the strength of the sheet. The retention of cooked starch is fairly low and the benefits from its use seem at times to be decidedly questionable. It might be regarded as insurance against weak or fuzzy paper, or as a substitute for part of the beating which would be needed to increase the strength of the paper, but for which there is insufficient time or beater capacity.

Nearly every paper contains coloring matter of some sort and amount, and the final paper may vary from some tone of white to deeper colors of all kinds prepared for special printing and decorative purposes. Such coloring is usually done by adding the necessary material to the stock in the beater. Even white papers generally contain small amounts of blue and pink to offset the naturally yellow tone of the fibers, and to give the blue tone which is generally considered to be "white."

Soluble dyes are used more generally than pigment colors and are always added to the stock after dissolving. Such dyes are of three types, the so-called "acid," "basic" and "direct" dyes, which have to be handled differently in the beater and which impart different characteristics to the paper. Acid dyes need to be applied on sized stock as they do not take well on unsized fibers. They give even colors of good fastness to light, but are not so intense or brilliant as the next class. Basic dyes give much brighter colors of greater depth, but most of them are not very fast to light, so are seldom used where great permanence of color is needed. Direct dyes take well on unsized fibers, but are not of great brilliance, and are used only where that property is unimportant. Different fibers take up the coloring matter differently and if the fiber furnish contains both bleached and unbleached fibers uneven dyeing may result, some fibers being much more deeply colored than the rest. The use of dyes in a paper mill is not usually given the expert attention that it is in a textile mill, but the job requires a high degree of skill and a keen eye for slight color differences.

Pigments are not as often used as formerly, except for those synthetic pigments which are extremely fast to light and which are so finely divided in suspension that they partake largely of the character of dyes. These are increasing in their use for special papers, but the common pigments, such as ultramarine, ochres, oxide reds, umber, and chrome yellow are not so important as formerly. These pigments, if used in large amounts, act as fillers as well as coloring matters and are subject to the laws affecting retention.

These older pigments are notable for their permanence to light, but are not particularly brilliant.

Another characteristic which is developed by adding certain materials to the stock in the beater is that known as "wet strength." A number of resins will cause this, among them being those formed from melamine and formaldehyde, or urea and formaldehyde, and others for the same purpose are being introduced frequently. These resins attach themselves to the fibers, and once the paper is dried it no longer becomes as weak when soaked in water as the untreated paper would be. While a paper with no resin will retain almost none of its dry strength on soaking, one treated to develop wet strength and correspondingly soaked may retain 30—40 per cent. This difference is very appreciable in the use of the paper and makes it much more serviceable for blue prints, military maps and all purposes where it might become wet.

While all these operations have been described as carried on in the beater some of them may be effected at other points, and in fact there is quite a tendency to add size, alum and wet strength agents after the stock has passed the jordan and as it approaches the paper machine. When so used, quicker changes and adjustments can be made, but the control of quantities used may be slightly more difficult or complicated. In some mills where the same grade of paper is made day after day it has been found advantageous to meter all these materials to the stock at the paper machine headbox. It is sometimes the case that there is no other place to add them when no beaters are used and the fibers are metered to the machine.

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