SHEET CALENDERS
Fine writing-papers may be finished in a variety of ways. A plain, smoothed surface is obtained by passing the sheets, which are automatically fed, by a system of tapes, through calender stacks, called sheet calenders.
Plating.—Plating was first resorted to as a means of smoothing paper in the sheet, but when a linen, or pebbled, or any other special finish is desired, it is also accomplished in a plating machine. This consists of two heavy rolls. The sheets of paper, with a metal plate top and bottom, are passed through the rolls under heavy pressure. If a linen finish is desired, pieces of linen are placed between the plates on both sides of the sheets so that the linen texture is embossed into the paper. Similarly any other substance may be used for other effects.
Embossing.—Embossed papers are usually finished from the roll by running between iron rolls with embossing patterns engraved upon them. An extra strength is required of paper for this purpose, otherwise the pattern will cut through the sheet.
Supercalendering.—Supercalenders are machines, apart from the paper-machine itself, for making high-finished paper. The rolls vary in number. Each alternate roll is made of hard paper. In treating uncoated stock there are also one or two steam boxes to moisten the paper before it is calendered. This softens the surface fibers, and they can then be rolled flatter and hence take a shinier surface. The alternate rolls in a stack for calendering coated papers are made of cotton, and no steam boxes can be used, because the moisture would injure the coating.
The paper is run through the calenders in the web. All smooth, or special, finishes are gained only at added cost. Where the process takes place on the machine, more breakage is occasioned and more paper has to be sorted out, as the hard-finishing accentuates spots in the paper, and little lumps of fibers, which would pass unnoticed in an uncalendered or antique paper, are squashed down and blackened by calendering. Hence the higher cost of such papers.
Supercalendering and plating bring into play different workman, so that the labor cost is increased, and any finishing, sheet by sheet, is necessarily slow and more costly than that accomplished in a continuous process from the roll.
Combining.—Many kinds of papers, as photo-mounts, double-thick covers and cardboards, are made by pasting two or more thicknesses together. This was formerly done in the sheet, but most of the pasting is now effected in the web. The papers are run over a paste roll, combined, and passed either through a drying chamber or over a battery of driers like those of the paper machine. The pasted paper is lastly made into rolls and taken to the finishing room to be sheeted.
Coated Papers.—Coated papers are made by covering the surface of ordinary paper with a veneer of clay, mixed with some adhesive, as casein or glue, and suitably colored.
The process is done from the roll; the paper first goes through the machine where the liquid coating is brushed onto the surface, passing directly in automatically formed festoons through a long, heated room to dry, and finally is rewound. The rolls are then taken to the supercalender room and the paper is given the desired finish.
Dull-finish coated papers require a special kind of coating and receive very light calendering after being coated.
High-finished coated papers of the best grades are double-coated and run several times through the calenders.
Another method of producing a high finish is known as “flinting.” In this process the paper is mechanically polished by smooth flint stones and gains a very high luster. Such papers are most widely used as box covering. A similar effect is obtained by friction calenders, which consist of two chilled iron rolls with an intermediate roll of hard paper. The top roll rotates at a higher speed than the others.
COATING ROOM, APPLETON COATED PAPER CO.
This view of the “wet end” of the coating machines shows the rolls going through the coating process, the web of paper traveling along the drying racks appears in the background.
The coating may be dyed to any color desired, so that coated and glazed papers are obtainable in a wide variety of shades.
FINISHING-ROOM, CRANE & CO.
The machine on the right is a plater.
Gummed Papers.—Gummed papers are made by passing the web through a machine, which coats it with glue, after which it passes over drying apparatus and is wound into rolls ready for finishing.
Gummed paper for labels is usually finished in sheets, while for sealing tape and box stays it is ordinarily made up into rolls.
Waxed Papers.—Waxed papers are made by applying a coating of parrafin. This renders the stock water proof, and it is used largely as a wrapper for food products.
Glassine Paper.—By a special treatment in the beaters and jordans cellulose fiber is so treated as to become hydrated. This hydration makes the paper produced grease proof, and by heavy supercalendering the character of the sheet is again greatly altered, it becoming almost perfectly transparent. In this state it makes a most attractive and hygiene wrapper.
CHAPTER EIGHT
THE PHYSICAL AND CHEMICAL ASPECTS OF PAPER
The size and weight of a sheet of paper of any given quality and finish are its most obvious features, and when we speak of the weight of a sheet of paper we refer not to the one sheet, but to the weight of one ream of similar sheets. Most papers are ordered on a basis of ream weight for a specified size, as, for example, 25 by 38, 50-pound. Blanks, cardboards and cover-papers, especially the first two, are more frequently ordered on a basis of bulk, as two-ply, three-ply, etc., and thick or double thick in the case of covers. The thinner covers are usually designated by their ream weight, though frequently quoted, as are the heavy-weight covers, the blanks and cardboards, in price by the hundred sheets.
The reason for this difference is probably that such stocks are sold in comparatively small lots, so that it is simpler to bill them in accordance with the number of sheets than to figure the weight of a small number of sheets and multiply by the pound price.
Another thing which facilitates the system is that these kinds of paper are carried in standard stock sizes, as the majority of orders are too small to be made in special sizes.
The relation between thickness and weight of a given paper is approximately a direct ratio. For example, given a sheet of machine finish 25 by 38, 50-pound, four sheets of which bulk .011 of an inch, the bulk of the same finish and quality, in 25 by 38, 60-pound, can be approximately ascertained by the equation 50 : .011 :: 60 : x, the answer of which is .0132.
The difference in bulk between two papers of the same weight depends on:
1. The finish.
2. The percentage of mineral filler.
3. The nature and treatment of the fiber.
For example, on a bulk of .015 of an inch to four sheets a supercalendered paper would weigh about 65 pounds, a high machine finish about 60 pounds, a text or medium finish about 50 pounds, an antique about 40 pounds. In other words, the density of any given piece of paper is proportionate to the amount of calendering it receives. Naturally, the antique paper, lightly pressed and uncalendered, is loose for texture and full of minute air pockets, so that it is light for bulk, while the supercalendered paper is squeezed to a hard, dense sheet containing little air space.
If the proportion of mineral filler is great, the weight will be still greater in proportion to the bulk, as the specific gravity of the mineral is greater than that of the fiber, and the fine particles tend to fill completely the small interstices between the fibers, so that the air space is reduced to a minimum. If, in addition, a surface coating is added, we get a paper with the highest possible percentage of filler, and consequently a glazed coated paper has less bulk in proportion to its weight than any other kind. Such paper contains from 30 to 40 percent of mineral.
The nature of the fiber brings about a difference, in that some fibers have thicker walls and smaller canals than others. The treatment causes a variation, in that a quick beating with sharp knives leaves the fibers more nearly in their original shape than a prolonged beating with dull knives, which breaks down the structure of the fibers and draws them out into minute fabrillæ.
The strength of a paper of given quality will also to a certain extent be proportionate to the duration of beating, as well as the amount of pressing and calendering received. The amount of sizing and the drying also affect its strength.
An antique paper, having large air spaces and loosely knit as it is, has not the tensile strength it would possess if pressed and calendered to a greater density.
The addition of loading adds to the weight without increasing the strength, as it has no binding properties. Moreover, the bulk, in proportion to the weight, is lessened by the introduction of filler.
Consequently it is axiomatic, that of two given papers of equal weight, finish and quality of fiber, the one containing the less filler will be the stronger, as well as bulkier. The addition of filler, however, increases the opacity, gives mellowness, and improves the printing quality by equalizing the texture of the surface.
The addition of sizing tends to increase the strength of paper, owing to its adhesive properties, but if liberally used it detracts from the mellowness and gives the sheet a tinny “character.”
The length of the fiber also affects the strength, as long fibers give greater strength and better folding quality than short. It is not possible to get as close formation with long as with short fibers.
Hence occasions frequently arise wherein customers ask for characteristics which are somewhat contradictory.
A desires a light, bulky paper with a high finish, but a bulky paper with high finish must, in the nature of things, be heavy.
B desires a very strong, thin, but opaque paper. It is obvious that the strength of a thin, opaque paper can be but a relative factor, while thinness and opacity are irreconcilable features.
C inquires for a closely formed sheet, with good folding qualities, but the first characteristic is only to be gained at the expense of the latter.
D wishes to print half-tones on an antique paper. In this case modern printing inventions have bridged over some of the obstacles of the past, and the offset press and extra-deep engravings have brought this last requirement within the realms of possibility, but unless resort is had to these new methods, the requirements again are irreconcilable to each other.
It is evident, however, that only through technical paper information can one solve such problems as necessitate a compromise capable of giving the maximum possible satisfaction.
The structure of paper, machine made, results in the greater proportion of the fibers in the formed sheet lying in the direction of the flow of the stuff. This determines what is called the “grain” of the paper. When paper is in the roll the grain of course is lengthwise of the web, but in the sheet the cutting and slitting may be arranged so as to leave the grain either lengthwise or crosswise of the sheet. This is an important consideration for a number of reasons.
In the first place, it is easier to tear the paper with the grain than across, as the fibers are parted rather than fractured in this way. This is a point which might be utilized by printers when printing detachable coupons.
Perhaps the most important consideration is the great difference in folding qualities. Many a paper will fold very nicely with the grain and crack badly if folded the other way.
Again, a great difference is noticeable in the flexibility of books, dependent largely on whether the grain runs parallel or at right angles to the binding. If flexibility is desired, the grain should run parallel to the back of the binding. Occasionally a wide-paged pamphlet, especially of light-weight paper, is improved by the rigidity to be gained from having the fibers run at right angles to the binding. It is also true that this increases the strength of the binding, as the sewing or wire stitching passes around more fibers than if the grain ran up and down the page.
Not infrequently does the middle signature of a pamphlet pull loose from the binding. Usually in such cases the paper is not strong anyway, but it could have had more resistance had the grain run at right angles to the binding.
The tensile strength of a strip of paper is greater with the grain, but its elasticity is greater across the grain.
A convenient way to ascertain the direction of the grain in papers that do not show it clearly by folding is to cut two narrow strips a few inches long, hold them by one end so that they coincide. When held horizontally, if the loose ends do not part, it indicates that the lower paper has its grain in the long dimension. If the lower paper has its grain crosswise, the loose end will sag away from the top strip, because, as above remarked, a paper is more flexible across the grain. This test may be applied either to sized or unsized papers.
Another test is to cut a small square and moisten one side; the paper will curl into a little cylinder and the grain runs parallel to the length of the cylinder. This test cannot be applied to an unsized paper.
This leads us to a consideration of the effects of moisture and humidity on paper.
It will be recalled from the chapter on Paper-Making (No. [VI]) how plastic paper is in its moist stage, and how tenacious of water are the cellulose fibers. It will also be recalled that there is considerable shrinkage across the web of the paper from the time it leaves the wire to the moment it is reeled. In fact, the very thing which makes paper-making a possibility is the shrinking of each individual fiber, occasioned by the expulsion and evaporation of the water, which has served as a carrier from the machine chest to the wet end of the machine.
This propensity of each individual fiber does not cease when the paper is made, but persists forever. A cellulose fiber will absorb moisture from the air in proportion to the relative humidity, just as the hair in a barometer is continually shrinking or expanding as the weather changes.
A definite percentage of moisture is normal to a cellulose fiber in proportion to the moisture in the air. The fiber swells as it absorbs, and shrinks as it gives off water.
Herzberg gives as the results of investigation with a good writing-paper made of rags, sized with rosin, the following report of the percentage of moisture retained under various degrees of relative humidity:
| Relative humidity of the air, percentage | Moisture contained in the paper, percentage |
| 100 | 21.5 |
| 90 | 13.5 |
| 80 | 8.9 |
| 70 | 8.4 |
| 60 | 6.5 |
| 50 | 5.6 |
| 40 | 3.4 |
| 30 | 2.3 |
In a sheet of paper, where thousands of fibers lie side by side, the combined expansion is distinctly noticeable in the changing dimensions of the sheet. This gives rise to difficulties in securing accurate register in color-printing, owing to atmospheric changes. The manufacturer may minimize this difficulty by a careful formation of the paper and the regulation of the drying, so as to turn out the paper as nearly as possible containing an average normal percentage of moisture.
The same conditions are responsible for wavy edges, which occur principally along the cross-grain dimension of the sheets. The ends of the fibers, being exposed, easily absorb moisture as paper lies in a pile, but the moisture seldom permeates more than a few inches into the pile. Therefore, the larger part of each sheet is unaffected, but the fibers exposed to the air expand when absorbing moisture increasing the area of the exposed end and, consequently, causing it to assume a wavy formation which is suggestive of a ruffle.
When feeding such sheets to a cylinder press, much trouble may arise if the waves occur along the “gripper edge,” which is usually on the longer dimension of the sheet. In some instances the difficulty may be avoided by ordering paper with the grain running the long way of the sheet, which also offers another advantage in relation to securing close register, namely this: the area of the sheet in square inches will increase least through atmospheric expansion which occurs across the grain if the cross-grain dimension is the lesser.
SUPERCALENDER STACKS, APPLETON COATED PAPER CO.
For a description of the Supercalendering process, see pages [55] and [56].
CHAPTER NINE
APPRAISING AND TESTING PAPER
The appraisal of a specimen of paper differs from testing in that an appraisal comprehends the value of an object in relation to its usefulness and marketability, whereas testing is merely an arbitrary method of expressing the chemical or physical properties of the object. The knack of appraising can be acquired only through practical experience; and the ability to make tests is gained only by careful technical training.
In the majority of cases a satisfactory appraisal may be given without chemical or physical tests, but these are cases when the superficial characteristics, such as color, finish, feel, etc., are the prime qualifications, and such considerations as fiber contents, freedom from impurities, exact tensile strength etc., are of negligible importance.
Although experience, only, leads to the knack of appraising paper, certain points might be suggested with benefit to the beginner which would assist him to an earlier acquirement of the art.
Color.—Color being a purely relative term as applied to the variations in so-called “white” papers, it is necessary to make comparisons with accepted standards of the various grades in order to arrive at conclusions.
In common parlance, white papers may be described as natural, light natural, white, blue-white, pink-white. Natural papers are those in which a minimum of artificial coloring has been added, and the brilliancy of shade depends entirely upon the quality of the stock.
Almost all paper is colored to some degree while the stock is in the beater, and the minimum quantity of order of paper, which any mill will make on a special run is usually limited by the contents of one beater, and, on account of the time required to wash up, the cost of special colors is increased. Rose-pink and blue are the colors used in modifying the natural color of any beater of pulp to produce a white paper. The so-called “white color” of the cheaper grades of papers is ordinarily gained by a comparatively heavy use of blue, and by comparison with a white paper of good quality the blueness is decidedly noticeable. In judging color, it is well not only to look at the surface, but also to examine the paper when held up against the light, making comparison with some acceptable standard, also noting the clearness of the stock, as indicated by the sharpness of definition of the shadows of the fingers which hold the sheet. This comparison is affected, of course, by the bulk of the paper, but two papers of about equal bulk may be fairly compared in this way. Any judgment as to shade is, in part, only a question of taste. Permanency of color may easily be determined by exposing a portion of a sheet to sunlight for a few hours and noting any alteration in color.
Formation.—While examining a paper for color and clearness, the formation of the sheet should also be observed. In general, a close, even formation is to be desired. Fibers of the same approximate length may be loosely or evenly formed, according to the skill of the machine-tender. The longer the fiber, the harder it is to get a close, even formation, and it should be remembered that these two qualifications are to a greater or less extent contradictory.
Finish.—Whatever the finish of paper, the two sides of an ideal sheet would look exactly the same. In most papers made on a Fourdrinier machine the impress of the wire is discernible, and there is a perceptible difference in texture between the “wire,” or bottom, side and the “felt,” or top, side, the one tending to reproduce the texture of the wire cloth, and the other the weave of the felts.
Some manufacturers have perfected their processes to a degree that renders these differences imperceptible. Papers made on cylinder machines of more than one vat are apt to be more even-sided, as the contact with the wire of the molds is less protracted and there is considerable pressing of the web between two felts as it is carried along.
The evenness of the finish, and the fineness of texture over all parts of a sheet, may best be judged by holding it aslant to the light. This also discloses whether the paper is “fuzzy” or free from lint.
Fuzz, or hairiness, usually occurs on the wire side of the sheet. This is due partially to the stock, soda pulp being especially likely to fuzz. It is also due to overdrying, and sometimes to the action of the suction boxes, which if worked too hard cause the surface fibers to stand on end.
“Hairiness,” or fuzz is more apt to occur on antique and other light finishes, but calendering will not entirely overcome it, and such papers as would be fuzzy uncalendered, become fuzzy with handling.
In fact, the durability of the surface may well be tested by rubbing the paper between the fingers. In this way, too, one judges the “feel,” which of all qualities of paper is perhaps the most difficult to express, but usually described as hard, soft, mellow, harsh, rough, smooth.
In highly calendered papers, well closed and evenly finished, the light will be reflected uniformly, as from a well-polished table-top; but if the formation is “wild,” there will be a blotchy look as the small knots of unevenly distributed fibers cause thick and thin areas, and the thick ones get harder squeezing through the calender rolls and, consequently, a higher finish.
Another cause for unevenness in finish is a variation in the thickness of the paper as it is made on the machine. This unevenness runs lengthwise in streaks, and may originate on the wet end of the machine if the pulp is not deposited uniformly.
Again, the pressing may be faulty at the press rolls, causing a thin streak. Naturally, the thin part of the paper dries more readily than the thick, and as even surfacing depends partly upon even dissemination of moisture in the sheet, a poorly pressed sheet would have a faulty finish. Dirty felts also cause uneven drying, as water can not be evenly squeezed through a felt the pores of which are partially choked. Lastly the unevenness may be caused by the calender rolls themselves being in poor condition.
It is easy to detect thin areas by examining paper in a pile, as a pile of papers of uniform thickness will be practically level on top.
Papers for half-tone printing, whether coated or uncoated, should be even in formation, thickness and surface, otherwise the printer’s “make-ready,” which is designed to offset inequalities in the plates, will be discounted by inequalities in the paper.
There are some special papers in which unevenness in formation and finish are intentional, on account of the unusual effects thus gained; and other papers, such as wrappings, where such niceties of the paper-makers’ art are of little importance.
Opacity.—Opacity may easily be judged, although it is difficult to express it in any accurate terms, by placing the papers to be compared side by side over a printed page, the relative merits in this respect may be immediately perceived.
Sizing.—Sizing may be approximately judged by moistening the stock and noting the rapidity of the absorption, or tested by drawing lines with ink and watching to see if they spread afterward. Absorbency in blotting-papers may be measured by submerging two strips equally and noting how high the ink is drawn up into the strips. Such papers as are made without any sizing and are ordinarily called “water-leaf.”
The sizing of coated papers should be neutral, but is frequently alkaline or acid, since alkali is used to neutralize the lactic acid of the casein. This may be detected by taste. The retention of a piece of coated paper in the mouth for a few minutes will reveal through the taste any tendency of the coating to sour.
Weight and Bulk.—Weight and bulk may be closely approximated by a practiced hand, but they must also be considered in relation to finish, as pointed out in the [preceding chapter].
There are many convenient forms of micrometer gauges for measuring the thickness of paper and any one who has much to do with paper should be provided with one, as it is unsafe to depend entirely upon judgment when a thousandth part of an inch may account for ten pounds difference in the weight of a ream of paper or cause serious variations in the bulk of a book.
Quality and Strength.—Quality and strength may be approximately judged by tearing the paper in both directions of the grain and observing the fractured fibers, but these matters are to be more accurately estimated by mechanical and chemical tests.
It will be observed that cleanliness in paper, and most of the foregoing characteristics of paper, do not lend themselves to mechanical tests, but are properties which require the judgment of an expert.
Cardboards.—In judging thick papers, such as bristol boards, it is customary to see if they are snappy. An idea of their fibrous strength may be had by folding in various directions. Pasted cardboards may be distinguished from unpasted by burning, for if paste has been used the layers of paper will split apart as the paper burns. This burning will also give a slight idea of the amount of filler in the stock, as the ash will be greater as the filler is increased.
Paper-testing.—Tests applicable to paper may be divided into three classes—microscopical, physical and chemical.
The purpose of microscopical tests is to determine the kind and character of the fibers, and the proportion of each kind, also to assist in determining the nature of mineral filler and of impurities. It is also used in estimating the percentages of the various kinds of fiber. Chemists are able to estimate this within five per cent. A minute sample of paper is prepared by boiling in a one per cent solution of sodium hydroxid, in order to remove everything from the fibers themselves. The resulting mite of pulp is placed on a slide with a dissecting needle, the excess moisture is removed and a stain is added. This stain gives different characteristic hues to the different kinds of fibers. The color and form of the fibers as observed through the microscope disclose their character to the trained eye.
By counting the different kinds of fibers under observation, the analyst estimates the proportions in which they existed in the sample of paper.
The physical tests are more familiar to most persons, and include (1) weight per ream, (2) thickness, (3) bursting strength, (4) tensile strength, (5) folding endurance, (6) absorption, (7) expansion.
1.—There are two kinds of paper-scales. The most common kind gives, directly, the ream weight from weighing a single sheet, and is of such convenience that almost all paper-users could well afford to have one.
Sensitive paper-scales for small samples, 4 by 4 inches in size, are of great assistance also, and should form part of the equipment of every paper-dealer.
2.—The thickness is determined by a micrometer gauge measuring to one-thousandth of an inch. In gauging thin papers it will prove more accurate to take four thicknesses, as the error in reading is thus quartered. The following table of bulks, which shows the number of pages per inch from a gauge of four sheets, will be found convenient:
| Thickness of four sheets in thousandths | Number of pages to one inch. |
| 8 | 1,000 |
| 8½ | 941 |
| 9 | 889 |
| 9½ | 842 |
| 10 | 800 |
| 10½ | 762 |
| 11 | 727 |
| 11½ | 696 |
| 12 | 667 |
| 12½ | 640 |
| 13 | 615 |
| 13½ | 593 |
| 14 | 571 |
| 14½ | 552 |
| 15 | 533 |
| 15½ | 516 |
| 16 | 500 |
| 16½ | 485 |
| 17 | 471 |
| 17½ | 457 |
| 18 | 444 |
| 18½ | 432 |
| 19 | 421 |
| 19½ | 410 |
| 20 | 400 |
| 20½ | 390 |
| 21 | 381 |
| 21½ | 372 |
| 22 | 364 |
| 22½ | 356 |
| 23 | 348 |
| 23½ | 340 |
| 24 | 333 |
| 24½ | 326 |
| 25 | 320 |
| 25½ | 314 |
| 26 | 308 |
| 26½ | 302 |
| 27 | 296 |
| 27½ | 291 |
| 28 | 286 |
| 28½ | 281 |
| 29 | 276 |
| 29½ | 271 |
| 30 | 267 |
| 30½ | 262 |
3.—Bursting strength is determined by a variety of testing-machines, constructed so as to record the pressure per square inch which may be exerted before rupturing the paper.
In a government bulletin, Report No. 89, United States Department of Agriculture, the following criticisms of this test are made: “This pressure is generally believed to represent the mean strength of the paper—that is, an average of the strength across and with the sheet. This is not true however, experience indicating that strength as thus determined more nearly agrees with the strength of the paper in the cross direction, with the minimum strength rather than with the average strength of the paper.
“Among other objections to testers of this type, is that to a certain extent the operator can influence the results at will, and even with the greatest care there is quite a wide difference between different tests of the same paper.”
4.—Tensile strength is determined by clamping a strip of paper of standard dimensions in a machine which exerts a uniform tension until the strip breaks. The breaking strength is shown on the recorder, and the amount of stretch before breaking is also registered, thus indicating the elasticity of the paper. The best known instrument of this sort is the “Schöpper,” but the machine is very costly, hence is rarely found except in well-equipped laboratories.
5.—Folding endurance is determined on a machine which folds a strip of paper back and forth in a slot, the strip being clamped at either end to a spring device which maintains a uniform tension. The number of double folds which the strip withstands is automatically registered. This test is favorably regarded as an indicator of durability, but the apparatus is expensive and not easily available, hence this test fails of frequent use.
6.—The absorption tests are applied principally to blotting-paper, and consist in suspending equal widths of paper so their ends are submerged in a beaker of colored water. The height the water rises in a given time demonstrates the capillary attraction.
7.—Expansion is estimated by taking strips of uniform dimension, dipping in water and measuring the expansion.
Chemical tests are for the determination of (1) the percentage of mineral filler; (2) the percentage and nature of sizing materials; (3) qualitative test for starch, acid, sulphur, chlorine, glue, filler material, dyes, ground wood.
The amount of filler may be determined by incinerating a piece of paper of known weight. As the filler is non-combustible, the weight of the ash determines the percentage of filler, although allowance must be made for the amount of water of crystallization driven off from the mineral.
Tests for acids are important in papers used for mounting tarnishable substances, such as jewelry.
Tests for sulphur or chlorine are important in determining the chemical purity of the paper, since such residues militate against the permanency of color and strength of paper.
The presence of ground wood is easily determined by a drop of either strong nitric acid, which turns the paper brown, or a drop of phloroglucine, which gives a reddish-brown tint from contact with ground wood. Aniline sulphate produces a yellow tinge.
The presence of starch may be ascertained by using a dilute solution of Iodine which leaves a black stain in contact with starch.
Note.—For more technical information see “Paper Technology” by R. W. Sindall.
CHAPTER TEN
PRESSROOM DIFFICULTIES
Technical difficulties with paper in the pressroom arise from many sources. They may be conveniently classified into three groups: Difficulties for which the manufacturer is responsible; difficulties for which the printer is responsible, and difficulties due to atmospheric and other natural conditions not entirely within human control. Let us consider some of the first group.
Uniformity.—Probably the most frequent source of trouble is lack of uniformity, either in weight, thickness or finish. This is chargeable to carelessness on the part of the paper-machine tender. A run of paper which varies in weight will naturally vary in thickness, and, obviously, this could account for uneven color in presswork. These variations would not necessarily be accompanied by a variation in finish. To make paper uniform in all three respects necessitates, firstly, uniform consistency of the pulp—or “stuff,” as it is technically called—at the point where it flows onto the machine. A uniform volume of stuff and uniform speed of the machine are also demanded. The speed of the machine and the volume of stuff are quite readily controlled, but as the amount of water used by the beater-man in preparing the stuff is usually judged by the appearance of the pulp in the beater, there are always such variations as are peculiar to this human factor.
The difficulties of the machine-tender may often be traced to the beater-man, not only on account of the amount of water in the mixture, but also because of the irregularity in the length of fiber from one beaterful to another.
Assuming that the stuff is right and the formation on the machine is good, the pressing of the paper next demands close attention. It is obvious that any unevenness of pressure will result in the water being expelled unevenly from the web of paper, with a consequent variation in thickness. In this case there would also be a damp streak in that part of the web where the pressing was too light.
The result is that such paper can not be dried evenly all the way across the machine because this damp streak will still have an excess of moisture after the adjacent areas of the web have become properly dried.
Finish.—In running through the calenders the damper portion will take a higher finish. It may even be so damp as to cause a blackening or crushing of the paper; whereas, if the moisture is sufficiently evaporated from this streak, the rest of the paper may be so dry that it will not finish smoothly enough.
On the other hand, there are cases where the pressing and drying may be perfectly uniform, but the whole web vacillates from being too dry to being too moist, while between times the manipulation is exactly right.
The result, obviously, will be a variation in finish over the whole width of the paper instead of over a portion. Moreover, too much drying makes the paper fuzzy and likely to become wavy, besides weakening the fibers.
Another result of uneven pressing is to make the paper thinner where the pressing is hardest. Such a defect is quite obvious in a pile of paper, as the top will not be as level as it would be in paper that is uniform in thickness throughout.
Assuming that the paper is perfect as it leaves the driers there is still a chance that one or more of the calender rolls may get out of true, especially when starting a run after they have been idle long enough to get cold. Under such conditions they often heat up and expand unevenly so that the pressure is harder in some sections than in others. The result is a thin streak in the paper. Whether the thinness be caused by poor pressing or calendering, it can easily be detected in a roll of paper, as the thin streak makes a soft spot in the roll which can quickly be located by tapping the roll all the way across. A muffled rather than a ringing sound discloses soft places.
This defect, if bad, may cause considerable trouble on a web press, as no amount of manipulation will make the paper draw evenly as it runs into the press if the edge of the roll is slack.
Occasionally, segregated areas in paper are found to vary in finish, and when these do not run in continuous streaks they may often be caused by the felts which carry the paper through the press rolls having become clogged up in spots so that the water can not pass out evenly from the paper through the felt. This must be guarded against by occasionally stopping the machine and washing the felts, or changing them, as the occasion dictates.
Such damp spots in the paper crush in the calendering and make blackened areas in the paper. Uneven drying may also have been occasioned by slackness of the drier felt which holds the paper against the driers. In sheeting the cheaper grades of book paper it is customary to cut off from a number of rolls simultaneously, which often accounts for a variation in finish or bulk in sheets from the same case.
Of course, when any of these symptoms appear it is the duty of the men on the machines to correct them, and in the continuous course of paper-making it is inevitable that felts become filled up and require washing or changing, or that the variations of consistency in the stuff should call for some form of regulation. Stuff which runs too moist on the wire will often “crush” under the couch roll, producing a curdled appearance. Stuff run with insufficient water will not form evenly. The skilful machine-tender avoids these extremes.
Trade Customs.—In recognition of the many variable elements in paper-making, trade customs have been established, such as allowances for a normal variation in the weight of paper above or below the nominal ream-weight, and reasonable allowance should be made for normal variations in other characteristics.
Eternal vigilance and alert judgment are certainly required for setting high standards in the manufacture of paper. It is a matter of common observation that mills using practically the same raw materials vary widely in their reputation for uniformity and excellence of product. The reason for this is to be found in the human element.
Calender Defects.—A number of difficulties may arise from much less excusable causes than those mentioned. For example, the paper sometimes may run slack through the calenders, with the result that it wrinkles and cuts in diagonal jags called “calender cuts.”
Sheets containing such defects sometimes elude the finishers, and on the printing-press such a sheet may crack and go around one of the ink rollers. On a web press the trouble from such a defect would be even worse, causing breaks and necessitating delays on the press. It is more difficult to exclude calender cuts from roll paper, as it is not always easy to see them in the fast-running paper, so that an occasional cut is not an unforgivable sin.
Among other defects arising on the calenders are little scarlike depressions in the paper, made by small scraps of paper which have become lodged on a calender roll and are embossed into the web at each revolution of the roll.
Holes, Dirt, Etc.—In very light papers, holes are sometimes found, the most likely cause of which may be picking under the dandy roll or grease spots on the wire cloth. Of less frequent difficulty are the so-called pinholes, caused by sand or grit, while slime spots, or spots caused by slight bundles of fibers, are also occasionally noticed.
Dirt and specks originate from careless handling of rags or paper stock, and are also derived from shives of undigested wood in the wood-pulp.
Streaks in the paper may originate from a crease in the wire, and mottled effects denote some fault in the handling of the paper in the wet stages of making.
Again there are times when sheets are not cut quite square, which is, of course, inexcusably careless. Likewise, the packing of paper may be done in a careless manner, and cases too loosely packed, if set on end, often cause a wave in the paper, which sags in the case instead of remaining tight and flat. It is desirable that cases of paper be kept flat in storage and not set on end. Cases should be made from well-dried boards, and waterproof lining-paper should be used to exclude all dampness. When paper is finished in rolls it is fair to demand that the rolls should be wound evenly and hard, and all breaks should be carefully spliced and flagged.
Color.—The foregoing troubles are mechanical. Other difficulties may exist, even when the paper is handled well on the machine, owing to errors in composition. The color may vary, and the term “color” includes the various shades called white, as well as tints. Color is affected by water conditions. In the case of mills which depends upon river water, the water sometimes becomes so dirty that it severely handicaps the paper-maker, in spite of his filtering apparatus, and at such times it becomes difficult to get as bright and lively shades as under favorable conditions.
Dyestuffs do not always work uniformly, and, therefore, absolute matches of color from run to run are not to be expected. Shortcomings of this nature should be regarded with some lenience.
In this class of difficulties, discrepancies in sizing are the less pardonable and are more apt to be noticed when hard sizing is requisite, as in writing-paper or index bristol. In such cases a lack of sufficient sizing is an incurable fault, for which the manufacturer is responsible. There are occasions when excess of sizing would be troublesome—for example, in a smooth-finished book-paper it would be likely to cause offsetting, but this trouble may be alleviated by using less ink, or, if necessary, by slip-sheeting.
It would be difficult to catalogue all possible sources of trouble, but we have at least covered the principal defects of uncoated papers.
Packing.—Occasionally troubles may be charged to faulty packing—cases too loosely packed when stood on end permit the paper to sag, thus causing a curling tendency at one end of the sheets.
The use of unseasoned case lumber or cases and inferior case lining give access to moisture, the effect of which is discussed herein at length.
The susceptibility of coating to picking may be determined by applying hot sealing wax. If the wax after cooling is pulled off with only the coating adhering it may be assumed that a “tacky” ink would work the same, whereas if the paper tears out with the wax—it proves conclusively that the coating is well sized.
Coated-paper Troubles.—Coated papers have their characteristic shortcomings. The picking of small particles of the coating is perhaps the most common fault, and is caused by insufficient adhesive elements in the coating mixture. Other troubles are traceable to some of the defects of the body stock. Irregularity of the finish is sure to come from faulty application of the coating or careless calendering. Grit or bubbles in the coating is likely to result in a porous surface. The sour odor of some coated papers is due to decomposing casein or glue.
Casein used as an adhesive in most coated papers is a product from skim milk. It contains lactic acid which must be neutralized in preparing the coating mixture. For this purpose an alkali such as soda or ammonia is used, and when properly handled the coating should be neutral. An alkaline coating will cause re-etching on lithographic plates or stones.
Starch coatings or combinations of starch and casein are cheaper than full casein and do not yield as high a finish and when improperly used have often been the cause of picking.
The Printer’s Responsibility.—The second group of difficulties, or those for which the printer is to blame, may originate with the improper storage of the paper. As pointed out, the standing of cases on end is conducive to wavy paper. Dampness is a prime cause of trouble, as will be sufficiently shown later on, but it is elementary to say that paper should never be exposed to moisture.
Engravings.—The troubles of ignorant or inefficient pressmen and foremen are often laid to the paper, especially where half-tone printing is involved. In the first place, too little attention is given to securing proper originals for the half-tones. Retouching is omitted in a fit of false economy, for at this very stage of the game it was never truer that “An ounce of prevention is worth a pound of cure.” Too much care can hardly be given to securing good engravings.
Secondly, the selection of a proper half-tone screen is frequently overlooked. While no hard-and-fast rules may be set, the best one, when in doubt, is to include with the engraver’s order a sample of the paper on which the cuts will be printed. He can judge, taking into consideration the subject and the stock, which screen is advisable. In general, it may be affirmed that 120 or 133 line screens are best for uncoated smoothly finished papers, and 150 or 175 line screens are most satisfactory for coated stock.
Inks.—Next comes the suitability of the ink, and there again the ink manufacturer’s advice, always available, is often neglected; but experience proves that certain papers yield best results with certain inks. Such matters can only be determined by actual experience, but when in doubt consult the ink-man.
Make-ready.—Finally, the make-ready should be intelligently varied according to the subject and the paper. The best printers agree that different papers to some extent require individual treatment. A make-ready suitable for a coated paper is not necessarily equally satisfactory for an uncoated half-tone paper or even a dull-finished coated stock. But it is not within my province to go further than to emphasize these warnings.
Grain.—The question of the grain in paper is certainly, in many cases, within the control of the printer when ordering his paper, but its importance is very frequently overlooked. In machine-made papers there is a distinct grain that is caused because a majority of the fibers point in the direction that the stuff flows on the machine, just like logs floating in a river.
This grain direction is noticeable in folding, the crease being smoothest with the grain, because folding across the grain encounters the most resistance and breaks many of the fibers. This is especially noticeable in fairly heavy book-papers, in bristol boards and cover-stock, all of which should be scored for folding.
Cut cards, to have the maximum stiffness, should be so trimmed out of the sheet as to have the grain run in the long direction of the card.
Even in book-papers, where flexibility is desirable, it is necessary to have the grain run up and down the page. There are occasional cases when the grain is deliberately arranged to run across the page to acquire more rigidity. A wide page of light-weight paper might otherwise be too limp. Moreover, this arrangement makes for stronger bindings, as the stitches or wires pass around the bundles of fibers instead of cutting between them. The English books are mostly made up in this way, but they do not open so easily as when the grain runs parallel to the binding. Paper is materially weaker across the grain and can withstand only about half the tensile strain that it could bear with the grain, although crosswise it is more elastic.
There is one very serious objection to making books or catalogues “cross-grained.” This is on account of the way fibers are affected by moisture. The cellulose fiber expands in diameter on absorbing moisture, for which it has a great affinity. Indeed, a cellulose fiber is only stable under uniform atmospheric conditions. The expansion of each fiber in diameter makes paper expand much more across than with the grain. Obviously, the total expansion of a sheet equals the amount each fiber expands times the number of fibers that side by side go to make up the sheet.
When the glue is applied to a book in process of binding, it causes an expansion of all the moistened fibers.
If the grain runs parallel to the shelf-back no harm results, as the paper is free to expand toward the side margin, but if the grain is at right angles it usually makes a cockle in the binding because the moistened edges of the leaves expand while the dry portion beyond where the moisture penetrates retains its shape and resists the elongation of the wet edges. Consequently the expansion of the fibers expresses itself by cockling.
Register.—In all printing, when close register is necessary, the danger of poor register from the expansion of paper is minimized when the dimension across the grain is the shorter. Lithographers invariably prefer to have the grain run the long way of the sheet on this account. Moreover, they rack the paper before printing in order to get it thoroughly seasoned. To protect it from atmospheric changes that may occur during the printing process, they use slip-sheets of considerably larger dimensions, so that there is a generous margin of slip-sheet around each printed sheet, which helps to exclude the air from the edges of the printed paper.
Moisture in Paper.—It is true that some papers are more prone to expand than others, especially if they have been run too dry on the machine. Paper is not naturally bone-dry. Under average weather conditions, it contains six or seven per cent of moisture.[D] When in the making it is turned off far below its normal moistness, it seeks to obtain this moisture from the air at the first opportunity, and in acquiring it expansion takes place. Unless the expansion pervades the entire sheet, wavy edges will result. Similarly when the air becomes dry exposed edges of paper give off some moisture and shrink accordingly leaving a boggy center to the sheets.
[D] See Herzberg table, page [64].
Seasoning.—This process of acquiring normal moisture is usually called “seasoning.” As paper is probably never turned off at its full normal moisture, it is most desirable that it should be allowed time to season. It is not unusual to have people speak of new paper being too “green.” This may not have been an uncommon condition of hand-made papers which were dried entirely naturally, but, so far as machine-dried paper is concerned, I doubt if it is ever too green, though it is frequently made too dry.
Conditions Beyond Absolute Control.—Believing it to be impracticable to leave the precise normal moisture in machine-made paper, I have deliberately refrained from classifying this difficulty with faults chargeable to manufacture, and the general recognition of this circumstance indicates the wisdom of ordering paper long enough in advance to permit of a period of seasoning. In fact, this phenomenon of expansion or contraction of cellulose fibers places difficulties originating from this source in the class of conditions beyond absolute human control, but a study and understanding of the subject will enable one to prevent, or at least to minimize, such troubles. It consequently becomes the business of the printer to inform himself as thoroughly as possible on the subject. Static electricity is an element beyond absolute control and the source of much trouble. Both phenomena could be controlled by proper atmospheric conditions in storage and press rooms, but it is an expensive matter to equip rooms and install the necessary apparatus. The amount of trouble arising from these elements is often sufficiently costly in time and material to warrant investigation as to the expense involved.
The least a printer can do is to maintain hygrometers in his pressroom so as to keep track of atmospheric variations, and be guided accordingly.