Appraising And Testing Paper
( Originally Published 1920 )
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 impurties, 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 it 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 aver 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 formaton 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 them-selves 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 in-equalities 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 pre-ceding 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 them-selves 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 percent-ages 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:
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 deter-mined 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 "Schopper," 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 con-tact with starch.