  There are two points of view from which a paper may be tested: first, of physical or mechanical properties; secondly, of material composition. We shall consider the subject according to this division. (I.) Quantitative measurements of such properties as resistances to breaking and tearing strains are seldom made by English paper-makers. In Germany, on the other hand, the matter has been very thoroughly investigated in connection with the work of the KÖnigl. Techn. Versuchsanstalt, Berlin, and through the agency and influence of Prof. Sell, and C. Hofmann, a department has been organised exclusively for the work of paper testing. The results of the tests are becoming widely recognised by practical men and the trade in that country, as affording a true index of the quality of a paper. It is therefore of importance to give an outline of the methods employed. The determination of the strain or weight which a paper is capable of supporting is a very obvious measure of the strength of the paper. Observations of the limiting strain or breaking weight are sometimes made by paper-makers, but the apparatus and method employed are usually crude. The simplest means consist in clamping the paper—a strip of standard length and breadth, arbitrarily chosen—at one end, the clamp being firmly held in a fixed support, and to the other attaching by means of a similar clamp, an ordinary scale pan, the whole arrangement hanging vertically. Into the pan, weights are added in due succession until fracture of the strip is determined. It is scarcely necessary to point {194} out that the errors of experiment with such a method are very great: indeed it has been found that even with the refined apparatus about to be described the errors are not inconsiderable. However, by exhaustive investigation, according to the well-known “law of errors,” these have been quantified, and a careful operator can therefore obtain results which are trustworthy. The apparatus in question is the Hartig-Reusch machine. The principle will be readily grasped by inspection of the diagrammatic representation of its essential parts—Fig. 77. The strip of paper is held horizontally by the clamps a and b, a being held by the fixed support A, b by the movable carriage B. B is connected by means of a swivel with the spiral spring F, and this again is similarly connected with the screw, which is made to rotate by the wheel D. By turning D, therefore, the spiral may be extended, and a corresponding strain communicated through B and b to the paper. The paper undergoes a certain elongation under the strain, and the carriage B moves from right to left in consequence. The rotation of the screw is continued, and the extension of the spiral proceeds until the paper is fractured. At this point it is required to determine, (1) the elongation of the spiral which is the measure of the breaking strain, and (2) the distance through which the carriage has moved, i.e. the elongation of the paper. Both these effects are communicated to the pencil G, the latter directly, since the pencil-holder is in rigid connection with B, the former through the rod I, from which, by a special arrangement, the horizontal is converted into a vertical motion of the pencil. This, therefore, traces a curve, of which the ordinates represent the strains, and the abscissÆ the elongations of the paper produced by the strain. The scale shown in Fig. 76 indicates the exact position of the clamp A. {195}
{196} The values for the spiral spring—i.e. extension for a given load—having been determined by previous observations in a special apparatus, the curve obtained is at once a measure and a permanent record of these cardinal factors, breaking strain and elasticity. As with all other such instruments, the recording apparatus introduces certain errors, which, however, by careful investigation and modification in accordance with the results, have been reduced to a minimum. Nevertheless, the director, Dr. Martens, has recently adopted a simpler instrument, altogether similar in principle, but based upon a direct reading of the two movements, in which of course these errors do not appear. For the student, however, the recording instrument is the more instructive, and we have given it preference for description here, more especially as no difference in essential parts is involved. Those who wish to pursue the matter into the most interesting details of the investigations made upon the subject, are referred to the papers published by the Institute for 1885. In testing the strength of papers by this or similar machines, it is important to observe the hygrometric state of the atmosphere at the time the trials are made, as this has been found to exert a considerable influence on the results, a paper being weaker the moister the atmosphere. The results of the tests are expressed in the following terms:—The elongation is given directly in percentage of the original length. This is uniformly taken at 180 mm., a length arrived at after laborious investigation, as minimising the errors of experiment; in other words, as giving mean value with the minimum of variation. For the breaking strain an ingenious expression has been arrived at, viz. the length of the paper which suspended vertically, with one end hanging freely, the other fixed, would determine fracture at the fixed end. As the breaking strain would vary with the thickness, the numbers obtained in {197} units of force or weight for strips of constant breadth, would need correction in order to admit of strict comparison with one another. By substituting an expression in terms of the paper itself—since a paper of greater thickness, and requiring therefore a proportionately greater force to fracture it, weighs more per unit of area, and in the same proportion—all the numbers for breaking strains are strictly comparative one with the other. In the same way also the question of width may be disregarded. A further mechanical test, forming a part of the scheme of investigation, is the resistance of the paper to rubbing. This test is an altogether empirical one, as the following brief description will show:—A piece of the paper, about 6 inches square, crumpled by successive folding in two directions at right angles, is grasped by the thumb and forefinger of each hand, at a distance of 3–4 inches apart. It is then rubbed upon itself across the thumbs a given number of times (seven is the number chosen) and held up to the light. If no holes are visible, the rubbing is repeated. The number of times necessary to repeat the rubbing until holes appear is the measure of the resistance. A sufficient uniformity in the results of this test has been observed to make it the basis of a classification of papers, in regard to their resistance to such disintegration; they are divided into the following eight groups, beginning with the lowest:—
The classification of papers on the results of these tests cannot be more lucidly given than in the following scheme, under which the results are officially recorded:—
{198} This classification is based on the results of some hundreds of observations. It is interesting to note the differences observed in the numbers for a and b according to the direction for the test in which the paper is cut, i.e. in the direction in which it was run on the paper machine, or at right angles (see Chap. XI. p. 171). The mean ratio for the breaking lengths (strains) may be taken as 1:1·6, i.e. the paper is about 40 per cent. It is also of interest to note the influence of the glazing process (p. 167) upon the quality of the paper as determined by these tests. First, we must notice the effect of the treatment upon the substance of the paper itself. The mean reduction of thickness is 23 per cent. On the other hand, the reduction of weight, calculated per unit of surface (square metre), is 6·7 per cent., whence we may infer an increase of surface, flattening out, in the process. These quantities, but more particularly the latter, will doubtless vary with the various methods of glazing and with the materials of which the paper is composed. The breaking length (strain) shows a mean increase of about 8 per cent.; the elongation under strain, on the other hand, a diminution of 6 per cent. For an interesting discussion of the question of the relative strengths of machine and hand-made paper see ‘Paper,’ by Richard Parkinson. FIG. 78. The thickness of a paper may be determined by means of an ordinary micrometer, such as is shown in Fig. 78. The paper is placed in the jaws of the instrument, and the screw {199} advanced until it touches the paper. The thickness is then read off on the scale. Other forms of apparatus are sold for the same purpose. In making a determination of the thickness of a paper it is necessary to take the mean of a series of observations at different points of the sheet, as the thickness may vary somewhat. (II.) The analysis of a paper naturally divides itself into two parts:—(a) The determination of the nature of the fibrous material of which it is composed; and (b) the identification of such adventitious substances as size and filling material. ( a) This again is divided into two sets of observations—microscopical and chemical.A fragment of the paper is soaked for some time in glycerine, and is then carefully teased out with a pair of needles, and the fragments laid on a glass slip with a drop of glycerine. A cover-glass is then laid on and lightly pressed down so as to spread the fibres in a thin layer. The microscopical features of the different fibres have been already described, and it is only necessary now to summarise the chief characteristics of the more important materials. Cotton.—Flat riband-like fibres, frequently twisted upon themselves. The ends generally appear laminated. The fibres are frequently covered with numerous fine markings (see Frontispiece).Linen.—Cylindrical fibres, similar to the typical bast fibre (see Fig. 6). The ends are frequently drawn out into numerous fibrillÆ (see Frontispiece).Esparto.—Esparto pulp consists of a complex of bast fibres and epidermal cells. These serrated cells are, as has been already pointed out, characteristic of esparto, straw, and similar fibres. Certain differences exist between those of esparto and straw, and even between the different species of straw, which enable the microscopist to identify their source. The most characteristic feature of esparto pulp is the presence {200} of a number of the fine hairs which line the inner surface of the leaf (e, Fig. 10), some of which invariably survive the boiling and washing processes, although the greater portion passes away through the wire-cloth of the washing engines. The presence of these hairs may be taken as conclusive evidence of the presence of esparto.Straw.—Straw pulp very closely resembles esparto-pulp in its microscopical features. The hairs above alluded to are, however, absent. On the other hand, a number of flat oval cells are always present in paper made from straw (b, Fig. 13). It should be borne in mind, however, that bamboo and similar pulps also contain these cells.Wood (Chemical).—Flat riband-like fibres, showing unbroken ends (see photographs, Frontispiece). The presence of the pitted vessels (Frontispiece, and a, Fig. 15) is eminently characteristic of pulp prepared from pine-wood. The fibres of other woods are not sufficiently characteristic. They much resemble those of pine-wood, with this difference, that the pitted vessels are absent.Wood (Mechanical).—Mechanical wood-pulp may be recognised by the peculiar configuration of the torn ends of the fibres, and from the fact that the fibres are rarely separated, but are generally more or less agglomerated (see Frontispiece). Pulp from pine of course shows the pitted vessels already referred to. They are usually more distinct than in chemical wood-pulp. Occasionally fragments are to be met with connected together with portions of the medullary rays.The microscopical examination of a paper is a matter of very great difficulty, and one requiring much practice. The student is recommended to study closely for himself the microscopical features of pulps obtained from authentic specimens. Some approximate idea of the relative proportion of the various fibres present in a paper can be obtained from a careful microscopical examination. VÉtillart maintains that a quantitative determination within a fair limit of accuracy is possible. On the other hand, the {201} Berlin PrÜfungsanstalt do not profess to do more than merely identify the fibres. In examining a paper under the microscope, it should be observed whether the fibres appear as fragments, or whether they consist of whole bast cells in which the tapered ends appear. Cotton and linen, owing to the great length of their ultimate fibres, yield, when beaten, fragments showing where the fracture has taken place. From the appearance of this fracture it is possible to ascertain whether or not the beating operation has been properly conducted. If the beater-knives have been too sharp, or have been let down to the bed-plate too quickly, the fractures will appear as clean cuts, whereas when the operation has been properly conducted the fracture will appear ragged and drawn out. The bearing of this on the strength of the finished paper is considerable. Esparto, straw, and wood, whose ultimate fibres do not exceed 1–2 mm., should, in the majority of cases, appear as whole bast fibres with two tapered ends; the beating, when properly conducted, being confined merely to the separation into these ultimate fibres. ( b) For the chemical identification of the fibres in a paper, the only available reactions are those with aniline sulphate solution. The majority of fibres (celluloses) give no reaction, straw and esparto celluloses and mechanical wood-pulp being the only ones that can be identified by its means. The authors have found that when a paper containing straw or esparto is treated for some time with a boiling 1 per cent. solution of aniline sulphate, a pink colour is produced. Esparto gives the reaction with greater intensity than straw. In this way the presence of a very small quantity of these pulps can be detected with certainty.Mechanical wood-pulp, when treated with a solution of aniline sulphate, develops, even in the cold, a deep yellow colour. If a paper containing mechanical wood-pulp so treated be examined under the microscope, the fragments of wood will be found to be deeply stained, whereas the other {202} fibres remain colourless or nearly so. It must be borne in mind that cellulose obtained from lignified fibres, if the boiling and bleaching processes have not been carried sufficiently far, will give with aniline sulphate a more or less intense yellow coloration. Various other reagents have been suggested for the identification of mechanical wood-pulp, all based upon the production of a colour with lignose. The reaction of lignose with chlorine and sodium sulphite solution already referred to (p. 19) may also be made available for the detection of mechanical wood-pulp in a paper. Imperfectly boiled or bleached pulps sometimes give this reaction faintly. Quantitative Estimation of Mechanical Wood-pulp.—The determination of the amount of mechanical wood-pulp present in a paper is sometimes a matter of some importance, and it is also a matter of great difficulty. Some idea of the amount present can be obtained by observing the depth of the yellow colour produced with aniline sulphate or the intensity of the magenta reaction with chlorine and sodium sulphite. It is also possible to calculate approximately the percentage from the percentage of cellulose contained in the specimen. Mechanical wood-pulp (pine) may be taken to contain 60 per cent. of cellulose. If, therefore, a paper ascertained to contain a pure cellulose in addition to this constituent, yield 75 per cent. of cellulose on the ordinary test, it may be assumed that about 62·5 per cent. of mechanical wood-pulp is present.The authors have proposed a method of estimating the amount of mechanical wood-pulp present in a paper, based upon the absorption of iodine in definite proportions by wood in a finely divided state, under strictly regulated conditions. The paper is carefully reduced to a fine pulp, and is then left in contact with a standard solution of iodine in potassium iodide. At the end of twenty-four hours the amount of free iodine is determined by titration with sodium thiosulphate and by deducting this from the amount originally taken, the amount absorbed is ascertained. As this amount, under {203} strictly comparative conditions, always corresponds to a definite amount of mechanical wood-pulp the amount present can be readily calculated. 2. Loading, Sizing Materials, &c.—The determination of the amount of loading material in a paper has been already described (p. 134). The identification of the material can only be arrived at by a careful chemical analysis. The principal loading materials are china-clay and pearl-hardening (calcium sulphate). The ash from a paper containing china-clay is insoluble in boiling dilute hydrochloric acid; that from paper containing calcium sulphate is soluble: the solution on cooling deposits long needle-shaped crystals of CaSO4 2 H2O and gives with barium chloride a copious precipitate of BaSO4 (barium sulphate) insoluble in acids, and with ammonia and oxalate of ammonia a precipitate of calcium oxalate.The presence of starch in a paper can be readily ascertained by its behaviour with a solution of iodine. If starch be present the well-known blue colour of the compound of iodine and starch will be produced. The determination of the amount of starch present is a matter of some difficulty, the details of which are somewhat beyond the scope of the present work. It is based upon the conversion of the starch into sugar, and the estimation of the latter with Fehling’s solution. The nature of the material with which a paper is sized may be ascertained in the following way:— The sample, cut up into small fragments, is warmed for a few minutes with alcohol containing a few drops of acetic acid. The alcohol is allowed to cool, and is then poured into four or five times its bulk of distilled water. If any precipitate or cloudiness is produced, it indicates that the paper has been sized with rosin. The alcohol dissolves the rosin, which, being insoluble in water, is thrown down on dilution. The alcohol used should be previously purified by distillation, as some samples contain a small quantity of shellac, which would itself be precipitated with water. {204} The paper after treatment with alcohol should now be boiled for some minutes with water: the solution allowed to cool, and then filtered. To the filtrate a few drops of a solution of tannic acid are added, when, if the paper has been sized with gelatine, a white curdy precipitate will be formed. The estimation of the amount of sizing material in a paper is a very complicated process and one which demands considerable chemical experience for its proper conduct. The amount of gelatine present is best ascertained by determining the amount of nitrogen present by combustion with soda-lime, and from this, calculating the amount of gelatine. Pure gelatine contains 18·16 per cent. of nitrogen (Muntz). The comparison of one paper with another with a view to ascertain the relative degree of sizing, is usually performed in a more or less rough and ready way by moistening the samples with the tongue for a certain time, and noticing the degree of transparency produced, which is of course inversely to the degree of hardness. A more accurate method consists in placing a drop of a mixture of alcohol and water containing some colouring matter in solution and determining the time necessary for the colour to make its appearance on the other side. In this way more trustworthy comparisons can be made. Colouring Matters.—The chemical reactions of the chief colouring matters have been already described (p. 141). |
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