Straw.
Wood (Chemical).
Wood (Mechanical).
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
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 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 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.
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CHAPTER XIV. GENERAL CHEMICAL ANALYSIS FOR PAPER-MAKERS.
A knowledge of the methods usually employed for the qualitative and quantitative analysis of the various chemical substances met with in paper manufacture is of considerable importance. The scope of this work will not allow us to enter fully into the necessary details of manipulation; for these the reader is referred to standard works on analysis; we will merely indicate the methods.
Caustic Soda, Soda Ash, Recovered Soda, &c.
In testing recovered soda, it is necessary to boil the finely powdered ash for some time with water; unless this be done, the amount of alkali will probably be under-estimated, as part of the soda is present as a difficultly soluble silicate.
It is sometimes desirable to determine the amount both of sodium hydrate and carbonate in a sample of alkali. For this purpose an excess of a solution of barium chloride is added to a solution containing a known quantity of the substance under examination, and the mixture made up to a definite volume. It is then filtered, and an aliquot portion of the filtrate titrated with standard alkali. The filtrate contains only the alkali that was originally present as hydrate, the carbonate of soda having formed with the barium chloride, insoluble barium carbonate, and sodium chloride.