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.
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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.