CHAPTER XII FIBROUS MATERIALS USED IN PAPER-MAKING General classification-Linen, cotton, esparto, straw, hemp, jute, wood pulp, other fibres -Microscopic features-Dimensions-Colour reactions. Vegetable Fibres. THE number of vegetable fibrous substances suitable for the manufacture of paper is comparatively small. It is true that in the laboratory paper can be made from plants of every description, but the practical application of such fibres for commercial purposes is determined by the cost of manufacture, the supply of raw material, and the quality of the final product. These latter considerations limit the choice of material in a marked degree. Fig. 136a.-The Fibres of a Magazine Paper, chiefly esparto. In the library of the British Museum may be seen a Dutch book, printed as far back as 1772, containing over sixty specimens of paper prepared from different materials; but it is merely a curiosity, a facsimile of which could readily be prepared by any one possessing a few simple chemical appliances. Hence, while such widely differing vegetable substances as straw, cotton, linen, hemp, hay, nettle, sugar-cane, wood, cabbage-stalks, cocoanut-matting, grass, peat and many others may be converted into paper of some description, yet the list of fibres actually in common use as raw material for paper-making is a short one. A broad general classification of paper and paper products, and the raw materials employed in manufacture may be made as follows: Writings and printings Rags, esparto, and wood pulp. However varied in form and appearance the ordinary plants suitable for the manufacture of paper may be, yet when subjected to chemical treatment they appear to lose their distinctive form and physical structure, yielding a white fibrous substance which is called cellulose, and it is not easy by mere superficial examination to determine the source of this white cellulose when thus isolated from the plant. But though similar in appearance and chemical composition, yet the various celluloses may be distinguised by microscopic examination of the ultimate fibre. The microscope at once reveals very important and characteristic differences in structure, and largely explains the great differences to be found in the paper produced. The effect of the processes used for isolating the pure paper-making fibre is confined therefore to the removal of non-fibrous constituents, a term which may be conveniently used as comprising all the elements in the plant. which have no value as fibre. A fibrous plant may be regarded as a structure consisting of fibres associated with cells which are built up together to form a tissue more or less complex, according to the nature of the plant and the family to which it belongs. The fibres are associated with the cells somewhat after the fashion of a building in which bricks and mortar are found; but the analogy must not be pushed too far, since the association in the plant is by no means so simple as the mere mechanical contact between bricks and mortar. The commercial application of the plant itself for the purpose of the papermaker requires a breaking down of the plant structure into fibres and cells. The valuable portion of the plant is the fibre, but in some cases the cells are also useful. The classification of the paper-making fibres, or celluloses, from the various available plants can be made when the plant structure is looked at from this point of view. In cotton no separation of fibres from cellular matter is necessary, since the growth of the plant results in the formation of a mass of fibres free from any cells or cellular matter. In flax, hemp and jute the plant is a typical fibrous mass, in which the individual fibres are more closely associated with one another than with the surrounding cellular matter. In the case of straw, esparto and wood the fibres are more regularly distributed through the substance, and are more intimately associated with the cellular mass than in the case of flax and hemp. In the case of plant seeds, such as those of the cereals and many vegetables, the cellulose compounds are not fibrous in character, but consist almost entirely of cellular matter, and these substances are therefore of little interest to the paper-maker. The plants which are of interest to the paper-maker can therefore be arranged under some scheme of classification which commences with the typical cotton fibre characterised by the presence of a very high percentage of fibre, with a complete absence of cellular matter, and which ends up with plant structures containing a large proportion of cellular matter with the absence of useful fibrous constituents. Considered from the point of view of resistance to chemical treatment the typical paper-making fibres may be roughly classified under two headings. (1) Those of greatest resistance to the action of chemicals: cotton, flax, hemp, ramie, sunn hemp, &c., in which the average yield is over 75 per cent. of cellulose. (2) Those of less resistance to chemical treatment: esparto, straw, jute, wood, bamboo, &c., in which the average yield is from 40 to 60 per cent. of cellulose. These striking differences in the nature of the several plants available for the manufacture of paper are sufficient to explain why rag papers come nearest to the ideal paper. Some rough-and-ready idea of the permanence and durability of papers may be obtained by submitting samples of papers for eighteen to twenty hours in anoven at a temperature of about 110° C. The colour of a high-class rag-paper will not appreciably alter. A paper made from esparto or straw will turn a light 1 3 3 Fig. 137.-Linen Fibres, according to Herzberg. yellowish brown. An ordinary printing paper containing mechanical wood pulp will take on a decided yellowish brown colour, the changes being due to the condition of the cellulose in the paper. Rags. All rags have their peculiarities of quality and suitability, their value being in proportion to cleanliness, whiteness and strength. The different grades are usually supplied to the paper-mill roughly sorted out according to their condition and colour, as the term "rags" is used in the trade to include all kinds of Fig. 138. Cotton Fibres, according to Herzberg. 1, General appearance of a raw fibre 2, Fibre as occasionally seen after textile fabrics which have already been utilised for other purposes. The list of materials which should be mentioned under this heading is a fairly long one: New linen cuttings, new cotton pieces, fine white linens or cottons, inferior white rags, coloured rags, blue rags, gunny, rope, sailcloth, canvas, hemp and flax waste, manila, bagging. Cotton and linen rags, sailcloth and canvas are used in the manufacture of high-class hand-made and machine-made writing papers. Rope, gunny, bagging and textile wastes are utilised chiefly for wrappers and boards. Linen. For the manufacture of high-class papers the bast cells of flax are regarded as being the most suitable material. The linen fibre in its raw state is readily distinguished from cotton, under the microscope, by a narrow central canal of small diameter, and by the repeated thickening of the cell wall which forms knots in the fibre at regular and frequent intervals. The fibre is also more cylindrical in shape and of smaller diameter, the ends tapering gradually to a fine point. Many of these distinguishing characteristics are greatly modified in the process of manufacture, so that it often becomes a matter of some difficulty to determine the proportion of cotton and linen in a given paper, while the identification of linen and hemp in a mixture of the two with a view of finding out the relative amounts present is in many cases impossible owing to the strong resemblance of the ultimate beaten fibres. The application of suitable reagents to the raw fibre when examined under the microscope serves to bring out the details of structure very clearly, but with paper containing the well-boiled and beaten material these details are by no means so distinct. The curious brownish colour found with the knots in the raw fibre is almost entirely absent in the beaten stuff and can only be identified with a higher magnification than is usually employed. The fibre ends are generally blunt and frayed out, the pieces cut off being torn into more or less structureless particles. Some fibres are flattened out and resemble cotton. The identification of linen involves a careful search for the central canal, the traces of knots, and the presence of numerous pores in the cell walls appearing as dark lines across the fibre. Cotton. Under the microscope raw cotton presents a characteristic form which is easily recognised. The fibre is a transparent tube, usually flattened and collapsed, always more or less twisted and tapering off to a blunt point. The twisted or corkscrew form of the fibre serves to distinguish it from other vegetable fibres and is an easy means of identifying cotton. The fibre is quite devoid of pores and knots. The treatment necessary for the conversion of cotton rags into paper alters the general nature and appearance of the fibre considerably. The curious twist in the raw cotton is largely removed by the action of the caustic soda used for boiling the rags, while the mechanical reduction of the fibres by the process of "beating" also affects the physical structure. The natural ends of the fibre are seldom found in paper; the fibres are often flattened to such an extent as to be mistaken for wood cellulose, though in the latter case the coloration produced by the use of zinc chloride and iodine solution serves to identify the wood-cells with certainty; the fibres are sometimes split lengthwise by prolonged beating which gives them a partially striated appearance, somewhat resembling wellbeaten hemp; the fibres frequently exhibit, after the process of beating, a complete absence of structure, owing to the removal of those characteristic features which distinguish the raw fibre. Amongst papers containing cotton fibre, ordinary blottings preserve the original structure best, except that the fibres are cut very short, while high-class T.S. writings exhibit the greatest deviations from the original structure, on account of the severe treatment to which the fibre has been subjected. Esparto. This fibre was introduced into England in 1852 by Thomas Routledge. It is a grass, Stipa tenacissima, growing freely in Spain, Algeria, and other parts of Northern Africa, which has proved to be eminently suitable for magazine papers and soft printings. The leaf grows to a height of three or four |