# Paper and Its Uses/Chapter 12

CHAPTER XII
DURABILITY OF PAPER
Paper is used for many publications and jobs of an ephemeral character, and for these the permanence of paper is never in question. On the other hand, ledgers, leases, agreements, share certificates, must be upon paper which is to all intents and purposes permanent and capable of resisting a good deal of handling. Printed records, too, must be preserved on paper that will, with ordinary care, be indestructible.
The constituents of paper, as shown in the first chapter, are vegetable fibres, mineral filling, colouring matter, and vegetable or animal sizing. The fibres producing the paper which approximates most nearly to a pure cellulose material, with the minimum of chemical and mechanical treatment, are, of course, the best possible. Classified with that in view, cotton, flax, hemp, chemical wood, esparto, and mechanical wood is the order of merit. Cotton is, more than any other material, the ideal fibre. It contains 91 per cent, of pure cellulose, has a comparatively small amount of incrusting matter, and its fibre is easily bleached, and easily prepared for papermaking. Consisting as it does of seed-hairs, cotton is a free fibre from the first. It consists of a long tube, of dumb-bell section, with a tendency to twist upon itself. Prolonged beating produces numerous fibrillæ, and the softness of the original fibre is preserved until over-beating is reached. ​The twisting, the division into fibrillæ, make for strength, good felting, and, with the softness in addition, the best and most durable papers are those of cotton.
The flax fibre is a bast fibre. Its yield of pure cellulose is 70 to 80 per cent. The fibre consists of a thick walled canal, which is easily seen in the unbeaten state. Beating tends to crush and remove the early characteristics. The fibres are regularly rounded or polygonal, and easily split into numerous fibrillæ, the ends of the fibres beat out into bunches of small fibres, and these, together with the nodules which occur on many of the fibres, produce strength in the paper. The flax fibre is straighter than the cotton fibre, and so linen papers are stiffer and harder than cotton papers.
Wood, produced as fibres by chemical means, consists largely of tracheids, long ribbon-like cells, which are easily broken into shorter lengths. It is not possible to subdivide the fibres longitudinally by prolonged beating. This only tends to shorten the fibres. Hence Mitscherlich, or similarly produced wood pulp, gives strong tough papers, unattainable by those pulps which are strongly bleached and much reduced. The tracheids, being smooth and flat, do not tend to make soft papers. But, blended with rag or esparto fibres, excellent papers may be produced. Only 50 per cent. of fibre is produced from the original wood.
Esparto gives a smooth, cylindrical fibre, pointed, short, with small canal. Being small, the fibres do not receive much treatment in beating. Separation and cleaning are the principal ends of the preparatory stages. Esparto is, to the papermaker, synonymous ​with bulky papers. The best of printing papers, litho. papers, and featherweights are composed largely of esparto. It blends well with the preceding fibres, and especially with chemical wood for printing papers. Unfortunately esparto is liable to deterioration, and thus is not suitable for permanent papers. Its yield of cellulose is low—42 to 47 per cent.
Mechanical wood is lowest in the scale of paper-making materials. Chemically it is impure; structurally it consists of chips and fragments, seldom complete fibres. Ground into short lengths, it consists usually of short bundles of short pieces of fibre. It does not felt well, and requires the addition of other fibrous material to hold the pulp together as paper. Ten to 40 per cent. of chemical pulp is usually added to mechanical pulp to make it more lasting and less brittle.
In 1898 a committee appointed by the Society of Arts reported upon the deterioration of papers after extensive investigation. Their conclusions hold good to-day, and may be summarised in the next five paragraphs.
The deterioration of paper may be by discoloration only, or disintegration may also occur. Discoloration may be caused simply by the action of the atmosphere, and is to be seen in the margins of books and in coloured papers. The outer margins of books are more susceptible to oxidation than the interior, and in gaslit rooms most books will in time suffer from discoloured margins. Chemical residues from the manufacturing processes, if left in the paper, will bring about changes in colour, engine-sized papers being more liable to change than papers which are tub-sized. Papers which contain esparto, straw, or mechanical wood, will in chemical laboratories certainly ​become discoloured, as aniline and other coal-tar bases stain the papers yellow or pink. There are but few colouring matters which are absolutely fast, therefore most tinted and coloured papers will change in time.
Loss of strength may be due to impurities in paper, such as residues of the chemicals used in the preparation of the pulp, to the impurities in the pulp itself, or to the use of gas as the agent for lighting and heating. The use of china clay for the improvement of the surface of the paper and for the increase of opacity, tends to weaken the paper, not by any chemical reaction, but merely by rendering the paper less resistant to wear. The attainment of extreme whiteness by bleaching is sometimes obtained at the expense of durability, as products are sometimes left in the fibre which will cause deterioration and discoloration of the paper.
The classification of the fibres has been referred to, and the four classes are: (i) cotton, flax, hemp; (ii) chemical wood; (iii) esparto and straw; (iv) mechanical wood.
For written documents of permanent value the paper should be all rag fibre, without starch and loading, tub-sized with gelatine. For printed books to be preserved as of permanent value, not less than 70 per cent, of the fibre should be rag, the loading should not exceed 10 per cent. as shown in the ash of the paper, and the sizing should be effected by not more than 2 per cent. of resin.
The wearing qualities of paper are affected by the method of manufacture as well as by the constituents. Blotting paper, which is an all-rag paper, will soon wear away, owing to the fact that the fibres are cut short and loosely held together without sizing. If the paper were heavily rolled it would reduce its usefulness as an absorbent paper. Featherweight ​papers are made entirely of esparto, finished to produce as bulky a paper as possible, consequently the fibres are not well rolled together, and the books printed on such paper are anything but durable. Imitation art papers give a mineral residue of 25 to 35 per cent. on ashing, and have very little strength, owing to the large proportion of china clay present.
Art papers do not fold or stitch well, as the mineral coating, although firmly fixed to the paper, behaves as a non-fibrous material might be expected to do, breaking down, and the paper beneath tends to give way too. If kept in a damp place art papers absorb moisture at the edges, and in the presence of a large amount of moisture the sheets will stick together. Rag art papers are procurable (the body being a rag paper), and possibly it will be found that such papers, kept from air and moisture, will be very durable.