Page:Paper and Its Uses.djvu/76

64 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