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
