[CHAPTER XII]
THE DETERIORATION OF PAPER
Recent complaints about the quality of paper and the rapid decay of manuscripts and papers have resulted in arousing some interest in the subject of the durability of paper used for books and legal documents, and in the equally important question of the ink employed. The Society of Arts and the Library Association in England and the Imperial Paper Testing Institute in Germany have already appointed special committees of inquiry, and from this it is evident that the subject is one of urgent importance.
It is sometimes argued that the lack of durability is due to the want of care on the part of manufacturers in preserving the knowledge of paper-making as handed down by the early pioneers, but such an argument is superficial and utterly erroneous. The quality of paper, in common with the quality of many other articles of commerce, has suffered because the demand for a really good high-class material is so small. The general public has become accustomed to ask for something cheap, and since the reduction in price is only rendered possible by the use of cheap raw material and less expensive methods of manufacture, the paper of the present day, with certain exceptions, is inferior to that of fifty years ago.
The causes which favour the deterioration of paper are best understood by an inquiry into the nature of the fibres and other materials used and the methods of manufacture employed.
The Fibres Used.—Cotton and linen rags stand preeminent amongst vegetable fibres as being the most suitable for the production of high-class paper capable of withstanding the ravages of time. This arises from the fact that cotton and linen require the least amount of chemical treatment to convert them into paper pulp, since they are almost pure cellulose, cotton containing 98·7 per cent. of air-dry cellulose, and flax 90·6 per cent. The processes through which the raw cotton and flax are passed for the manufacture of textile goods are of the simplest character, and the rags themselves can be converted into paper without chemical treatment if necessary. As a matter of fact certain papers, such as the O. W. S. and other drawing papers, are manufactured from rags without the aid of caustic soda, bleach, or chemicals. The rags are carefully selected, boiled for a long time in plain water, broken up and beaten into pulp, and made up into sheets by purely mechanical methods.
The liability of papers to decay, in respect of the fibrous composition, is almost in direct proportion to the severity of the chemical treatment necessary to convert the raw material into cellulose, and the extent of the deviation of the fibre from pure cellulose is a measure of the degradation which is to be expected. The behaviour of the fibres towards caustic soda or any similar hydrolytic agent serves to distinguish the fibres of maximum durability from those of lesser resistance. It may be noted that in the former the raw materials, viz., cotton, linen, hemp, ramie, etc., contain a high percentage of pure cellulose, while in the latter the percentage of cellulose is very much lower, such fibres as esparto, straw, wood, bamboo, etc., giving only 40-50 per cent. of cellulose. The two extremes are represented by pure cotton rag and mechanical wood pulp. Other things being equal, the decay which may take place in papers containing the fibre only, without the admixture of size or chemicals, may be considered as one of oxidation, which takes place slowly in cotton, and much more rapidly with mechanical wood pulp. Experimental evidence of this oxidation is afforded when thin sheets of paper made from these materials are exposed to a temperature of 100° to 110° C. in an air oven. The cotton paper is but little affected, while the mechanical wood pulp paper soon falls to pieces.
The order of durability of various papers in relation to the fibrous constituents may be expressed thus: (1) rag cellulose; (2) chemical wood cellulose; (3) esparto, straw, and bamboo celluloses; (4) mechanical wood pulp. The rate and extent of oxidation is approximately shown by the effect of heat as described. The differences between the celluloses are also shown by heating strips of various papers in a weak solution of aniline sulphate, which has no effect on wood or rag cellulose, dyes esparto and straw a pinkish colour, and imparts a strong yellow colour to mechanical wood pulp and jute.