Flax is a typical compound cellulose, to which has been given the name pecto-cellulose on account of certain properties. Other well-known plants of this class are ramie, aloe, “sunn hemp,” manila.
Esparto.—The cellulose isolated from esparto differs in composition from cotton cellulose:—
| Carbon | 41·0 |
| Hydrogen | 5·8 |
| Oxygen | 53·2 |
| 100·0 | |
It is regarded as an oxycellulose, being readily oxidised by exposure to air at 100° C. Other oxycelluloses familiar to the paper-maker are straw, sugarcane, bamboo.
Wood.—The difference between wood and the plants already mentioned is expressed by the term lignified fibre or ligno-cellulose. This term is used to indicate that the wood is a compound cellulose containing non-fibrous constituents, to which has been given the name lignone. Jute is another example of this class.
These distinctions may be exemplified by reference to a simple experiment. If three papers, such as a pure rag tissue or a linen writing, an ordinary esparto printing, and a cheap newspaper containing about 80 per cent. of mechanical wood, are heated for twenty-four hours in an oven at a temperature of 105° C., the first will undergo little, if any, change in colour, while the others will be appreciably discoloured, the mechanical wood pulp paper most of all.
This change is due to the gradual oxidation of the constituents of the paper, the ligno-cellulose of the mechanical wood pulp being most readily affected by the high temperature, and the pure cellulose of the rag paper being least altered.
The process of oxidation, brought about rapidly under the conditions of the experiment described, takes place in papers of low quality exposed to air in the ordinary circumstances of daily use, but of course at an extremely slow rate. The deterioration of such paper is not, however, due to the simple oxidation of the cellulose compounds, because other factors have to be taken into account. The presence of impurities in the paper on the one hand, and of chemical vapours in the air on the other, hastens the decay of papers very considerably.
Percentage of Cellulose in Fibrous Plants.—The value of a vegetable plant for paper-making is first determined by a close examination of the physical structure of the cellulose isolated by the ordinary methods of treatment. If the fibres are weak and short, the raw material is of little value, and it is at once condemned without further investigation, but should the fibre prove suitable, then the question of the percentage of cellulose becomes important.