Wood is converted into pulp suitable for the manufacture of paper by methods which produce two distinct varieties. The first is mechanical wood pulp, so called because it is made by a purely mechanical process. The second is termed chemical wood pulp from the fact that the material is submitted to chemical treatment.
Ground Wood and Cellulose.—The two varieties of pulp are sometimes distinguished by the use of the terms ground wood and cellulose. In the former case the description implies a product consisting of pulp obtained by grinding wood into a fibrous condition, while in the second the word suggests a purified chemical product freed from the resinous and non-fibrous constituents found in wood. This is, in fact, the essential difference, for mechanical wood pulp consists of fibres which have been torn away from wood by means of a grindstone; it differs but slightly in chemical composition from the original raw material and contains most of the complex substances natural to wood. Chemical wood pulp, on the other hand, consists of fibre isolated from wood in such a manner that the complex non-fibrous substances are more or less entirely removed. The difference between these two pulps is shown in the following approximate analysis of spruce wood, and of the pulp derived from it. The composition of the mechanical pulp is practically identical with that of the wood itself.
Composition of Spruce Wood, and of Chemical Wood Pulp (Spruce).
| — | Wood (Spruce). | Chemical Wood Pulp. |
| Cellulose | 53·0 | 88·0 |
| Resin | 1·5 | 0·5 |
| Aqueous Extract | 2·5 | 0·5 |
| Water | 12·0 | 8·0 |
| Lignin | 30·5 | 2·5 |
| Ash | 0·5 | 0·5 |
| 100·0 | 100·0 | |
The use of mechanical wood pulp is generally confined to the manufacture of news, common printings and packing papers, cardboards, and boxboards. It possesses very little strength, quickly discolours when exposed to light and air, and gradually loses its fibrous character. The chemical wood pulp is a strong fibre, from which high-class papers can be manufactured, the colour and strength of which leave little to be desired.
Species of Wood.—The woods most commonly used for the manufacture of wood pulp belong to the order Coniferæ, or cone-bearing trees. In Europe the spruce and silver fir are the chief species, while in America spruce, balsam, pine, and fir are employed. The harder woods, such as hemlock, beech, larch and others, are not converted into pulp by the mechanical process.
Timber Operations.—The trees are cut down in the early part of winter by gangs of men specially trained to the work. The organisation of a lumber camp when the operations are of an extensive character is very complete and carefully arranged, every detail being attended to in order to get out the wood as cheaply and expeditiously as possible. The branches and small tops are removed from the trees when they are fallen, and the trunks cut into logs of 12, 14, or 16 feet in length, and afterwards piled up on the banks of the nearest river, or on the ice, ready for the breaking up of the winter.
As soon as the ice breaks up and the rivers become navigable the logs are floated down to their destination, in some cases hundreds of miles from the scene of operations. Where rivers are not available the timber is brought out by horses or bullocks, or by means of a light railway.
Log Cutting.—As the timber arrives at the mill it is carefully measured, both as to its diameter and length, in order that a record may be kept of the quantity used. Some of the logs are piled up in the storeyard for use in the winter, and the remainder converted into pulp day by day. The logs are first cut into short pieces about 2 feet long by means of a powerful circular saw, the arrangements for this work being devised so as to keep down the cost of labour as much as possible. All waste pieces are thrown aside to be utilised as fuel.