Timber for building
The different uses to which timber may be put are determined by its characteristics – its grain pattern, density, colour, hardiness and so on. For example, balsa is ideal for making model aeroplanes because of its lightness, whereas heavy woods – teak, ebony and box – are more suitable for furniture-making. Cedar wood is often used for the outside of houses because of its attractive appearance and resistance to rot. Elm is useful for lock gates and the bottoms of barges because it does not rot easily under water, and is extremely resistant to the attacks of pests such as termites.
However, before any wood can be used in building or furniture manufacture it has lo be dried in controlled conditions. Fresh wood contains water (which is why logs crackle in a fire) and, as it dries, it shrinks, warps and cracks. Obviously, this cannot be allowed to happen to wood that has been incorporated into a building or a piece of furniture, so it is dried out before being used. This used to be done by leaving the logs or cut planks out in the open, but nowadays they are seasoned in humidity-controlled kilns to produce a large quantity of workable timber in the fastest possible time.
The last 30 years or so have seen marked changes in the pattern of timber use. The value of timber has increased so much that the appearance of an end product is now often more important than the materials that go into its manufacture. A familiar example is chipboard which, when coated with a plastic laminate, is used for kitchen work surfaces and utility units in the home.
The advantage of chipboard is that it can be made from all sorts of wood, particularly trees too small to be cut into planks, and it can be made to any size and be manufactured by machines.
Plywood is a combination material of a different sort. Thin sheets of wood are glued together with the grain of each layer lying at right-angles to the grains of the layers immediately above and below. The ‘ply number’ such as 3-ply or 6-ply refers to the number of sheets. Plywood is much stronger than a plank of wood of the same thickness and the arrangement of its sheets makes it resistant to warping.
Timber for pulping
Today, vast quantities of wood are pulped for paper. To give just one example, a top-selling national newspaper will consume up to 15.000 trees in a single edition.
Wood pulp is produced either mechanically or chemically. In the former process sections of logs are gradually pulped on a revolving grindstone and the fibres removed by water. In chemical pulping, the logs are chipped into pieces and then digested in either soda, calcium or magnesium sulphate or sodium sulphite. The resulting slurry is then beaten and chopped in a huge cylinder equipped with a rotating set of knives. China clay or starch is added to fill in pores in the wood fibres and give weight to the paper, and dyes and resins are added to reduce absorbency.
The mixture is then pumped into a paper-rolling machine, where it is passed through a series of rollers that squeeze the pulp into paper. Finally, after being dried, the paper is passed through chilled rollers to give it a finish. Fine-quality papers may be glazed with China clay or some other material to give them a glossy finish.