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Seasoning of Timber
From day to day, most people have some contact with "seasoned" timber. From childhood days - wooden cots and toys, to school desks and, eventually, to wooden furniture and flooring in homes or places of employment - seasoned timber is to be found. Yet how many people really understand what seasoned timber is? Only when cracks appear in furniture or floor, or when a door shows some degree of warping, is any thought given to this concept. It is to be regretted that even some people associated with the timber trade have little knowledge of what seasoned timber is and the best method of obtaining it.
What is "seasoned" timber?
The process of drying out the water from "wet" or "green" timber is termed "seasoning", or more simply "drying". Water is just as essential to the life of a tree as it is for all living matter. Together with the various minerals, it enters through the roots of the tree and is carried in the sapwood - the outer woody part - to the leaves. The food, that is the sugars and starch, are made in the leaves by photosynthesis and are transported in solution down the inner bark to the growing cells. The whole trunk of the tree is made up of cells, which are like small tubes, having walls of cellulose and a more or less hollow' cavity filled with water and other materials known as sap. Consequently, when the tree is felled and the resulting log is sawn into timber, the sawn sections consist of innumerable small cells containing water. Drying the moisture out of wood enhances its properties to such an extent that the resulting timber is given the special name "seasoned" rather than "dried" although the terms are identical.
In order to understand what is meant by seasoned timber the term "moisture content" must be understood. This is simply the weight of water contained in a piece of timber compared with the weight of actual woody substance in the same piece. This is usually expressed as a percentage.
Expressed as a formula
Moisture content= Weight of Water/Weight of wood substances * 100
Consider an ordinary sponge. This could weight only 100 g when dry, but when it is saturated with water it could weight 500 g. Its saturated moisture content could then be said to be 400/100 x 100 per cent or 400 per cent. In other words it holds four times its own weight in water. The green moisture content, that is the moisture content of a freshly sawn log, varies with the density of timber. Balsa, a very light porous timber, can have a green moisture content of 400 per cent, but ironbark, a very heavy timber, has a green moisture content of only about 40 per cent. In iron bark, there is so much woody tissue that there is very little free space to hold water. This water not only is contained in the hollow spaces in the woody cells (i.e. in the cell cavities) but also saturates the walls of the cells.
“Free” and “bound” moisture
"Free moisture" is the name given to the water in the cell cavities in timber, and the moisture saturating the cell walls is termed the "bound" or "combined" moisture. Although the moisture is exactly the same in either position, its effect on the timber is quite different. As timber dries, the free water evaporates first, and the effect produced is principally a loss of weight. As the bound water is removed, however, the properties of the timber become noticeably changed.