The improvement in dimensional stability is marked, since the superior stability and modulus of elasticity along the grain provide an enhanced restraint for the cross-grain dimensional change. It is appropriate in concluding this discussion to refer briefly to the durability of wood. As wood is organic in nature it can be attacked and destroyed by micro-organisms of various kinds. The risk generally becomes serious only when wood is maintained at a high moisture content in excess of 20 per cent but less than saturated. Treatment with preservatives is usually desirable in order to extend its life under these conditions.
Wood can have a very long life when continuously immersed in water.
At the other end of the moisture scale, it can last almost indefinitely if protected from sunlight and from excessive moisture contents and rapid or large changes in moisture content. Sunlight slowly attacks and changes the exposed surface, but its main deteriorating effect is probably associated largely with the rapid large moisture changes and corresponding dimensional changes that can result in serious checking when exposed wood is wetted by rain and then dried by sun heat. Even under these conditions wood can function very well as long as it is always arranged in such a way that it can drain and dry freely and is not held at high moisture contents for prolonged periods of time. This discussion has tended to emphasize some of the potentially challenging aspects of wood and its applications. This bas been done intentionally, not with any thought of being critical of wood as a material, but in order to promote a better understanding of its nature and of the ways in which it will perform in particular situations.