Knots are formed where branches of the tree join the trunk (Fig. 4.11). Where the wood fibres of the branch are continuous with the trunk, then a live knot is produced. If, however, the branch is dead, or bark becomes incorporated into the trunk, a dead knot is produced. This is liable to be loose, lead to incipient decay and cause structural weakness.
Knots are described as face, edge, splay, margin or arris, dependent upon how they appear on the faces of converted timber. Additionally, knots may appear as clusters, and range in size from insignificant to many millimetres across. Frequently they are hard to work, and in softwoods contain quantities of resin, which will continue to seep out unless the wood is sealed before painting.
Many minor defects occur to varying degrees in different varieties of timber. Bark pockets occur where pieces of bark have been enclosed within the timber as a result of earlier damage to the cambium or growth
layer. Pitch pockets and resin streaks, containing fluid resin, are frequently seen along the grain of softwoods; their extent in usable timber is limited by BS EN 942: 1996.
Trees leaning owing to sloping ground, or subject to strong prevailing winds, produce reaction wood to counteract these forces. In softwoods, compression wood is produced which is darker in colour due to an increased lignin content. In hardwoods, tension wood is produced, which is lighter in colour owing to the presence of an extra cellulose layer in the cell walls. Both types of reaction wood have an abnormally high longitudinal shrinkage, causing distortion on seasoning; furthermore, tension wood tends to produce a rough surface when it is machined.
The width of the growth rings is an indicator of the growth rate and timber strength, with the optimum ranged around five rings per centimetre for softwoods and three rings per centimetre for hardwoods depending on the species. Excessively fast or slow growth rates give rise to weaker timber owing to a reduction in the proportion of the stronger late wood or its production with thinner-walled fibres.
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