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State the effects on production of the following timber defects.
a) Knots.
b) Case hardening.
Defects are structural faults within the timber which may impair its strength, resilience or durability and should not be confused with surface blemishes. Features of some timbers such as knots may be termed NATURAL DEFECTS whilst others such as Case Hardening are termed ARTIFICIAL DEFECTS, because they could have been reduced with more careful conversion (sawing of the log into useful sizes) or seasoning (the reduction of the moisture content in the timber.)
Knots are the remains of outgrowing branches of a tree. Their grain runs at an angle to that of the main timber and where the knots are large and numerous the fibres are distorted and the strength of the timber much reduced. Knots that are "TIGHT" and those that are intergrown do not fall out and leave holes that need filling.
Case Hardening Case hard timber cups and bows badly when ripped down its length on a rip or band saw. These twisted and miss-shaped timbers are difficult and wasteful to get straight for use in the joinery workshop.
Case Hardening is induced by poor seasoning. The kiln operator can test for case hardening (the inducing of stresses in the timber by poor seasoning) by taking a sample of the timber from the kiln, cutting a thin 'slice' and cutting the centre portion out of the sample on a band saw. What happen next to the sample will indicate if the timber is case hard. If the 'fingers' of the sample stay straight then no stresses are being induced. If the 'fingers' cup in or bow out then the stresses are being induced and the operator needs to recondition the timber in the kiln
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