Distortion
Much of the distortion that occurs during drying is a direct result of the
natural differential shrinkage which occurs in a piece of wood. Normally
shrinkage in a direction tangential to the growth rings is greater, and
sometimes more than twice, than that which occurs in the radial direction. The
effect of this is best illustrated with flat sawn where differential shrinkage
results in a tendency for timber to cup (see Figure 13, below). Because
the face nearer the heart of the tree is more nearly radial it shrinks less than
the outer face with the result that cup develops with a curvature which is in
the opposite direction to the curve of the growth rings. Similarly it may be
shown that twist, bow and spring (Figure 13, below) can be attributed to
differential shrinkage in pieces of wood containing distorted or curved grain.
Differential shrinkage is also responsible for distortion in and around knots
and for the 'diamonding' of square material (Figure 13, below) when this
has not been cut on the quarter.
Normally the shrinkage of timber along the grain is very small and may be
treated as negligible in most cases. Certain woods, however, contain zones of
reaction wood (tension wood) which result in appreciable longitudinal shrinkage,
occasionally as much as 1 or 2%. Such shrinkage often results in excessive
longitudinal distortion, either during drying or when the timber is subsequently
resawn.
Poor quality material, and wood species in which the grain direction is
seldom straight, have a marked tendency to distort and these need to be piled
very carefully and in some instances weighted down if distortion is to be
minimised. Wood is a semi-plastic material and suitable restraining forces will
largely or wholly prevent the development of distortion, particularly at kiln
operating temperatures. Restraint during drying can result in some increase in
splitting, but generally this is far outweighed by the overall improvement in
shape. It should be noted that the greater plasticity of wood at higher
temperatures can result in severe distortion even with good quality timber if
this has been incorrectly piled.
Higher drying temperatures appear to increase the total amount of shrinkage
which occurs and low temperature kiln schedules are normally recommended for
those timbers, especially many hardwoods, which have a tendency to distort badly
during drying.  Figure 13 Various
forms of distortion: cross-sectional and longitudinal
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