Casehardening
If permanent set of the outer layers occurs during drying and if, at the
conclusion of drying, the core remains in tension and the outer layers in
compression, then the wood is referred to as casehardened. Although universally
used to describe wood in this condition the term casehardened is slightly
misleading because the hardness and permeability of the wood are unaffected.
Casehardening stresses are very variable in intensity and can occur to some
extent under even quite mild drying conditions, including air drying. There may
be not obvious signs of degrade with casehardened wood, but if it is sawn or
heavily machined, the balance of stress within the material is disturbed and
some distortion, often cup, will usually occur.
The existence and intensity of casehardening stresses can be determined by
cutting cross sections about 15mm thick, not less than 200mm from one end of a
piece which is being dried, and then cutting out the centre portion to within
about 25mm of one end, leaving a prong-shaped test piece (see Figure
2). When freshly cut the behaviour of the prongs will usually be governed by
the moisture gradient across the piece, and to determine the amount of permanent
set which has occurred it is necessary first to equilibrate the moisture content
within the test prong by drying it under normal room conditions for 24 hours.
Newly cut prongs from a piece in the early stages of drying will at first
curve outwards but as they are allowed to dry to a uniform moisture content they
will either become straight, indicating that the timber under examination had
not been severely stressed during drying, or they will curve inward indicating
that tensile forces during the drying schedule have resulted in permanent set in
the outer layers of wood. Later in the drying process this permanent set in the
outer layers will continue to cause the conditioned test prongs to curve inwards
even though by this time the outer wood will be in compression rather than in
tension.
Prongs cut from heavily casehardened material will tend to spring inward on
the saw and press tightly together after conditioning. If drying has resulted in
only minimal permanent set and the development of only small stresses within the
piece, the test prongs will remain straight on cutting and become only slightly
curved on drying.
Casehardening can be relieved by applying a high temperature, high humidity
treatment. The treatment must be moderated to avoid 'reverse casehardening'
which is detected by the outward curving of the equilibrated test prongs.
Sometimes casehardening can be present in wood which has been partially
dried. This can result in excessive cupping if the timber is then sawn into
boards or planks for final drying in a kiln. It is not possible to remedy this
cupping during kilning, indeed it will become worse as drying proceeds because
the damper inner wood of each piece will shrink to a greater extent than the
casehardened outer zone.
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