Series editors' preface - Wood Tools

figure or behaviour. Tangential (flat or slash
cut) sawing is used to obtain growth ring
figure in timbers such as cedar, Douglas fir,
ash and pine with distinct growth rings. The
timber seasons quickly but does not retain its
shape well. Quarter-sawing (radial or rift
cutting) is used to reveal medullary ray figure
in oaks, beech and mahogany, and ribbon
grain figure in Queensland maple and walnut.
For a given surface area to volume ratio, it
seasons slowly but retains its shape well. Rays
constitute planes of structural weakness in
wood which may lead to checking and splitting
during drying. Areas of weaker ray tissue
may also tear out during planing. Sapwood
has a higher moisture content than heartwood.
If left to dry out naturally, the sapwood will
shrink more than the heartwood and planks
will warp accordingly. Controlled drying or
seasoning is therefore used to promote even
drying.
Seasoning defects
The idea of seasoning is to create a controlled
moisture gradient steep enough to cause
moisture to be lost but not so steep that
damage occurs. However, during seasoning,
differential shrinkage across the radial and
tangential dimensions causes boards to cup
away from the heart, rounds to become ovals
and squares to become diamonds. Drying
begins immediately after felling so care needs
to be taken to prevent damage by uncontrolled
drying due to wind or sun. Case hardening
(usually associated with kiln drying) may occur
in which the outside dries out before the inside
so that the case or outside sets hard while the
core becomes honeycombed with shrinkage
checks. Case hardening causes re-sawing difficulties
and leads to warping. Collapse is the
term used to describe flattening or distortion of
cells during drying showing excessive and
uneven shrinkage on the surface of the timber.
Checks, in which separation of wood along the
grain occurs, usually in the direction of the
rays, are generally the results of shrinkage
stresses set up by the drying effect of the wind
or hot sun on freshly sawn surfaces or during
seasoning. Checks are measured in inches,
shakes are much larger – both reduce shear
strength of the timber. Types of checks
include: end checks, heart checks, internal
checks, ray checks, surface checks, through
Deterioration of wood and wooden structures 289
(a)
(b)
(c)
(d)
Figure 7.2 Warping describes any deviation of
converted timber from a flat surface. Common types
include: (a) bow; (b) crook; (c) cup and (d) twist
(arrows indicate direction of planar deviation). A
warped board may have a combination of these faults
checks. Stresses which build up during seasoning
and which exceed the strength of the
material may also result in splits. A split is a
separation of the wood fibres from face to face
in a piece of wood while a check is a longitudinal
shake (cleavage or split) that does not
go through the whole of the cross section. Sap
stain is a fungal infestation of sap wood
common in pines resulting from poor hygiene