Degrade During Kiln and Air Drying
Although the shrinkage which occurs when timber is dried is not normally
regarded as a form of degrade, there are several timber defects and blemishes
which are specifically associated with the drying process. It may not always be
possible to eliminate these, but the adoption of correct drying techniques will
nearly always reduce the extent of degrade and therefore minimise any loss in
value of the timber. This section discusses the main types of drying degrade
which can occur. The reader is referred to Appendix C
which is a summary list of the types of degrade and the ways in which these can
reduced.
Surface checking and splitting
As mentioned in 'The kiln drying of a load of Timber', air and kiln drying
depend on the formation of a moisture gradient within the timber. However, many
drying defects can occur when this gradient is allowed to become too steep. The
gradient which can be tolerated will vary between timber species but generally
if there are significant differences in the amount of shrinkage at the surface
and that at the centre then stresses will develop within the piece. If these
stresses become extreme, the fibres in the drier outer layer can be torn apart
and surface checking and splitting of the wood may occur.
Since shrinkage in the direction of the growth rings is greater than that at
right angles to them, checking is most likely to occur on the faces of
tangentially cut pieces and on the edges of quarter-sawn material, usually along
the rays which form planes of weakness. Wood is more likely to suffer this
defect during the early stages of drying, when moisture gradients may be
considerable, than during the stages when the material is at a lower moisture
content throughout.
Splitting and checking can both be minimised by controlling the air
conditions to prevent the formation of steep moisture gradients in the timber.
In kiln drying this is done by maintaining the kiln air at comparatively high
relative humidities during the early stages of drying. With air drying it is
possible to control the slope of the moisture gradient to a very limited extent
by selection of an appropriate piling stick thickness.
- End Checking and Splitting
The fact that sawn timber is liable to develop end checks and splits may be
attributed to the relative ease with which moisture moves in the longitudinal
direction and out of the ends. Wood near the ends has a tendency to dry and
shrink in advance of the centre section and this may lead to stresses
sufficient to cause end splits to develop and to extend an appreciable
distance along the pieces.
Even with great care during drying it may not always be possible to prevent
the formation of end checks or splits and boards and planks are usually cut
rather longer than required to allow for this fairly common form of degrade.
In the drying of special high quality dimension stock, such as oak furniture
squares, it is uneconomical to allow for wastage from the ends and these are
usually sealed with a moisture-resistant coating to reduce end drying and
minimise end splitting. In general the end coating of boards and planks is
recommended for the reduction of end checking and splitting but the added
labour and expense may often limit the use of this procedure.
End cleats attached to boards or planks to prevent them splitting after
conversion or during air drying are of doubtful value in kiln drying. Cleats
should be removed before kilning because if they are rigid, strong and firmly
attached they will restrain natural shrinkage and actually cause end splits to
develop or extend.
- Effect of Timber Quality
The tendency to check and split is influenced by the quality of the timber
being dried and by any previous drying treatment which has been carried out.
Therefore, checking and splitting should always be monitored during kiln
drying and air conditions adjusted if an unacceptable level of degrade is
occurring. In general, milder conditions will normally help to reduce the
splitting and checking which can be expected to be more pronounced in lower
quality timber.
Little can be done during kiln drying to reverse splitting and checking
which has already occurred during preliminary air drying, but careful choice
of kiln conditions can limit the further development of this degrade. A
particular problem here is that checks induced during air drying can later
close up so that the extent of initial degrade is not apparent at the start of
the kiln drying process.
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MTC’s
