nger in
material checking comes with the March winds and the April showers,
and not infrequently in the South it occurs earlier than that. In
other words, as soon as the sap begins to rise, the timber shows signs
of checking, and that is the time to take extra precautions by careful
piling and protection from the sun. When the hot days of summer arrive
the tendency to check is not so bad, but stock will sour from the
heat, stain from the sap, mildew from moisture, and fall a prey to
wood-destroying insects.

It has been proven in a general way that wood will season more slowly
in winter than in summer, and also that the water content during
various months varies. In the spring the drying-out of wood cut in
October and November will take place more rapidly.

SECTION XI

KILN-DRYING OF WOOD

Advantages of Kiln-drying over Air-drying

Some of the advantages of kiln-drying to be secured over air-drying in
addition to reducing the shipping weight and lessening quantity of
stock are the following:

1. Less material lost.
2. Better quality of product.
3. Prevention of sap stain and mould.
4. Fixation of gums and resins.
5. Reduction of hygroscopicity.

This reduction in the tendency to take up moisture means a reduction
in the "working" of the material which, even though slight, is of
importance.

The problem of drying wood in the best manner divides itself into two
distinct parts, one of which is entirely concerned with the behavior
of the wood itself and the physical phenomena involved, while the
other part has to do with the control of the drying process.

Physical Conditions governing the Drying of Wood

1. Wood is soft and plastic while hot and moist, and becomes
"set" in whatever shape it dries. Some species are much more
plastic than others.

2. Wood substance begins to shrink only when it dries below
the fibre-saturation point, at which it contains from 25 to
30 per cent moisture based on its dry weight. Eucalyptus and
certain other species appear to be exceptions to this law.

3. The shrinkage of wood is about twice as great
circumferentially as in the radial direction; lengthwise, it
is very slight.

4. Wood shrinks most when subjected, while kept moist, to
slow drying at high temperatures.

5. Rapid drying produces l