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 less shrinkage than slow drying at high temperatures, but is apt to cause case-hardening and honeycombing, especially in dense woods.
6. Case-hardening, honeycombing, and cupping result directly from conditions 1, 4, and 5, and chemical changes of the outer surface.
7. Brittleness is caused by carrying the drying process too far, or by using too high temperatures. Safe limits of treatment vary greatly for different species.
8. Wood absorbs or loses moisture in proportion to the relative humidity in the air, not according to the temperature. This property is called its "hygroscopicity."
9. Hygroscopicity and "working" are reduced but not eliminated by thorough drying.
10. Moisture tends to transfuse from the hot towards the cold portion of the wood.
11. Collapse of the cells may occur in some species while the wood is hot and plastic. This collapse is independent of subsequent shrinkage.