bre Saturation Point in Wood

Water exists in green wood in two forms: (1) As liquid water contained
in the cavities of the cells or pores, and (2) as "imbibed" water
intimately absorbed in the substance of which the wood is composed.
The removal of the free water from the cells or pores will evidently
have no effect upon the physical properties or shrinkage of the wood,
but as soon as any of the "imbibed" moisture is removed from the cell
walls, shrinkage begins to take place and other changes occur. The
strength also begins to increase at this time.

The point where the cell walls or wood substance becomes saturated is
called the "fibre saturation point," and is a very significant point
in the drying of wood.

It is easy to remove the free water from woods which will stand a high
temperature, as it is only necessary to heat the wood slightly above
the boiling point in a closed vessel, which will allow the escape of
the steam as it is formed, but will not allow dry air to come in
contact with the wood, so that the surface will not become dried below
its saturation point. This can be accomplished with most of the
softwoods, but not as a rule with the hardwoods, as they are injured
by the temperature necessary.

The chief difficulties are encountered in evaporating the "imbibed"
moisture and also where the free water has to be removed through its
gradual transfusion instead of boiling. As soon as the imbibed
moisture begins to be extracted from any portion, shrinkage takes
place and stresses are set up in the wood which tend to cause
checking.

The fibre saturation point lies between moisture conditions of 25 and
30 per cent of the dry weight of the wood, depending on the species.
Certain species of eucalyptus, and probably other woods, however,
appear to be exceptional in this respect, in that shrinkage begins to
take place at a moisture condition of 80 to 90 per cent of the dry
weight.

SECTION VII

WHAT SEASONING IS

Seasoning is ordinarily understood to mean drying. When exposed to the
sun and air, the water in green wood rapidly evaporates. The rate of
evaporation will depend on: (1) the kind of wood; (2) the shape and
thickness of the timber; and (3) the conditions under which the wood
is placed or piled.

Pieces of wood completely surrounded by air, exposed to the wind and
the sun, and protected by a roof from rain and snow, will dry out