uses, it loses most (but not all) of its moisture. Third, the loss of moisture
affects many properties: for example, it increases strength but decreases
dimension (i.e., causes shrinkage). Fourth, after initial drying to an equilib-
rium with its environment, wood remains hygroscopic and will continue
to adsorb or desorb moisture, and consequently change dimension or
other properties, in response to changes in relative humidity (RH). Fifth,
wood can remain dimensionally responsive to humidity-related moisture
changes indefinitely.Moisture content
The amount of moisture in wood is usually expressed quantitatively as
moisture content (MC). The MC of wood is defined as the ratio of the
weight ofwater in a given piece of wood to the weight of the wood when
it is completely dry. The water-free weight of wood is also referred to as
its oven-dry weight, determined by drying a specimen at 100–105 ºC until
it ceases to lose weight (loss in weight is taken as moisture loss). MC is
expressed as a percentage and is calculated as follows:where: MC 5 moisture content, in percent; Wi 5 original weight; and
Wod 5 oven-dry weight.Forms of water in wood
Water exists in wood in two forms: bound and free. Water adsorbed and
held within the cell walls by hydrogen bonding is called bound water. Any
available moisture will be adsorbed by the cell walls until they reach satu-
ration. Water in wood in excess of cell-wall saturation exists as liquid
water in the cell cavities; it is called free water. The hypothetical moisture
condition ofwood wherein the cell walls are completely saturated with
bound water but the cell cavities are devoid of free water is called the fiber
saturation point (FSP). The FSP is usually expressed as a numerical value
ofmoisture content. For common species of wood, the FSP is approxi-
mately 28–30% moisture content.
The sap contained in living trees is primarily water, with small
amounts of dissolved minerals and nutrients. In living trees, the moisture
content of the wood is always above the FSP, but it can vary from as low
as 35–40% in some woods to 200–300% in others. When trees are harvested
and the timber is seasoned for use, all the free water and some of the bound
water is dried from the wood. As drying progresses, the FSP has special
significance to wood properties. For example, loss of free water has no
effect on strength or dimension of wood. In any portion of the wood tis-
sue, bound water is not lost until all free water is dissipated. Only when
wood is dried below the FSP does the loss of bound water effect an increase
of strength and a reduction ofdimension.Hygroscopicity
Cell-wall substance is hygroscopic—that is, wood has the capability of
exchanging bound water in the cell walls by adsorption or desorption
directly with the atmosphere. When wood is seasoned, the amount of
bound water that is lost, as well as the amount that remains in the wood,
is determined by the RH of the atmosphere in which the drying is com-MC 5 WiW^2 W od 3100
od12 Hoadley