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A piece of wood allowed to reach an equilibrium moisture content (EMC) in a certain environment, if placed in a different environment with a different combination of humidity and temperature will adsorb or desorb moisture until it reaches a new equilibrium. Wood is not significantly dependant on particle size for its sorption characteristics. A large and a small piece of wood will adsorb a similar proportion of its total mass in water, as water is transported through capilliaries and stored in the cell walls or in the cell cavities. It is unclear whether this property extends right down to single wood fibres. The fibre-saturation point is the point at which the cell cavities contain no liquid water, all the moisture is stored in the cell walls (commonly 25 to 30% MC) [Stamm, 1964].
Sorption isotherms, such as Figure 
are used to demonstrate the
relationship between EMC of wood and the RH of its
environment. Sorption isotherms are different for each timber and wood product
although the sigmoid shape of Figure is typical for
wood. This study will only examine the effect of RH on MC. Other factors such as temperature,
stress and sorption history (sorption isotherms usually show
hysteresis with a higher MC during desorption compared to adsorption)
which affect MC were kept constant.
Moisture content also has an effect on the mechanical properties,
as stated earlier,
with strength generally decreasing with increasing MC (Figure
). But above the point
there is no drop in strength with increasing MC
[Tiemann, 1906]. Thus it is reasoned that only cell wall water
exerts an effect on strength. The dip in strength properties at
humidities below 5% is probably due to the wood becoming more rigid
at low moisture contents with stress concentrations building up that can not be transfered to
adjacent sections. The effect of RH on mechanical properties and MC varies for
different mechanical properties and with different timbers. Figure
should be regarded as only a general model of the effect
of MC on mechanical properties.
Figure: A moisture adsorption
curve as typified by radiata pine [Rijsdijk, 1994].
Figure:
Graph showing a typical plot of the ratio of strength
at a given MC to that of the 12% MC value [Kollman,1940].