pleted. After initial drying, wood remains hygroscopic. It responds to
changes in atmospheric humidity and loses bound water as RH decreases,
or regains bound water as RH increases.
The moisture condition established when the amount ofbound
water is in balance with the ambient RH is called the equilibrium moisture
content (EMC). The extremely important relationship between EMC
andRH is shown in Figure 7. The figure contains average data for white
spruce, a typical species, shown as having an FSP of about 30% of the
moisture content. The FSP varies somewhat among different species: for
woods having a high extractive content, such as rosewood or mahogany,
the FSP can be as lowas 22–24%; for those low in extractives, such as
beech or birch, the FSP might be as high as 32–34%. Temperature also has
an effect on EMC. The curves shown are for 21 ºC, but at intermediate
humidities the EMC would be about one percentage point lower for every
14–16 ºC elevation in temperature. The EMC curves always converge at
0% RH and 0% EMC, so variation due to extractives and temperature will
therefore be most pronounced toward the FSP end of the relationship.
Under conditions in which the RH is closely controlled, as in labo-
ratory treatments or experiments, the curve for wood that is losing mois-
ture (a desorption curve) is significantly higher than the curve for wood
that is gaining moisture (an adsorption curve), as illustrated in Figure 7.
This effect is called hysteresis. During the conditioning of wooden objects
under precisely controlled laboratory conditions, the hysteresis effect may
be apparent. Under normal room or outdoor conditions offluctuating
RH, an averaging effect results, usually referred to as the oscillating curve.As with most physical solids, wood responds dimensionally to thermal
changes—expanding when heated, contracting when cooled. However, the
coefficient ofthermal linear expansion for wood is relatively quite small—
about a third of the value for steel. For most uses of wood, such minute
dimensional change is insignificant to an object’s performance and is usu-
ally ignored; therefore, thermal expansion or contraction of wood will not
be covered here. Moisture-related shrinkage and swelling of wood, how-
ever, is of critical importance and is the major contributor to warping andMoisture-Related
Dimensional ChangeC P P W 13RH (%)0 25 50 75 1003024181260Oscillating
humidityEMC(%)DesorptionAdsorptionFigure 7
Relationship between environmental RH
andEMC for wood, with white spruce as an
example. The hysteresis effect is indicated
by the different curves for desorption and
adsorption in thin specimens under carefully
controlled conditions. In the natural environ-
ment, with its fluctuating humidity, wood of
lumber thickness attains average MCs as indi-
cated by the oscillating humidity curve.