Series editors' preface - Wood Tools

depending upon species, whether sapwood or
heartwood, and so forth. When trees are harvested
and the timber is seasoned for use, all
of the free water and some of the bound water
is dried from the wood. In any portion of the
wood tissue, bound water is not lost until all
free water has evacuated.
2.4.1 Hygroscopicity
Wood is hygroscopic, that is, it has the capability
of exchanging moisture by adsorption or
desorption directly with the atmosphere. When
wood is seasoned, how much bound water is
lost – and how much remains in the wood – is
determined by the relative humidity of the
atmosphere in which the drying takes place.
After initial drying, wood remains hygroscopic.
It responds to changes in atmospheric humidity
and loses bound water as the relative
humidity decreases or regains bound water as
the humidity increases. The moisture condition
established when the amount of bound water
is in balance with the ambient relative humidity
is called the equilibrium moisture content
(EMC). For example, for interior woodwork in
most parts of the United States a moisture content
of 5–10% is recommended, but in the
damp southern coastal regions a range of
8–13% is suggested and in the dry southern
regions, 4–9% (Desch, 1973). In the United
Kingdom and most regions of the United
States, thoroughly air-dry timber seasoned
under the most favourable conditions contains
15–18% moisture.
The relationship between the equilibrium
moisture content and relative humidity is
shown in Figure 2.14. The figure represents
average data for white spruce, a typical
species, shown as having a fibre saturation
point of about 30% moisture content. The FSP
varies somewhat among different species. In
woods having a high extractive content such as
rosewood or mahogany, the FSP can be as low
as 22–24%; for those low in extractives, such as
beech or birch, the FSP might be as high as
32–35%. Temperature also has an effect on
equilibrium moisture content. The curves
shown are for 70 °F (21.1 °C), but at intermediate
humidity levels the EMC would be about
1% lower for every 25–30 °F elevation in temperature.
The EMC curves always converge at
0% RH and 0% EMC, so variation due to extrac-
Equilibrium Moisture Content (EMC) %
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30
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Wood and wooden structures 77
0 5 10 15 20 25 30 35 40 45 50 55 50 65 70 75 80 85 90 95 100
Relative Humidity (RH) %
Figure 2.14 The relationship between relative humidity
(RH) and equilibrium moisture content (EMC) in white
spruce is shown by the solid line. Most other species
fall within the shaded band. The curve shows EMC as a
function of RH at constant temperature and is called a
moisture sorption isotherm. The moisture content of
wood is directly related to the humidity and
temperature of the surrounding air (RH). As RH rises or
falls, the wood absorbs or desorbs moisture until it is in
equilibrium with its environment (hence equilibrium
moisture content)
tives and temperature will be most pronounced
towards the FSP end of the relationship.
Under conditions where the relative humidity
is closely controlled, as in laboratory treatments
or experiments, the curve for wood that
is losing moisture (desorption curve) is slightly
higher than that for wood which is gaining
moisture (adsorption curve). This effect is
called hysteresis. Under normal room or outdoor
conditions of fluctuating relative humidity,
an averaging effect results, as indicated by
the oscillating curve.
2.4.2 Measuring moisture content of
wood
Any property of wood that varies in some
known and predictable way with moisture content
could in theory be used to measure moisture
content. According to Skaar, there are as
many as fifteen methods that have been used
(Skaar, 1984). Methods of moisture measurement
commonly used or potentially useful in
conservation are based on one or other of the
following: changes in mass; changes in electrical
resistance: change in the dielectric constant.