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

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.

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