<Li> strength (strengths generally increase consistently as the wood is dried below the FSP (Desch and Dinwoodie, 1996), except for impact - bending strength and, in some cases, toughness); </Li> <Li> electrical resistivity, which increases very rapidly with the loss of bound water when the wood dries below the FSP . </Li> <P> Wood is a hygroscopic substance . It has the ability to take in or give off moisture in the form of vapour . Water contained in wood exerts vapour pressure of its own, which is determined by the maximum size of the capillaries filled with water at any time . If water vapour pressure in the ambient space is lower than vapour pressure within wood, desorption takes place . The largest - sized capillaries, which are full of water at the time, empty first . Vapour pressure within the wood falls as water is successively contained in smaller capillaries . A stage is eventually reached when vapour pressure within the wood equals vapour pressure in the ambient space above the wood, and further desorption ceases . The amount of moisture that remains in the wood at this stage is in equilibrium with water vapour pressure in the ambient space, and is termed the equilibrium moisture content or EMC (Siau, 1984). Because of its hygroscopicity, wood tends to reach a moisture content that is in equilibrium with the relative humidity and temperature of the surrounding air . The EMC of wood varies with the ambient relative humidity (a function of temperature) significantly, to a lesser degree with the temperature . Siau (1984) reported that the EMC also varies very slightly with species, mechanical stress, drying history of wood, density, extractives content and the direction of sorption in which the moisture change takes place (i.e. adsorption or desorption). </P> <P> Wood retains its hygroscopic characteristics after it is put into use . It is then subjected to fluctuating humidity, the dominant factor in determining its EMC . These fluctuations may be more or less cyclical, such as diurnal changes or annual seasonal changes . </P>

To what level of dryness can air-dried lumber reach