One of the properties of wood is its ability to modify its own moisture. This property is called hygroscopicity. Humidity varies depending on the environment – if the wood is wet, then dry air, it emits moisture, if the air is humid and dry wood, it takes moisture. Thanks colloidal substances in the wood cells percentage of bound moisture in her could not be more specific maximum (30%), which depends on temperature and wood species. If the total moisture content greater than this maximum, then the timber is also free water, whose volume is calculated from the ratio between the walls cells and their cavities (from 50-60 to 200-250% depending on the breed). There are so-called stable moisture content.
Her material gets, if present in the air with a steady state (temperature, humidity, etc. characteristics) in two ways – sorption and desorption. If the air condition will change, the moisture content of wood will either increase or decrease. Sorption – the absorption of moisture from the air timber. Desorption – drying and, consequently, the release of moisture in the environment. When measuring the moisture content of wood is used as an indicator of sorption hysteresis. It is derived from the difference between a stable moisture content for desorption and sorption. The value of this index is largely dependent on the sample size – about 2.5% moisture content for samples more 15h100 (ShhD).