No other area of wood science and technology is more important to object conservation than wood-moisture relationships. The moisture condition of wood is related to properties ranging from thermal conductivity and strength to adhesive bonding and fungal development. However, the most telling influence of moisture in wood is upon dimensional behaviour. Solving and preventing the array of problems related to dimensional movement in wooden objects begins by recognizing the fundamental relationships involving wood, moisture and the atmosphere. It is customary to express the amount of water in wood as moisture content. The moisture content (MC) of wood is defined as the ratio of the weight of water in a given piece of wood to the weight of the wood when it is completely dry. The water-free weight of wood is also referred to as its oven-dry weight, determined by drying a wood specimen at 100-105 °C until it ceases to lose weight. Moisture content is expressed as a percent and is calculated as follows:
MC = moisture content, expressed as a percentage
Wi = initial weight
Wod = oven-dry weight.
Water exists in wood in two forms, as bound water and as free water. Water adsorbed and held within the cell wall by hydrogen bonding is called bound water. Water in wood in excess of the fibre saturation point exists as liquid water in the cell cavities and is called free water. The fibre saturation point (FSP) is the moisture condition of wood wherein the cell walls are completely saturated with bound water but the cell cavities are devoid of free water. It is usually expressed as a numerical value of moisture content and is generally in the range 25-30%.
The sap contained in living trees is primarily water, with small amounts of dissolved minerals and nutrients. Wood in living trees is always above the fibre saturation point, although the moisture content may vary from slightly above the fibre saturation point to 200-300%,
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)
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.
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