For easy reference, this glossary contains a list of words and phrases used in treating wood for furniture building.
Air Drying: a method of stacking wood outdoors to dry. Boards are separated by stickers arranged one above the other and close enough together to prevent sagging of the drying boards. Dimension of the stickers controls the rate of airflow through the stack. Ends are usually coated to prevent over-rapid moisture loss. Typically, a cover protects the stack from direct sun and rain and an open shed wall deflects strong winds.
Bound Water: the water in the cell walls.The way water is held in and between the fibrils that comprise the cell wall is complex. Consequently, removing it is a complex and relatively slow process which, if done incorrectly, can spoil the usefulness of the wood.
Case Hardening: when the outer layers have shrunk due to over-rapid drying and the core is not yet dry, the outer shell goes into tension. As the core dries, the shell, which is set in a permanently expanded condition, prevents normal shrinkage of the core. When the core eventually dries, it goes into tension and the shell goes into compression.
Differential Shrinkage: the nature of wood to shrink about twice as much in the tangential direction as in the radial direction in a cut board or billet; the sole reason why wood distorts while drying.
Equilibrium Moisture Content: the moisture content at which the wood is neither gaining nor losing moisture; a dynamic equilibrium that changes with relative humidity and temperature.
Fiber Saturation Point: a theoretical state at which all the free water has been dried from a sample of wood and all the bound water remains.The MC at this state is about 30%.
Free Water: the water in the cell cavities.
Hygroscopic: the property of exchanging (absorbing/releasing) moisture with the atmosphere.
Kiln Drying: the drying of wood in a closed container where an operator with the aid of computer program controls the air circulation, humidity and temperature. In a modern kiln, each species is dried according to a schedule — the cycle according to which the three variables are regulated to speed drying with a minimum of degrade as shrinkage occurs while reducing MC to about 8%.
Moisture Content (MC): the weight of water in a sample of wood expressed as a percentage of the dry weight of the sample.
Oven-dry Method (to determine MC): a wood sample is weighed, then dried in an oven at about 215°.Weigh it every four hours. When the last two readings are the same, the sample is dry. Its initial weight minus its dry weight is the weight of water. Its MC is determined by the standard formula.
Relative Humidity: at every temperature there is a proscribed maximum amount of water vapor the air can hold. Relative humidity is the actual amount of water vapor present, expressed as a percentage of the maximum possible for that temperature. At maximum, the air is saturated.