Woodworker's Journal 2008-32-6, страница 42
^ Differential shrinkage is the sole reason why wood distorts, ^ A finish may retard the rate of change, and one finish, or the number of applications, may be temporarily more effective than another, but if humidity and temperature changes in the atmosphere persist, wood movement is unavoidable. Kilns Regulate Drying Variables Kiln operators have a vested interest in minimizing any sort of degrade to the charge of wood being dried. They strive to avoid distortion, splits and case hardening. The early days of kiln drying often produced batches of less than satisfactory quality. As operators learned how to better regulate the three drying variables, they developed and applied a rigorous series of adjustments over given periods of time. Operators now follow a kiln schedule for every species of commercial __ lumber. More than one species may have the same kiln schedule so they can be dried together. Kiln operations today are so well understood that results are predictable. The outcome is that we have better and more consistent lumber products to work with than pur predecessors. Kiln drying has become a science; air drying is still an art. Computer technology is central to Llgnnmat meter systems. Shown here, a wireless transmitter with probes passes MC readings onto a receiver connected to a personal computer that displays the data. Wireless probes can also measure temperature and EMC values. Xmorg on thg wgb Visit our home page at www.woodworkersjournal.com anA click on this icon to see Ian Kirby's article "Marking Up for Success: Prepare the Stock." What's In a Name? For easy reference, this glossary contains a list of italicized words in the text. In some cases, the definitions are expanded. Air Drying: a method of stacking wood outdoors to dry. Boards are separated by stickers arranged one above the other arid dose 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-rap id 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 21 5°.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 (see p. 37). 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. Ian Kirby is a wood scientist, designer and master woodworker. In addition to his frequent contributions to Woodworker's Journal, he is the author of many books. Woodworker's Journal December 2008 41 |
