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in softwoods, resin not set) are unfortunately all too common. Many times, drying too rapidly or too slowly and poor stacking are operational errors too. All operational errors are preventable if the operator has enough information and enough time, and if the operator has the support of management. Short courses in lumber drying are offered throughout the country and throughout the year. Many associations, such as the National Hardwood Lumber Association, Southeastern Lumber Manufacturers Association, and Hardwood Manufacturers Association, sponsor these seminars. Further, computerized controls are now available that virtually eliminate operator errors, and further can accurately convey management objections and can provide process control of drying conditions. Drying too hot (over 85°F), will drive off the cedar oils and therefore the cedar smell is lost. Miscellaneous drying defects are those defects which are inevitable due to the properties or characteristics of the tree. Drying too quickly, drying at too high a temperature, or drying at too low an RH will accentuate these defects. Processing defects, except for saw pinching and bad odors, are defects that result when the MC is not at the correct level (i.e., within 2% of the EMC in storage, manufacturing, and use). Saw pinching is an indication of casehardening. Bad odors are an indication of the presence of bacteria in the tree. Types of Drying Degrade Surface checks are a result of using too low an RH and/or too high a velocity, especially during Stage I. They are more likely to occur with bacterial infected wood or lumber sawn with a dull circle saw. The checks will close and become invisible from the surface, but will appear when the lumber is planed. Checks can also develop into serious degrade if the lumber is cycled between humid and dry conditions, such as caused by long fan reversals cycles at high humidities. End checks are a result of too rapid drying of the ends. Nearly all end checks can be prevented with end coating on the logs and on freshly sawn lumber ends. (A delay of just a few days in applying the coating to lumber reduces the benefit by 50%.) When a check goes through from face to face, it is called a split. Internal checks, or bottleneck or honeycomb checks, are almost always (99%) a result of an end check or surface check penetrating inward. Once surface or end checks develop, the drying rate must be slowed or else the checks will develop into internal checks. Collapse is common in low density woods with high green MCs. The water is pulled out of the cells so fast that the cells collapse inward, reducing the lumen size. Very slow drying is the only potential preventative measure. However, almost all collapse can be recovered at the end of drying by steaming the lumber at 100% RH. Fungal stains are a result of slow drying above 40% MC. They are easily prevented by immediate stacking after drying and then placing the stacked lumber in a location where the RH 53PDF Image | Principles and Practices of Drying Lumber
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