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Drying Degrade Causes of and Cures for Drying Degrade At times there is a great deal of mystique regarding the causes of degrade. However, it is really quite straightforward. Drying too fast (i.e., temperature too high, humidity too low, and/or velocity too fast), as indicated in Table 11, will result in failure of the wood (checking, splitting, honeycomb, etc.). In 99% of all cases, these failures will occur or become inevitable during Stage I - the loss of the first 1/3 of the lumber's "tree green" MC. (Recall that many times the wood is damaged in Stage I, but the damage doesn't show up until later.) Often the problem with "drying too fast" is that the operator doesn't realize that the lumber being dried is not of as high quality as expected and therefore requires slower drying. For example, it may be bacterially infected; or it may have been slightly checked prior to receipt. Such factors mean that standard practices must be modified to accommodate the weaker lumber. Drying too fast may also occur in those scattered areas where velocities are too high. Drying too slowly (i.e., humidity too high, velocity too low, and rarely temperature too low) will result in staining and discoloration, as well as warp. The risk is greatest above 50% MC. Below 30%, the risk is probably nil. Drying too slowly, when conditions oscillate between good drying and very slow drying, especially when the cycle is over 3 hours, result in aggravation of surface checks and end splits. Drying too slowly also includes the time after sawing but before stacking. Three days of tight piling before stacking at temperatures over 70°F, even when the lumber is dipped in an antifungal chemical, can result in severe staining. Depressions initially in the kiln, even for 12 hours, less than 10°F can result in staining. All staining will typically not develop the actual color change until the MC is below 20% Further, often the stain is not visible on the rough surface. It is only after the lumber is planed that it shows up. (Note: Slow drying and therefore a risk of staining may also occur in those scattered areas in a dryer where the velocity is much below the average.) Poor stacking (i.e., having stickers out of alignment, having bolsters out of alignment or not under every sticker or not level, or having too few stickers) can accentuate warp, especially waviness or bow along the lumber's length. Crook, twist, and cup are not too sensitive to stacking, except when stacking is very poor. Humidity and natural factors (tension wood, compression wood, and juvenile wood) dominate the causes of warp in most operations with "average" stacking. Because a kiln or predryer for hardwoods will have higher humidities than in air drying, it is not unusual to see an increase in warp when drying "green-from-the-saw" compared to air drying. 51PDF Image | Principles and Practices of Drying Lumber
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