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is below 92% (throughout the lumber pile). Mold, mildew, and decay are caused by fungi, which again means that the lumber was dried too slowly at high MCs. Chemical dips may not always control these forms of degrade. Iron stain is a result of the tannic acid in the wood reacting with iron. The iron can be in the steam or water pipes or can drip from a fixture in the dryer. Chemical stain is a result of naturally occurring chemicals in the wood changing color via an enzymatic oxidation reaction. The reaction occurs at very high MCs whenever the lumber is dried too slowly. The final discoloration step often occurs under 20% MC, but slow drying must be done at the high MCs. The kiln schedule must start with an initial 10° to 12°F depression to prevent staining in light colored woods. Warp can be a result of drying at an excessively high RH. Warp, especially bow, is also a result of poor stacking. Much warp is a natural occurrence in the tree and is difficult to control. Warp can be a result of increasing the RH of partly dried lumber - never steam a load of partly dried lumber when it first goes into the kiln. Uneven final MC is usually a result of varying air velocity in the dryer at high MCs. Varying incoming MCs can also cause a significant problem. Poor stacking also contributes to this problem. Too wet or too dry lumber is usually a result of poor practices in sampling the lumber. Sometimes, poor instructions from management can contribute. Casehardening in dry lumber even after a conditioning treatment is usually a result of a conditioning EMC that is too low. (Remember, it is what you get, not what you set for temperatures and EMC that is important!) Poor sampling or poor equalizing can also be important causes. Resin exudation will occur with softwoods that are not dried at a high enough dry-bulb temperature. Temperatures over 160°F are essential in order to evaporate those chemicals in the resin that will flow at room temperature. Loss of aromatic odors is a result of a dry-bulb temperature being too high. Usually 85°F is the maximum temperature when odors must be retained. Ring failure, or shake is a natural occurrence in the tree and is seldom a true drying defect. Checked, loosened, or fallen knots are a result of drying too fast at too high a dry-bulb temperature, but the characteristics of the wood also contribute to the cause of this degrade. Heart splits are considered inevitable whenever the pith is in the piece. Raised, chipped, fuzzy, and torn grain are related to the presence of tension wood in hardwoods 54PDF Image | Principles and Practices of Drying Lumber
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