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The tension inside is usually not as large as the tension levels in Stage I. Nevertheless, it is still important to keep the wood temperature fairly low to maximize its strength and prevent small surface checks from developing into internal checks or honeycomb. However, as the risk of forming new checks has greatly diminished, the relative humidity can be lowered slightly without risk IF THERE ARE NO CHECKS UP TO THIS POINT. As the interior continues to dry, the shrinkage of the interior will pull the checks on the surface closed - they will become invisible from the rough surface. (This is also called "stress reversal.") Yet they are open inside and may increase in depth, width, and length inside the wood with no indication from the outside. This increase in internal checking will only occur if the drying rate is too fast at this stage, however. Therefore, when checks are seen after planing lumber, but were invisible in the rough, most often the error was made during Stage II. Once stress reversal occurs, it is essential to prevent the wood from regaining moisture as such regain will increase the internal tension and worsen checking and honeycomb. If we want to call Stage II by another name, we might call it the stage that determines whether there will be subsequent internal checking or honeycomb. But bear in mind that almost all interior defects are a result of surface checks or end checks penetrating inward and not the spontaneous creation of interior failures. If we can't control checking in Stage I (which may occur with thick oak where a small amount of checking might be considered normal or inevitable), then we must treat the lumber gently enough in Stage II to prevent further growth of the checks. (See also Checking the Quality of Incoming Lumber on Page 36.) If checks in Stage I were subjected to alternate wetting and drying or were especially severe, then they will not close when stress reversal occurs. In fact, they will be open at the end of drying, while "ordinary" checks will be closed at the end. Stage III Stage III exists from 30% MC to the final MC. Drying time is 30 to 50 days for 8/4 red oak. In this Stage, it is nearly impossible to initiate new degrade (except with the most severe drying conditions or if water is added back to the wood). In this stage, the temperature of the wood can be raised as high as 200°F for some species and the humidity can be lowered to very low levels (50°F depression). For optimum quality, however, the maximum temperature should not exceed 160°F and the depression should not exceed 40° to 45°F for all species and thicknesses. It is in this stage that small checks will develop into honeycomb; but such development is usually inevitable due to improper drying in Stage I or II. It is seldom, if ever, due to errors in Stage III. It is, in most cases, impossible to fix in Stage III or IV what has been done wrong in the previous stages. 23PDF Image | Principles and Practices of Drying Lumber
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