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How Wood Dries - Shrinkage, Stresses, Moisture Gradients Wood does not dry uniformly, but dries from the outside first and then eventually the inside dries. This drying gradient creates stresses and many drying defects. The following discussion is based on drying 8/4 red oak, but represents what happens for every species and thickness. It even applies to vacuum drying. Therefore, this description should be studied over and over again until it "makes sense" and is thoroughly understood. Such an understanding will make drying with zero defects "a piece of cake." Stage I Stage I is also called the surfacing checking stage, as it is during this stage that new surface checks are formed. Stage I exists from green (i.e., "tree green") to 2/3 of green. For red oak, with a green MC of 75%, Stage I is from 75% to 50% MC. For white oak with a green MC of 66%, Stage I exists from 66% to 44% MC. Stage I may require 20 days in the kiln for 8/4 red oak lumber. During this Stage, the cells on the surface rapidly dry and drop below the fsp, which means that they are trying to shrink. However, the cells below the surface are still above fsp and therefore resist the shrinkage force from the surface cells. This surface layer is also called the shell; the interior is the core. As the shell tries to shrink, it exerts a compressive force onto the core. According to Newton's Third Law, "For every action there is an opposite, equal reaction." Therefore, if the shell creates a compressive force on the core, then the core exerts a tensile force on the shell. If this tension force exceeds the strength of the wood, then "Bang!", a surface check. The tension on the shell is greatest on the fifth day (approximately) of kiln drying green-from-the-saw with 8/4 red oak. To create a surface check requires a tension in the shell and it is only during Stage I that there are any large tension forces on the shell. Hence, it is only in Stage I that surface checks are created. This fact is important in deciding who or what is the cause of surface checking. When wood is checked, something went wrong at very high MCs. Note that although surface checks may close as drying continues (Stage II) and may worsen and go deeper later (Stage III), the situation is analogous to jumping out the ninth story window. The damage from improper actions at the ninth floor shows up much later. Further, the damage is hard to control toward the end. The best method of damage control is exerted at the ninth floor. Likewise the best method of surface checking control is exerted at very high MCs, not later. To prevent surface checking in Stage I, it is necessary both to minimize the stress development by controlling humidity and velocity and to maximize the wood's strength by keeping the wood cool. (See also How Wood Dries - Environmental Effects on Page 25.) Note that because 99% of all honeycomb is originally a check that then grows internally, controlling checking in Stage I will also control honeycomb. In fact, it is accurate to state "The most critical time in drying is during the loss of the first 1/3 of the moisture." (The second most critical time is at the 20PDF Image | Principles and Practices of Drying Lumber
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