Third, what all this means is that on the clear face cuttings, there cannot be incipient decay. However, on the reverse of a clear face cutting, there can be incipient decay, as the reverse side of a clear cutting only needs to be sound. Note that some species (such as sap gum and aspen) and some grades (such as No. 2B Common) admit stain in clear cuttings or require only sound cuttings. In this case, incipient decay is permitted on both sides of the cutting.

Fourth, rot is when the decay has gone far enough to soften the wood. Rot is never permitted on the face or reverse side or on the interior of a cutting, either a clear or a sound cutting.

Fifth, the area of the lumber that is outside of the cuttings used to establish the grade can have incipient decay.

will be more severe.

In my experience, cupping is often the result of mixing MCs in a kiln. The high RH used to protect the wetter lumber will weaken the fibers on the drier lumber and the drier lumber will cup.

Cupping is also accentuated if the lumber is over-dried. Even if the MC is increased after over-drying, the cupping does not moderate much. Over-drying can occur when MCs are mixed

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in the kiln and the drier lumber’s MC is not monitored.

So, as a short answer, check for anything that you are doing to rewet the partly dried lumber and check to see if some of the lumber is being over-dried. ●

 

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Q:We are getting a lot of cupping in some of our lumber. What is causing this and what can we do to fix it?

A:Cupping, which is warp across the width of lumber, is a natural event for wood, with the bark side of lumber shrinking more than the heart side (or side closest to the center of the tree). This means that perfectly flatsawn lumber cups the most; perfectly quar-tersawn does not cup at all. Also, the closer the lumber is to the center of the tree, the greater the difference in shrinkage between the two sides and, therefore, the greater the cupping tendency. This means that lower grade lumber, which is usually closer to the center of the tree, will cup more than clearer lumber from further away.

When drying lumber, if the wood is dried fairly quickly, meaning at a low RH, then the outer fibers will be dry, which means they will be fairly strong. These dry strong fibers resist cupping. However, if partly dried lumber is rewetted, which means the fibers become weaker, or if the drying conditions have a high RH, meaning that the fibers are not as strong, then cupping

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Gene Wengert, “The Wood Doctor,” is presently an extension specialist emeritus at the University of Wisconsin-Madison. Fax your questions to him at 847.390.7100 or e-mail to wooddoc@ uwalumni.com.

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