Q. We are seeing some end checking, and rarely some face checking, in the solid oak panels that we manufacture. The checking is right after finishing. We are thinking that our ovens might be too hot, or maybe the residence time is too long. Any guidance before we start changing things?
A. I suppose that it is indeed possible that the oven is too hot or the residence is too long, but I do believe that this is a very rare reason that we see end and face checking. Here is why: The tension strength of dry wood is around 10,000 pounds per square inch. So, somehow, we are going to have to develop a huge amount of tension force within the wood while it is in the oven.
We do know that an oven is typically about 1 percent relative humidity, so it is indeed possible that the wood in the oven will lose a little moisture from the surface and from the ends, which means a little attempted shrinkage. But, in most cases, it is not a huge amount of shrinkage if the wood going in was already quite dry and if the residence time is not excessive. In short, the bottom line is that there is not enough stress that will develop to break the strong, dry wood.
So, the checks you are seeing are because a) either the wood is weaker than normal or (b) if the end checks are at the glue joint, the glue joint is weaker than normal.
If the wood is weaker than normal, we would also see that the weight of the wood is much lighter, maybe 50 percent lighter, than normal. This is indeed a rare event, but it is worth checking on. When you get a failure, check the weight.
The wood can also be weaker than normal due to a bacterial infection in the tree that destroyed some of the strength. In this case, we almost always notice a foul odor and checks (sometimes called shake) that run parallel to the growth rings rather than across the rings. Again, this is not common.
So, here is your answer: The most common reason for weak wood is that there is a preexisting check, oftentimes an end check from drying that was not totally eliminated during the cutting of the blanks. The very end of an end check (looking at the end of the lumber) is oftentimes easy to see in rough lumber, especially on a dry day (On a humid day they may swell closed and become temporary invisible.), but the lengthwise extent of the end check is often hard to see precisely, especially in humid weather. Our emphasis on high yield can also discourage end trimming from being a bit excessive, just in case. In fact, sometimes computer scanners will automatically take and inch off the end of the lumber, but sometimes the check is indeed longer.
Now, what happens in the dry oven, is that there is a small amount of drying and shrinkage, but the strength of the wood right at the end where there is a closed end check is zero, so the stress opens this preexisting check. In my experience when analyzing rough mills, this is reason for end checks in wood 99 percent of the time.
Note that this is the same scenario that happens with a face check. During drying, some small, barely detectable checks developed on the face. When planing these small checks, sometimes they are not fully removed. So we have a tightly closed opening on the face. The dry oven dries the wood, the wood shrinks and with zero strength in the vicinity of the check, we see an open check.
Weak glue joint
Another reason for an end check opening, especially in the wintertime, is that the glue line is weak. A properly made joint will be 1-1/2 times stronger than wood, but if the ends are not properly glued, then the strength of the joint is much lower. A little drying in the oven means the ends dry and shrink a little bit, creating stress that exceeds the strength of the weak end joint.
The #1 reason for poor end joints, is that after the individual staves for a glued panel were prepared, the ends shrank a small amount so when we made the joint, there was a small gap (maybe only 1/100 inch) at the ends; most adhesives do not bridge this size gap and maintain their strength.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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