Q: I have just received some glued-up oak panels that we will be using for tabletops. One of our people noticed that the ends of the panels have developed some cracks and more cracks are showing up every 10 minutes it seems. Most of the cracks are at the glue joint at the end of the panels. The supplier says that our shop is too dry (25 percent to 30 percent RH is what we measure now during the wintertime). My people just checked the MC of many panels, including some that have yet to be unstacked and unwrapped; they are getting some readings as high as 10 percent MC (after correcting for temperature), with a lot over 8 percent MC too. We need an outside person to give us an accurate evaluation.
A: There are several items that need to be discussed.
First, the wood cannot lose or gain much moisture in just a few hours, so the MC readings you are getting (if taken properly) are a good indication of the MC of the lumber when it was shipped to you.
Second, your shop is indeed much drier than the wood is; that is, your shop is about 5 percent EMC, while the lumber is 10 percent MC. I suggest that the maximum difference between the two numbers is 2 percent, with 1 percent or less being better. The basic question: "Is your shop too dry or the lumber too wet?" I know that if you raise the RH in your shop to 55 percent RH (10 percent EMC) or higher, cracking problems in your shop will be gone. What will happen, however, is that the pieces will dry and crack when they get to the customer's home. The customer's home is probably around 30 percent RH (6 percent EMC) in the wintertime. In other words, by humidifying your shop over 40 percent RH (over 7 percent EMC), you will be postponing the cracking problem until the customer gets the table. So, I think that you can understand why I come to the conclusion that the panels are too wet. I do think that you should raise your shop's humidity up to the 30 percent to 35 percent RH range, however.
Third, the cracking does indeed indicate that the wood is shrinking, or trying to shrink and developing stress. When the stress exceeds the strength of either the glue or the wood, a crack results. However, every adhesive we use on wood is actually stronger than the wood itself (except hotmelts and mastics). Therefore, if the wood panels had been properly made, you should have noted failures in the wood more than in the glue line. So, I believe that the glue joint is not the best. In fact, I suspect that the wood strips used for the panels were too wet for the gluing EMC. Therefore, after the strips were ripped and then sat around for a while, the ends shrank before they were glued, which in turn means that there was a gap between the individual strips (at the ends) that exceeded 0.006 inch. The result is a weak glue joint.
Fourth, wood dries fairly slowly. Therefore, for the cracks to be seen within just a few hours means that it is likely that the joints may have actually cracked open before you received them. That is, the operation where they were glued had cracks develop, but they might have missed them because they didn't examine the panels closely a day or two after manufacturing. Further, when moisture was added, such as when they were stored or when they were shipped on the truck (typical outside conditions are 12 percent EMC), the slight moisture regain closed the cracks tightly. However, when the panel is exposed to the dry conditions in your shop, the cracks are RE-opening (versus developing for the first time). Look inside the cracks (that is, break them open and look at the exposed surfaces) to see if you can see any debris, such as sanding dust, stain, etc., or glue puddling. This indicates that the cracks were open during the initial processing.
Finally, the basic problem is improper kiln drying of the lumber for your use. Often the kiln operator does not have the special instructions needed to dry lumber intended for a moisture sensitive product such as a tabletop. Often the kiln operator samples only eight pieces of lumber from a kiln load of 5,000 to 8,000 pieces! The odds of getting the wettest pieces of lumber with only eight samples are worse than the odds of winning the Pick-3 lottery! My suggestion to anyone processing lumber that will be used for moisture-sensitive products (in this case, my advice is for the panel gluing operation) is, "Check the moisture on every piece of lumber." You can use a hand meter or a stationary in-line meter. The damage or cost of getting a wet piece of lumber into production can be quite high when rejects show up further down the processing scheme, much higher than the cost of metering every piece and pulling the wets initially.
Special note: Your employees' measurement of RH in your shop and MC of the panels, as well as prompt inspection and employee action, make this diagnosis easy. Congratulations on a good job.
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