Surface cracks

Q: I have recently launched a problem-solving effort that is focusing on small surface cracks and glue joint cracks in our final product. Most of what I have read takes me to the kiln drying process; however, we do not dry lumber and we receive material in several different species from domestic and overseas suppliers. I am looking for a method to understand what causes these failures to better establish moisture content tolerances on our incoming material. Our raw material warehouse and plant both have a humidification system. Material generally is turned over within 10 days after receipt. Oftentimes, I do not see the cracks until after finishing. I am also subjecting the material to convection heat in the finishing process - is this a factor? As we have several variables within our process, what would the best approach be to establishing incoming tolerances and/or internal controls that protect us from splits?

A: I believe that you have two different events happening. First, the small cracks, which we usually call surface checks, result when the wood is pulled apart. It is unlikely that you could develop enough shrinkage forces after drying to create such failures, as the wood is so strong when it is dry and shrinkage forces are not that big. Therefore, I believe that your checks are actually preexisting checks that developed early in drying, and went too deep to be surfaced off. Your high RH in the warehouse and plant actually causes these checks to swell closed so tightly that you cannot see them during processing. It is only when the wood is subject to dry air, especially in the finishing process, that shrinks the wood surface a bit and causes the checks to re-open. You do not create the checks; you merely re-open existing checks.

The cure is to prevent such checks in drying by avoiding excessively fast drying when the wood is at very high MCs. You are not causing the checks. In your plant, the high RH in storage and in the plant is merely delaying the re-opening of the checks. I would suggest that 38 percent RH is as high as you ever want to have your facility. This lower RH should open any checks earlier in the process before you spend a lot of money on processing.

Incidentally, you can sample your incoming lumber for surface checks by cutting a one-inch-along-the-grain piece from your lumber (not near the ends however; flatsawn lumber is more prone to checking, so sample this lumber) and then cutting a thin wafer off the surface. If there are checks, the wafer will break in two while you are cutting it.

Your open glue joints are a result of overall shrinkage of the wood. Basically, the wood is too wet for the plant's environment, especially in the dry finishing room. In most cases, the problem is that between ripping and gluing the ends of the individual pieces shrink slightly - the end grain dries faster than face grain.

When you assemble a panel with the ends being slightly scant (just 0.006 of an inch is too much), the glue joint is not very strong. Any shrinkage subsequent to gluing will create stress and this stress will cause the glue joint to break at the ends where it is weak. To control this problem, sample incoming MC. During the heating season, any pieces over 7.5 percent MC should probably be pulled out of production. Either dry them, then remachine the edge to be glued, and glue; or save them until summer and then remachine and glue. If this is unacceptable, then rip and glue within 15 minutes. This rapid handling will prevent serious shrinkage before the panel is manufactured. In fact, try a few tests where you rip and glue immediately. You will note that gluing failures drop to zero, proving that the issue is high MC in the incoming lumber.

Incidentally, your humidified warehouse is unnecessary and actually is hurting you slightly. Keep the warehouse at no higher than 37 percent RH and no lower than 25 percent MC. In a tight bundle, however, you will not get any substantial moisture change within 10 days no matter what the warehouse RH is, except for the piece on the outside of the bundle.

Although you cannot fix the incoming MC problem (it needs to be fixed in the kiln), you can certainly check the incoming MC. An in-line MC meter would be ideal. I know of one door manufacturer that eliminated complaints from the field by eliminating wet lumber with an in-line meter. They went from over 100 calls to just three during the colder months.

Let me know if you have further questions, as this is a short answer for a complex problem.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.