Surface checking, edge gluing
November 13, 2014 | 6:00 pm CST

Q:I have recently noticed that our incoming lumber is about 4 percent MC. Our kiln operator says it is higher and that he could not dry the wood that dry. What do you think?

A: It would be impossible for lumber to be dried under 3.3 percent MC at the surface in most dry kiln operations. In fact, many could not achieve under 3.8 percent MC. The core MC would be 1.5 percent MC higher, unless drying was continued for weeks and weeks.

So, this means that 4.5 percent MC would be the absolute lowest average MC value you would see. Further, at the end of drying, equalization and conditioning are done in the kiln. These processes would add about 1 percent MC to the lumber. Therefore, it would be nearly impossible to see any piece under 5.0 percent MC average, and most of the drier pieces would be a little higher (5.5 to 6.0 percent MC). I suspect that you are using a pinless-type moisture meter (the pin type could not measure under 6.5 percent MC), so my suggestion is that you send it back to the manufacturer for recalibration. You might also buy a calibration standard for your meter so you can quickly verify proper, calibrated operation yourself at any time.


Q: I am having a little problem with quartersawn red oak pieces that I am edge gluing into panels. I rip the individual strips for the panel from regular flatsawn lumber, mainly 8/4 thickness stock. I then turn them 90 degrees, giving me 2-inch-wide strips with a quartersawn face. I use the quartersawn face (the surface that I just ripped) as the face of the panels. When I sand the panels after gluing, I notice that often the ray fleck is loose and it is easy to catch a finishing rag on the tips of the flecks. We might have a lot of problems in one load and then none for a long time. What is this?

A:When drying flatsawn lumber, especially 8/4, once in a while surface checking will develop and then this checking goes deeper into the interior of the lumber. It is then called interior checking, bottleneck checking or honeycomb. All surface checks and interior checks will follow the edge of a ray. If we rip a piece of lumber with a small amount of interior checking, we will notice the exact situation you describe. So, you have honeycomb lumber, from time to time. Work with your drying people or lumber supplier to avoid this problem.


Q: We are loading our predryers with red oak that has been sitting on sticks three to six days before loading in the predryer. We are noticing some surface checks on the top layers as we are loading the predryers. We are going to predryers with moisture readings from mid 50 percent MC to low 70 percent MC. The first 10 days in the predryer we are pulling 2 percent MC per day, and after that 1 percent MC per day. In samples the checks are closing. When we pull a section to go to the kilns we are ranging from 18 percent to 25 percent MC. We start the kiln out at 110 on the dry bulb and 90 on the wet bulb. Lumber is in kilns four to five days.

When the lumber is entering the plant and being surfaced and machined we are noticing some checks that have been driven deep and closed on the top, and some honeycomb also. Do you have any advice that might help?

A:There are three problems with the way you are currently handling this lumber. First, it is critical to avoid air drying in lumber before it goes into the predryer. Why? To avoid surface checking, which originates during the first few days of the drying process (above 50 percent MC). The fact that you notice some checking is indeed a red flag that tells me you are air drying much too rapidly; your air drying is out of control.

Second, the green MC of red oak is 75 percent MC or higher. If some of your oak is going into the predryer at the "mid-50 percent MCs" then I suspect you are air drying the oak longer than six days, or else, during the six days of air drying, the lumber is drying extremely rapidly. After having been exposed to such dry conditions in air drying, the humid predryer 70 percent RH will cause the surface to swell slightly, closing the checks, but also driving them deeper. Although I never suggest air drying before predrying, if you must do this, then make sure your predryer is running at the same RH as the outside conditions. (Of course, this is hard to do in a predryer without affecting the rest of the lumber adversely. So, the bottom line is do not air dry.)

Third, if the wettest piece of lumber going in to the kiln is 25 percent MC and you dry it in five days, you are drying way too rapidly. I would expect six days plus perhaps another half day or more for equalizing and stress relief. Also, the starting kiln conditions should be just slightly drier than the predryer; if the predryer is 70 percent RH, then start the kiln at 65 percent RH. Your starting conditions are too dry and will drive small checks deeper into the wood.

Being quite blunt, the person in charge of drying needs to attend an advanced drying class ASAP to learn some of these fundamental principles, as well as some of the more subtle principles and procedures.


Q: We have a new customer for our oak furniture panels that insists that our panels be evaluated with a boil and drying test. We need to boil a section of the panels in water for 30 minutes and then dry them in an oven at 200 F for 24 hours. The glue joints must stay intact. We find that we are getting too many failures. Can you help us out?

A:The boil-dry glue line test is a specific test used to evaluate adhesives, but it would be inappropriate for evaluating interior adhesives that we commonly use for wood. Why? The PVA adhesives (and even the UF adhesives to some degree) are not designed to stand up to high temperatures or long-term exposure to water, especially when the temperature is 212F. The glues soften and lose their strength. There are glues that will pass the test, but they are more expensive and most will leave a colored glue line. In short, unless you plan to have the furniture made from your panels exposed to boiling water and hot ovens, this test is ridiculous, far from reality.


Q: Does improperly dried pine lumber cause sandpaper to gum up?

A:One of the reasons for gumming of sandpaper (some people might call it loading the sandpaper) is that the resin in the lumber is very soft. When heat is generated by the sandpaper rubbing on the lumber, this heat melts the resin (or at least some of it). The resin is picked up by the belt, the belt then cools as it loops around, and with cooling, the resin hardens.

In drying of pine and other resinous softwoods, it would be essential to run the temperature up to 170 or 180 F (even 190 F sometimes) for about 24 hours. This heat will evaporate much of the resin that would be liquid at room temperature and at some of the temperatures encountered during machining. This process is called "setting the pitch." The resin left in the lumber will soften and melt at high temperatures however, So, even if the pitch is set, when the sanding operation generates excessive heat (for example, slow feed, worn paper, excessive stock removal), then this excessive heat will soften and melt the resin and load the belts.

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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.