Q. We just received some 4/4 white oak lumber and we tested the MC and found that it was between 4.3 and 5.0 percent MC using the oven test. This is too dry. Our spec is 7.0 percent MC. Two questions: What is the risk if we use this overly dry lumber? And how can we increase the MC? We do have kilns.
 
 

KNOWLEDGE CENTER

Find more Wood Doctor at FDMCdigital.com

Gene Wengert, aka The Wood Doctor, troubleshoots wood related problems, and explores lumber and veneer qualities and performance, species by species, in Wood Explorer, inside FDMC's Knowledge Center.


A: You are indeed correct, assuming that your oven tests are accurate—the wood is over-dried. So, let’s go over this situation one step at a time.
 
 
Moisture Test. When testing the MC of dry lumber, the piece you cut and then oven dry must be left in the oven until weight loss stops and not for a specific time. If you weighed the piece too soon, the calculated MC would be lower than the actual, true MC. I am glad you are using an oven test, as the pin-type moisture meter does not work well at these low MCs and the pinless requires following the instructions closely to get good readings.
 
 
 
Over-Dry Problems. Lumber that is over-dried does indeed present some serious issues for the subsequent manufacturer. Here are some of the main issues.
 
-Overly dry lumber has more cup.
 
-Overly dry lumber absorbs the liquid in the adhesive quickly, requiring very rapid  assembly and pressure to avoid a weak, starved glue joint.
 
-Overly dry lumber is more brittle and so machines more poorly with chipped  grain being much more common. Planer or roller splits are also more likely. Machine knives dull more rapidly because the wood fibers require more effort to cut. If feed into the machine is too slow, the surface will burnish and will not glue well.
 
-Overly dry lumber will absorb a significant amount of moisture in the summertime, resulting in swelling, which can cause some issues in some products.
 
 
 
Increasing the MC. When considering how to increase the MC, the first question is “What is the core MC?” If the core is also over-dried, it will take longer and will require more effort to increase the MC.
 
 
 
To increase the MC, the lumber should be put back into a kiln, on stickers. The temperature of the kiln should be around 130F initially. The humidity in the kiln should be set for the upper limit of the moisture content you want. Let’s say 7.6 percent MC is the highest MC value you want. The kiln operator would look up in a table and find that at 130F, the humidity should be 48 percent RH (which is also 7.6 percent EMC and a 22 F wet-bulb depression). This setting will bring the surface fibers up to 7.6 percent MC in a day or less. The lumber is held as this humid condition for as long as it takes for the core MC to reach the lowest acceptable MC. After the first day, the heat can be raised to 150F to speed up the process. So, typically, at the end of this process, the surface will be at 7.5 percent MC and the core at 6.5 percent  MC. Then let the lumber rest for several days (in a warm spot, but not necessarily in the kiln) so that the core and shell MCs blend to gather a bit.
 
 
 
Drying Stress. This increase of moisture does not add stress (or casehardening) to the lumber. However, a prong test for stress can incorrectly show stress due to the MC gradient. Once the gradient is reduced, the stress test will be fine.
 
 
 
Kiln Operation. Special notes for the person drying the lumber: The kiln operator should always use a process called equalization to avoid over-drying the lumber. If low MCs are found, as in your case, it is clear that the operator did not use the proper equalization procedures. When equalization is properly done, it is impossible to over-dry the lumber.
 
 
 
To avoid over-drying in the future, the kiln operator needs to make sure that he has taken a sample from the kiln that represents the driest lumber. When this driest sample reaches two percentage points below the target MC, then the kiln humidity must be increased to a point where this driest sample will not lose any more MC. Technically speaking, the wood’s MC must equal the EMC of the air. Again, a table is used to establish the correct RH. For example, to achieve 5.5 percent  EMC, the conditions are 35 percent  RH (5.6 percent  EMC and 30 F depression at 130 F dry-bulb temperature).
 
 
 
 

 

Have something to say? Share your thoughts with us in the comments below.