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.
A: You are indeed correct,assuming that your oven tests are accurate—thewood is over-dried. So, let’s go over this situation onestep at a time.
Moisture Test. When testing the MC of dry lumber, the piece you cutand then oven dry must be left in the oven until weight loss stops and not fora specific time. If you weighed the piece too soon, the calculated MC would belower than the actual, true MC. I am glad you are using an oven test, as thepin-type moisture meter does not work well at these low MCs and the pinlessrequires following the instructions closely to get good readings.
Over-Dry Problems. Lumber that is over-dried does indeed present someserious issues for the subsequent manufacturer. Here are some of the mainissues.
-Overly dry lumber has more cup.
-Overly dry lumber absorbs theliquid in the adhesive quickly, requiring very rapid assembly and pressure to avoid a weak,starved glue joint.
-Overly dry lumber is more brittleand 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 tocut. If feed into the machine is too slow, the surface will burnish and willnot glue well.
-Overly dry lumber will absorb asignificant amount of moisture in the summertime, resulting in swelling, whichcan cause some issues in some products.
Increasing the MC. When considering how to increase the MC, the firstquestion is “What is the core MC?” Ifthe core is also over-dried, it will take longer and will require more effortto increase the MC.
To increase the MC, the lumbershould be put back into a kiln, on stickers. The temperature of the kiln shouldbe around 130F initially. The humidity in the kiln should be set for the upperlimit of the moisture content you want. Let’s say7.6 percent MC is the highest MC value you want. The kiln operator would lookup 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 settingwill bring the surface fibers up to 7.6 percent MC in a day or less. The lumberis held as this humid condition for as long as it takes for the core MC toreach the lowest acceptable MC. After the first day, the heat can be raised to150F to speed up the process. So, typically, at the end of this process, thesurface 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 shellMCs blend to gather a bit.
Drying Stress. This increase of moisture does not add stress (orcasehardening) to the lumber. However, a prong test for stress can incorrectlyshow stress due to the MC gradient. Once the gradient is reduced, the stresstest will be fine.
Kiln Operation. Special notes for the person drying the lumber: Thekiln operator should always use a process called equalization to avoidover-drying the lumber. If low MCs are found, as in your case, it is clear thatthe operator did not use the proper equalization procedures. When equalizationis properly done, it is impossible to over-dry the lumber.
To avoid over-drying in thefuture, the kiln operator needs to make sure that he has taken a sample fromthe kiln that represents the driest lumber. When this driest sample reaches twopercentage points below the target MC, then the kiln humidity must be increasedto 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 isused 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-bulbtemperature).
ASK THE WOOD DOCTOR 1024
Q: We have an RF gluer. As you know, when we have a wet piece of wood,we tend to get some arcing and smoking, which of course, ruins currentproduction and takes time to get back into production. So, we see two options: First,run the kilns longer to avoid wet lumber or, second, use an in-line moisturemeter to eliminate the wet pieces (maybe 1 to 3 percent of a load might be wet)and then refrying these wet pieces when we collect enough for a kiln load. Whatdo you think?
A: Obviously, you have done some studying and also know yourequipment. So, without any background info, I will give you my thoughts.
The first approach is a good solution if you are drying differentspecies or thicknesses. It may take an extra day or two in the kiln to get thecorrect final MC and eliminate the wet pieces. As a rough rule of thumb, Isuggest that a kiln has an operating cost of $10 to $15 per day per MBF. So,two extra days in the kiln increases the cost of the incoming lumber by about$25 per MBF. In a 50 MBF kiln, that is $1250. This approach does worktechnically, but it can be expensive. (Rather than running the kiln an extraday or two, we might also address the cause of MC variation, which is oftenuneven air drying prior to kiln drying. So, money spent in the air yard to geta uniform MC for lumber going into the kiln can have a big impact on costs andgluing production too.)
The second approach is very effective when the incoming rawmaterial is the same from hour to hour and day to day. Stated another way, thesecond approach will not work well (money-wise) when there are variations inraw material (other than moisture variations). I have had several clientsinstall in-line meters to identify wet lumber (and even give clues as to whichkiln and what history may be encouraging the wide range of MC). One clientreduced the end splitting problem in the wintertime to well under 1 percent afterthey pulled out the wet pieces from production. Although each operation willhave different numbers, several clients have found that rejecting 1 to 3percent of the pieces due to high MC is more cost effective than running thekilns for several extra days.
I had a client making kitchen cabinets and they were able toreduce their rejects in the wintertime to under 4 percent by pulling the wetterpieces and then holding them in storage until the summertime.
As a final comment, even with the first approach, an on-linemeter will help in identifying the incoming MC, the variations in MC andpotentially the causes of such undesired variations or high MCs so that changescan be made to improve overall processing. If you cannot afford to measure theMC of every piece with an in-line meter, there are valid ways to estimate theaverage MC and the variation using a handheld meter and specific samplingtechniques (such as 6-Sigma, SPC, and TQM). Your local community college orCooperative Extension Small Business office often can provide on-the-groundassistance.
Gene Wengert, [email protected]
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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