Regaining MC in overdry wood
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Is machinability lost when lumber has been overdried, then had moisture put back in? May 17, 2003
Question
I work at a kiln drying and manufacturing plant and am having a debate with one of our lumber suppliers. We ran several tests previously on material that exited too dry from the kilns and we tried to put moisture back in. We went by the core sample until it reached 6%, but it still machined the same as if it was over dry. I know once a piece has gone below approx 5.5% at any time it tends to behave as if it was 5.5% for the rest of its life, even if the samples say 8.0% core after treatment. I have tried to reason with textbook theory and even remember reading an article on it… I assume it is because the damage is already done, i.e. hairline cracks and you never gain back that bound water. Can anyone help?
Forum Responses
(Sawing and Drying Forum)
From contributor E:
If you dried it too much or conditioned it too long and it has "reverse case hardened" you will never get moisture back in. I’m assuming what you tried to do was condition the lumber again and if you try and add too much back to it, you will ruin the lumber. When the lumber is machined, if it pinches the saw it has not been conditioned long enough. If you rip the lumber and it peels like a banana, then you are drying too low an MC and conditioning too long. Correct me if I’m wrong, someone...
From the original questioner:
I understand the above comments, but let me give an example. Lumber arrives at core @4.3% Prongs okay. It is restacked, put into kiln to add moisture. After, the core is 6.5%, but it still machines like it is 4.3%. Why?
From contributor E:
I’m assuming that (depending on where you are located) you may be drying the lumber too dry and it is sitting in a very dry shed with a very low relative humidity. You need to check your humidity throughout your facility and sheds to determine this. Or you only need to dry down to 8% MC. Then if the lumber sets for awhile it will (eventually) equalize to the RH of your facility. I feel that between 4 and 6% there wouldn’t be much difference on how it is machining. 4 to 6 percent is very dry and as we all know even after conditioning and dry kilning lumber, you will have some lumber that is dryer than others. Where I used to work we had the same problems. We actually had to install misters throughout the facility set on timers to spray moisture in the air to keep this problem from happening.
From the original questioner:
This is the odd thing - we have humidity control. It is the lumber after regained moisture that is still behaving badly.
From contributor M:
Is there any way to buy lumber at 10% to 12% MC and then condition it to 6%? Once the core gets under 6.5%, it is a bugger to get it back. What species are you talking about? Red oak can be a pain when it comes to over dry.
From contributor F:
Are you having trouble getting the wood at the moisture content you want or is the trouble that once you get the moisture content you want it still behaves as if it is too dry? How is it behaving? Wood is damaged when it is over dried.
From contributor M:
What kind of damage occurs when wood is over dried? I have the over dry problem from time to time.
From contributor F:
I'm not sure anyone knows what the mechanism is, but the mechanical properties are reduced when it is over dried. In softwood this damage occurs as high as 14%MC. (See Dave Green's and Dave Kretchman's work at FPL.) Even the old publications for all species show a decrease in mechanical properties as the MC goes below ~6%.
It's not casehardening and it happens even if there are no checks or collapse.
From contributor M:
I sometimes see oven samples at 3 and 4%. Can this be true or do you think I’m doing something wrong with the oven test?
Wood loses machinability after over-drying (under 8% MC for softwoods and under 6% MC for hardwoods). There was a good paper about this from Mississippi State University. Bringing the MC back up does not restore the machinability.
It is difficult to have lumber regain moisture to a higher MC, but it can be done. See DRYING HARDWOOD LUMBER for the procedure. Also, when trying to redry lumber, see the same book in about the same place.
Lumber that is rewetted quickly has a very high risk of reverse casehardening.
Remember - *important* - any stress tests on lumber with a moisture gradient will not be accurate. Stress tests must be done on lumber without a gradient. If you have a gradient, then you must wait until the gradient is gone before reading the results of the test. That is where 15 seconds at high in a microwave for the prongs can help; longer when using a water spray instead of steam spray.
Contributor F, is the paper you site for wood that is overdried just a few percent MC (say down to 4% MC for hardwoods, over 8% for softwoods) and then the MC is restored to a higher value? I thought it was for well over-dried wood rather than slightly over-dried and that the paper did not discuss machinability or the properties when the MC was restored to a higher value? Did it apply to all species? Southern pine or…? If it applied to southern pine, then I wonder why it is dried in a high temperature kiln with some pieces, and the outside of other pieces, well over-dried. Sounds like an example of research not agreeing with reality, as Koch showed that there was not an important effect.
It would be nearly impossible to see oven samples at 3% MC, as the driest conditions used in a kiln would typically not be drier than 3.3% EMC (often higher than that). Exception: High temperature drying. So, the shell would achieve 3.3% MC, but the core would take so long that it would not achieve it in a normal kiln run. If you get readings that low, I suspect that you are not thoroughly oven-drying. How long do you oven-dry? What temperature? How much do your samples weigh? How many digits in your scale or balance?
Gene Wengert, forum technical advisor
From contributor F:
It is possible to have some pieces down to 3-4% but as Gene pointed out, there are numerous other possibilities that could give an erroneous value.
From the original questioner:
I have also experienced samples at 3-4% core moisture content on some purchase loads but average overdry is 4.5%-5%. You are hitting the hammer so close to the nail it is killing me. We have followed the procedure for regaining moisture but the material still machines as if it was the original overdry moisture even though the core says 6%. I believe that when it went down the first time the damage was done and is irreversible cellularly or mechanically.
