Over-Drying and Wood Quality

Kiln operators discuss what happens to lumber when you overshoot your moisture content target. February 14, 2006

Question
What happens to wood when it dries too much? I almost did it today. From what I understand, 6% is as low as I would ever want to go and I usually shoot for around 8%. I am told that irreversible damage occurs to the structure of the wood when it gets too dry. Does that apply to all species? Reclaimed heat pine (longleaf southern yellow pine) specifically? Mine showed around 6.5% to 6.7% when I checked a few boards in the kiln today.

I turn the kiln on in the evenings when I'm going to be here to keep an eye on it, and leave it off at night and during the day. This evening, I turned it on and forgot about it, but it had already run the previous 2 evenings and the charge was getting close to dry. I think I almost ruined a charge, which got me to wondering what exactly happens when it dries too much.

Forum Responses
(Sawing and Drying Forum)
From contributor J:
The desired MC depends on where you are going to use it. In most of the US the desired MC is 6-8%. If there is no checking or warp then there is no danger in going too low. It will take on more water once put in the environment that it will be used in.


From Gene Wengert, forum technical advisor:
Most pine would be over-dried if less than 10% MC. It does become brittle and raising the MC does not return the wood to a satisfactory state all the time. Unfortunately, 6 to 8% MC is too low. Did you make a temperature correction when you used the meter? For hot wood this means that the MC is lower than the meter indicates. What type of kiln do you have?

From contributor D:
A couple weeks ago, I dried some spotted gum from Australia. I over dried it to the 4% area thinking that it was going to be more difficult than it was. I just checked and it's now around 10%. It's been that hot and humid.


From the original questioner:
It's an electric kiln we built at the beginning of this year. It heats with three 20 watt heat strips and a large squirrel cage fan. A full charge will run around 1200 to 1500 bd ft. We start a charge off at 130 degrees, and bump up the thermostat gradually to 160 by the end of the cycle. I did make temperature corrections. I just checked it again, and it is about 7% after temp and species corrections. (it has cooled down to 85 degrees now).

The reason I usually shoot for 8% or so is that is what has been requested most of the time. I have tested wood in homes in my area that have been in the home long enough to fully acclimate, and homes in my area seem to keep wood in the 9% to 11% range. I haven't ruined this wood, have I? I suppose I shouldn't be drying less than 10%?

Contributor D - If I understand correctly, you can dry wood to 4% as mentioned, and you can raise it back up by introducing it to moisture, but the damage has been done and is not reversible. I am told cell structure of the wood becomes damaged and the cells are obviously dead now, so they can't be repaired by introducing moisture. Kind of like you can turn iron into rust, but you can't turn rust into iron. So just because the meter says it's back to 10% doesn't mean the wood is the same as it was when it was at 10% the first time around.


From contributor D:
To the original questioner: I believe that machinability will return. If you are talking about 'strength', more damage is done by high temperature than low MC, if I'm not mistaken.

From the original questioner:
That could be. What the difference is in the wood after it has been over dried and then returned to optimal MC is what I don't know. I'm not sure it even matters but that's what I'm trying to find out.

In fact, I may be misinformed all the way around. It seems to be hard to find real fact when dealing with this kind of stuff. There is so much conflicting info out there. My main concern right now is determining whether the wood I have at 7% is still acceptable product. If so, should I wait until it gets to a higher MC before milling it? It's for flooring.


From Gene Wengert, forum technical advisor:
For reclaimed heart pine, you want to dry only to 10% MC and no lower than 9% MC. Because this pine shrinks so little, you can be (in fact it is best to be) a little wetter than for hardwoods. Further, as you go under 10% MC with most softwood species, you will lose machinability and it will not return when you increase the MC slightly to 10% MC.

You have 60 watts of power? Did you know that it takes about 15 kWh to dry 1500 BF of pine by 1% MC. That is about 10 days with 60 watts running full time. Of course, you are only running part time. Something does not seem to add up here. I really have a hard time figuring how you could actually dry the wood, let alone over-dry it.


From the original questioner:
Sorry, I said watts but meant amps. It dries the wood very quickly. I can go from wet to the touch to under 10% in 2 days with the kiln turned off overnight.

I ran a little of the 4/4 through the moulder today with no problems at all. Machinability was the same as always, but this particular wood had a good bit more resin/pitch in it than usual. I think that had a lot to do with the good results. It sure does gum up a machine in a hurry though. I'll run the rest through and see how it does. Most of the wood isn't as sappy as what we ran today, so I'll have a better idea of machinability soon.

So machinability is the main concern about over drying, correct? Structural integrity isn't compromised?


From Gene Wengert, forum technical advisor:
Strength and stiffness is not affected. In fact, the drier it is, the stronger it is at that low MC; stiffness changes little with this small change in MC.

From contributor D:
Gene, I've been wondering about machinability since reading your post that softwoods don't recover. Let’s go back to hardwoods for a moment. Suppose someone came to me and said their red oak was over dried because they were having trouble with tear-out. Suppose I metered the wood and it was 7 to 8%. Are you saying that the tear-out still could have been caused by over drying because the wood might have been over dry at one point?

From Gene Wengert, forum technical advisor:
Yes, that can be one cause of excessive tear out indeed. High temperature is another. Of course, knife angles, knife sharpness, feed rate, and amount of stock removed are also important

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