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Hardness and Kiln-Drying Temperature

Higher drying temperatures affect the hardness and machinability of wood. December 12, 2008

Question
One of our customers says that our 8/4 cherry is harder than that of a competitor. However, they like the color of our cherry better. Is the hardness due to a denser cherry, or from being too dry, or both? Would equalizing at a higher EMC help alleviate some of this hardness? I am currently equalizing at 5% EMC when my driest sample reaches 5% MC. The customer has no problem with stress.

Forum Responses
(Sawing and Drying Forum)
From contributor A:
It would be nice to find a good hardness scale on the web for different types of wood. When I run cherry through my planer it screams like no other wood that I have planed. Though, the other woods are walnut, oak, poplar, maple, cedar, and pine.



From Professor Gene Wengert, forum technical advisor:
Dry wood (by a few % MC) is slightly harder. Wood density also affects hardness, but overall, cherry does not change density much from site to site.


From contributor J:
I haven't had the good luck to work with cherry much. Oak and maple that has been kiln-dried seems harder and more brittle than wood air dried all the way to low MC. Am I imagining this?


From contributor S:
No you aren't... I know a guy that does a lot of carving and he won't use kiln dried wood... too hard and brittle, he says. He will only use air dried that has been inside in his shop 2-3 years. He's a patient guy. Lumber that is kiln dried in a low temp DH kiln or a solar kiln is supposed to be less brittle, but this may be BS.


From Professor Gene Wengert, forum technical advisor:
In most of the USA, air-dried lumber reaches 12% MC at the lowest and usually a bit higher. Kiln drying, especially in a steam kiln, can reach 5% or even 4% MC. Even though in the kiln the MC is raised, the wood behaves as though it was still at this very low MC... That is, it is brittle and hard. Anyone that runs a planer commercially can tell the difference between lumber dried at 160 F maximum and 180 F maximum temperature.


From contributor D:
I can feel the difference between commercially dried lumber and the lumber from my low temp (120*) kiln just by handling it. I firmly believe you get a better product when you dry at lower temps.


From Professor Gene Wengert, forum technical advisor:
We do know that lumber dried at 160 F or under machines better than 180 F. But other than that, the shrinkage (and density) is the same. What often happens with a commercial steam kiln is that a good deal of the lumber is over-dried due to operator carelessness and ignorance about the effect of over-drying on quality. Production is of more concern than quality. I suspect that you do a better job with your kiln and that is what you are seeing.
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