Twisting lumber
What causes lumber to twist in a dehumidification kiln? March 20, 2001
Q.
I have a small dehumidification kiln and have problems with wood twisting, mostly cherry and poplar. What causes this?
Forum Responses
One major cause is the sawing pattern--whether they saw parallel to the bark or not. Another is tree genetics.
Gene Wengert, forum technical advisor
Couldn't the twisting be because the drying schedule is used in the second phase of the drying...high temperature? In addition, what about sticker placement? We are drying 2 and 3A com lumber and we see some twist due to sawing and defects, but it is minimal if handled properly.
Twisting is not caused by drying, although with slow (humid) drying and warmer temperatures, twisting will be more.
Gene Wengert, forum technical advisor
I have been drying at a very low temperature--about 42 degrees C. I have always been told the lower the temperature, the better the quality. Am I wrong? I also place my stickers about 24" OC. I always air dry to about 20% before I place the lumber in the kiln.
Everything you state is okay if you do not let the lumber regain moisture when you start the kiln. The 24" spacing is widely used.
Tension set (also called casehardening) will help prevent warp--so using a harsh schedule (low RH) is indeed a good idea (so long as you do not check the wood).
Gene Wengert, forum technical advisor
You can test for a tendency to twist. I split a length of log into quarters. I split off a tangent piece and a quarter piece. I flatten one side with a jointer. I then band saw a piece off each that is about 1/4" thick. I dry these in a warm, dry area without restraint. If the wood has any tendencies to warp or twist, they will show up quickly. I have had some wood samples from good trees that dried almost perfectly flat without any restraint. When I milled the logs and air dried the lumber, it also dried without warp.
