Even though I'm using the correct schedule, the last couple of times I dried air dried cherry in my kiln, the cherry had a lot of hairline cracks in it. Not anything like checks in oak, but long, going mostly down the grain. What am I doing wrong? I need to dry some more now, but it is someone else's wood. I will be mixing cherry, walnut and red oak in the kiln. All well air dried.
From Professor Gene Wengert, forum technical advisor:
Any checks in well dried wood are developing in air drying and not in the kiln.
I believe I read in one of your past answers on kilning that a lot of the schedules people are using are for a larger volume setup and they need to be tweaked to their individual size kiln due to heat/air movement/insulation thickness/etc. Wouldn't the same tweaking also be needed for the amount of prior air drying time/temp and humidity ranges/MC for air dried wood being entered into kiln?
As mentioned, the stress that is created when drying too quickly exceeds the strength of the wood. The strength of the wood can be lowered due to the heat, bacteria activity and so on. Obviously, in a posting like the original one made here, none of us are able to evaluate the wood to see if it is bacterially infected, has an unusual growth rate, or other factor that might lower the strength.
In any case, the shrinkage and stresses will be greatest and the wood will be the weakest when it is wet. Hence, the checking is an air drying defect. The wood is too strong and the shrinkage too small with air dried wood that is put into a kiln to develop new checks. Of course, if wood is put into the kiln and then allowed to regain moisture, we might indeed create or worsen checks. I hope this was not done here. The kiln EMC should never be higher than what the lumber has been used to. (We typically measure the surface MC and then start the kiln with an EMC that is numerically a bit lower than the surface MC.)
It is hard to cover a topic completely in a brief posting, but more details and discussion are in Drying Hardwood Lumber.
Wouldn't a slower/longer initial startup help the transition from a slow release state (as air drying has been done) to a faster release of MC under controlled environment (kilning). I think the wood is taking a great shock, making a quick change from a slow to a fast MC release state.
Kinda like a rubberband, if you stretch it a lot when it's fresh (KD), it can handle the changes, but if you let it age (AD) then stretch it fast, it will break (check), but if you gently start the stretching, it usually will handle it.
Incidentally, the idea that wood is a like a rubberband that can be stretched at first but then the band gets brittle does not apply. Wood is not at all like that. Wood actually gets stronger as it dries.
So, fine hairline checks that exist after drying were caused and developed at high MCs, which in this case means during air drying. They do not develop under about 50% MC, although they can worsen a bit.
The wood cell starts shrinking when it reaches 30% MC. In air drying, the average outside EMC is 12% EMC. So the outer fibers are drying more and shrinking three times more than when they are in the kiln and dry from 12% MC to 7% MC. Plus the wood is stronger at lower MCs.