Hairline Cracks in Kiln-Drying Cherry

      Checking in Cherry appears as a fine crack, and is the result of too-fast air-drying, not kiln-drying. April 15, 2012

Question
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.

Forum Responses
From Professor Gene Wengert, forum technical advisor:
Any checks in well dried wood are developing in air drying and not in the kiln.



From the original questioner:
But these don't look like typical checks to me, more hairline cracks down the length of the board. Or is that what checks in cherry look like?


From contributor T:
If it's "well dried," and I'm assuming you mean processed correctly, including the air drying, then how can you say it's in the air drying? Without more info I would call it "stresses in wood" and that covers all the steps.

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?



From Professor Gene Wengert, forum technical advisor:
Some species like oak develop large checks and we call them honeycomb. Other species, like maple and cherry, develop fine hairline checks. They are caused by the same event... drying too fast at high MCs.

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.



From contributor T:
Thanks Gene. I understand drying too fast is causing the check and the wood is most vulnerable at the higher MC, but it doesn't mean that it was dried too fast in the air drying stage if the checks are showing after the kilning process, not prior. This leads me to think (in theory) that at the beginning of kilning the air dried wood, it may start out at too fast or too hot, creating the stress/shock.

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.



From Professor Gene Wengert, forum technical advisor:
It is during the initial process (air drying) that the stress in the outside develops. When you reach 35% MC or lower, the surface fibers are in compression and not tension. Some people call this stress reversal. This stress reversal occurs because of tension set (also called case hardening) that develops. It develops because the wood is plastic as well as elastic. (There is a discussion in Drying Harwood Lumber about the four stages of drying that would help your understanding.) That is why it is impossible (without rewetting) to create these checks at MC under 35% MC. At these lower MCs, even if the conditions are severe, the shrinkage must first offset the compression stress that exists. Then, because the outside fibers are nearly twice as strong, the stress would have to be twice as big as when drying first starts. This just cannot happen.

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.



From contributor C:
Gene, if the lumber came from cherry logs that ranged in age from 1 week to 6 months, would the older logs produce lumber at a low MC and have already started checking in the log form? Could the lumber be more susceptible to checking coming from old logs?


From Professor Gene Wengert, forum technical advisor:
If the bark is on, the logs will not lose much MC except at the ends. However, we do know that oak logs check more with 6 months of storage.

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