Checking in Oak
A discussion of kiln conditions and other factors that can cause Oak to check while drying. October 29, 2008
Question
I had a nice oak fall in my woods. We had this oak, possibly red, or hybrid, cut to 7/4. It was then placed in the solar kiln over the winter where the MC went from 29% to 22% in an almost four month time period. We never see the sun here in northern OH during the winter. When it was taken out to dry further in the Nyle, some of the boards had bad checking parallel to the edges. These checks are 4-5" long and very thin. Some boards had none, though. I thought checking like this was caused by drying too fast? A local sawmill operator said it was because the boards were cut wrong, but we blocked the center, and stayed away from the heart of the logs. I dry 8/4 maple all the time without ever seeing checking like this. Luckily there was no money involved, other than my time!
Forum Responses
(Sawing and Drying Forum)
From contributor J:
The mill I grade and saw at pushes out about 65,000 board feet a day. Most of the oak has very fine season check in it. We don't grade against it in the mill, but after it is dried, a lot of the lower grade boards develop the check that you mention. I just thought that it was a characteristic of red oak.
From Professor Gene Wengert, forum technical advisor:
Checking in oak is caused by drying too fast. It could be that this checking is due to weaker wood than normal because it is bacterially infected. The solar kiln is designed for drying 4/4 and 5/4, so it will dry 7/4 too fast, even on warm wintertime days.
Green oak is normally about 75% MC. The fact that you had 29% MC indicates that it dried before you got it into the kiln, as checking occurs above 50% MC. How did you measure 29% MC? If it was with a meter, then the reading is incorrect.
Oak is sawn with all sorts of grain patterns, so sawing is not an issue. However, pieces of lumber that contain the pith will check in virtually all cases. Normally we would not saw 7/4 containing the pith as it would be low grade, and there is no use or market for low grade 7/4.
From the original questioner:
Thanks for the info. The tree was laying on the ground for a while, but I'm not sure exactly how long, as it was on the other side of my property. I have a pin meter that I calibrated with two other moisture meters my friends have. The months we had it in the kiln were some of the coldest, cloudiest of the year. We actually broke records for cold and snow in March. I am not exaggerating when I say we do not see the sun for weeks at a time up here along Lake Erie in the winter. Lots of lake effect snow, and clouds. These were the months it was in the solar kiln. I took them out the first week of April. A few days before I took them out, I plowed a foot of snow out of my driveway! Perhaps with the log lying on the ground, it picked up some fungi or bacteria? There may have even been checking in the boards after cutting, but the sawdust was frozen to the boards, and it was brutally cold out, so we were not looking at the boards very closely. They may have opened up after drying a little? Probably will never know, I guess. Next time a big oak log falls I will use it to heat my house instead.
From Professor Gene Wengert, forum technical advisor:
Pin meters cannot read accurately over 28% MC, so your wood was likely much wetter, but the meter did not read that.
From the original questioner:
I was under the illusion that pin meters were more accurate than pinless types. I should buy myself a new meter then? Also, why would some of the boards check very badly, some with some checking and others not at all? All from the same tree, so you would think they would all behave somewhat alike? Trying to learn as much as I can on drying. One thing I have learned is you cannot make money DH drying thick oak! It takes too long, too much in electricity, and sells low.
From contributor N:
It is my understanding that no moisture meters work well over about 30% moisture. It has nothing to do with the pin or no pin.
From Professor Gene Wengert, forum technical advisor:
Contributor N is correct.
Bacterial problems can cause variation in checking as well as variation in drying conditions, and natural variation of wood.
From contributor B:
With some of the boards being checked and some not, I would speculate that the checks had happened while the tree lay on the ground before it was sawn or drying attempted -perhaps last summer if it were down then? I sometimes do not get to cut my logs for a while after I get them and if I wait too long, I get checks on the boards from the outside of the log.
From the original questioner:
Thank you everyone for taking the time to respond. I learned a lot. The tree fell hard as it was on the edge of the wood line with no other trees to help its fall. The butt end looks like it did not check a lot. The sections above that did. Tree was big enough that it had to be whittled down to fit on the saw.
