Hi Allan. Length warning - only for those interested in the topic. Reading some of the Gene Wengert and USDAFPL material it seems as though the position is pretty clear- and the causes of stress/case hardening in wood are well understood. Going back at least to the 60s, and probably much further. This is another long one - it's an attempt to briefly summarise what's going on. Important to go to the original documents if working in this area - no guarantees here.
The problem in a nutshell seems to be that either due to operating error, or cutting corners/skimping kiln operators don't always properly equalize or condition wood - with the result that it can end up highly stressed.
Wood ends up stressed after kilning because of the moisture gradient that develops during the drying process. As in the outer skin of the wood dries first and tries to shrink, but is prevented from doing so by the still wet and swollen core. This causes the outer shell to 'take a set' in the 'stretched' condition. The core subsequently dries and forces the stretched outer shell to contract around it. Which causes the stresses that cause our problem. (known as case hardening) They can act lengthwise, or crosswise, and produce differing symptoms.
The material below describes fairly simple tests for both types of stress. It involves cutting out a sample of wood to form a forked shape, and observing what direction it distorts in if any. i.e. properly equalized and conditioned wood should be stable. The trick is perhaps to use these tests before working the wood - because it's still possible to stress relieve (condition) it at this stage.
The equalization step is very important, because unless all the wood in the kiln is very close to the correct moisture content the conditioning/stress relieving step won't work consistently. Equalising entails holding the kiln conditions very close to the required equilibrium moisture content (it's necessary to have moisture content numbers for the wettest and driest wood in the kiln to calculate this - the result is a very slow drying step compared to previously when the kiln was hauling ass to get the moisture out quickly), and waiting the time needed for all of the wood in the load to reach the correct moisture content. (the cost of this time is why it gets skimped)
Once the wood is equalized its conditioned/stress relieved by raising the temperature, injecting steam and holding the wood in this condition for a period. Not all kilns or kiln types have this capability - another reason apart from the time why it gets omitted. This conditioning cycle reverses what happened during the main drying cycle - the outer shell ends up wet and hot, while the core is largely unaffected. The result is that the shell now relaxes from it's previously stressed condition, and assumes a state very similar to that of the core.
Short paper describing the issues:
https://www.extension.purdue.edu/ext...NR/FNR-132.pdf
Gene Wengerts summary on Woodweb:
http://www.woodweb.com/knowledge_bas...Condition.html
The USDAFDL drying kiln manual which contains the information on cycles etc:
http://www.esf.edu/scme/wus/document...torsManual.pdf
The processes involved are decribed in the paper, the conditioning and equalizing process is fully described in the manual. (it sets out the kiln cycles and procedures) The temperatures are not that high - 160 - 180 deg F Going much higher seemingly can stablise the wood, but it reduces its strength by degrading the lignin - and causes colour changes.
It's worth noting that the problem (presuming the wood is properly dried and has ended up with equal moisture content throughout) seems very much a consequence of fast/aggressive/commercial kiln drying - the steep moisture content gradient that causes the problem wouldn't otherwise arise. Air drying it seems largely avoids the issue - because it's a slow process, and because high night and morning humidity (not to mention wet weather) have a natural conditioning effect. (they wet the surface of the wood, and prevent in from becoming dry enough in advance of the core to stop stressing/case hardening from arising.
Projecting tentatively from what the professionals have to say, it seems that most woods (but they vary a good bit in the time required) become quite plastic when heated. The steam/moisture seems to be used primarily as a means of applying the forces required to cause the wood to de-stress. Hard to know if it would work, but if wood has already been resawn and checks out as being of uniform moisture content and is very thin it might be worth trying heating it (for the time in the equalising schedule), maybe (?) wetting the concave sides and closely sticking it flat under pressure to get the stresses out.
The problem with overdoing the wetting or steaming is that it may raise the moisture content of the wood high enough to become a problem. Against that maybe if it's sticked in warm and dry enough conditions (at the correct EMC) it might not take too long for it to dry again….
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Originally Posted by David Ragan
