So two comments about case hardening same day, I'll step up to those here directly.
Construction is on hold for now because my charity cord is homeless. Every year I stack one cord of the 8-10 that I process to give away. You would think giving away a cord of seasoned wood in Alaska in December would be easy. Usually the men's group from my church is here within a week when I tell them the cord is ready, with a big trailer and a bunch of guys and away it goes. It is still here this year and it is in the way of my new construction project. I guess everyone has enough wood this year ;-) In the fullness of time, in the fullness of time.
In the meantime, case hardening.
When I open an air dried piece of primo seasoned birch, I expect to find about 16% MC down close to the bark half way between the ends of the split. Smack in the middle of a freshly split face, maybe 14ish %, and around the edges of the freshly split face, 12%.
Like in this picture:
So lets back up and think about case hardening. In our lifetimes, 1900 to present, the most common place to see the words "case hardened" was on a padlock. Like on our lockers in high school. What the lock makers were doing was building the locks out of relatively inexpensive steel, and then case hardening the surface of the lock. It is a relatively simple, repeatable heat treatment that makes the surface of the metal bar, or the lock shackle much harder than the original steel was.
Pre 1900 case hardening was used to strengthen firearms. Even in the mid 1700s the lock parts of a flintlock would be case hardened before final assembly so the rubbing surfaces wouldn't wear each other out as quickly. Hard on hard instead of soft on soft. On civil war era revolvers, case hardening is the process (using bone chunks and charcoal in a small furnaces) that causes the mottled colors on the receivers.
Case hardened wood is the same concept - a heat treatment that hardens the surface - but the end result is water (potentially a lot of it) trapped inside a board that feels dry on the outside. As above.
It sounds to me like FionaD got into some case hardened cord wood splits in this thread,
https://www.hearth.com/talk/threads/seriously-thinking-about-giving-up-on-burning-wood.149234/ , specifically:
FionaD said:
I've measured some 'kiln dried' splits up to 45% at their core!
In general, any split that explodes like popcorn inside my stove when water trapped inside flashes to steam is a bad thing. Likewise, any water trapped inside above the fiber saturation point is a bad thing.
Using the general variables L sub r, L sub t and L sub l to represent the movement of water radially, tangentially and lengthwise, the quickest way to get to fiber saturation point is to maximize L sub l, lengthwise motion of liquid water through the existing tubules via capillary action. Once the tubules (vertical in tree and lengthwise in the split) are empty, all further drying is by diffusion and the lengthwise water loss is no longer 10x the radial and tangential - because it is all diffusion now. The cellulose fiber is all still saturated with water, but the tubules are empty. It's the capillary action that makes L sub l so fast.
So once I get to FSP in my kiln all further drying will be by diffusion, capillary action is done and case hardening is no longer a significant concern. That is to say once capillary movement is done, it doesn't matter how small the tubules shrink at the ends of the splits in a hot kiln because all further drying is via diffusion anyway.
Cranking the kiln up and shrinking the ends of the tubules before capillary action is finished is where the risk of popcorn splits arises.
