An Example of Case Hardening

[Brian Gustin]

I have seen a few posts (and participated in a few) that mention and describe Case Hardening, but I don't recall seeing any photos or examples. I came across this piece of Firewood (in a box of purchased mixed offcuts) that might be worth posting as an example of what can happen when buying lumber or turning blanks.

In the following photos, the FIRST TWO are what the whole blank (8/4 walnut) looked like before resawing (Ignore the fact it has already been resawn!) - from the outside, there is NO indication at all of case hardening - no checks, cracks or splits and looks like a nice, beautiful piece of walnut .. So I resawed it, thinking it would make a nice box (along with a some Maple lumber I already had) in flatwork.. and the third picture shows the disappointment - consigned to the firewood pile now... so I am glad I didn't decide to try and turn something from it (which would have meant worse disappointment after all that work)

 

[Richard Coers]

When wood is case hardened, it looks like a potato chip when you open up the wet inside. Walnut dries so incredibly easily, I've never heard of it being case hardened. It would have to be a major kiln error to get that to happen. What I see looks more like ring shake, or damage from dropping the tree. Case hardening, in most cases, doesn't show cracks as soon as you open it up. It would also be case hardened over the entire length and be full or cracks from end to end if that is really what happened. And it would definitely not be sitting flat on the bench.

 

[Brian Gustin]

If it was ring shake, it would follow growth rings, these splits and checks go across the rings - It sits flat because outsides had already been planed, I am betting it DOES move and goes wonky if I leave it set a few days.

I have another piece out of the same box that you can see the large split (Looks like a tear in a pair of jeans) on the end grain right in the middle , split goes across growth rings, and only exists on the inside of the wood (the outer surfaces placed flat and "look" nice)
- I have had identical case hardening patches in other species of wood (Maple and Oak for a couple) - basically the inner grain separates as it dries while the outside of the board which dried too fast "locks" it in place - with no room for the wood to move as it shrinks inside, it simply separates exactly as in the pictures - it doesn't follow growth rings like ring shake would... Thus, you'll find those many varied checks and pockets of splits in case hardened wood when you open it up.

 

[Kevin Jenness]

Looks like drying induced honeycombing to me, an extreme example of case hardening where the tension caused by the shell setting with a severe moisture gradient to the core results in the core fibers actually separating. Usually case hardened wood bows when resawn without the internal cracks. I saw this once in a stack of kd 8/4 walnut. The distributor's rep blamed it on thick walnut being hard to dry, which is bogus. The kiln operator just screwed up in a big way.

 

[Brian Gustin]

Looks like drying induced honeycombing to me, an extreme example of case hardening where the tension caused by the shell setting with a severe moisture gradient to the core results in the core fibers actually separating. Usually case hardened wood bows when resawn without the internal cracks. I saw this once in a stack of kd 8/4 walnut. The distributor's rep blamed it on thick walnut being hard to dry, which is bogus. The kiln operator just screwed up in a big way.

Yeah, looking closer at the pictures, it is harder to see , perhaps I should go take some more pictures to get a better "view" of what it actually looks like. in fact, I may take the other piece and do a "before" and several
"after" and cut it cross-cut several times about an inch apart before resawing it (I resaw on table saw, over multiple passes, so any warp that happens while cutting a piece open doesn't bind down on the blade - learned that lesson when I did the same thing with a piece of ambrosia maple that was fine for about 65% of its length until I hit those pockets of case-hardened "honeycombing" as you call it)

Either way I recognize it from having seen it in several other species, and just thought it may be worthwhile for future reference for other users to see actual examples that otherwise look like beautiful turning blanks from the outside, but end up as firewood. Case hardening can happen to ANY wood if it is improperly dried.

 

[Kevin Jenness]

I usually trim lumber back until end checks disappear before milling, generally 1-3 inches. With that batch of walnut you could cut the entire length into 1" slices and still have checks.

 

[Richard Coers]

"these splits and checks go across the rings" Really? I'm having a real tough time seeing that.

 

[Brian Gustin]

"these splits and checks go across the rings" Really? I'm having a real tough time seeing that.

Yep - Photo 1 - I circled a couple most obvious spots in red - Next 2 photos are the other piece , first "as-is" and second after I cross cut a few sections - This one actually the case hardening cracks did not go very deep (as far as I can tell) and actually squared up into a decent bowl or platter blank (which I plan to turn) - Long story short, you never know what you'll find when encountering this sort of drying damage.

Photo 1: The above referenced board, I cross-cut through the case hardening splits to get end grain shots (since those splits didn't show through on the end grain of the whole board - at all)


Photo 2: This is the "other" board I mentioned that shows obvious case hardening check on end grain - before I went and did some cross-cutting into the board.


And, photo 3 - same board (next to the cross cut sections I cut away, turned out to what appears an otherwise decent turning blank... )

 

[Gerald Lawrence]

Take a look at this article for both honeycombing and case hardening
https://www.rpwoodwork.com/blog/2019/03/17/wood-woes-the-dread-honeycomb/

 

[Brian Gustin]

Nice link, Gerald. All I ever knew it as was Case Hardening, Honeycombing is a new term to me, but even then, Honeycombing appears to be a *symptom* of case hardening (as in, you don't get Honeycombing without Case hardening, right?)

I actually live diagonally across the street from a lumber mill (they specialize in what amounts to spindle turning blanks, and rounds/dowels, so all their lumber is pretty much 8/4 to 12/4 to 16/4 and they have their own kiln. I frequently wander over to grab some offcuts when I want Cherry or Maple spindle blanks...)

 

[robo hippy]

Worst ever honeycombing I ever saw was a big Madrone burl that had dried for 20 years in the shop. Outside looked fine. Inside was shattered. Some times that is from drying too fast. Some times it is just the wood and it would have done that no matter what. Walnut is fairly easy to dry, so I would guess that they tried to dry that stuff too fast.

Brian, in your first/top picture, the 2 cracks that are lined in red would be normal shrinkage related cracks. The 2 in the center could be related to ring shake since they point along the rings, but they look more like shrinkage/stress cracks, and again, I would guess they tried to dry it too fast.

robo hippy

 

[Bill Boehme]

I’m not convinced that the defects are casehardening. The pictures aren’t close enough to show fine detail, the focus seems to be a bit soft, and I think that brighter lighting (sunlight) would also help to improve contrast. The description also makes me think that casehardening may not be the problem. Here is a very good article about casehardening on the Wisconsin Department of Natural Resources website.

 

[Brian Gustin]

I’m not convinced that the defects are casehardening. The pictures aren’t close enough to show fine detail, the focus seems to be a bit soft, and I think that brighter lighting (sunlight) would also help to improve contrast. The description also makes me think that casehardening may not be the problem. Here is a very good article about casehardening on the Wisconsin Department of Natural Resources website.

Don't know what else to call it - this sort of defect has always shown for me in Kiln Dried lumber only, but like you said the pictures leave a lot to be desired (taken with cell phone camera with flash) - when I searched the web when I first encountered it, only thing that came close to describing what I was seeing (I think it was over at lumber jocks) called it case hardening, which is where I learned HOW that happens, after reading up more on the several articles from several states (and universities)

My main point is, the fact that this defect only shows up when you start cutting into the wood - the defect was not evident *at all* on any of the 6 surfaces of the board before it was re-sawn, so it wasn't simple checks from drying, definitely not natural flaws from cutting, wind, or lightning, so don't know what else to call it.... - for the second board (in later photos) the reverse happened - The defect was only about 4 inches into the board before it was cut away entirely (and leaving me with what so far appears to be a good bowl blank)