THE SCOURGE OF WOODWORKING
Case hardening. What is it? This occurs when the wood has been kiln-dried on an overly aggressive schedule. Essentially, the process is rushed: they crank up the heat, drop the humidity, and speed up the drying to push more lumber through the kiln and to market faster.
This results in an imbalance in how the board dries. The outside dries quickly, causing those fibers to lose moisture and shrink. However, the inner core of the board hasn’t dried enough to keep up, so the fibers inside remain swollen with moisture. The rapidly shrinking outer shell can’t shrink further because it’s constrained by the wet inner fibers. This creates a lot of tension on the outside of the board and compression on the inside.
Eventually, the inner core begins to dry, but by now, the outer shell is already set. As the inside of the board tries to shrink, it can’t pull the outer fibers inward to accommodate its reduction in size. This reverses the stress: the outside of the board is under compression while the inner fibers are under tension, and this imbalance leads to problems.
How Case Hardening Affects Your Wood
This problem is particularly common with pine, especially construction lumber. If you’ve ever ripped a 2x4 and found that the kerf closed, pinching the back of the blade, you’ve likely experienced case hardening. But it’s one thing when it happens to a $5 2x4. It’s another thing entirely if you’ve spent money on a pile of expensive hardwood that’s now useless.
Case-hardened wood doesn’t just cause the saw blade to bog down or leave scorch marks. It can lead to warped or bowed boards after you try to cut them straight. If you’re resawing the wood into thinner pieces, you might find that it cups immediately after cutting. In some cases, you might crosscut a piece only to discover cracks inside the board that weren’t visible before. All of this can quickly ruin your day—and your project.
Spotting Case Hardening
Case hardening is a serious defect that, sadly, isn’t that uncommon. Fortunately, there’s a simple way to check your boards for case hardening before you begin cutting them into project parts. Here’s what you can do:
Cut a small piece from the end of the board.
Make a kerf through the center of the end grain. If you don’t have a bandsaw, a handsaw will work just fine for this.
The goal is to remove some of the core material so the outer shell of the board can release its built-up stress. If the board has been properly dried, the remaining outer layers should remain straight. But if the board is case-hardened, the outer layers will bend—either inward, closing the kerf, or outward, widening it.
You might notice the results immediately, but I recommend giving the board some time to show the full effect. A full 24 hours of air drying should do the trick. Alternatively, you can speed up the process by microwaving the pieces for 20 seconds and allowing them a few minutes to cool down.
What to Do Next
Just because one board is case-hardened doesn’t mean the entire pile of lumber will be affected. It’s a good idea to repeat this test on other boards as well. If you find defective wood, take it, along with your test samples, back to the dealer for a refund.
If the dealer looks at you like you’ve lost your mind, don’t mention that you learned about this issue here—it probably won’t help your case. Instead, I’ll include a link below to a document from the U.S. Forestry Service that explains case hardening, how the test works, and why it’s a defect that you shouldn’t have to pay for. Print it out and take it with you.
One thing to remember is that case hardening happens during the kiln-drying process. If you’re polite and approach the dealer in a respectful way, you’ll have a better chance of getting a refund. Simply explaining that case hardening happens at the kiln will work better than accusing the dealer of selling bad wood. If the dealer operates their own kiln or if this is a persistent problem with the source of their lumber, you may need to consider shopping elsewhere in the future.
Conclusion
All of us encounter case-hardened lumber from time to time, but few woodworkers truly understand what’s happening and what they can do about it. I hope this post helps you identify the issue and deal with it when it arises. Happy woodworking!
Link promised in this video:
US forestry service article: https://www.fs.usda.gov/treesearch/pu...
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