Case Hardening & Burns!

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Forum topic by tturner posted 07-08-2013 11:34 AM 3206 views 0 times favorited 11 replies Add to Favorites Watch
View tturner's profile


63 posts in 2026 days

07-08-2013 11:34 AM

Topic tags/keywords: saw table saw walnut

We’ve all experienced it. Getting a nicely figured piece, ripping in to it only to get burns and warping/twisting-case hardened lumber, which, as I understand, is usually the result of poor drying. But, is it really? Lumber processors are probably just like any other manufacturer today-do it in mass quantities as fast as possible to reduce labor costs, beat budgets, etc etc. The faster lumber is dried, the more likely case hardening can occur-as I have read and heard. And, the bad part-there is no way to know if the piece youre looking at is CH.

The species I use most are Walnut, Oak, Maple, and Jatoba. All of these tend to bend, warp, and twist after being ripped.

My question is, What do you guys do to prevent, or avoid, or work with case hardening?

I use a 24 tooth Freud glue-line rip blade. It is a very good quality blade that leaves a very good, glue-ready finish. What do you guys use? If i use a 20-tooth will I not get the twist and warp? This is one of those nuances of woodworking I am sure and I just want to get a little better handle on it. Thanks for all the info here!


-- I'm him

11 replies so far

View Charlie's profile


1100 posts in 2283 days

#1 posted 07-08-2013 11:39 AM

My personal take on this is that you’re not going to somehow relieve the tension in the wood, which is what causes it to warp, bend, twist after cutting, by changing your blade.

Wood is wood.

View fredj's profile


186 posts in 1815 days

#2 posted 07-08-2013 12:02 PM

I haven’t heard the term case hardened wood before. Over heating wood while drying ? You might try getting your wood from another source, or letting it stabilize to the humidity and temp in your shop for a few days before you rip it.

I agree with Charlie, using a different saw blade isn’t going to make any difference. If there is tension in the wood, it’s going to move. I’ve gone through a great deal of oak, walnut, and maple. I’ve seen your problem rarely, but most often with 8/4 and 12/4 hard maple.

-- Fredj

View Roger's profile


20928 posts in 2801 days

#3 posted 07-08-2013 12:26 PM

I’ve been riping some Walnut that is over 25 years old, and was air dried in a barn. A few boards have bent a little. It all depends on the grain, I’m sure. No saw blade in the world is gonna stop this from happening. It’s just the wood movement… .02

-- Roger from KY. Work/Play/Travel Safe. Keep your dust collector fed.

View treaterryan's profile


109 posts in 2284 days

#4 posted 07-08-2013 07:45 PM

Case Hardening is real. It happens when lumber is dried to quickly (generally in a kiln) and the outside surface dries, shrinks, and compresses in on the wet interior of the lumber. This applies stresses to the lumber that you generally cannot see until you work it. When you expose the more damp interior, warping, twisting, and every kind of undesirable motion will occur. This is the reason air dried lumber is so sought after. Rarely can you case harden air dried lumber. Case hardening is more likely to occur in thicker timbers than 4/4. Almost all lumber has some form of light case hardening, and you can see that when you plane a piece of lumber, let it sit overnight, and go to the shop the next morning to discovered a slightly out of square work piece.

As for working with it, good luck. It will do what it wants to. Plane it and joint it to rough dimensions and let it sit for a few days before planing to final dimensions. Maybe try to find a new supplier who air dries lumber. You can not fix case hardened lumber. Even if the interior dries out, the damage has been done and the stresses have been applied (though, I would think somewhat reduced when the whole board met equilibrium moisture content). Properly Kiln Dried lumber will be fine, as they constantly monitor the humidity and drying rates.

-- Ryan - Bethel Park, PA

View SASmith               's profile


1850 posts in 2984 days

#5 posted 07-08-2013 10:44 PM

You can test for case hardening, but not before you buy.
It is called the “prong test.”

-- Scott Smith, Southern Illinois

View REO's profile


928 posts in 2071 days

#6 posted 07-09-2013 02:28 AM

when I worked in the mill they called it casing the wood not case hardening. It is a prcess where steam is introduced at the beginning of the drying cycle. This elevates the temp of the charge without drying to reduce stress and ultimately checking. It does affect the final outcome of the cells when drying is complete. boards appear a bit more ridged. Improperly done I would suppose it could cause what has been described previously but I think the natural lay of the grain has much more to do with it. Even working with stress relieved metal products close attention has to be paid in order to maintain a flat surface when machining.

View Don Broussard's profile

Don Broussard

3547 posts in 2249 days

#7 posted 07-09-2013 02:39 AM

When I first saw the title, I thought it was a discussion about a law firm. But I did learn about a new condition from kiln-drying wood. Regular LJ visits are a part of my higher education now.

-- People say I hammer like lightning. It's not that I'm fast -- it's that I never hit the same place twice!

View TopamaxSurvivor's profile


18269 posts in 3673 days

#8 posted 07-09-2013 02:44 AM

I thought it was going to be about case hardening metal on tools and burning handles in ;-))

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View richardwootton's profile


1699 posts in 1952 days

#9 posted 07-09-2013 02:03 PM

I remember reading on here (I think) about a member taking his QS white oak to a kiln that basically ruined his wood. Poor kiln operation was the culprit and case hardening destroyed some beautifully flecked white oak.

-- Richard, Hot Springs, Ar -- Galoot In Training

View Grandpa's profile


3259 posts in 2673 days

#10 posted 07-09-2013 04:35 PM

I am like many of the others and thought this applied only to metals. I went to Wikipedia and read what they had to say. If he is right it is a condition that occurs when the wood is dried as a log in most instances. Then REO comes in with his “casing” terminology. That made me think of when we baled alfalfa and we waited for it to “case-up”. This was when the dew came on it in the night time and the moisture made the leaves stay on the stems. If they fell off you baled sticks. If you waited too long and it got too wet then it would spontaneously combust in the bale. Most interesting when it is related to wood. I think when you cut into any larger piece of wood (and some smaller pieces) the internal stress is going to rear its ugly head and you have a problem. Just my experience.

View Nomad62's profile


726 posts in 2955 days

#11 posted 07-11-2013 11:02 PM

treaterryan has it right, but falls a bit short… after the outside wood is hard and develops its (incorrect) shape, the center will dry out but will be held in its wet shape by the case-hardened outer shell. When it dries anyway, the center will pull apart at small levels, causing “honeycomb”. All of this is bad, and is caused by a kiln operator pushing for dollars. Getting wood from 10% to 8% can cost a lot of money, more than going from 15% to 13% due to the need to more forcefully pull the moisture from the center; and these percentages are the average of the load, not necessarily of a given piece. The outside may read 10%, while the inside is still 20% and a seller will call it kiln dried to 10% while really it averages to 15%. Money is the bottom line, and if they can get a days more drying time out of an adjustment, they will often try. Once the wood is dry to their specs, and found to be case hardened, they will inject steam into the kiln again to remoisten the outer shell and relieve the stress; this does not help the honey combed wood. There are lots of people with kilns that have little knowledge of what they are doing. If you have continuing problems with your wood I’d think it a good time to find a distributor with a different kiln operator. Oak is the worst, I’m surprised you have problems with walnut.

-- Power tools put us ahead of the monkeys

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