Furniture Making

You are not logged in. [ Login ] Why log in
(NOTE: Login is not required to post)

casehardened chair seat

Elia Bizzarri


Not sure if this is the correct forum, but I'll try to be brief.

I just carved a Windsor chair seat out of a 20" wide 8/4 walnut board. The board turned out to be casehardened and cupped towards the excavated side of the seat. Tried spraying the concave side of the seat with water, waiting till it flattened out and putting it in my little light-bulb powered kiln at 120 degrees, with minimal success.

I know that kiln operators can relieve casehardening in their kilns some way, but not sure how that works or if I can reproduce that in my little low-tech kiln.

Any help would be appreciated,

6/2/15       #2: casehardened chair seat ...
Dave Nauman  Member


It's always been my understanding that case hardening is caused by drying wood too fast, and kiln drying will do that if it's not well air dried first. Maybe steaming or soaking the seat would reverse it, but I've never tried it. I'd stick with air dried material for slab seats.

6/6/15       #3: casehardened chair seat ...
Gene Wengert-WoodDoc

Case hardening is a stress issue, so you can try to relieve the stress. The side that you machined has had much of the stress relieved, as you removed the casehardened wood. So, it is the bottom side that has the stress that is causing the warp. What we want to do is have this side try to expand rapidly with the addition of water. It cannot do so, however, and so it develops compression set (technical term) that the offsets the tension set that developed early when the wood was drying fast and could not shrink enough. After wetting, dry the water slowly. Ok?

6/6/15       #4: casehardened chair seat ...
Keith Newton

Gene, When you said, "It cannot do so, however, and so it develops compression set (technical term) that the offsets the tension set" I was wondering if he should have the seat under pressure from clamps from the sides, as well as pressing the cup out? I've never tried this, so don't know.

However, I would like to ask how Elia came to the conclusion that the wood was case hardened, and what the MC is, and how it had been stored?

My first attempt to flatten, would be to place it out in the Sun, with the convex side up for an hour or two while preventing any airflow to the other, if the wood possibly had gained moisture after it had been dried.

6/6/15       #5: casehardened chair seat ...
Gene Wengert-WoodDoc

Casehardening is a stress that results in immediate warp when machining, while moisture content issues cause warp over time.

To get rid of the stress or to offset it, we need to create the opposite type of stress and a lot of it. Note that Casehardening stress is only in the outer portions of the wood, so trying to bend it flat will not work for this reason as well. Further, once dry, the wood is so strong that takes twice as much bending to correct the stress. It is much easier to just bend or try to bend the outer shell with water instead of the entire piece. Ok?

6/6/15       #6: casehardened chair seat ...
Elia Bizzarri

Great questions and answers!

First, about the board. It was bought a month ago, prior to which it had been stored in a shed, then upstairs in my shop.

I excavated the seat and it immediately cupped, the excavated side of the board going concave. I tried the sunlight trick, without any permanent effect. I then made a fork test (correct term?) on a scrap and the forks bent in and have stayed that way for a week now.

After I posted my first query here, I found this article: <
P>I screwed battans to the seat, figuring that if I wet only one side of the seat I would simply cup the board, without permanently effecting the tension-set. After heating my kiln up as high as it would go (140F), I put a soaking wet blanket against the convex side of the seat and left it in the kiln overnight.

Left it out of the kiln two days and just took the battans off a half hour ago. Seems to have worked. Need to do it again to one end where the screws popped loose, but assuming it doesn't move, I think it is a success.

Question: What effect does the heat have on the process? I know heat plasticizes lignin, but does that help with these cross-grain issues?

A somewhat unrelated question: Are there any sources for info on what happens to wood in a kiln that could make it seem brash and hard to work with hand tools?

Thanks for your help!

6/6/15       #7: casehardened chair seat ...
Gene Wengert-WoodDoc

The wood cells that are casehardened are only the outer layers, maybe outer 20% of the lumber. So, if you machine one side, you remove the Casehardening on that side and now the stresses are unbalanced...that is, the bottom in machined side still has all the casehardening stress. That is why working on the machined side does not help as there are no big stresses on that side. We can relieve the stress on the back side by planing or routing that side, or like is done with flooring, cut several grooves in the backside, removing some stresses.

The forces are very large...large enough to bend the piece, as you noted. In fact, if you cut the prong test, but before cutting the prongs measured the length, you would see the prongs, after cutting will be 1/8 to 1/4" longer and the center prongs will be shorter. It takes a lot of force to stretch wood that much.

Now what happens when you wet the un machined side with liquid water, the surface cells want to expand quickly, but they cannot due to the dry, unmoving core. So the outer layer develops compression set or reverse casehardening. This offsets the tension set or regular casehardening.

Heat makes things go faster.

6/6/15       #8: casehardened chair seat ...
Gene Wengert-WoodDoc

Brash wood (that is indeed its name, but some people say the wood is brittle) can be from growth factors...mainly tension wood in hardwoods, but also very slow growth for woods like oak...and from drying factors...over drying (under 6.0% MC)and from high temperature drying (over160 F).

6/6/15       #9: casehardened chair seat ...
Elia Bizzarri

Thanks! That makes sense and confirms what I gleaned from that article. This second wetting of the seat, I think I'll try clamping it to a bench before wetting it and see what happens without heat.

Do you have a recommendation for a book that explains wood behavior related to drying and bending wood? I have read Hoadley's 'Understanding Wood' a number of times, but that only goes so far.

Thanks again for your help,

6/6/15       #10: casehardened chair seat ...
Gene Wengert-WoodDoc

The early chapters of DRYING HARDWOOD LUMBER has what you need. It is in the archives here.

6/8/15       #11: casehardened chair seat ...
Keith Newton

I left out "before wetting" in my question above, "I was wondering if he should have the seat under pressure from clamps from the sides, as well as pressing the cup out? I've never tried this, so don't know. " making it sound as if I was suggesting just clamping it dry.

What I was wondering, was that since there is already compression on the back surface, which wetting would probably increase, wouldn't there be a chance of internal splitting, since tension perpendicular to the grain is normally the weakest property in most woods?

6/9/15       #12: casehardened chair seat ...
Gene Wengert-WoodDoc

You have it perfect...the bottom is in compression. Water will add to that compression. But here is the key...we want to have enough compression in the bottom to exceed the proportional limit so that we get "compression set" which will offset the tension set (which we commonly call casehardening).

Your concern about getting some damage is also correct. If we wet the bottom fairly deeply, then there would be a whole lot of the wood that would be expanding or trying to expand. (Isaac Newton correctly stated that for every force there is an equal but opposite force. So, with lots of compression, there has to be lots of tension.) However, casehardening (or tension set) is only in the outer layer and primarily in the outermost layers...let's say the outer 1/8" to 3/16". That is why planing the top causes so much warp, as you remove a lot of the set from one side. SO...this means that when wetting, whether with steam or water, we only want to wet the outer region and do it quickly. Long wetting (like with a very wet cloth for hours or long, slow steaming for over 12 hours) will not remove stress very well, but will increase the MC above the desired level.

Does this all make sense?

6/9/15       #13: casehardened chair seat ...
Elia Bizzarri

Interesting! I read somewhere that roughly 20% of a case-hardened board was compression-set and somehow I assumed that was from both sides, for a total of 40%.

I bet I've been wetting it way too long!

Any guess for how long I should put a soaking wet sheet on it in a 140F kiln? An hour or two?


P.S. I about had the cup removed, but I put it back in the kiln to thoroughly dry it and all the cup returned. Obviously I shouldn't do that. Any idea what happened?

6/9/15       #14: casehardened chair seat ...
Gene Wengert-WoodDoc

A wet rag technique is not well documented as a process, so it is just a guess but a soaking wet rag might be needed for only 15 to 30 minutes. It depends on how wet, temperature, wood density, amount of casehardening, etc.

Pieces cup (or flatten) for two reasons... casehardening stress and normal shrinkage or swelling. Normal shrinkage caused the cup in drying.

6/9/15       #15: casehardened chair seat ...
Elia Bizzarri

The cup I got when drying in the kiln (and the cup I got from casehardening) made the side of the seat closer to the pith go concave, which is counter to my understanding of drying cup.


6/10/15       #16: casehardened chair seat ...

Elia, in addition to being a brilliant chairmaker you are clearly becoming well versed in wood drying.

The cupping towards the pith is something I have never seen. Like you I use local wood, mainly air dried and occasionally I dry some in my attic or a hot shed.

Let us know what else you learn - do you know whether or not that particular tree leaned when it was growing? There has to be something to account for how it is behaving now.

  • Post a Response to this thread
  • notify me of responses to this topic
  • To receive email notification of additions to this forum thread,
    enter your name and email address, and then click the
    "Keep Me Posted" button below.

    Please Note: If you have posted a message or response,
    do not submit this request ... you are already signed up
    to receive notification!

    Your Name:
    E-Mail Address:
    Enter the correct numbers into the field below:

    Date of your Birth:

    Return to top of page

    Buy & Sell Exchanges | Forums | Galleries | Site Map

    FORUM GUIDELINES: Please review the guidelines below before posting at WOODWEB's Interactive Message Boards (return to top)

  • WOODWEB is a professional industrial woodworking site. Hobbyist and homeowner woodworking questions are inappropriate.
  • Messages should be kept reasonably short and on topic, relating to the focus of the forum. Responses should relate to the original question.
  • A valid email return address must be included with each message.
  • Advertising is inappropriate. The only exceptions are the Classified Ads Exchange, Machinery Exchange, Lumber Exchange, and Job Opportunities and Services Exchange. When posting listings in these areas, review the posting instructions carefully.
  • Subject lines may be edited for length and clarity.
  • "Cross posting" is not permitted. Choose the best forum for your question, and post your question at one forum only.
  • Messages requesting private responses will be removed - Forums are designed to provide information and assistance for all of our visitors. Private response requests are appropriate at WOODWEB's Exchanges and Job Opportunities and Services.
  • Messages that accuse businesses or individuals of alleged negative actions or behavior are inappropriate since WOODWEB is unable to verify or substantiate the claims.
  • Posts with the intent of soliciting answers to surveys are not appropriate. Contact WOODWEB for more information on initiating a survey.
  • Excessive forum participation by an individual upsets the balance of a healthy forum atmosphere. Individuals who excessively post responses containing marginal content will be considered repeat forum abusers.
  • Responses that initiate or support inappropriate and off-topic discussion of general politics detract from the professional woodworking focus of WOODWEB, and will be removed.
  • Participants are encouraged to use their real name when posting. Intentionally using another persons name is prohibited, and posts of this nature will be removed at WOODWEB's discretion.
  • Comments, questions, or criticisms regarding Forum policies should be directed to WOODWEB's Systems Administrator
    (return to top).

    Carefully review your message before clicking on the "Send Message" button - you will not be able to revise the message once it has been sent.

    You will be notified of responses to the message(s) you posted via email. Be sure to enter your email address correctly.

    WOODWEB's forums are a highly regarded resource for professional woodworkers. Messages and responses that are crafted in a professional and civil manner strengthen this resource. Messages that do not reflect a professional tone reduce the value of our forums.

    Messages are inappropriate when their content: is deemed libelous in nature or is based on rumor, fails to meet basic standards of decorum, contains blatant advertising or inappropriate emphasis on self promotion (return to top).

    Libel:   Posts which defame an individual or organization, or employ a tone which can be viewed as malicious in nature. Words, pictures, or cartoons which expose a person or organization to public hatred, shame, disgrace, or ridicule, or induce an ill opinion of a person or organization, are libelous.

    Improper Decorum:   Posts which are profane, inciting, disrespectful or uncivil in tone, or maliciously worded. This also includes the venting of unsubstantiated opinions. Such messages do little to illuminate a given topic, and often have the opposite effect. Constructive criticism is acceptable (return to top).

    Advertising:   The purpose of WOODWEB Forums is to provide answers, not an advertising venue. Companies participating in a Forum discussion should provide specific answers to posted questions. WOODWEB suggests that businesses include an appropriately crafted signature in order to identify their company. A well meaning post that seems to be on-topic but contains a product reference may do your business more harm than good in the Forum environment. Forum users may perceive your references to specific products as unsolicited advertising (spam) and consciously avoid your web site or services. A well-crafted signature is an appropriate way to advertise your services that will not offend potential customers. Signatures should be limited to 4-6 lines, and may contain information that identifies the type of business you're in, your URL and email address (return to top).

    Repeated Forum Abuse: Forum participants who repeatedly fail to follow WOODWEB's Forum Guidelines may encounter difficulty when attempting to post messages.

    There are often situations when the original message asks for opinions: "What is the best widget for my type of shop?". To a certain extent, the person posting the message is responsible for including specific questions within the message. An open ended question (like the one above) invites responses that may read as sales pitches. WOODWEB suggests that companies responding to such a question provide detailed and substantive replies rather than responses that read as a one-sided product promotion. It has been WOODWEB's experience that substantive responses are held in higher regard by our readers (return to top).

    The staff of WOODWEB assume no responsibility for the accuracy, content, or outcome of any posting transmitted at WOODWEB's Message Boards. Participants should undertake the use of machinery, materials and methods discussed at WOODWEB's Message Boards after considerate evaluation, and at their own risk. WOODWEB reserves the right to delete any messages it deems inappropriate. (return to top)

  • Forum Posting Help
    Your Name The name you enter in this field will be the name that appears with your post or response (return to form).
    Your Website Personal or business website links must point to the author's website. Inappropriate links will be removed without notice, and at WOODWEB's sole discretion. WOODWEB reserves the right to delete any messages with links it deems inappropriate. (return to form)
    E-Mail Address Your e-mail address will not be publicly viewable. Forum participants will be able to contact you using a contact link (included with your post) that is substituted for your actual address. You must include a valid email address in this field. (return to form)
    Subject Subject may be edited for length and clarity. Subject lines should provide an indication of the content of your post. (return to form)
    Thread Related Link and Image Guidelines Thread Related Links posted at WOODWEB's Forums and Exchanges should point to locations that provide supporting information for the topic being discussed in the current message thread. The purpose of WOODWEB Forums is to provide answers, not to serve as an advertising venue. A Thread Related Link that directs visitors to an area with inappropriate content will be removed. WOODWEB reserves the right to delete any messages with links or images it deems inappropriate. (return to form)
    Thread Related File Uploads Thread Related Files posted at WOODWEB's Forums and Exchanges should provide supporting information for the topic being discussed in the current message thread. Video Files: acceptable video formats are: .MOV .AVI .WMV .MPEG .MPG .MP4 (Image Upload Tips)   If you encounter any difficulty when uploading video files, E-mail WOODWEB for assistance. The purpose of WOODWEB Forums is to provide answers, not to serve as an advertising venue. A Thread Related File that contains inappropriate content will be removed, and uploaded files that are not directly related to the message thread will be removed. WOODWEB reserves the right to delete any messages with links, files, or images it deems inappropriate. (return to form)