Cupped Slab help

[Brendan Prichard]

Hello all! This is officially my first post/thread in the sawmill community. While I have read and gained much knowledge over the past couple of years, by simply reading existing material, I finally came across a problem I couldnt seem to figure out without asking specific questions.First let me say that I am relatively green in the wonderful world of woodworking. I am completely self taught, but have been progressively devoting more and more of my time, over the past 3 years, to this craft. On to my questions! Over the past year I have made a few live edge slab cheeseboards as gifts. I have exclusively used walnut slabs to date. I live in Jacksonville, Florida, where there very few retail outfits selling anything other then pine, cedar, poplar or red oak, so most of my hardwood lumber is mail ordered, including the previously mentioned slabs. The previous slabs I purchased all measured approx 1"x12"X48", live edge, kiln dried,skip planed, but both very flat. None of the slabs ever moved significantly, only requiring minimal sanding before seasoning. The slabs came from Indiana, and I received them in Dec-Jan of this past year. Everything was great. Well I recently purchased 3 more walnut slabs with almost identical descriptions as the previously mentioned pieces. The only differences are: the new slabs are 3/4" thickness, and they came from Tennessee, and I received them in August, as opposed to December. the were skip planed and flat before leaving Tennessee, but when I unpacked them , the both came out with pretty significant, full length cupping. So my real question is: is cupping usually associated with wood gaining moisture, or losing it? Mind you, August in Florida where I live is pretty brutal. Stepping into my non-climate controlled garage/workshop is equivalent an experience to what Marlow must have felt cruising up the Congo River. It was 96 degrees with 80+% humidity the day I received the slabs. Any suggestions on what to do to help remedy this problem, would be much appreciated. Being that the slabs are only 3/4" thick, I dont really have the leeway to joint them down to flat , especially considering I had planned on incorporating the "live" edge on both sides. Thanks in advance for any replies, I truly appreciate it!

[Sam Babbage]

It's related to change in moisture level either In either direction. If you flatten something when it's at a high moisture content and then it dries out it will cup, the same goes for the opposite (but it will cup in the opposite direction). Wood will also cup if its faces are unequally exposed to a difference in moisture, but that is a related kettle of fish. Quartersawn wood will cup much less than plainsawn, but in Australia at least, all walnut is plainsawn.

[Matt Mackinnon]

What Sam has said is mostly right, but he has skipped a few important bits of the explanation. The grain of the wood has a direct effect on the warping of the wood. As you will know, a tree grows in rings around the center pith of the tree, so how the board is cut from the tree really makes a difference. Quartersawn wood has all the grain rings in the board all going in the same direction so as they expand (then they gain moisture) the width of the board will expand but the board will stay mostly flat. But if the board is flat sawn, and you get a piece they goes through just off the middle of the tree, if you look at the edge of the board, you will see the grain rings at the outside edge of the board point up and down, but in the middle of the board they point left and right. And in between the direction slowly changes between the two.

What that does is as the grain expands, it does so width wise and not length wise. but as your rings are going in all different directions, as it expands the direction of the rings will pull the once flat board out of flat as they grow wider.

[Al Bacon]

Hi Brendan

Both Sam and Matt are right on the mark with their descriptions of why the wood cupped. Since it has cupped I think I would allow the slabs to acclimate for at least two weeks in your climate. I would then rip them, joint them, and glue them back together. If you are careful when gluing the joint will be hard to see. This will save most of the thickness of the would as opposed to trying to plane the cup out of the full board width.

[Brendan Prichard]

Thanks guys! I really appreciate the help/information. I am posting a couple of pictures of the pieces. The boards look plain sawn.268643268644 The first two pictures are opposite ends of the same board, the 3rd and 4th are opposite ends of the other piece. 268641268642While I let the boards acclimate for the next couple of weeks, is there anything I can do to minimize the cupping? Would applying light pressure the length of the board to the peak of the bow be an exercise in futility?

[bill tindall]

They cupped because they lost or gained moisture. If they cupped because they gained, exposing both sides to ambient conditions will likely make them flat again. If they cupped because they dried out, in my experience they are likely to remain cupped.

Straightening theory: If the cup is "set" the only way to uncup is to bend the wood straight. To bend wood it must be softened. The least complicated way to soften(plasticize is the technical term) wood is to get it wet and hot by steaming. If you figure out some way to steam the cutting board sized pieces, like in a canning sized pressure cooker, until the interior of the boards is at steam temperature, you can then take the piece out, clamp it flat between some straight sturdy cauls and it will then dry flat and stay that way. It may take a 3-4 weeks of drying. Don't accelerate the drying with heat or the ends will split.

Once I was highly motivated to rescue a critical piece of mahogany that was bowed along its length. It was too long (30") to steam in any container I had. I clamped it so that I forced an opposite bow. The board was now under tension the opposite of the original bow. I then covered the convex side with a wet towel and steamed it with a steam clothes pressing iron. The theory was to soften the stretched wood fibers to enable them to relax and stretch to relieve the bow. It took about a half hour of steaming to get the exposed side of the wood hot and wet. At this point I left it to dry under tension opposing the original bow. It worked to straighten the bow and this piece of lumber is now the top of a Federal wall cabinet.

[Steve Jenkins]

If you are going to let them acclimate don't leave then lying flat with just one face exposed. prop them up on edge so air can circulate all around them. An easy way to hold them up is to use a couple short clamps as a base.

[Andrew Hughes]

Brenden, your end grain work bench is sic.I would love to have one for my living room!

[Brendan Prichard]

Thanks again guys for all of the input/advice! I am definitely going to give it a few weeks for acclimation before I freak out and do anything drastic. I can only hope that being that I live in Florida with the extreme humidity and heat, the slabs are taking on moisture and will eventually(hopefully) settle back down. I truly appreciate the info and expertise you all are displaying! And thanks Andrew Hughes for the comment on my bench. I put in a good portion of this year into it. I have a few more things to add, including leg/end vises but I am happy with it right now. Ill post a few more pics of the top for a better perspective. Thanks again everyone! 268657268658268659268660

[Sam Stephens]

i'm going to offer a different opinion. if I were to buy wood this way (i.e. mail order), I would expect it to be relatively flat when I received it. If it moves, after a week or so in my shop, then it's acclimating to my conditions and is my problem, but not when I receive it. My guess is that it cupped w/in 24hrs of being skip- planed. but was probably packaged up for shipping. Needless to say, I'd want a few words w/ the supplier, and would not use them again if they are unwilling to remedy your situation. Was it KD or AD? Most folks air dry walnut b/c the color is much richer. FWIW, I'm guessing Tennessee is pretty darn humid like the rest of the SE (I'm from GA and now live in NC).

[Brendan Prichard]

Sam, I was kind of thinking along the same line as you. Mainly because of exactly what you just stated, it wasnt sitting in my possession for a week, all the while cupping. I received the package in the afternoon and within an hour I had unpacked it. And it was definitely already cupped right out of the packaging. I tracked the package and it only took 3 days from purchase to delivery, so it wasnt sitting in a warehouse or shipping container for 2 weeks. I did contact the seller and he assured me it was kiln dried, and said he would get back to me with some more specific information on monday. So I guess I will just have wait and see what he says. Thanks for your input!

[Scott T Smith]

Thanks guys! I really appreciate the help/information. I am posting a couple of pictures of the pieces. The boards look plain sawn.268643268644 The first two pictures are opposite ends of the same board, the 3rd and 4th are opposite ends of the other piece. 268641268642While I let the boards acclimate for the next couple of weeks, is there anything I can do to minimize the cupping? Would applying light pressure the length of the board to the peak of the bow be an exercise in futility?

These photo's are very interesting, and the orientation of the growth rings tells an interesting tale. When lumber dries, due to the cellular orientation it is the natural tendency of the board to "try to straighten out the growth rings". In essence, boards tend to cup towards the bark as they lose moisture.

The board in photo 1 cupped because it gained moisture after surfacing; that's why it cupped in the direction of the growth rings. The boards in photo's 3 and 4 lost moisture after surfacing, as in them they are cupped opposite the growth rings. Photo 2 is hard to tell because of the crotch section at the end of the board.

I've seen this occur with boards that were resawn from a thicker plank (which had a higher MC% in the center of the plank versus the faces); however from looking at your ends these were not resawn from the same plank.

I would suggest that you rewet the cupped side with a damp rag (or you can lay it outside in damp grass with the cupped side down on a sunny day), and then stack/sticker/weight the boards after they have cupped a bit the opposite way. It usually only requires 30 - 60 minutes in the yard for this to happen. Often times you can straighten a cupped board back out.