Cupped Walnut Slab

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Forum topic by CaseyJones posted 03-12-2012 04:05 AM 10102 views 0 times favorited 5 replies Add to Favorites Watch
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7 posts in 2302 days

03-12-2012 04:05 AM

Hi All,

Does anyone have any advice on dealing with this issue? I’m assuming it’s pretty common, but I’m a novice. I bought a real nice 2” thick Black Walnut live edge crotch slab to make a desk out of. It had been kiln dried but I don’t have a moisture meter to verify MC. It’s been in my house over the winter while I’ve dealt with making the base (metal). I haven’t had much time, and there’s a learning curve with metal fab for me so it’s been there a couple months. It’s been laying flat on saw horses at the eventual height of the table so I can play around with the legs and how they look. I heat with wood and the room is small so it’s likely pretty dry in there. One end of the slab has about 1/2”-3/4” worth of cupping going on now that I’m ready to put the legs on.

My questions are:

Is there a way to remove this cup by introducing moisture back into the wood, and stickering it with a lot of weight on top? Or some other trick I don’t know of? I’d prefer not to sand or plane any more away because it’s pretty much at finished thickness now.

Also, I’m bolting on the metal bases, using threaded inserts in the wood. Do I need to drill out the holes in the metal significantly wider than the bolts to allow the bolts to move side to side with possible expansion and contraction of the wood or is this a non-issue with metal legs? To clarify, the legs are big, solid chunks of C-channel (like half of an i-beam) that will span most of the width of the board and be fastened in on both edges and the middle.

I plan to use multiple coats of Waterlox to finish it on all sides. The slab has some of the most vivid purple color I’ve ever seen and I’d like to preserve that if possible….I was wondering if there are any considerations on using a tung based wiping varnish on walnut and losing some of that purple color to yellowing? Any better suggestions? I don’t have a spray rig.


5 replies so far

View fussy's profile


980 posts in 3049 days

#1 posted 03-12-2012 08:34 AM

Go here to level that bad boy; . I am afraid you will have to level it as it won’t be coxed back easily if at all. Looseing a quarter or 1/2” isn’t a big problem with that chunk. Watch offerman do his thing a settle that piece. As to preserving the purple in it, try offcuts with blo or tung, but both will diminesh it over time. I have heard soy oil is ok for this, but a good soaking with clear lacquer seem most promising. LLet me know how it works, If you want to junk it, I’llcome get it and dispose of it for a modest charge that’s negotiable.

wHILE TOOLONG AROUND fWW’s fine sie, look up articles on finishing by Jeff Jewiit or some others. Read first.

-- Steve in KY. 44 years so far with my lovely bride. Think I'll keep her.

View HalDougherty's profile


1820 posts in 3236 days

#2 posted 03-12-2012 11:18 AM

A router sled is the way to flatten your slab and here’s the one I made. It doesn’t have to be this elaborate to flatten the slab. A simple 2×4 frame with a plywood base long enough to span the slab and frame and some 1” boards on the side to firm up the plywood so the router doesn’t sag and thin the middle too much.

Click for details

But, you can adjust your base to almost level the slab and use it like it is. I’ve had several slabs that I was either too lazy or they were to thin to flatten and after they were “leveled” enough to look flat sitting on the floor, no one has ever noticed they weren’t flat and level. If it’s cupped instead of twisted, just say you hollowed the center so it would be more comfortable. I’ve also sculpted seats like you see in a Maloof style rocker and the bench made from a twisted slab looked great. I carved the seats facing different directions like a love seat. Those two flat places were the only places on the bench that was level and straight. Getting the legs level was a huge problem, but when I finished it was only an inch or so shorter than I wanted. Nobody ever noticed.

-- Hal, Tennessee

View Pete Mohr's profile

Pete Mohr

75 posts in 3087 days

#3 posted 03-12-2012 12:47 PM

Most folks say you can’t flatten a warped board but . . .
I got a 4/4 walnut slab for cheap that was warped and twisted. I decided to experiment. I soaked it, clamped it flat and kept it wet. I re-clamped it a few times adding shims to “over-bend” the warp. After a month of so it started pretty flat. Then I laminated it to some junky, but flat, 4/4 walnut boards to bring the thickness to a bit over 2”. I only had to take off 1/4” or so to get it flat (mostly via belt sander). It’s not pool table flat but it’s close enough for me.

You’re starting with a 2” slab so that might make it tougher to get flat but if your not in a hurry it might be worth a try.

Here’s how mine came out:


-- "Man is so made that he can only find relaxation from one kind of labor by taking up another." -Anatole France

View markplusone's profile


81 posts in 2954 days

#4 posted 03-12-2012 03:54 PM

Cupped slabs…I see them in my nightmares! If its cupping like you say chances are that it is a top of the flitch cut. Where the board comes from in the log and how big the log is determines how it reacts and moves to the addition and release of moisture. The farther from the center of the flitch, or heart of the log, the more movement. Look at the end of the board. If the cup is in the opposite arch as the growth ring, you have taken on moisture. So if you orient the board that the growth rings are sad face direction and the cup is in the happy face direction, you need to take the moisture back down. Probably with some heavy weight on it to help. As for the attachment of your legs heres how I deal with it. I have a tight point at the center of the board (the point closest to the heart. As I go away from that point on either side I add wiggle room. Depending on how wide the slab is and how far I have to go dictates how much room I have to add. So the first from the cent. will have 1/4” the next will have 1/2” (the expansion from the first plus the expansion from the second) and so on and so forth. I would try to deal with the cup by drying it first and then I would worry about flattening it if there were some left over.

-- Dont carry that which you dont hold with.

View Loren's profile


10396 posts in 3647 days

#5 posted 03-12-2012 04:09 PM

Don’t bet on it.

Dovetailed battens on the bottom can control cupping.

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