# Walnut slab cupping after flattening?!



## Tishman (Mar 8, 2016)

Hi all,

I got some great walnut slabs from a friend and am working on them for some coffee tables. Pretty big stuff, maybe 60" long, 24" wide, and 2"+ thick. Some big checks and splits, but working around those with some bow-tie keys/epoxy/etc. I built a router flattening jig and got both sides nice and flat. A couple days later there is not a pretty noticeable cupping in the center of the piece. I checked and moisture level is around 7%, so this seems strange that this should be happening. Anyone have any advice/thoughts? Help!


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## RRBOU (Feb 15, 2012)

I am guessing that you laid them on the bench, the under side did not get any air and this is why thy cupped. It is very important to after removing thickness to let air get to both sides.


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## Unknowncraftsman (Jun 23, 2013)

It could be several reasons.like RRBOU mentions they didn't acclimate evenly.
Or it could be the slab has tension in it.If you look at the sides can you see the way the grain runs.Does it match the cupping.
Or is there a oval cathedral grain on the surface that matches the cupping.
These are the most common one that I find.

Aj


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## gargey (Apr 11, 2016)

Higher than usual neutrino shower last few days could have contributed.


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## soob (Feb 3, 2015)

2" lumber that shows 7% on the outside might be 20% in the center. Thick slabs take a long time to dry.


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## rwe2156 (May 7, 2014)

Big checks and splits indicate internal stress.

or what he ^ said.


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## Tishman (Mar 8, 2016)

Thanks, everyone!

@RRBOU you are correct. I flipped it and it seems to be flattening back out. If it flattens back out, can I continue working on it (belt sanding, finishing, etc.) and just leave it on its edge when not working it?


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## WDHLT15 (Aug 15, 2011)

Yes, you need to let it get air on all sides. I suspect that the slab is not as dry as you think it is, as a dry slab would not cup.


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## Mahdeew (Jul 24, 2013)

Instead of putting it on its edge, put some 2×4s under it.


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## dbray45 (Oct 19, 2010)

One of the problems with walnut is the stress. I have had blades stop from the stress locking on the blade and pieces fly off when cutting. Watch the grain carefully.

I had a local mill (he moved away) that kiln dried everything. For the big stuff, he would let it sit for 2 years - covered from the elements and stickered to dry before putting it in the kiln. Since he moved away, all wood that I get, I let it sit in my basement for 2 months with a dehumidifier running 24×7. This has worked well.


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## dbray45 (Oct 19, 2010)

If it doesn't flatten out, and you are not in AZ, lay it on the ground, cup side down for a few hours. It will pull moisture from the ground on that side.

If it is white oak the rules are different. This wood can take up to 4 times longer for slabs to dry.


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## Tishman (Mar 8, 2016)

Thanks everyone! Slab has started to flatten back out. Will keep it on 2X4s on the bench while working on it, installing bowties, sanding, etc. Appreciate all the advice!!


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## KelleyCrafts (May 17, 2016)

> If it doesn t flatten out, and you are not in AZ, lay it on the ground, cup side down for a few hours. It will pull moisture from the ground on that side.
> 
> If it is white oak the rules are different. This wood can take up to 4 times longer for slabs to dry.
> 
> - dbray45


Cause in AZ our wood is dry 10 minutes off the tree. Instant kiln dried wood here!


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## dbray45 (Oct 19, 2010)

Yep - setting it on the ground in the sun will scorch it.


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## Tishman (Mar 8, 2016)

Thanks again, everyone. I reflattened both sides. It's at about 13% moisture content. Should I put it on 2×4s and clamp it to the workbench so it won't cup or what? Please advise!


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## logboy73 (Jul 23, 2009)

You need to stop working on it and wait until the dang thing is dry or you'll just be chasing your tail. It will continue to just cup and warp until it finally reaches 6-8% moisture.


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## newwoodbutcher (Aug 6, 2010)

What he said


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## daddywoofdawg (Feb 1, 2014)

sticker it and put it away for a couple months.it needs to dry,and relax after you flatten it.


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## unbob (Mar 10, 2013)

> One of the problems with walnut is the stress. I have had blades stop from the stress locking on the blade and pieces fly off when cutting. Watch the grain carefully.
> 
> I had a local mill (he moved away) that kiln dried everything. For the big stuff, he would let it sit for 2 years - covered from the elements and stickered to dry before putting it in the kiln. Since he moved away, all wood that I get, I let it sit in my basement for 2 months with a dehumidifier running 24×7. This has worked well.- dbray45
> 
> This is worth repeating, I have been working some really nice local walnut. I have had some incidents where pieces have broken off, and kicked back out. Best not to stand in front of the blade!


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## Gene01 (Jan 5, 2009)

> Cause in AZ our wood is dry 10 minutes off the tree. Instant kiln dried wood here!
> 
> - ki7hy


You said it. Where are you?


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## Tishman (Mar 8, 2016)

Understood and thanks for the advice. It's in the attic, where it's about 20% lower humidity and 20° hotter than the rest of the house stickered and weighted. I'll check in periodically. Thanks everybody!


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## Papa3 (Oct 18, 2020)

New to woodworking so any help appreciated. I'm making cutting boards and charcuterie boards and having issues with cupping. I am using slabs that have been dried but that's all I know. They were 2" and I took them back and had them re sawn to 1" give or take. What do I need to be aware of. I am using black walnut, cherry, and rainbow poplar. Any help appreciated!


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## therealSteveN (Oct 29, 2016)

> New to woodworking so any help appreciated. I m making cutting boards and charcuterie boards and having issues with cupping. I am using slabs that have been dried but that s all I know. They were 2" and I took them back and had them re sawn to 1" give or take. What do I need to be aware of. I am using black walnut, cherry, and rainbow poplar. Any help appreciated!
> 
> - Papa3


When wood is cut it dries quickest at it's ends, then it follows across the side, and face grain. Last to go is the interior wood which even if done in a kiln, may not dry as completely as the exterior. Air dried is even slower to dry, unless evidently you live in Arizona 

So when you cut a board open, especially if you open it lengthwise you are getting to moister wood in most cases. Probably not dripping wet, but often wet enough to actually feel it if touched right after you resaw it for thickness.

Because of this, if you can, it's best to remove wood from both sides in an attempt to "balance" this moisture. Then right after cutting, to lay it out on some stickers so air can flow all around the wood. The OP had an issue where he laid it flat, and the side down didn't get air, and the board cupped. Fortunately the fix is flip the board, and allow air over the entire board to balance the drying. Allowing time for the wood to acclimate is the next step.

Some folks put a piece of wood, sticker it, another piece of wood, sticker that, and build a pile, then clamp the entire pile to try to keep any movement from occurring. I do not, I just sticker, and most of the time end just fine.

For your boards, it's a fact smaller pieces dry faster, and sometimes more completely. If you cut your strips from the resawn slabs, cut them oversized, so you have stock left OVER what you plan for final size. Allow that wood to dry for several days, to a week, stickered is good. Then when you are close to glue up, joint, plane and cut to final size, and glue right after you prep. Cutting them more, and allowing them to sit, and you will likely see more minute movement.


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## AlaskaGuy (Jan 29, 2012)

Never in my 76 years have I seen a piece of wood @ 6-8 %. Unless of course it just came out of a kiln. I have wood in my shop that's 20 years old and none of it gets to 6-8% .

Wood is hydroponics kind of like a sponge. It will adjust (in time) to the environment its store in. Right now the humidity is 95 percent outside and 43% inside.

About the best I've seen in my shop it 10-12%. I build with that all the time. One last note, maybe my moisture meter is off.


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## ibewjon (Oct 2, 2010)

Take it to Prudhoe Bay for the winter. When I worked there, the winter humidity was less than 5%.


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## CaptainKlutz (Apr 23, 2014)

> Never in my 76 years have I seen a piece of wood @ 6-8 %. Unless of course it just came out of a kiln. I have wood in my shop that s 20 years old and none of it gets to 6-8% .
> - AlaskaGuy


Location is everything.
Have 7% RH this afternoon in Arizona. Single digit RH is normal here. Was 22% RH this morning. Most of my stored wood measures 4-6% or less 9 months of year. Only during monsoon season, or Dec/Jan cloudy/gloomy season will it climb higher. Have some air dried slabs I checked a couple weeks ago by cutting off a slice; measured 5% on outside and 9% on inside.

Most sawyers in AZ keep fresh slabs in shade for 6 months before using kiln to equalize the moisture levels and kill bugs, as kilns get too hot to fast. Don't dare put fresh cut oak in AZ sunshine kiln, it cracks/splits and surface hardens; making it firewood in week or two.

Reference materials that might help:
https://www.wood-database.com/wood-articles/wood-and-moisture/

Chapter 4 of USDA lumber handbook: https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_04.pdf

Entire Wood Handbook if you want all 15MB of PDF file:

Cheers!


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## therealSteveN (Oct 29, 2016)

Location, location, location. In SW Ohio I would think average for wood is probably around 11%, higher in the Summer, lower in the Winter. But what AG said is gospel. Wood is Hydroscopic, and even cut down the cells can still drink it up.


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