resawing of air dried vs kiln dried stock

[Steve Hubbard]

My resawing experience is limited to 8 or 10 projects. All of it is with domestic species of kiln dried wood - maple, walnut, cherry, poplar. Each piece has bowed considerably as it came off the saw, no cupping, just bowing the length of the board. I now have the opportunity to work with a piece of air dried walnut that is 3 inches thick and 12 inches wide. It has been stickered and stored in a warehouse with good airflow for 3 years. The current moisture content is not known. Location is Nashville, TN. I would like to resaw and bookmatch it to build a 24 inch wide table. Should I expect the same amount of bowing that I have experienced with kiln dried stock, or will the air dried walnut be more stable?
Thank you.

[Dan Hahr]

You never know till you start cutting.

Dan

[Andrew Seemann]

I agree you won't know until you start cutting, but I would expect it to bow along the length of the board to some extent. Cupping isn't out of the question either.

3 inches is pretty thick, and I doubt the center of the board is at the same moisture content as the outside, no matter what season it is, or how long it has been stored. You could luck out and it be close enough for it to not move or not move much, but don't be surprised if it does. Tabletops always move anyways, so your aprons should/need to be stout enough to take out the bow either way.

[George Makra]

You probably will have some wood movement.
Having done a similar table top earlier this year the wood moved a bit when first cut then again when it went from the shop to the house.

If you can cut it to the rough dimensions and then store it inside for a month or so to let it find its final resting shape.

[Steve Hubbard]

Thank you Dan, Andrew, and George. Very helpful replies.

If I were to pull the boards flat will stout aprons, is it likey that the mechanical fasteners would pull out of the top over time?

[Jim Becker]

"How" the wood was dried doesn't really matter..."how well" it was dried is what counts. Hidden internal stresses can appear no matter what drying method was used, too. You can end up with pretzel boards when resawing kiln dried material and perfectly straight and true sticks when resawing air dried material. As has been noted, you may not really know what's coming until you cut into the wood. Further, you should not be in a rush to use the resawn stock, either. No matter what, you really need to stack and sticker it for at least a few days so that moisture can equalize between what was originally the outside and the fresh cut surface from the inside. When you do process it to final dimension, you have to be sure to remove material from the original outside surface, too, to help avoid moisture imbalance. It's a dance...

[Keith Outten]

It's all to common for hardwood boards to bow and cup if they have been kiln dried to fast at high temperatures. There is a reason for drying schedules but they impact the bottom line. If you can find lumber dried in a de-humidification kiln per the drying schedule you will most likely have the best quality lumber you have ever had, but it will be the most expensive.

[John TenEyck]

Air dried in Nashville likely means not very dry compared to what's needed for indoor furniture. I'm in WNY and air dried means 12 - 14%, at best; I have to believe it's at least that high where you are and likely higher. Modern heated and AC controlled houses need wood at 6 - 8% in most parts of the country in order to avoid problems. So go ahead and resaw those boards but then sticker them in your shop until they stabilize in moisture. If they bow or cup when you resaw them so be it. Just sticker them and wait; many times they will straighten out again as the moisture equilibrates.

Once the moisture content has equilibrated in the lumber you can build the top. However, you need to plan for the movement that's going to happen if the moisture content is still substantially different from 6 - 8% so that the top can move without cupping and cracking. Good luck.

John

[Alan Gage]

I have a sawmill and use air dried almost exclusively. It will air dry to 11-12% here. I haven't checked the interior wood in my house in the summer (no AC) but in the winter it gets down to around 9%. The wood can move considerably in that last 3%.

What I do is first stack and sticker boards in my basement during the fall and winter when I'm running the wood stove down there. Usually a month down there will bring them down to 9-10% and they'll match the rest of the house. Then it's out to the shop for further processing. If I have thick stock that I intend to resaw I'll resaw it first before stacking in the basement.

For wood that I'm not in such a hurry about I stack and sticker already air dried lumber in the shop for further drying.

I'm relying on most of the drying happening during the winter when the heater is running and humidity is low. If you're running AC you should be able to do the same thing year round. Just make sure you're not stacking in a humid location.

Another thing with a 3" thick slab is does it contain the pith (the center of the log)? If so splitting the pith is a bad thing and will almost guarantee uncontrolled wood movement, not to mention a large split. You learn this quickly when you start sawing lumber on a sawmill. You'll notice that most of the lumber at a lumberyard either has the pith centered in the board or doesn't contain it at all. The boards where the pith is close to an edge, or where it was split, are the ones most likely to be bowed, crooked, cracked, or twisted.

Always make sure the boards have good (and equal) air flow all the way around them. If you lay a 12% board flat on a bench with only one side exposed to the air it will start to bow or cup quickly.

And one last thing is that kiln dried doesn't mean it's "dry". Typical construction lumber is only kiln dried down to 19% and even if a piece of wood was kiln dried to 7% it will start gaining moisture again as soon as it comes out of the kiln and is exposed to an environment with regular humidity levels. I've got lots of kiln dried lumber in my shop that has rebounded back to "air dry" levels of moisture content and it needs to be treated as such when using it.

Alan

[John TenEyck]

You are absolutely right, Alan, wood dried to 7% will regain moisture if you put it somewhere where the EMC is higher. But there was still a benefit to drying it to 7% to begin with. There is a hysteresis effect from drying; that is, once it's been dried to some value it won't gain moisture back as quickly and it will more easily dry out again because the initial drying removed water from the inside the cell walls while the water it's now gaining and losing is between them. Similarly, the wood will expand/contract in response to that change in moisture, but it's not to the same value as it was during initial kiln drying.

In the end it doesn't matter if it was AD or KD as long as the person using it knows what he/she is doing. That said, there will be less expansion/contraction to deal with if the wood was first kiln dried to the lowest average moisture content of where the piece will be housed. Where I live that's 6 - 8% MC and why I dry everything to 7%. Your drying strategy is a very good one for someone w/o a kiln. Actually, you are using your basement as your kiln.

John

[Andrew Hughes]

There is another way to help you succeed. But you will need a accurate jointer and bandsaw with long tables.
Rip the board in half with a thin kerf bandsaw blade. Then resaw the two and join them back together.
Ripping the board should release some tension if any is present you will also see how much the two pieces move.
You should get better yield from a shorter in height resaw.
I think it’s worth a try since air dried walnut is very nice to work and look at.
Good Luck

[al ladd]

The likely bowed or cupped state of resawn boards can be temporary and reversible, as it's often simply the result of moisture gradient. I'd resaw it and if it stays flat immediately weight the boards down , stickered, in a dry environment for a few weeks. If they bow or cup right after resawing, which is likely, I'd clamp the boards together with stickers between in a way to eliminate most or all of the bow, and then dry, and likely the boards will retain that flat condition once uniformly dry.
Feeding with substantial infeed pressure from a power feeder or strong stacked featherboards might restrain some of the cupping that might occur while you're cutting. The resulting boards will still cup, but will be more uniform in thickness, and if flattened after by clamping and drying you'll get more thickness out of them.
In my experience, air dried stock resaws more easily. Unlikely to find 12/4 KD stock that doesn't have a gradient.