How to "un-warp" wood?

[Rob Luter]

If you do try to press it out don't use laminate, it won't allow the water to escape so panel will stay wet for a loooooong time. Ideally you would want to press it allowing for maximum exposure to dry evenly. May be difficult to do. Alternatively you could press between two flat sheets but have some material that will absorb the extra moisture.....essentially what is done with veneers before gluing. I don't know if it will work.....but only one way to find out

good luck,
JeffD

I figured I'd use something firm but absorbent. I'll probably line the laminate with paper or cloth so it will wick the water out of the wood.

[John K Jordan]

...Logic at the time dictated that I orient the deck planks such that the rings on the end grain arched up. I was told that any cupping that would occur would cup towards the concave side of the rings and the boards would always shed water. In reality, just the opposite happened. I always wondered how I screwed that up? ...

Rob,

It sounds like someone gave you advice based on ignorance. The way boards tend to cup certainly is counter-intuitive until you understand how wood shrinks as it dries.

Since wood shrinks more in the tangential direction than the radial direction, all things being equal a board will cup towards the convex ring direction as wood shortens along the ring. This picture shows some typical shrinkage of pieces cut from various places in the log cross section.



Imagining that the growth rings are trying to "straighten out" may be a simplistic way to remember this but understanding the way wood shrinks is far more useful since you can apply it to all possible cross-sectional orientations. The tangential shrinkage is often close to twice the radial shrinkage. (Shrinkage in the long grain direction is usually insignificant.) If you look up "T/R ratio" you can find charts of typical shrinkage ratios for different species. The fact that wood shrinks less in the radial direction is what makes true quartersawn lumber less likely to warp.

Note that the T/R ratio does not tell the entire story when trying to predict shrinkage and warping. The total shrinkage of the wood species is important as are other properties of the wood. Some species are inherently more stable than others. Also, when considering the entire board, the simple cross-sectional T/R ratio must be considered along with the way the board is cut from the log relative to the long grain. For example if the board is not perfectly aligned with the grain relative to the pith (almost always the case) the board can twist, bow, or crook. Also, stresses in the wood from things like the tree growing at an angle to vertical can create "reaction wood" and the warping from that can overshadow other factors. The way the wood is dried can also cause internal stress and warping.

Anyone who works with solid wood can benefit from learning how wood behaves. My favorite reference is "Understanding Wood" by R. Bruce Hoadley. The author is not only a wood scientist but a a craftsman with real-world experience.
https://www.amazon.com/Understanding.../dp/B004WYO862

As for unwarping, I always apply moisture to one side somehow. Just last week I flattened a cutting board that had somehow cupped badly. It was completely dry. I dampened the concave side, stuck a wet paper towel to that side, then set it back on the counter with the wet side down. In less than a day it flattened perfectly. I propped it up to dry completely and after a week of use it is still perfectly flat. If it was not, I would have repeated the process as needed with decreasing moisture each time.

I have also been successful many times flattening boards by laying them out on the grass with the cupped side down, preferably out of the sun unless I'm in a hurry. The bottom side picks up moisture from the ground. I don't know if I'd recommend this for fine lumber intended for joinery.

JKJ

[mark mcfarlane]

The bottom panel is 1/4" thick. I don't have anything I can measure moisture content with.

Using the 1"/year rule of thumb for green lumber, that's 3 months to fully air dry 1/4" stock, depending on how saturated it was and the temp/humidity in the area where you are trying to dry it. It probably won't take quite 3 months, but a few days or weeks also probably isn't enough. Someone with more experience air drying may chime in...

[John K Jordan]

Using the 1"/year rule of thumb for green lumber, that's 3 months to fully air dry 1/4" stock, depending on how saturated it was and the temp/humidity in the area where you are trying to dry it. It probably won't take quite 3 months, but a few days or weeks also probably isn't enough. Someone with more experience air drying may chime in...

My experience is wood 1/4" thick will air dry very quickly, perhaps even in a week or so depending on the species and what it is finished with, if anything. One way to check is carefully weigh the board. Then weigh it again after some time, maybe a week for something this thin, repeating if the weight has changed at all. When the wood has quit losing weight it is at equilibrium with the environment.

JKJ

[Rob Luter]

@John K Jordan - Thanks for the thorough response. The graphic was especially useful. The uppermost example is what is happening, and what happened on my deck boards. I'm glad I use Quartersawn wood on almost everything I build.

[mark mcfarlane]

My experience is wood 1/4" thick will air dry very quickly, perhaps even in a week or so depending on the species and what it is finished with, if anything. One way to check is carefully weigh the board. Then weigh it again after some time, maybe a week for something this thin, repeating if the weight has changed at all. When the wood has quit losing weight it is at equilibrium with the environment.

JKJ

Thanks John.