Potato Chips

[Nelson Brown]

I have a beautiful mahogany back set with just the craziest potato chipping. I've been watching it in humidity controlled shop for a couple of years, and this thing snaps back and forth like you wouldn't believe. It's kind of fun to watch. It doesn't take any real effort to bend it flat. Just a little pressure and a little curvature, and it flattens-ish out enough.

My question - is this piece of wood worth putting in a guitar? A little glue would probably hold it to the rim fairly easily. OTOH, it moves so much over time, I wonder if I would just be setting myself up for headaches if I tried to do anything with it.

[Peter Wilcox]

As usual, I have no good advice. Do they bend along the grain, or across, or both? Assuming the pieces are book matched, will they bend in opposite directions when joined? If so, one might offset the other, and when braced and incorporated into the guitar might not be a problem. Or maybe that would set up a torsional effect to make things worse?

Or if they flatten with such little pressure, it might not be a problem at all.

[Nelson Brown]

It's a bookmatched set, and they both chip in the same direction. So, I guess when it gets into a guitar, they'll be opposite twists. I guess that could work itself out. With a little longitudinal bending around the curvature of a rim, I suppose it might just stay flat. Maybe I could sandwitch some carbon fiber in the braces to make them stiffer. It's a nice looking set with little to no figure. IIRC it's recovered Swietena.

[Barry Daniels]

If your humidity is stable the wood shouldn't be moving at all. I have to question how you are controlling humidity and how well your shop is sealed.

[Nelson Brown]

It's not fantastic, but it's not a garage. It's attached to the central air. Being in the mid-Atlantic, control will always be a struggle. We get some real extremes here.

There's a dehumidifier in there year round, and most of the wood doesn't really move much at all. This one set seems to have some internal tension that I think exaggerates the effect.

[Clay Schaeffer]

As it is taken to final thickness the force needed to hold it flat should be less. I would use wide /flatter (more flexible) braces in the lower bout as the older Martins used, so if it wants to move some it will be able to do so, rather than being restrained and cracking. Often tops and backs will lose some of the arching and flatten out in the drier winter months as the plates shrink across the grain.
I live in the mid Atlantic also (Maryland) and the indoor humidity swings can vary greatly as you mentioned. I try to control the humidity during glueups, and hope that will allow the instrument to live with the swings.

[Alan Carruth]

That sort of 'oil canning' is often caused by some sort of built-in stress, a small change due to humidity causes it to 'flip' to a different shape. There could be any number of reasons for this, not all problematic. Is the wood flat cut, or, if quartered, does it have a strong 'stripe' figure? Sides that do that can be hard to bend without crosswise ripple, but on the back you should be able to control it.