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I've read quite a few posts where people keep their building environment at about consistent 45% or so RH. Of course many problems can occur if you don't take any regard for the humidity. Bracing up a top when the humidity is at 60% can cause the plate to flatten or crack once the humidity drops back down. The opposite is also true, brace a plate while the humidity is low and when it rises the plate will have more arch. Everyone knows this already.

But does anyone here purposefully keep the humidity extra low before the body is closed up? I've read that is how Martin got most of the arch in their tops back in the day and that they would even store them by the stove to dry them further right before bracing them up. Any thoughts on this?

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Jeremy Douglas

Yup. When I was building we would bake the tops for a time in a home-made low-temp convection oven (for a time predetermined by testing - would certainly differ in different shops and procedures), then brace the top and back in to a shallow radius and allow it to swell after bracing and gluing to the sides.

I don't know if Martin used this practice, but it's centuries old none the less. Steinway has been doing their piano soundboards this way for at least a century, and they certainly weren't the first to come up with it.

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Eschew obfuscation, espouse elucidation.

Good question Jeremy and great answer thanks David.

Cheers

Kim

I was thinking since the wood shrinks and swells cross grain that one of the benefits to this method would be the formation of extra crosswise arch; which is where you'll need it if the humidity drops... I think



Can you give some general estimates of the temperature and duration? I guess the temp would have to be pretty low since you're dealing with a joined top. I don't have a recipe for spruce in any of my cookbooks

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Jeremy Douglas

I stumbled across this method a couple of years ago. Thought I was the only one doing it till I came across one of David's post in the archives.

Gluing braces flat at 30% gives me about a 35' radius parallel to the X at 45% and about 25' across the grain on the lower bout. It's hard to control the arch exactly and seems to depend somewhat on the wood.

David, did you let the top swell before gluing to the rims?

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http://www.chassonguitars.com

I should think you would still be sanding an arch into your braces and all though. There is likely a time difference for the braces to dry out as compared to the thin top or back plate. So I’d imagine the timing would be important.
So the change being described here is drying the plates before bracing and letting them come back to shop humidity before gluing to the rim? Is that correct?
Do you keep your braces at shop RH or do they get dried or heated too?
And, do you not want to drop the RH before closing the box?
I just want to be sure I have the recipe right.
Thanks,

Wade

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Wade
Nashua, NH
http://www.wadefx.com

What I was talking about was keeping everything(braces,sides,top,back) dry until after the box is closed. I think you would get more arch this way. I believe if you brought the top back up to 45% before gluing to the rim you wouldn't get as much arch because the top would be allowed to expand outwardly but if you wait till it's glued onto the rims to bring the humidity up the top can't expand outwardly and instead has to expand upward to increase the distance across the plate. That's just a guess from me though.

Also, I've heard of people who dedicate the winter to bracing and closing up bodies by building a few at the same time.

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Jeremy Douglas