Warped back wood pieces

Construction and repair of Classical Guitar and related instruments
KamHon Leung
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Warped back wood pieces

Post by KamHon Leung » Fri Mar 31, 2017 7:38 am

I have in storage of about 40 sets of back and side rosewood (of different species) and cypress. My room has air-conditioning and the relative humidity is tightly controlled at 50%. My room temperature is around 24 degree C (75 F).

Recently I have acquired a set of cocobolo back and sides. The back pieces were straight and flat when I received them. However, after a few weeks of storage in my room, the two back pieces are seriously warped. The warping is symmetrical, i.e. the left and right back pieces are exact mirror image.

Is there something wrong with my storage condition? Or is there something wrong with this set of back? The sides are straight and flat which I believe are from the same tree.

All my other sets of back and side do not have this problem.

Anything I can do to make them flat again?

Thanks.
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Euan Hannah
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Re: Warped back wood pieces

Post by Euan Hannah » Fri Mar 31, 2017 10:14 am

Judging by the pictures you could make propellers out of them but I think they are probably not much use for instruments. I doubt if it is your sotrage conditions, more likely the wood has been cut from the billet before it was properly seasoned and although you may be able to straighten it out to some extent it will always have the tendency to warp. It looks like a nice grain pattern too. I take it you keep the sets "stickered" to allow air circulation.

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Manuel Najera
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Re: Warped back wood pieces

Post by Manuel Najera » Fri Mar 31, 2017 2:31 pm

I was told once by a wood supplier, that cocobolo is a wood with a high resin content. That is why they need to give it “special” treatment, similar to what they use for regular pine, or, just wait a very long time before resawing it or sell it.
I also have had that problem when transporting my recently bought sets from the outside temperature (above 90°F) and then bring it them to the air conditioner. They wrap in a matter of seconds. I have to storage them immediately on the bottom of my other sets at least for a couple of months, “no stickered”.
If you want to try to save that set you can moist with a damp cloth the concave side (if there is one) of the billet and then place it under pressure for a couple of months. (you can use a couple of plywood pieces and clamps).
Check it then, and you may need to repeat the process a couple of times.

Regards,

Tom Sommerville
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Re: Warped back wood pieces

Post by Tom Sommerville » Fri Mar 31, 2017 3:24 pm

I was about to say you should send to me; I just happen to run an orphanage for misfit back and side sets, but it's cocobolo and would probably kill me.
So here's an honest suggestion: put it in the oven preaheated to 220 degrees f or 100 c and cook it a while, say 1/2 hour. Take it out and clamp it flat. Repeat as needed and vary time and temp if deemed advisable.

Also keep in mind that final thicknessing will greatly diminish the tension in the wood, to the point that gravity rules it.

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geoff-bristol
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Re: Warped back wood pieces

Post by geoff-bristol » Fri Mar 31, 2017 9:20 pm

Move it out of the room you put it in - then wait a while. It will likely return to its original cut shape ? Just give it a change of humidity.
If I can I always weigh wood when I get it - its surprising to see wood get heavier !
I would just put it aside in a warmish dry place - no humidity control - see what happens. I'm not a fan of constant himidity - but then I have an non-centrally heated house/workshop. The instruments are not going to spend their days constantly humidified if they are played a lot !

MessyTendon
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Re: Warped back wood pieces

Post by MessyTendon » Fri Mar 31, 2017 11:12 pm

I agree with Tom...The lignan in the wood can only change shape when malleable. Like steel, if you heat it bends. I would moisten the wood and then place it wrapped in a brown paper bag. The bag will protect the wood from being scorched.

I am no expert here. But I would hate to see that wood go to waste. I think Tom is right. But I be very judicious in my approach to clamping the heated wood...you can in theory contort the wood and lignan like you would a piece of steel, so long as you don't exceed the elastic limit it won't break.

Make a clamping jig, but don't force it. Acclimate the wood and repeat heat and clamping. Then season the wood. I think if you try to do it all at once, you will force it too much. I can't say that if you did it quicker it would be better. But in general things take time.

Gradually bending and seasoning will create more stability, provided it is kept relatively the same humidity. Wood is technically dead, but is very much alive like iron or soft steel.

Jim Frieson
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Re: Warped back wood pieces

Post by Jim Frieson » Sat Apr 01, 2017 4:05 am

Heat it in an oven , and then clamp it between flat boards . Soak it wrap it in paper or cloth and clamp it between flat boards .
The first could work but heating exposed wood results in rapid changes and possible cracking . The second method I never had much luck with .
Soaking ABW or cocobolo good luck .

But what I do , have used on Indian Brasilian ABW cocobolo and others , potato-chipped wood , and it does work :
You get two plates of mild steel 1 cm thick . As big as you need and no bigger .
Heat them up to 100 degrees C . Use a gas stove or any heater that will do it .
( I just heat them up till water sizzles on them , and as long as wood does not smoke and burn , but you can get a little bi-metal surface temperature guage cheap and it is good to keep records of what works best . It is better to use the lowest temperature that will be effective )
I use visegrips to handle the metal . It is heavy and care should be observed .
Put the offending back piece between the heated plates of steel and leave there till they cool down and the back will come out flat as can be .

One drawback :
Ever had a nice dark set and when you plane it down it is much lighter inside ?
Without discussing oxidation I have observed and other luthiers experts have confirmed same thing to me , that heating seems to draw the color to the surface .
So this fact should be taken into account when working up the wood .

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Chris Sobel
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Re: Warped back wood pieces

Post by Chris Sobel » Sat Apr 01, 2017 4:50 am

Jim Frieson wrote:Heat it in an oven , and then clamp it between flat boards . Soak it wrap it in paper or cloth and clamp it between flat boards .
The first could work but heating exposed wood results in rapid changes and possible cracking . The second method I never had much luck with .
Soaking ABW or cocobolo good luck .

But what I do , have used on Indian Brasilian ABW cocobolo and others , potato-chipped wood , and it does work :
You get two plates of mild steel 1 cm thick . As big as you need and no bigger .
Heat them up to 100 degrees C . Use a gas stove or any heater that will do it .
( I just heat them up till water sizzles on them , and as long as wood does not smoke and burn , but you can get a little bi-metal surface temperature guage cheap and it is good to keep records of what works best . It is better to use the lowest temperature that will be effective )
I use visegrips to handle the metal . It is heavy and care should be observed .
Put the offending back piece between the heated plates of steel and leave there till they cool down and the back will come out flat as can be .

One drawback :
Ever had a nice dark set and when you plane it down it is much lighter inside ?
Without discussing oxidation I have observed and other luthiers experts have confirmed same thing to me , that heating seems to draw the color to the surface .
So this fact should be taken into account when working up the wood .
This is the advice Dake Traphagen gave me for finicky Brazilian... he even made a steel press with heating elements for this exact purpose. Nifty.

Chris
CE Sobel Guitars

KamHon Leung
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Re: Warped back wood pieces

Post by KamHon Leung » Wed Apr 19, 2017 1:39 pm

Thank you everyone for your valuable input. I think I will try the steel plates method and see if it helps. Thanks again.

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