Fixing a concave top with humidity??

Construction and repair of Classical Guitar and related instruments
Olivier77
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Joined: Sat May 14, 2016 1:01 am
Location: Vancouver

Fixing a concave top with humidity??

Post by Olivier77 » Sat Mar 03, 2018 10:56 pm

In the middle of my build, my top was exposed to dry conditions long enough for it to shrink and become concave. The top was clamped to the solera without being attached to the neck or sides yet. The braces and harmony bars are glued and shaped, same for ring, fb patch etc.

The relative humidity went from around 50 to 30 when we moved and I criminally left my humidifier unplug in the chaos. The result is pretty much the opposite of a bombedo. Even the harmony bars raised a little bit from the solera. Would putting the top back under pressure on a bracing board at higher humidity fix the problem? Did anybody have positive result with this kind of solution. The only other option I see is to remove everything and brace again :chaud:

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Steve Ganz
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Re: Fixing a concave top with humidity??

Post by Steve Ganz » Sun Mar 04, 2018 5:11 am

Olivier,
You moved?
It bent backwards?
You have a humidifier?
We have had some drastic humidity changes in our nieghborhood recently, with a long stretch of low humidity.
Your top is behaving like a like the inside is hygroscopic more than the outside. If the outside was clamped int the solera, I suppose that might make sense, if it damp, not dry when you moved. Then you unclamped. I've never seen this drastic phenomenon as you describe it though, so I'm basically guessing.

Since you are a neighbor, I'll invite you to my place in Blaine, sometime. PM or email me when you are ready.
Steve

vesa
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Re: Fixing a concave top with humidity??

Post by vesa » Sun Mar 04, 2018 7:00 am

Hi Olivier

You cannot force it to anything, humidity chages will do the job - not physical force.
It may come back to the original shape if you return to humidity % where it was made and let the time do the trick (it can take days or weeks) in that case it should remain in the position (fastened in the solera?) it was when all this happened to get stabilized.
Other possibility is that you try to fiddle with different humidity levels.
Keep it simple as it is: if drier wood shrinks - if more humid it expands. Keep the other side covered and expose only one side to the humidity change. If concave it will need higher humidity, if convex it will need lower humidity to return to the original shape.
Everything happens with a delay, so if you have overdone it, turn the top upside down to even it.
It is possible, but do not rush - let the water do the job.

Vesa
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Antonio Marin nr. 813 1995 (Bouchet)
Vesa Kuokkanen 2016

Olivier77
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Joined: Sat May 14, 2016 1:01 am
Location: Vancouver

Re: Fixing a concave top with humidity??

Post by Olivier77 » Sun Mar 04, 2018 7:15 am

IMG_5021.JPG
IMG_5019.JPG
Hi Steve, Thank you for your reply!

I'm sorry my write up was confusing, I hope the pictures help.
The first one shows how I secure the top to my bracing board and solera when I work. This is how it was left when the humidity dropped and my humidifier was left unplugged.

The second is of the top now, after a day in the go-bar system, with the humidifier on. It still has a little bit of that bend I was talking about but It is a LOT better than it was this morning. What do you think?

I would really like to come and visit! I will let you know next time I come down in your area!
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Olivier77
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Location: Vancouver

Re: Fixing a concave top with humidity??

Post by Olivier77 » Sun Mar 04, 2018 7:26 am

vesa wrote:
Sun Mar 04, 2018 7:00 am
Hi Olivier

You cannot force it to anything, humidity chages will do the job - not physical force.
It may come back to the original shape if you return to humidity % where it was made and let the time do the trick (it can take days or weeks) in that case it should remain in the position (fastened in the solera?) it was when all this happened to get stabilized.
Other possibility is that you try to fiddle with different humidity levels.
Keep it simple as it is: if drier wood shrinks - if more humid it expands. Keep the other side covered and expose only one side to the humidity change. If concave it will need higher humidity, if convex it will need lower humidity to return to the original shape.
Everything happens with a delay, so if you have overdone it, turn the top upside down to even it.
It is possible, but do not rush - let the water do the job.

Vesa
Thank you Vesa!

It is reassuring to know it is possible it will go back. I will play around with these ideas, and maybe release most of the pressure I have on it now. I did put it in a go-bar today. I may have overreacted a bit!

simonm
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Re: Fixing a concave top with humidity??

Post by simonm » Sun Mar 04, 2018 11:16 am

The second picture looks pretty good. The first picture would make me nervous but I may be missing something. If for some reason I have to leave a braced top for a couple of days before I close the box, I leave it on the solera with a jig pressing the top into the solera. You could leave it in the go-bar system for storage too - would need very little pressure just for storage. When I am building I aim to get the box closed up within two days. If I know I won't be able to do that, I don't brace the top or back. It doesn't stop me preparing everything or doing other stuff, but when it comes to bracing the top and back and closing the box I like a clear run at it when I have suitable humidity conditions.

James Frieson
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Re: Fixing a concave top with humidity??

Post by James Frieson » Sun Mar 04, 2018 12:25 pm

If your top has gone concave all by itself due to experiencing of atmospheric relative humidity , it has been predetermined to do so , by the moisture condition of the wood at the time it was braced . There is no fix for this , with the exception of removing the bracing and rebracing it again under ideal conditions for doing so .

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Michael.N.
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Re: Fixing a concave top with humidity??

Post by Michael.N. » Sun Mar 04, 2018 1:03 pm

I don't think it's far from what it should be, going by the second picture. It may well be a combination of a slight dip (from the centre joint) and the edges of the lower bout curling up a touch. I'd give it a few more days in the solera with the go bars. You could also lock the shape in by placing heavy weights across the bridge patch when assembling sides and gluing on the back - you just need to be able to get the weights out through the soundhole. This might seem a little radical but Hauser supposedly forced his dome by gluing the soundboard to the curved bridge.
Historicalguitars.

Olivier77
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Re: Fixing a concave top with humidity??

Post by Olivier77 » Sun Mar 04, 2018 5:52 pm

simonm wrote:
Sun Mar 04, 2018 11:16 am
The second picture looks pretty good. The first picture would make me nervous but I may be missing something. If for some reason I have to leave a braced top for a couple of days before I close the box, I leave it on the solera with a jig pressing the top into the solera. You could leave it in the go-bar system for storage too - would need very little pressure just for storage. When I am building I aim to get the box closed up within two days. If I know I won't be able to do that, I don't brace the top or back. It doesn't stop me preparing everything or doing other stuff, but when it comes to bracing the top and back and closing the box I like a clear run at it when I have suitable humidity conditions.
Hi Simon,

Yes, I left my braced top for a while like it is on the first image. Maybe the biggest lesson I have to draw from this is like you said, brace and close the box faster.

Thanks for your feed back!

Olivier77
Posts: 10
Joined: Sat May 14, 2016 1:01 am
Location: Vancouver

Re: Fixing a concave top with humidity??

Post by Olivier77 » Sun Mar 04, 2018 6:05 pm

James Frieson wrote:
Sun Mar 04, 2018 12:25 pm
If your top has gone concave all by itself due to experiencing of atmospheric relative humidity , it has been predetermined to do so , by the moisture condition of the wood at the time it was braced . There is no fix for this , with the exception of removing the bracing and rebracing it again under ideal conditions for doing so .
Hi James,

I have to say the top was really good when I braced it and for a while after. This is my first guitar and I do this in my free time, so it does take me quite a while to get from one step to the other. It was only when the humidity dropped that the top shrank...and it seems that putting it back at normal humidity level does bring it back to normal as well, or close to.

Thanks for your reply.

Olivier77
Posts: 10
Joined: Sat May 14, 2016 1:01 am
Location: Vancouver

Re: Fixing a concave top with humidity??

Post by Olivier77 » Sun Mar 04, 2018 6:13 pm

Michael.N. wrote:
Sun Mar 04, 2018 1:03 pm
I don't think it's far from what it should be, going by the second picture. It may well be a combination of a slight dip (from the centre joint) and the edges of the lower bout curling up a touch. I'd give it a few more days in the solera with the go bars. You could also lock the shape in by placing heavy weights across the bridge patch when assembling sides and gluing on the back - you just need to be able to get the weights out through the soundhole. This might seem a little radical but Hauser supposedly forced his dome by gluing the soundboard to the curved bridge.
Hi Micheal,

This is very interesting. Although I wish my bombedo does come back to normal in a couple days! As I said, there was already a major improvement yesterday. I'll make sure I post a pic in a couple of days. In the mean time, I better get my sides ready!

Thank you for the historical perspective!

James Frieson
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Joined: Sun Mar 04, 2018 5:09 am

Re: Fixing a concave top with humidity??

Post by James Frieson » Mon Mar 05, 2018 2:47 am

Olivier77 wrote:
Sun Mar 04, 2018 6:05 pm
James Frieson wrote:
Sun Mar 04, 2018 12:25 pm
If your top has gone concave all by itself due to experiencing of atmospheric relative humidity , it has been predetermined to do so , by the moisture condition of the wood at the time it was braced . There is no fix for this , with the exception of removing the bracing and rebracing it again under ideal conditions for doing so .
Hi James,

I have to say the top was really good when I braced it and for a while after. This is my first guitar and I do this in my free time, so it does take me quite a while to get from one step to the other. It was only when the humidity dropped that the top shrank...and it seems that putting it back at normal humidity level does bring it back to normal as well, or close to.

Thanks for your reply.
I had many shops , and in most of them I had no way to control the RH % . I just waited for dry weather . I have had many tops and backs go flat or concave . But for a long time I work in a shop where I have a small sealed room with a dehumidifier in it . I can control the humidity and know what it is at the time of assembly . Tops braced in that room at usually between 30 and 40%RH , I can leave for a year hanging around in my shop and then go back to them later ; they never collapse . The likelyhood that the top in question , that you are concerned with , will encounter again in its life , a period of dryness , and thus be affected by it , is large ; attempting to "fix " the top with humidity cannot eliminate that likelyhood . It is an easy matter to remove the bracing and rebrace it , and then you can have confidence in it .

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