Humidity

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Peto
Gidgee
Posts: 6
Joined: Thu Sep 29, 2016 7:11 pm

Humidity

Post by Peto » Tue Nov 17, 2020 1:29 pm

Hello Everybody!
A perfectly normal Engleman spruce top that I made just a few days ago underwent a terrible metamorphosis.
It was fine 2 days ago and this morning when I picked it up to start assembling the box this is what I found: Check attachment
20201117_101801.jpg
Humidity in Melbourne has never been an issue before and to be honest this much deformation is very unusual.
I finished bracing this top more than a week ago and it was fine for many days, but this morning I found it as you can see in it the picture. It is more than 1" or 30 cm the distance you see from the spruce to the bench where is resting.
The amount of tension to force it back would risk the integrity of the guitar so it is a loss I guess.

Did anyone experience such dramatic changes? It has been very dry in the last few days, but this is horrible.

Ric

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Mark McLean
Blackwood
Posts: 870
Joined: Thu Apr 10, 2008 2:03 pm
Location: Sydney

Re: Humidity

Post by Mark McLean » Tue Nov 17, 2020 8:32 pm

Oh dear!
I just checked the BOM observations for Melbourne over recent days. On Saturday the RH was in the 70-80% range (was that when you glued the bracing?) and then on Sunday it was in the mid 20s. That kind of drying out will result in this potato chip phenomenon. Most of us have been here before. It really is remarkable how much a piece of wood can shrink when the water goes out of it, and the thin flat soundboard (with huge surface area to mass ratio) loses more than the braces, so it curls outwards like that. I am sorry to say that you are going to need to replace the bracing. Just rout or plane all of the bracing off. I know that seems brutal - but it is necessary. You can reuse the soundboard. Just make sure the RH is no higher than about 45% when you do the glue up. The same goes for the back bracing. And the other critical stage for humidity is when you glue the back or top to the sides. A good RH meter (or two of them) and a dehumidifier are essential tools in luthiery.

Peto
Gidgee
Posts: 6
Joined: Thu Sep 29, 2016 7:11 pm

Re: Humidity

Post by Peto » Wed Nov 18, 2020 1:08 pm

Thanks Mark for the advice, I should pick dry days for gluing work, but this is the first time for me. I only do two or three guitars a year, I must have been lucky so far.
Making braces for the top is the part of the build that I like the least to put it mildly.
I might just leave it like that and frame it with the inscription "Build and Learn in Melbourne - Four Seasons in One Day" :)
Thanks again,

Ric

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Mark McLean
Blackwood
Posts: 870
Joined: Thu Apr 10, 2008 2:03 pm
Location: Sydney

Re: Humidity

Post by Mark McLean » Wed Nov 18, 2020 4:04 pm

Hi Ric
I learned this lesson the same way you did, and not until my third guitar. I was just lucky with the first two as I paid absolutely no attention to humidity issues during those builds. But number 3 developed serious problems after I had closed the box. I build when I can find the time. A good day in the workshop is often a nice rainy day - when garden work or other outdoor activities are not so attractive. I have learned to be careful what tasks I do on those wet days. I run a dehumidifier most of the time in the shed and this increases the amount of time when conditions are suitable for the sensitive parts of the build. But my building insulation is not great and on a really wet day the dehumidifier still can't keep it dry. I just delay gluing braces that day and carve the neck or build a jig instead.

Your bracing does look very beautiful!

Peto
Gidgee
Posts: 6
Joined: Thu Sep 29, 2016 7:11 pm

Re: Humidity

Post by Peto » Fri Nov 20, 2020 2:45 pm

Thanks Mark, bracing takes me a disproportionate amount of time.
I copied the model from John Bogdanovich and just for the sake of testing his system as accurately as possible I sized each brace according to his plan.
Almost every brace is different in size. 5x5, 5x5.5, 5x6.5, 4x4 and so on, plus the 25ft radius on the glueing face to build the shape of the lower bout.
Anyway...I have better get on with the work and start planing them down to save the top :)

Regards,
Ric

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