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  #1  
Old 03-10-2021, 10:28 AM
buffalohunt buffalohunt is offline
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Default First build - Battling humidity.

Hi folks. I am in the middle of my first acoustic build (I've done many solid body and one semi-hollow electrics before).

I am fortunate to have access to a large woodshop (I am a woodwork teacher at a local middle school). However, the humidity in the shop hovers around 30% and I have no way to raise or lower it. I have been taking my guitar home to do some of the more humidity sensitive work on my kitchen table, which hovers around 40% humidity. I have just finished closing up the box. My top and back are glued to the sides and am now working on figuring out the neck angle and the bolt-on tenon.

My question is... aside from gluing the braces to the plates and attaching the top and back to the sides, what other jobs should I be concerned about humidity? Or is 30% okay for most things?
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Old 03-10-2021, 11:02 AM
charles Tauber charles Tauber is offline
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At what humidity to build depends upon the humidity levels where the finished guitar will reside. You don't state the typical range of humidities that your finished guitar will live in. Without that, one can't give a meaningful answer.

Wood shrinks as it loses moisture and expands as it absorbs moisture. While exapanding can cause the action on a guitar to rise and can make the guitar sound "off", shrinkage runs the risk of cracking wood. So, it is shrinkage that is the primary concern. For that reason, the ideal humidity at which to make a guitar is somewhat below the mean humidity levels for where the finished guitar will be kept.

For example, if you live in the desert where (indoor) humidity ranges, say from 15% to 30%, ideally, you'd want to build somewhere around 20%. It would be foolish to build at 45% for that intended environment. Similarly, if you live somewhere where the humidity ranges from 50 to 80%, probably building around 60 to 65% is ideal. If would be foolish to build at 45% for that intended environment.
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Old 03-10-2021, 11:34 AM
buffalohunt buffalohunt is offline
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Quote:
Originally Posted by charles Tauber View Post
At what humidity to build depends upon the humidity levels where the finished guitar will reside. You don't state the typical range of humidities that your finished guitar will live in. Without that, one can't give a meaningful answer.

Wood shrinks as it loses moisture and expands as it absorbs moisture. While exapanding can cause the action on a guitar to rise and can make the guitar sound "off", shrinkage runs the risk of cracking wood. So, it is shrinkage that is the primary concern. For that reason, the ideal humidity at which to make a guitar is somewhat below the mean humidity levels for where the finished guitar will be kept.

For example, if you live in the desert where (indoor) humidity ranges, say from 15% to 30%, ideally, you'd want to build somewhere around 20%. It would be foolish to build at 45% for that intended environment. Similarly, if you live somewhere where the humidity ranges from 50 to 80%, probably building around 60 to 65% is ideal. If would be foolish to build at 45% for that intended environment.
Thanks. That makes total sense. I live in British Columbia. The outside humidity ranges from mid 30's in the summer to mid 70's in the winter. Not sure what the indoors average is, but it is probably around 20-40%. I haven't had any issues with my other guitars stored indoors, on stands without any humidity control.

Early in the build I had some issues. I started in the end of August and began with the top and back plates first. As I put them aside while working on the rims, the back's 15' radius inverted. I figured this was due to a change in humidity. Unfortunately, I didn't put much thought into humidity when I first started. I ended up planing the braces off and re-bracing around 40% humidity and then glued them to the sides.
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Old 03-10-2021, 01:50 PM
charles Tauber charles Tauber is offline
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Originally Posted by buffalohunt View Post
Not sure what the indoors average is, but it is probably around 20-40%.
A lot of it depends upon how much heating you have to do in the winter, how big a swing in humidity you'll have indoors.

If that 20 to 40% is accurate, you probably do want to build around 30%. Building in an environment that changes by that much will, as you've experienced, cause issues. You need to have a pretty constant humidity level during the building process, else components change size and shape from one day to the next. It is impossible to create close-fitting joints with wood that is constantly changing size and shape: it is a moving target.

The controlled environment can be as simple as a large plywood enclosure or a closet in which you control temperature and/or humidity. Remove the components while you work on them, then put them back in the enclosure afterwards.
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Old 03-10-2021, 07:11 PM
buffalohunt buffalohunt is offline
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Originally Posted by charles Tauber View Post
A lot of it depends upon how much heating you have to do in the winter, how big a swing in humidity you'll have indoors.

If that 20 to 40% is accurate, you probably do want to build around 30%. Building in an environment that changes by that much will, as you've experienced, cause issues. You need to have a pretty constant humidity level during the building process, else components change size and shape from one day to the next. It is impossible to create close-fitting joints with wood that is constantly changing size and shape: it is a moving target.

The controlled environment can be as simple as a large plywood enclosure or a closet in which you control temperature and/or humidity. Remove the components while you work on them, then put them back in the enclosure afterwards.
That all sounds good. I was chatting with a local luthier about our local climate and he figured that building in around 30%(+ or -) is alright for around here. He suggested that I could keep my body and other parts in a guitar case with a regular sound hole humidifier while I'm not working on them.
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Old 03-11-2021, 03:04 PM
phavriluk phavriluk is offline
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I've done what Mr. Tauber articulates, and it's worked out nicely for me. That 40 percent number that gets mentioned religiously works fine for factories that ship product all over, but I don't think it's useful for a builder of one's own guitars.
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Old 03-11-2021, 03:59 PM
printer2 printer2 is offline
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Built this body about 20% with the back flat. It has been about 15% most of the winter (I'll get to installing the humidifier) and the back is a little concave.



But it probably will never crack for being too dry. When the RH gets to a reasonable value it gets an arch. The top was built with the braces curved but them glued on flat.



Need to get on the neck and finish this thing.
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Old 03-11-2021, 10:15 PM
buffalohunt buffalohunt is offline
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Originally Posted by printer2 View Post
Built this body about 20% with the back flat. It has been about 15% most of the winter (I'll get to installing the humidifier) and the back is a little concave.



But it probably will never crack for being too dry. When the RH gets to a reasonable value it gets an arch. The top was built with the braces curved but them glued on flat.

Need to get on the neck and finish this thing.
I'm starting to see the same issue. I glued my back with a 15' radius sanded into the braces and it is starting to flatten out.

Are you anticipating the back going back to normal on yours?
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Old 03-12-2021, 10:59 AM
redir redir is offline
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If anything erring on the side of low humidity is better. Bad things happen to guitars when they dry out more then when they plump up when the RH gets high. It sounds to me like you will be ok at 30% provided that measurement is accurate.
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Old 03-12-2021, 01:05 PM
printer2 printer2 is offline
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Quote:
Originally Posted by buffalohunt View Post
I'm starting to see the same issue. I glued my back with a 15' radius sanded into the braces and it is starting to flatten out.

Are you anticipating the back going back to normal on yours?
They return to normal. Actually normal is higher humidity as compared to how they were made but they do develop an arch. Wood is better at compressive forces so bracing at a lower humidity is less harmful.
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Old 03-14-2021, 09:02 PM
buffalohunt buffalohunt is offline
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Originally Posted by printer2 View Post
They return to normal. Actually normal is higher humidity as compared to how they were made but they do develop an arch. Wood is better at compressive forces so bracing at a lower humidity is less harmful.
That's good to know.
Mine started flattening out a bit after gluing it to the sides. It may be humidity, but I'm starting to wonder if it was because my back plate had a reverse dome before gluing the braces on. It was arching in the opposite way that the braces were. Maybe some forces in the wood are pulling the braces flat?

I would have glued the braces on the other side, but there was a big ugly blemish in the wood that I wanted to hide inside the guitar.
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Old 03-16-2021, 05:19 PM
Victory Pete Victory Pete is offline
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Quote:
Originally Posted by redir View Post
If anything erring on the side of low humidity is better. Bad things happen to guitars when they dry out more then when they plump up when the RH gets high. It sounds to me like you will be ok at 30% provided that measurement is accurate.
I agree, I used to keep my shop at 50% but recently decided on 40%.
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Old 03-17-2021, 10:24 AM
RHayes RHayes is offline
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I've read this thread a couple of times in an effort to learn something. My career carpenter background makes me question the MC of the wood used for the back originally. Sure, the RH affects the EMC of the wood, but if the wood changed from 16% to 6% MC, there would be some serious movement and distortion because of the attached braces. Movement of wood that is at 6% EMC would be minimal if the RH changed 20%.
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Old 03-17-2021, 10:36 AM
charles Tauber charles Tauber is offline
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It is possible, as you suggest that the moisture content (MC) of the wood was high, as in not properly/fully dried.

However, even wood that has been properly/fully dried to say 6 to 8% equilibrium moisture content (EMC) can still move considerably with changes of 10 to 20% relative humidity.

Many guitar makers, including me, have experienced having thin, wide pieces of wood, such as a guitar back, radically change shape with as little as 10% or so change in relative humidity. As others have reported, a fully braced, convex-arched back can become concave-arched with as little as 10% or so change in relative humidity. In my experience, it will return to its original shape once the humidity is restored to the level at which it was braced.

Working with thin, wide boards - about 1/16" thick x 16" wide - brings with it some experiences not often seen in general carpentry work. Sensitivity to humidity levels is one of those. Unrestrained, guitar tops or backs, depending upon the species of wood and cut, can see as much as about 3/16" of change in width with comon seasonal humidity changes. Since guitar tops and backs are restrained, they bulge, at one extreme of humidity, and split at the other.
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Old 03-17-2021, 11:28 AM
RHayes RHayes is offline
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Thanks Charles. Just one more thing to think about getting into guitar building and an important one.

One table I found indicated wood at 30% RH will reach 6% EMC and at 50% RH will reach 9%. Another table (using quarter sawn sugar maple) indicated a 3% change in MC results in a .06" change in width for a 12" panel. Hardly seems significant but when coupled with bracing that doesn't change in length is a strong force to recon with.
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