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I'm working on my second guitar. I'm building it for a friend. My first guitar was rosewood and spruce. This second guitar is all african mahogany.

So far, I'm building everything at just about the same time I built my first, last year. But this time, I'm experiencing shrinkage. It seems curious to me that I had no problem with humidity or dryness on my first build, but am having problems with the second one, while building it in the same place in my basement, at the same time of year, and everything. does mahogany change more drastically than spruce or EIR?

I glued the top and back bracing on in November. And some time around New Year's, the top and back lost all of their arch. The top is concave, with about a 25' inward arch, instead of its original 25'. The back is flat, instead of having its original 15' radius.

I'm wondering if I should plane all the bracing off and put new bracing on now, so that this guitar doesn't potentially have these same problems every year.

But, seeing as how this guitar will be going to the friend that lives in Kentucky, which is much more rainy and wet in the winter, and not nearly as cold, ... should I bring rh up, or wait for it, along with the top and back returning, and then continue with my build?

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

are you using a hygrometer?if your basement is extremely low humidity now , try rehydrating the instrument, my basment is only usable for building in the dead of winter , when the outside ground is frozen , and the moisture stops leaching through the cement . Jody

I haven't been using a hygrometer. but like you, I have so far only built during the winter. we started having freezing temperatures here in november, and that's when I glued my bracing to the top and back.

i'm still wondering what I should do? I have a couple of humidifiers but they aren't very strong. when I leave them running all day in the room with my workbench, no change occurs with my build. still reverse arched.

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

I would like to preface this with I am not a professional builder, but...I had the exact same problem. A friend of mine, and guitar maker for over 30 years, said just put the pieces (in my case the enclosed body) right on the humidifier for a day or two. Works just fine...don't soak the wood right in the steam of course. I also find when it gets real cold and dry my guitars suffer a bit, even in their cases with humidifiers...so, after a bit of playing, I stick them in the bathroom with me as I shower. The few minutes of humidity makes a world of difference.

When all else fails, I remember that while they are pieces of art, they are just guitars meant for playing. The quest for perfection will drive you crazy because you'll never get there...sorta like me trying to get my shop as clean as Hesh's.

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Jamie M

Just wanted to chime in and empathize with you. I am having an identical problem on my second guitar as well...Reversed radius, braced around November. I'm using a Sitka top, so I don't think it's just the Mahogany. I'll be following this thread closely, and I feel for ya!

Interesting thread,

Theres a number of things that can cause your back to be unstable in humidity including the tree having twisted as it grew.
I've begun making my flamenco guitars with a 25 ft back radius. My old conde has a similar back radius, and I simply follow that.
On the proviso that the timber was reasonably stable to begin with: every time that I have made a guitar with a 25' back radius the lower bout back brace gets pushed in to create a concave lower bout some time after the back is fitted to the guitar and before the guitar has shellac applied. Even though the braces were convex prior to fitting. This is because of moisture exchange with the atmosphere. It can move around quite alot prior to finishing. Apply some shellac and this seals the back on the outside. Every single time without fail the back relaxes and goes back to its original convex shape and becomes stable once I apply a couple of coats of shellac.
I worried me the first time it happened.
I've come to realise that this is the nature of that type of build.

Remembering again this is with stable timber. The message here is that even stable timbers can move around a bit.
I seal my guitars with french polish shellac, which i know to be a good sealer. But I only seal on the outside.

Sealing the guitar both the inside and outside of the back will probably make it more stable if you're dealing with a broad rh range. Some makers prefer to seal both the inside and outside of their guitars prior to assembly to make them more stable, and less prone to cracking etc. The issue there would be to keep a margin around the brace and lining glue surfaces free from shellac to keep ensure good glue adherance.

My guitar necks do the same thing. I don't use a truss rod, and I've noticed that I can get my necks truer if i profile the back of the neck, pore fill and finish the neck before levelling the fingeboard. I've watched nicely trued necks bow because I didn't seal them before levelling the fingerboard. Seal the back of the neck, let it stablise for a couple of days then re level the fingerboard and this fixes the problem.

Cheers,
Claire

Hey Martin!

I'll give you my thoughts......but let me state that I'm not experienced so take my answers with a grain of salt.

First, keep in mind that when humidity changes, wood swells and shrinks across the grain.......but along the grain, wood is stable. A brace glued across the grain of the back/top will not lengthen/shrink with humidity changes, but the top/back will expand/contract which is what causes a bow or warped shape.

Second, when you glue wood cross grain (like you do when gluing the braces to the top or back) while the wood is stable at the current humidity level, that becomes the "normal" or "neutral" hunidity level where the wood forms/maintains the radius at which it was glued. If the humidity rises above this level, the top/back will bow. If it drops below this level, the top/back will bow in the opposite direction.

With that in mind, maybe one should choose a humidity level somewhere in the middle of the humidity range to glue cross grain joints. Well, 50% humidity is right in the middle so is that the best level to glue across the grain? The biggest problem occurs if you glue a cross-grain joint while the humidity is high, then when humidty drops and the wood dries out the top/back plate will shrink considerably.....but the length of the braces change very little.......so the top/back can easily split. For this reason most folks would rather play it safe and make cross grain glueups at less than 50% humidity......say in the low 40s. A lot of folks try to maintain their shop humidity around 40 - 45% for this reason.....it's around the middle of the humidity range but errs on the low side so its less likely wood will split when it dries out due to low humidity.

I'm guessing your top/back will retuen to it's normal shape when the humidity returns to the level it was when you glued the braces to the top/back. It would probably be good to rehydrate your top/back till the radius is normal again. One way you could do this is to buy those large zip lock bags that are 20"x24" or 24"x30" and place the top inside with a damp sponge (not soaking wet). I found the large zip-lock bags at K-Mart last year. Give it 2 or 3 days for the shape to get back to normal. Don't set the sponge directly on the wood. Use stickers or something to allow air to get to all sides of the wood.

Personally, I would wait to glue your top/back to the rim. Wait till your top/back are stable with a normal radius and the humidity is low 40s to low 50s.

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Formerly known as Adaboy.......

OK guys this is the kind of thing that we always hear about this time of year and the reason for what you are experiencing is that the relative humidity has changed since you braced your tops and backs. Since it is winter the RH probably dropped and if that is the case that is why rehumidifying your tops, backs etc will reverse the problem... usually.

Consider this - Hesh finishes bracing his top in a humidity controlled shop with the RH at around 45%. Hesh leaves a hygrometer on his outside back deck and checks it and it reads 55%. Hesh decides in all his lack of wisdom that he is going to take said top out back on this nice summer day with the RH at 55% and do his final sanding and clean-up... on this top.

Once on the deck and while sanding the top goes flat, loses it's radius of 25' in 10 minutes..... Hesh notices but thinks that it will return when back in the shop. Hesh keeps sanding and 25 minutes into being on the back deck the top is now reversed it's radius..... Hesh notices and decides to take said top back in the shop and does so. An hour later top is now flat. Hesh weights it in a 25' dish and an hour after that the radius that was built into the top (which may not be 25' btw but that is the subject for yet another long winded diatribe.... ) returns and all is well.

The moron of the story other than me is that humidity control of some sort is really a must for guitar building. We really NEED to be aware of what the RH really is especially as indicated above when doing an cross grain gluing. You may get away with it for now, not having humidity control, but you will be at risk for something like a loud crack in the middle of the night when your guitar decides that something has to move and it can't so something cracks. BTW the loud crack at 3:00 has happened to an OLFer and he posted about it several years ago....

So what do you do now? If the guitar was cross grain braced in high humidity and you leave it as is the plates (tops/backs) need to be left to acclimate in high humidity until their radius returns and then glued at once onto a prepared rim. This guitar can never be permitted to dry out either. It will be subject to cracking from dryness for the rest of it's days and really should go to a humid climate with no forced air heating in the winter.

Or - you can perhaps wall off a section of your shops so that you can use vapor barriers and run dehumidifiers and humidifiers as required. Then you can build year around. You also need to know that digital hygrometers are NOT accurate and that only a wet bulb type test which is easy and cheap to do is an accurate way to know your RH.

Or - you can study the Luthiers of days gone by before the Internet, before electricity, before, well.... before lots of things... and find out how they managed the seasons and their building. Did they save certain operations for the time of year that the RH would naturally be what they needed? How did they know what the RH actually was? Many of the instruments that they produced a couple hundred years ago and more are still alive and kicking today. They had to have had an awareness and an affinity for the natural seasonal patterns of their respective climates.

Lastly if your guitars are for someone else and if someone else if paying for it you really have a duty and responsibility to do what is required to have the instrument be as tolerant of seasonal changes as any stinkin fact*ry instrument. I don't mean to sound harsh I just take RH control very seriously here at flushtone global headquarters and I am constantly schlepping water in one direction or the other as a result.....

Hope this helps guys.

With regard to finishing both the inside and outside of an instrument: in wooden boat building this is generally frowned upon due to the fact that completely sealing the wood can result in the growth of anaerobic bacteria, which are typically more harmful than aerobic bacteria. This is why some folks with bad respiratory illnesses are put into hyperbaric chambers - the oxygen rich environment actually helps kill a lot of bacteria.

In talking with my boat builder buddy he mentioned that good boats are built to get wet and that proper construction is what keeps them together even when they do get wet. The more I talk with him the more similarities I find between building acoustic guitars and wooden boats...interesting stuff!

Best,
Trev

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http://www.PeakeGuitars.com

Thanks, very much, everyone for the responses. Hesh, you're right, and you don't sound harsh at all. The guitar is for a friend. I've already given him much more than he's paid for! But the only correct thing to do going forward is to remove the bracing and re-do, at a known RH.

Another rough lesson in the anals, uh, annals of ... whatever this misadventure is called...

I've seen some that have removed bracing and reused. I think the only way to do this right is to plane them off and remake.

Ugh. But taking the trouble now will be easier than what will likely happen later.

Thanks!

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

Here's my plan.

I'll leave the guitar top and back braced, while I make new braces, to look like the old ones. (colonial hardwoods has red spruce pieces for $8)

Plane off the top and back bracing.

Buy a couple of lab thermometers (http://www.arborsci.com/detail.aspx?ID=62lab), in order to do the wet bulb/dry bulb thang (http://www.proulxguitars.com/test/shop_tips/rh.htm).

Wait until my shop's RH is in the 40's.

Glue on the new bracing.

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

I build in Albuquerque NM, very low humidity, most of the time my humidity gauage shows 15% !!
I'm thinking if I build with low humidity this "Collapsing top" wont be an issue ?? Yes / No ??

Gregg

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Gregg

hi Gregg, no it won't be. What I am unsure about, though, and I'd love to hear about, is if someone out there has built a guitar in low humidity (like yours) and sent the guitar to a location where the humidity was 60-70-80% for much of the time

was it a problem? a bad one? or just sumpin that a different saddle would remedy?

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

I'm building a 12 string for my Uncle in NJ, which as we know has a much higher relitive humidity than where I am in ABQ.
It should be interesting once this guitar lives there for a few months.

Gregg

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Gregg

As a rule of thumb I always brace at a lower humidity than my normal, working shop RH. Glue braces at low RH and when RH increases it will just increase your radius a bit but if you do that in reverse, glue braces at say 55% and RH goes down in winter to 35-40% you're going to have concave tops...and backs. This can really be a big deal when it's cold out and you cannot get your shop RH up enough to hit the 45-50% range. Your plates are destined to be concave or at least flattened out too much to use. If it's summer time I will dehumidify down to 35% when joining plates and glueing up braces. In winter I'll take it down even more to around 30%. My justification is most people will not judiciously humidify their instruments all the time. Higher RH is not nearly as destructive as low RH can be and I'm sure there are many threads here that have gone over that many, many times.

One thing to note: my first guitar has a slightly concave top due to my inexperience in the RH area but it is a killer sounding box. Most people that play it beg to have it. It's definitely not an ideal situation but I'm not selling it and it has been around for 13 years and made ALOT of music.

Best of luck.
Steve

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"We might not be big but we sure are slow"

Earlier I said I thought I was doing just the same thing on this build (my second) as I had done on my last build (my first). I built it the same time of year, one year ago, done the same way, and everything.

But I just had a look at my pics from the first build and the dates on them. On my first build I glued the back braces on in December, and top braces on in January. I glued the top and back to the rim in March. Very much a dry period here in Michigan. The first guitar has been great. Pretty lucky for a first timer unversed in these things.

On the second guitar I glued the top and back braces on in November. Even though we had snow then and the furnace was on, my basement must've still had a higher RH. Then, it was right around the first week of January, when my top and back sunk.

so the situations were different, and I got different results. live and learn.

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"...you have to get over your strict adherence to your largely imagined notion of absolute perfection..."

Martin,

I build in my house as my shop has no heater nor humidity control. The house has no humidity control either, so I figured out a way to work around it. While the humidity was high last summer, I placed my soundboard and braces inside a large ziplock bag and put silica gel in small bags made out of cheese cloth inside along with the wood and sealed it up. I placed a meter inside and the humidity gets down between 37%-40%. Then I glued all the bracing to teh top and back while it was inside the ziplock bag. Only bad thing is you can't get to the bracing to wipe off the glue squeeze out. Here is a picture:

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Formerly known as Adaboy.......

By the way, if you are unsure what the humidity was when you glued your braces to the top, I wouldn't be in a big rush to chisel off the old braces. I would wait till the humidity is back in the 40s for a few days and see if the radius gets back to normal before carving anything.

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Formerly known as Adaboy.......

The important thing you need is to get a Hydrometer so you can monitor your RH level in the shop. It is important on brace gluing days to know let you be aware if what RH you have . Some people will "bake" the top. While I don't do this I do have a heat blanket where I can place the top or back and heat it for about 5 minutes or so .
This helps to drive out the surface moisture. Then When it is still hot I glue it up in my disk. This helps to a degree but you are still at the mercy of RH swings. Often when you glue up a top you may see the water migrate to the plate and this can do funny things as the water migrates into the plate and can reverse the dome . In most cases it will return as the water escapes from the glue.
A friend once said , " building guitars looks hard , but it is harder than it look"

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it