Official Luthiers Forum!

Yes, I know. I need to control the humidity better.

However, that wasn't the question.

Last Saturday, I thinned down a Walnut and a Sapele back.
This week, I braced the backs.
This evening, I found that both of them were warped concave against the radius of the bracing! (I'm not freaking out, I just want to track down which way to compensate the climate of the shop to get them back to the original radius.)

The question I have is, which culprit was it, too much or too little humidity?

This is sort of like one of those Car Talk stumpers.
Normally, I could just look at both hygrometers I have in the shop and say difinitively that the humidity has dropped, which is exactly what both of them say. That WOULD make sense, because the door to the shop, which is normally open, was shut and the de-humidifier that usually controls the climate of the entire basement, was ultra-drying the room out.

So, lower humidity right?

I'd accept that as my problem, but I noticed that this being Mother Nature's first Minnesota mood change, we are experiencing rain, sleet, snow and 98% humidity. Lower shop humidity doesn't seem likely. So I tested the shop climate using a wet bulb test kit and found the humidity to be at around 60% according to that test.
The hygrometers say 40%. I try to keep the shop at 47%. Both of the dial hygrometers are usually within a couple points of each other and tested against the wet bulb, they are usually accurate enough.

But tonight, they are 18 points lower than the wet bulb. Or the wet bulb is ten points high.

So, I'm not trusting any of my equipment or instincts and wondering whether the experts (or the hacks like me who've been through this before) think a rapid rise of fall in humidity would cause this.

If you think you have the answer, write it on a bookmatched, old growth, sinker set of BRW back and sides, taped to quartered swiss moon wood and send it to me, or just reply to this post.

Thanks in advance,

Dave

I'd say it was Low humidity.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

I'll say low too. Only cause that's what Rod said, and he knows lot's more than me.

Darin

Do you know how a bi-metallic strip works in a thermostat?

That's what happens with backs and tops. The grain orientation of your back braces leaves them to expand or contract very little with moisture change along their length. The back or top is poised to move much greater along it's width. Brace dry then bring moisture up, it balloons up (convex). Brace wet then dry it out (it's all relative, mind you) and it collapses inward, or goes concave.

Do remember though, that simply measuring the relative humidity from today, yesterday, or the day before, isn't going to do much good. The wood's moisture content doesn't immediately conform to the humidity of the air it is placed in, and even well seasoned wood can take from days to weeks to acclimate.

_________________
Eschew obfuscation, espouse elucidation.

Sorry, straight to your point - it was braced with the back at a higher moisture content than it is at right now. If it is cupped in right now and the room is anywhere in the 40%-60% humidity, then you really, really don't want to simply rehumidify it to swell it back up and glue it on to the ribs. Do that and you'll simply be guaranteeing that it will collapse as a completed instrument. If the wood was braced too moist, it will always be ready to collapse when subject to even a reasonable humidity. Pull the braces, dry out the back to a reasonable environment, then rebrace. Better to do a little extra work now and save yourself some regret later.

_________________
Eschew obfuscation, espouse elucidation.

Along with a wet bulb psychrometer, I use an 18" (could be just about any length) x 1/4" square length of spruce clipped onto a piece of top cutoff of spruce. the cutoff piece is also about 18", 2" wide and about .110" thick, with the grain running across. Picture bookmatching a top, then cutting of two inches off its length. These are kept one on top of the other, with one end fixed together. The opposite end has some marks I made when the wood had settled at 70° F at 45% RH. I can look at this setup and know that, if the marks line up, I'm golden. It tells me when the moisture content is right, at varying temps too. If the weather is changing, it being spruce, it reacts quite fast; harder woods would take longer I'd expect. It's what's in the wood that we're concerned about, but I do still keep an eye on the wet bulb.

Pat

_________________
formerly known around here as burbank
_________________

http://www.patfosterguitars.com

Frack here...... BTW I love that show and listen to it every weekend.

Dave my friend it dried out since you braced it.

Now just like on Car Talk you have to imitate some stupid noise that you car is making and post a sound clip.....

Dave...This is no stumper...but you did show your good intent to achieve excellence by asking the question.

This should be a sticky because it is one of the most important concepts in building. Just to reiterate what had been said.

1. Perform any cross brace gluing operation in a low humidity state.
2. Assess the lowest possible humidity where the instrument will be and that environment should not be much higher in RH than when it was glued as to cause a concavity, or worse, a crack.

_________________
Ken

David and Hesh hit it. It's not the humidity itself, but the change in humidity that did it, more than likely. Another cause could be that the back had too much moisture to begin with, and dried out, but in your case it was the drop in humidity. The back more than likely shrank after it was braced due to the drop in humidity and loss of moisture content of the back wood. In theory, one could build a guitar in 90% humidity, and as long as the humidity where the guitar lives never changes, there would not be an issue....but, if a guitar is built in a high humidity setting, and sent to, say, Arizona to live, it would not live long. As it dried out, the plates would cave in as you mentioned, and then cracks would begin to open like the San Andreas fault!

The seasonal change, especially end of summer and into fall/winter (due to the propensity for humidity to trend into a lower value) is probably the most "dangerous" time to build if humidity in the shop is not strictly controlled. I shoot for 45% year round in the shop, here in the SE US.

edit...Ken posted while I was typing, and he brings up some great pearls.

_________________
Brad Tucker

Thanks all for the great replies.

Surprisingly (to myself), I'm extremely calm about this whole thing. Stuff happens. Especially when building guitars in a low tech shop in Minnesota. This happened last year at about the same time of year. I popped the braces off and redid things without issue but didn't think twice about it because it was an extremely problematic piece of Rosewood I was working with. That time I was much less relaxed about the deal though.

After giving this whole thing a ton of thought last evening, I think I came up with another contributing factor.
I cut up a bunch of braces a long time ago and put them in the basement shop. The wood has been part of a cattle yard for sixty+ years so I know it is well seasoned. However, the warpage almost entirely occurred on the long, wide brace across the lower bout. I ran out of brace stock of that dimension and milled some more this week on the band saw.

Of course, it was acclimated to the garage (in Minnesota at 90% humidity.)
So not only did the humidity of the back drop, but the braces dried out too.

Duh.

The sad part is, I need to get these instruments finished before X-mas and time is ticking.

Does anyone now have suggestions on how to dry out the garage wood in a timely manner? Could they be placed in a warm oven for an hour or two and then left in the shop for a day or two to acclimate?

thanks again,

Dave

Hmm ... Minnesota, 90% RH .... but what was the temp ??? RH 90% at 35 F is really low ammounts of moisture ... raise that air to 70 and the humidity will be way low ....

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Yes Dave you can do the oven but that seems pretty extreme to me. Since you noticed the potato chipping then the plate was in a low RH state and you can simply keep it there until you reglue the braces. But don't keep it in your shop for any length of time as to swell the wood in a high RH environment before re gluing.


Truth be know, it would be better to use a hair drier or space heater to heat the right before gluing. That will mitigate against the thing going concave. Make sure you dry out both sides evenly, but watch to see what happens when you dry it rapidly. If it cups in the wrong direction you will need to dry it more on the inside only.

Get the picture. Wood can handle the extreme change in RH and lose or gain moisture rapidly with out a problem, it is when this is restricted by cross bracing that causes the crack or weird shaped warping.

It would be best for you to try a dry run, no pun intended, and run it by the forum again.

good luck

_________________
Ken