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...my guitar build, that the top had sunk? (concave)

Well, I chucked it in a case and stored it upstairs, where the humidity wasn't controlled, but I knew was fairly stable?

I was going away on vacation for a bit and wanted to store it in a safe place. Also the guitar I am making will spend most of it's time stored upstairs when not being played. The bridge has not been glued on yet.

Last night I placed a straightedge along the lower bout and found that the guitar's top is convex again!!!!!

It was obviously humidity issues...I must have been harsh with dehumidifying the workshop.Sam Price38967.1501736111

I was running the shop between 45-55% RH...

if your shop was at that rh hen it is very unlikely that the shop was the problem. it might have been that the wood you used may not have been dry enough at the outset of your buiding adventure.

had you left it for several weeks to aclimate to the shop environment before beginning to build with it?

[QUOTE=crazymanmichael] if your shop was at that rh hen it is very unlikely that the shop was the problem. it might have been that the wood you used may not have been dry enough at the outset of your buiding adventure.

had you left it for several weeks to aclimate to the shop environment before beginning to build with it? [/QUOTE]

Yup, two weeks, as recommended by Stewmac.

Sam,

Maybe I'm a bit too conservative, but I'd be a little nervous about only two weeks ... especially since it may have gone through some interesting environmental fluctuations on the way over the pond and then customs.

You should take my approach to building ... build so slowly that all the wood you buy is fully aclimated, seasoned, over-seasoned, covered by cobwebs, and practically in decay by the time you use it!   

The 45% RH rule has always puzzled me. It seems that you would also need to know the room temp. as well. For example a 45% RH at a temp. of 95F (not uncommon in my shop) means there is a lot more water in the air than a 45% RH at a temp of 65F. I assume that would also translate to a different water content in the wood that has been allowed to acclimate. Am I thinking about this in the right way?

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53% of all statistics are made up on the spot
http://driftwoodguitars.blogspot.com/

Wayne I think it's supposed to be 45% at 75°F.

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Joe Breault
Merrimack, NH
Perpetual novice

Good question Wayne!
I've always assumed RH means relative to temp and 45%RH was safe regardless of temp. But I have heard an average temp of 75% (thanks Joe) is a good thing to try to maintain in the shop as well.

Do we need to be that picky?

WadeWade S.38967.4437847222

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Wade
Nashua, NH
http://www.wadefx.com

Sam
Thanks for the update on your guitars top
i was wondering about that.

Wayne
I think your thinking the right way, I check
the moisture content of my wood. I check the
shop by testing underside of my bench, shelves,etc.

Tom

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http://www.moriciguitar.com

Ah yes, Tom, I recall you recommending the re-hyration method. I tried it before, it didn't work (perhaps I didn't leave it for long enough.)


Basically the truth is that most of the year in my county 22 miles from Welsh Border (meaning lots of precipitation), the humidity runs from anything from 50%RH upwards. I was attempting to keep my shop as stable as possible during the build, although it would cost an incredible amount of money to keep my walk in closet strictly dehumidified just for one guitar. The guitar would naturally be stored the same as my others, with the exception of watching out for any dry spells...

As for the Stewmac recommendations of two weeks aclimatising...well I guess I'll know better not to listen to them next time, huh??

[QUOTE=Pete Licis] You should take my approach to building ... build so slowly that all the wood you buy is fully aclimated, seasoned, over-seasoned, covered by cobwebs, and practically in decay by the time you use it!    [/QUOTE]

Well, judging by those pics, apparently your method is a good one...

Two weeks seems way too short a period of time to me also...

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

[QUOTE=WayneC] The 45% RH rule has always puzzled me. It seems that you would also need to know the room temp. as well. For example a 45% RH at a temp. of 95F (not uncommon in my shop) means there is a lot more water in the air than a 45% RH at a temp of 65F. I assume that would also translate to a different water content in the wood that has been allowed to acclimate. Am I thinking about this in the right way?[/QUOTE]

It is not the amount of water in the air that is the issue. It is the movement of that water from air into the wood. That is a function of the relative humidity. If the RH is 45% at 65F, for example, the water in the wood will transfer to the air until it is at equilibrium. If you raise the temperature of the air and maintain the same RH, the air will not draw any more water from the wood. If, however, you raise the temperature only, the RH will drop. With a lower RH, the air is "drier" and will try to suck up water from any source that has a higher relative humidity, like your wood.

When the water leaves the wood the cells shrink and the whole board shrinks with them. Because of the physical structure of wood, the shrinkage is almost all across the grain with very little with the grain. Since the braces that are glued to the top and the sides themselves have the grain running perpendicular (more or less) the grain at the top, they don't shrink as much. They are trying to keep the top the same width as before but the top itself is trying to shrink. There will be some point in the structure of the top and the braces that is weakest and that is where a crack will form. One side of the crack will stick to one end of the brace while the other side sticks to the other end.

If you build in a dryer RH (43% seems to be just about ideal), then the changes you get from season to season aren't going to be enough to physically damage the top. If you build too dry, the top will expand and raise in the middle but not usually enough to cause physical problems with the top but does effect the action. Many players that I know have summer and winter saddles to deal with this.

But, I digress, When the RH is higher in the air than in the wood, the wood sucks the water out of the air until it is at equalibrium. It is the amount of water in the wood that effects the expansion. It appears (note my uncertainty here) that the cells in the wood are elastic over a certain range but will undergo structural changes if taken to extreems. This can be our friend. There seems to be a permanent effect if you dry out your wood to almost 0 RH in an oven. The cells seem to loose some of their ability to absorb water and the board will not expand or contract as much for a given range of RH. This makes the board better for building guitars. If the board doesn't change size as much it is less likely to crack.

As for how long should wood aclimatize to the shop, I'm inclined to think that it doesn't really need to be that long. The boards that we use are very thin and water can move pretty quickly from the entire thickness of the board. I think that it was Mario Proux that did an interesting experiment a few years back. He took a top panel and clamped it on the sides to a frame. This panel had been at the RH of his shop for some time. He then took the panel, while still in the frame, outside where the RH was much higher. Since the sides of the panel were restrained, any expansion of the board would cause the board to bend. If I recall correctly, the board did almost all its bending within hours or even minutes.

If all the RH related motion of the board happens within a day, than what other environmental factors are there in the shop that needs to be transfered to the wood. I haven't heard of any (chi?)

Well, what started out as a few lines explaining why RH and not temp was important turned into a somwhat meandering post. I do tend to get a little long winded. Sorry if I'm rambling.
Mike Mahar38967.6064814815

Mike,
These kinds of ramblings are very useful!
You answered most of the questions I had and then some.
Thanks so much!

Wade

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Wade
Nashua, NH
http://www.wadefx.com

MIKE MAHAR!!!!

I really enjoyed reading your post. Very informative. thanks!!!

Am I a confirmed nerd now?