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How quickly does change in humidity effect wood?
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Author:  Paul Burner [ Thu Sep 11, 2008 1:02 pm ]
Post subject:  How quickly does change in humidity effect wood?
Hey all... my "shop" is divided between a space in my garage, which is not humidity controlled, and my basement, which is.

When I need to do some sanding, sawing or routing I do it in the garage.

Chiseling, scraping and gluing happens in my basement.

There is occasionally a 20+% difference in the relative humidity. 48-50% inside. 68-70% outside.

How long before the extra 20% has an adverse effect on my wood that could cause problems down the road?

I guess I'm wondering if working 4-5 hours in my garage at the higher humidity will cause any problems (as long as I'm storing everything inside).
Author:  Kent Chasson [ Thu Sep 11, 2008 1:26 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
To get an idea of how quickly wood moves with humidity changes, place a top on your workbench and put a 100 watt light about a foot above it. The top will cup right before your eyes.

But that shouldn't cause any problems for the wood or any machining operations that I can think of. It's most important to have good humidity control when gluing something across the grain of something else, like braces to plates or plates to rims. As long as you let stuff sit in your assembly room for at least as long as it was out, you should be fine.
Author:  Michael Dale Payne [ Thu Sep 11, 2008 1:37 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Another point to this question and an important one is "How does a quick change in humidity affect wood?"

This side of the issue is seldom spoke of but can reall affect a build. Major quick swings are fare worse than gradual changes.
Author:  Todd Rose [ Thu Sep 11, 2008 1:39 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Here's a way you could test it. Make a wooden hygrometer, and take it with you to the garage when you go out to work on parts.

To make it, take a cut-off from a mahogany back or side and make a strip, maybe 1/2" to 1" wide x about 8" long, out of it. The grain should run lengthwise with minimal runout. Thin it down to something like .060". Make a similar strip out of a piece of spruce, or another hardwood, but cut this one so that the grain runs across the strip, rather than lengthwise. Again, thin it down to about .060". Store both pieces in your humidity controlled room for a couple days to make sure they're both at EMC in that environment. Hopefully, they are both still pretty straight/flat. Now glue them together. I'd suggest using a non-water based glue, such as epoxy. Mount one end on something, allowing the other end to move freely. If you mount it over a backing board, you can later add marks to the board to indicate humidity levels, i.e. calibrate it against a reliable hygrometer.

As the two woods absorb and release moisture, the cross grain layer will, of course, expand and contract a lot more than the long grain mahogany (it wouldn't really have to be mahogany, since all woods are pretty stable lengthwise), so the laminated strip will bend up or down. Not only does this give you a way to read humidity levels, but it tells you how fast the wood reacts, given that your laminated strip is of a thickness that approximates the thickness of tops, backs, and sides. Since the cross grain strip has a lot of exposed end grain relative to its size, it may react to humidity changes more quickly than your average top/back/side, but it will still give you an idea of how quickly humidity changes affect wood.

Clear as mud?
Author:  Hesh [ Thu Sep 11, 2008 1:39 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Paul my friend in an unsuccessful effort to attempt to keep up with Lance for having the cleanest box guts I took a top outside on my deck to do the final sanding. It was 65% RH outside and around 45% RH in my shop.

Within 15 minutes I noticed that my domed top was flattening out..... gaah

I returned it to the shop and placed it in the respective radius dish with the bunny eared guitar holder to weight it down and within an hour it was back to normal. It happened that quick!
Author:  Todd Rose [ Thu Sep 11, 2008 1:48 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
As Hesh and Kent note, sometimes the changes are pretty quick and easily observed in your actual guitar parts. But they might not always be so obvious, while the dimensional changes could still cause you problems.
Author:  Todd Rose [ Thu Sep 11, 2008 1:51 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Kent Chasson wrote:
As long as you let stuff sit in your assembly room for at least as long as it was out, you should be fine.


I think Kent makes the key point, here. Get the pieces back to EMC in your humidity controlled room before assembling them.
Author:  Sam Price [ Thu Sep 11, 2008 1:54 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Todd Rose wrote:
Here's a way you could test it. Make a wooden hygrometer, and take it with you to the garage when you go out to work on parts.

To make it, take a cut-off from a mahogany back or side and make a strip, maybe 1/2" to 1" wide x about 8" long, out of it. The grain should run lengthwise with minimal runout. Thin it down to something like .060". Make a similar strip out of a piece of spruce, or another hardwood, but cut this one so that the grain runs across the strip, rather than lengthwise. Again, thin it down to about .060". Store both pieces in your humidity controlled room for a couple days to make sure they're both at EMC in that environment. Hopefully, they are both still pretty straight/flat. Now glue them together. I'd suggest using a non-water based glue, such as epoxy. Mount one end on something, allowing the other end to move freely. If you mount it over a backing board, you can later add marks to the board to indicate humidity levels, i.e. calibrate it against a reliable hygrometer.

As the two woods absorb and release moisture, the cross grain layer will, of course, expand and contract a lot more than the long grain mahogany (it wouldn't really have to be mahogany, since all woods are pretty stable lengthwise), so the laminated strip will bend up or down. Not only does this give you a way to read humidity levels, but it tells you how fast the wood reacts, given that your laminated strip is of a thickness that approximates the thickness of tops, backs, and sides. Since the cross grain strip has a lot of exposed end grain relative to its size, it may react to humidity changes more quickly than your average top/back/side, but it will still give you an idea of how quickly humidity changes affect wood.

Clear as mud?


That's a great tip!
Author:  Pat Foster [ Thu Sep 11, 2008 2:55 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Yes, Todd, a great tip. Try it with spruce and a rosewood. They react at different rates.

I made one with fresh spruce three years ago. It hopped all over the place with changes in humidity. Now, three years later, it moves much less. Says something for seasoning wood, especially tops, no?

Pat
Author:  Frank Aarre [ Thu Sep 11, 2008 5:48 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
warping is one thing, but do you really think a couple hours in 20+ humidity will result in a dimensional difference?

i have a room in my basement where i've got my guitars and where i sit to practice that i've put a dehumidifier, this is also where i store my wood, but i do my work in the garage. My garage will need some work before i think its worthwhile to put a dehumidifier in it.

so far i've only joined the top and back of one guitar, but i've glued it up in the garage and brought it back to the controlled room after 30-60min of clamping pressure. of course i'm not making guitars for sale(yet anyways).

Frank
Author:  Hesh [ Thu Sep 11, 2008 6:56 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
Frank buddy several years ago an OLFer who I can't remember now posted a story where he had finished a first or second guitar. It had a few coats of nitro on it waiting for more coats and was being stored without RH control. If I recall correctly it was built with no RH control either.

Anyway he posted that at 3:00 AM he heard a very loud crack in the house and it came from where the guitar was sitting...... You guessed it, the top cracked.

Needless to say the very next day I purchased a hygrometer and a dehumidifier.........
Author:  Todd Rose [ Thu Sep 11, 2008 7:10 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
frank777 wrote:
warping is one thing, but do you really think a couple hours in 20+ humidity will result in a dimensional difference?


Dimensional change is what causes the warping. It warps because one surface is shrinking or expanding (changing dimensionally) more than the other (that's a bit of an oversimplification, but that's the basic principle).
Author:  PaulB [ Thu Sep 11, 2008 7:51 pm ]
Post subject:  Re: How quickly does change in humidity effect wood?
That story sounds similar to mine Hesh. I built my first without RH control, we had a hot westerly change in the weather - dropped from around 80% to 15% in less than twenty minutes. The loudness of that top splitting was impressive, that was a resonant box! Minutes is all it took.

I've also seen thicknessed and joined plates that were laid flat on a bench start to cup within only a few minutes.
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