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salmonella
Lokahi
240 Posts |
 Posted - 07/19/2010 : 12:41:05 PM
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For those of you who are into stressing over properly humidifying your guitars (or other wooden objects like furniture) here is a nice link to an explanation of how Relative Humidity and Temperature affect the moisture content of wood which, in turn, causes shrinking or swelling which can damage it. If you are currently not stressing over it, I would not read any further.
http://montgomerycountywoodturners.org/Documents/Tip29%20-%20Moisture%20Content%20and%20Wood%20Shrinkage.pdf
A link in the middle of this article will take you to a chart of EMC (Equilibrium Moisture Content) of wood as it changes with Temperature and Relative Humidity.
Here is an example of what I found.
My guitar was manufactured at 70 degrees and 40% relative humidity (the luthier knows this). According to the table that means it had a moisture content (EMC) when it was glued together of 7.7%. If I live where it is dry, and do not humidify it, and the guitar is allowed to equilibrate with air that is 75 degrees and 15% relative humidity (heated houses in a winter area), it’s moisture content will drop to 3.5 %. These values are pretty easy from the tables. Here is the part that I interpolated….
The wood will shrink proportional to the difference in EMC below about 28% and will shrink a maximum of about 7% (varies with the wood and a lot of other factors). So my guitar wood in my example above will shrink about 1 % from the size when it was glued together. ((7.7 - 3.5)/28) X 7% So if the lower bout is 15” (grand concert) when it is glued, it will try to be 14.85 inches wide in my heated house…. More than a tenth of an inch. Yowza. Seems like a lot and I hope my calculations are wrong. If not, it is easy to see how you could get cracks and other stresses. It is also important to note that wood shrinks very differently along different grain axes. Braces and side panels will not shrink in the same direction as top and bottom pieces. The top may not shrink the same as the back if it is sawn at a different angle. Complicated stuff.
Hope this is interesting to you.
Oh, and if you live in, or visit Hawaii, and move wooden objects from there to the mainland, the shrinkage can be extreme depending on the destination environment.
Let me know if you know of any mistakes in my calculations. thanks
Dave
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maggie
Aloha
USA
40 Posts |
Posted - 07/19/2010 : 4:24:58 PM
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| Erm...I normally don't associate with numbers (math phobic), but I found out about the wood shrinkage problem when I moved a couple of koa chests and a koa rocker, and a few other items from Maui to Southern Oregon. Darn School of Hard Knocks left some bigs cracks on our furniture. So the other day when I noticed my string pegs popping out of my guitar, I freaked. How could that be? My guitar was made in the high desert of Oregon, we don't use air conditioning, and the windows are always open. Rummaged through my desk drawer, found the "thingy" that measures humidity and temperature and discovered that my office had a relative humidity of 5% and the temperature was 95 degrees! Autumn can't come soon enough. |
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Allen M Cary
Lokahi
USA
158 Posts |
Posted - 07/20/2010 : 09:06:57 AM
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It's really nice to live in the Bay Area (SF not Tampa) where the temperature and relative humidity stay right about where guitars like them, all year. May get a little wet in winter, but it counteracts the furnace.
Allen |
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guitar humidity and shrinkage
