I'm very new , very inexperienced to this , but even so I'm attempting making an acoustic guitar. I've been playing for over sixty years, but because of a nerve problem I can no longer play,
So, I have made back and top, I've been extremely careful, braces well positioned etc, I was pleased with the work so far!
I decided to take finished back and top from my cold workshop. (15c. 74%humidity) to my house , a disused room,18c 58%humidity, to my horror the wood had become concave! I took them back into workshop clamped them back into dish . They have both gone back into shape and remained perfect for a week!
The wood is surfaced to 2.5mm. What have I done wrong?
Very new guitar maker
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Alan Carruth
- Posts: 1288
- Joined: Sun Jan 15, 2012 1:11 pm
Re: Very new guitar maker
The culprit was the humidity change. Wood changes dimension a lot across the grain with changes in humidity, but there's hardly any change in length along the grain. The braces running across the grain stayed the same length, while the top tried to get narrower in the lower humidity, so it went concave. Once you got it back in the shop and it had a few days to acclimate it went back to the proper shape. So long as you can hold the R.H. pretty well constant you won't have those problems, However....
Once the guitar is assembled the plates won't be able to change shape as much, but that won't eliminate the stress from differential shrinkage. Going from your shop humidity at 74% to indoor humidity at a lower level will cause the top to try shrink in width, but with the edges glued down there's no place for it to go. This puts some stress on the wood. If the indoor humidity gets much lower, as it can with central heat in the winter around here, there's that much more shrinkage and stress, and something has to give.
I'm in New England, which is legendary for fast weather changes and wide swings in humidity over short periods of time. It's 'guitar hell'. Back when I did repairs February and March were the big season for top cracks, after the wood had had plenty of time to get really dry in indoor conditions that often go as low as 20% R.H. or lower.
Most makers around here try to keep the R.H. in the shop around 40%-45% year 'round. This is on the low side of 'comfortable', and the instrument will probably spend most of it's life at a slightly higher humidity, but it's still low enough to accommodate brief periods of very low humidity.
Once the guitar is assembled the plates won't be able to change shape as much, but that won't eliminate the stress from differential shrinkage. Going from your shop humidity at 74% to indoor humidity at a lower level will cause the top to try shrink in width, but with the edges glued down there's no place for it to go. This puts some stress on the wood. If the indoor humidity gets much lower, as it can with central heat in the winter around here, there's that much more shrinkage and stress, and something has to give.
I'm in New England, which is legendary for fast weather changes and wide swings in humidity over short periods of time. It's 'guitar hell'. Back when I did repairs February and March were the big season for top cracks, after the wood had had plenty of time to get really dry in indoor conditions that often go as low as 20% R.H. or lower.
Most makers around here try to keep the R.H. in the shop around 40%-45% year 'round. This is on the low side of 'comfortable', and the instrument will probably spend most of it's life at a slightly higher humidity, but it's still low enough to accommodate brief periods of very low humidity.
Re: Very new guitar maker
My workshop is in the basement, and it was around 50 percent and 68 degrees from June through September. Upstairs it was 70-85 degrees an who knows how humid. Now it is 58 with 44 percent humidity. I checked it upstairs, and it is 68, but the humidity is 49. Not bad, but weird.
I like to torture my backs and bellies before they get finished. Get them wet, let dry, heat them up, repeat. Sometimes they move around, sometimes they don't. My archtop back, slab maple, got a little dip just off center a couple inches long. It's stable now, the action is still the same since last Christmas.
The belly did rise after stringing it up,but stopped. I switched to nylons too.sounds good..
Just roughing out violins can sometimes make the wood curl up like a potato chip. Now I rough them, torture them, and then finish.
I've had a day when pegs are all loose, strings off, even finger boards pop off. All overnight. One day. There were black spots on some wood, and potatoes went bad. That was strange. Something going on.
Have to make them for the war.
I like to torture my backs and bellies before they get finished. Get them wet, let dry, heat them up, repeat. Sometimes they move around, sometimes they don't. My archtop back, slab maple, got a little dip just off center a couple inches long. It's stable now, the action is still the same since last Christmas.
The belly did rise after stringing it up,but stopped. I switched to nylons too.sounds good..
Just roughing out violins can sometimes make the wood curl up like a potato chip. Now I rough them, torture them, and then finish.
I've had a day when pegs are all loose, strings off, even finger boards pop off. All overnight. One day. There were black spots on some wood, and potatoes went bad. That was strange. Something going on.
Have to make them for the war.
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Alan Carruth
- Posts: 1288
- Joined: Sun Jan 15, 2012 1:11 pm
Re: Very new guitar maker
"Just roughing out violins can sometimes make the wood curl up like a potato chip."
They'll do that to some extent if the humidity is changing fast: the surface has picked up or lost moisture but the inside hasn't had time to catch up. Most wood also has some level of built-in stress, which can be relieved to some extent by lots of moisture cycling. The violin wood suppliers used to store their stock in huge sheds with tight roofs but open spaces in the siding (like a tobacco barn). This allowed for weather changes but kept the wood from actually getting wet.
They'll do that to some extent if the humidity is changing fast: the surface has picked up or lost moisture but the inside hasn't had time to catch up. Most wood also has some level of built-in stress, which can be relieved to some extent by lots of moisture cycling. The violin wood suppliers used to store their stock in huge sheds with tight roofs but open spaces in the siding (like a tobacco barn). This allowed for weather changes but kept the wood from actually getting wet.