Humidity, geography, and guitars

[phavriluk]

This element of discussion came up in another post, and rather than hijack the post with an extraneous question, I've started a new topic. Someone replied to a would-be builder and I'm addressing the comments: You've raised a question - - - our builder is in darkest Texas. I don't think it gets to 40% humidity there in a rainstorm. Our builder is advised to get a work area up to 40%-50% humidity before commencing building. And this is my question - - - where is the error in building a guitar in the ambient humidity of its build location? If OP's location humidity is 10% year round, can the guitar not be built of wood that has acclimated to that 10% humidity? And wouldn't the instrument stay stable if the humidity it was stored in post-building was the same as it was when it was in raw wood state? I think I don't understand the underlying need for getting wood acclimated to a 40%-50% ambient humidity, build it in that humidity level and store it evermore at that humidity level, even if the only place that humidity level existed was the shop the wood was stored in and the instrument built in, lest it crack itself to death. And I'm not trying to ambush anyone with my question, I really don't understand what's in play with regard to humidity and instrument making. For example, 40% is average in my location, I have to humidify in the winter and be aware of excess humidity in the summer, the transition seasons pretty much taking care of themselves. But if I lived in Nevada, would that 40% rule apply for an instrument constructed when the RH was 10% and the wood was acclimated to that RH?

Thanks, folks.

[B. Howard]

If the guitar was kept in a bubble that contained that RH for the rest of it's useful life it would indeed be fine. But guitars must go out into the real world, they will travel. Guitars built at the extremes of RH and then subjected to real world conditions will suffer a fairly quick and gruesome death. When wood is so much different in MC than what the RH it is currently in dictates it will start to move rapidly to reach equilibrium. This movement will slow as it gets closer to stasis but the farther off it is the faster the reaction will be. These rapid movements in the wood stress all the glue joints and even the wood fibers themselves. To appreciate the movement involved let's look at a 12" wide piece of mahogany. Let's say our RH changes from 30% to 70% at 70 degrees F, that would equal a change in MC of about 7%. That piece will expand 9/64" under that change in conditions. That's a lot of movement...and that's the real world that guitars live in.

[phavriluk]

Brian, thanks for the explanation. So that's one reason why guitar backs are domed...