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Wood species compatibility -- reaction to changes in enviroment.

PostPosted: Sun Nov 05, 2017 11:20 am
by Mark Wybierala
In this case, I'm building a harp but the same question applies to multi-piece guitar neck blanks and solid body guitars. I've made multi-piece neck from maple/mahogany/maple and have had no problems with some of these guitars being ten years old now. However, I have seen lutherie projects from others that have gone south from either poor gluing technique or from what I fear may be a problem with expansion and contraction of dissimilar wood species. Is this an issue? are there species of wood that should not be used together? Are there certain species that just expand and contract with changes in humidity and temperature that despite looking favorable on a wood species strength chart should not be used for lutherie? It is also possible that the failures I've seen are due to the wood(s) not being properly dried at the time of making the instrument.

Re: Wood species compatibility -- reaction to changes in enviroment.

PostPosted: Sun Nov 05, 2017 12:34 pm
by Alan Carruth
It pays to pay attention to this issue alright. Different woods change dimension in different ways with changes in humidity, and there's also usually a difference in the same wood between flat and quarter cut pieces. There are tables that give the shrinkage rate: Hoadly has one in his book, and there should be data on line on this. You could try the US Forest Product Lab, which is part of the USDA, iirc.

Another issue for the long term is 'shrinkage hysteresis'. Hemicellulose is one of the structural components of wood, and is the stuff that actually absorbs moisture from the air. It breaks down slowly with age, which is one reason old wood tends to be more stable than newer stuff. As the hemicellulose breaks down and the wood loses the ability to pick up as much moisture it tends to shrink a little bit more with every moisture cycle, and not swell back as much. This seems to depend partly on the density of the wood, so that ebony, for example, just keeps getting smaller over time. An ebony fretboard can get narrow enough to pull the surface of the neck concave if it was made flat to begin with, and that makes barre chords very difficult.

As if that were not enough, wood can also have rather large built in stress. This also tends to work it's way out over time, but when the stress level is high to begin with it can take a lot of time. I got some spruce from a beam that was about 300 years old which contained reaction wood, and it warped after cutting even though it had been in the shop for months.

The bottom line is that making stuff as precise as we need to out of wood is a challenge, and it's worth some effort to get it right.

Re: Wood species compatibility -- reaction to changes in enviroment.

PostPosted: Mon Nov 06, 2017 9:43 pm
by Mark Wybierala
I hear you about the built-in stress and I suppose you never really know with certain wood. I had the opportunity to salvage wood from a 200 year old barn and the roof rafters were all 2" thick rough cut hemlock. I went through the trouble of using a metal detector to find hidden nails on about 150 feet of true 2" X 12" planks and put them through a thickness planer which yielded some beautiful lumber only to have more than half of it twist and split when I cut it into 6 and 8 ft lengths. I thought I had some great wood for solid body guitars and was heartbroken. I later found out that hemlock is known for this.