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Old 09-13-2021, 12:52 AM
Russ C Russ C is offline
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Default Resistance to bellying

I’m asking about the relative resistance to bending between new, old and torrefied wood (I say “resistance” rather than stiffness in case there is a difference). I’m thinking specifically about spruce soundboards bellying with string tension as the years pass. I’m assuming “identical” prior to becoming 80 years old, or torrefied.
And secondly, whether that elasticity reaches a limit over time where providing it doesn’t break, it will not distort further.

Thanks in advance for any opinions you’re willing to share,
Russ
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Old 09-13-2021, 12:04 PM
Fathand Fathand is offline
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Isn't the bracing the important part to prevent bellying?
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Old 09-13-2021, 12:31 PM
Alan Carruth Alan Carruth is offline
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In the short term 'resistance to bending' is the same thing as 'stiffness' by definition. No matter what sort of wood you're using, of whatever age, you can predict the short term bending under load by measuring the Young's modulus if the material and knowing the structure.

Wood does 'cold creep' over time under sustained bending loads. From what I understand it appears to deform in shear, probably by deformation of the lignin 'glue' that holds the structure of the wood together, and takes a permanent set'. Lignin is the only structural part of the wood that is thermoplastic; deforming more easily when heated, and thermoplastics commonly do cold creep.

Torrefaction is supposed to stabilize wood by removing the hemicellulose that absorbs moisture from the air, or, perhaps, converting it to a non-absorbing state. Hemicellulose is a sort of 'filler' in the lignin 'glue', and initially makes up about the same 25% or so of the structural weight as the lignin. Hemicellulose breaks down naturally over time, which is why well 'seasoned' wood tends to be more stable than newer stock. Since the wood takes up less moisture as the hemicellulose breaks down it also shrinks less over time. It still changes dimension with humidity, but less and less as it ages, and it ends up a little smaller with every moisture cycle.

Wood also seems to lose toughness as it ages, and I have noticed that Torrefied wood is less shock resistant and more likely to crack than wood that was not so treated.

One can imagine that Torrefaction could have a number of different effects on the cold creep properties of the wood in the long term, depending on just what happens to the lignin and hemicellulose in the treatment. If, for example, the heat in some sense 'cauterizes' the hemicellulose, Torrefied wood might well show less creep over time than non-Torrefied, as the retained hemicellulose might serve to 'lock' it in place. OTOH, other changes in the chemistry of the lignin might produce more. At the moment I don't think we really have enough long term data to be able to say which is more likely.

I gather that Torrefaction has been around as a wood treatment for a while, but perhaps not in uses that are as highly stressed as a guitar top. I think we'll just have to wait and see, maybe for a few decades, before we can form any useful opinions.
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Old 09-13-2021, 12:37 PM
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rick-slo rick-slo is offline
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Wood creep can persist for years though less and less as it ages and it becomes so glacial (ignoring global warming for the moment) that it often becomes a non issue. Do avoid larger humidity swings however.
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Old 09-13-2021, 04:39 PM
Russ C Russ C is offline
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Quote:
Originally Posted by Alan Carruth View Post
In the short term 'resistance to bending' is the same thing as 'stiffness' by definition. No matter what sort of wood you're using, of whatever age, you can predict the short term bending under load by measuring the Young's modulus if the material and knowing the structure.

Wood does 'cold creep' over time under sustained bending loads. From what I understand it appears to deform in shear, probably by deformation of the lignin 'glue' that holds the structure of the wood together, and takes a permanent set'. Lignin is the only structural part of the wood that is thermoplastic; deforming more easily when heated, and thermoplastics commonly do cold creep.

Torrefaction is supposed to stabilize wood by removing the hemicellulose that absorbs moisture from the air, or, perhaps, converting it to a non-absorbing state. Hemicellulose is a sort of 'filler' in the lignin 'glue', and initially makes up about the same 25% or so of the structural weight as the lignin. Hemicellulose breaks down naturally over time, which is why well 'seasoned' wood tends to be more stable than newer stock. Since the wood takes up less moisture as the hemicellulose breaks down it also shrinks less over time. It still changes dimension with humidity, but less and less as it ages, and it ends up a little smaller with every moisture cycle.

Wood also seems to lose toughness as it ages, and I have noticed that Torrefied wood is less shock resistant and more likely to crack than wood that was not so treated.

One can imagine that Torrefaction could have a number of different effects on the cold creep properties of the wood in the long term, depending on just what happens to the lignin and hemicellulose in the treatment. If, for example, the heat in some sense 'cauterizes' the hemicellulose, Torrefied wood might well show less creep over time than non-Torrefied, as the retained hemicellulose might serve to 'lock' it in place. OTOH, other changes in the chemistry of the lignin might produce more. At the moment I don't think we really have enough long term data to be able to say which is more likely.

I gather that Torrefaction has been around as a wood treatment for a while, but perhaps not in uses that are as highly stressed as a guitar top. I think we'll just have to wait and see, maybe for a few decades, before we can form any useful opinions.
Alan, thanks. Your knowledge, logic and way of explaining things is so good!
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  #6  
Old 09-13-2021, 04:41 PM
Russ C Russ C is offline
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Originally Posted by Fathand View Post
Isn't the bracing the important part to prevent bellying?
Yep sure. I was thinking of a soundboard as a finished article - bracing and all .. I should have said that.
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Old 09-13-2021, 04:42 PM
Russ C Russ C is offline
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Quote:
Originally Posted by rick-slo View Post
Wood creep can persist for years though less and less as it ages and it becomes so glacial (ignoring global warming for the moment) that it often becomes a non issue. Do avoid larger humidity swings however.
Thatís what I felt without really knowing. Thanks.
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Old 09-13-2021, 08:12 PM
John Arnold John Arnold is offline
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I was party to an experiment to determine the effect of torrefaction. One half of a bookmatched red spruce top was torrefied. The two halves were then thicknessed and trimmed for width so that they were identical. The torrefied half was both lighter and stiffer than the nontorrefied half. I think this would translate to less bellying, but only if the top was left at the same thickness.
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Old 09-13-2021, 08:59 PM
printer2 printer2 is offline
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But is a little bellying bad?
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Old 09-14-2021, 02:06 AM
Russ C Russ C is offline
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Quote:
Originally Posted by John Arnold View Post
I was party to an experiment to determine the effect of torrefaction. One half of a bookmatched red spruce top was torrefied. The two halves were then thicknessed and trimmed for width so that they were identical. The torrefied half was both lighter and stiffer than the nontorrefied half. I think this would translate to less bellying, but only if the top was left at the same thickness.
That is interesting, thanks John. As Alan said, we need the decades to know the rest.
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Old 09-14-2021, 02:13 AM
Russ C Russ C is offline
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Originally Posted by printer2 View Post
But is a little bellying bad?
I see a little as a healthy part of a guitar growing into its own skin. My question came from wondering how that (and folding in half between the bouts) would continue with the years. There must come a point where all that makes a guitar into a rebuild project, unless it stops .. set ups and neck resets being the first jobs that we just consider normal maintenance.
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Old 09-14-2021, 06:31 AM
redir redir is offline
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Quote:
Originally Posted by Alan Carruth View Post

Wood also seems to lose toughness as it ages, and I have noticed that Torrefied wood is less shock resistant and more likely to crack than wood that was not so treated.
This may not be related to 'toughness' but when I move about 7 years ago to a new place and built my shop up it was built in an old barn, probably about 80 years at this point, which had siding of huge White Oak timbers. I found it next to impossible to hammer nails into it and my guess was that it was because it was old.

Quote:
Originally Posted by John Arnold View Post
I was party to an experiment to determine the effect of torrefaction. One half of a bookmatched red spruce top was torrefied. The two halves were then thicknessed and trimmed for width so that they were identical. The torrefied half was both lighter and stiffer than the nontorrefied half. I think this would translate to less bellying, but only if the top was left at the same thickness.
This sort of confirms my intuition. I only started using T-Wood a few years ago but it always seemed to me to be the stiffest tops I have handled. And my defection testing indicated this as well. For example the tops I build for my small parlor guitars are usually around .09 to .1 inch deflecting at .4in. The last one i built with a T top deflecting at the same .4in was .07in thick.

It also feels brittle. That might have some relationship to the stiffness.
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Old 09-14-2021, 09:14 AM
Alan Carruth Alan Carruth is offline
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Lotsa variables.

I don't have any test data on Torrefied spruce, and only limited data on 'old' wood. Hemicellulose degradation is said to reduce both the density and Young's modulus of wood, with the density falling a bit faster, so that overall the Young's modulus/density ratio goes up a bit. If Torrefaction works the same way that would tend to produce a top that is lighter at a given stiffness, all else equal, but the top itself would need to be a bit thicker than an un-treated one. It's similar to swapping in a low density piece of wood, such as WRC, instead of an average piece of, say, Red spruce.

The question I don't think we can answer yet is whether the Torrefied top will see greater long-term creep, or less, as compared with a non-Torrefied top of the same species if both are worked to the same thickness and braced the same. It should be possible to do some experiments over a period of a year or two that might be able to address that if you have access to a lot of cut offs of tops. You could make a bunch of go-bars of the same size, spring them in into a deck of some sort, and leave them. Take them out after six months or a year and see how much of a 'set' they've taken, and whether they relax back to straight over time.
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Old 09-14-2021, 12:44 PM
redir redir is offline
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Quote:
Originally Posted by Alan Carruth View Post
Lotsa variables.

I don't have any test data on Torrefied spruce, and only limited data on 'old' wood. Hemicellulose degradation is said to reduce both the density and Young's modulus of wood, with the density falling a bit faster, so that overall the Young's modulus/density ratio goes up a bit. If Torrefaction works the same way that would tend to produce a top that is lighter at a given stiffness, all else equal, but the top itself would need to be a bit thicker than an un-treated one. It's similar to swapping in a low density piece of wood, such as WRC, instead of an average piece of, say, Red spruce.

The question I don't think we can answer yet is whether the Torrefied top will see greater long-term creep, or less, as compared with a non-Torrefied top of the same species if both are worked to the same thickness and braced the same. It should be possible to do some experiments over a period of a year or two that might be able to address that if you have access to a lot of cut offs of tops. You could make a bunch of go-bars of the same size, spring them in into a deck of some sort, and leave them. Take them out after six months or a year and see how much of a 'set' they've taken, and whether they relax back to straight over time.
So if one does deflection testing and say for example they found that for any average piece of spruce on an OM build that a deflection of .25in gives them the sound they want, would they want to make the T-top deflect less (hence generally speaking thicker) to achieve the same desired tonal palate? Given that all the tops are around average and not unusually stiff or unusually floppy.
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Old 09-15-2021, 05:17 PM
Alan Carruth Alan Carruth is offline
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As somebody who doesn't use deflection testing as an acoustic indicator I couldn't say. I use a vibration test to determine the Young's moduli along and across the grain, and the associated damping factors. I use the modulus along the grain to determine the proper thickness for structural reasons, and then use Chladni testing to optimize the brace profiles for the best acoustic response. You can, of course, use static deflection tests to get the Young's moduli, and use that data for structural determinations, but IMO stiffness and mass distribution are important, and that's what the patterns are about.

David Hurd does use static deflection maps of tops to guide him in brace shaving, and says my system gives deflection results that are similar to his, but that's a fair way from simple wood property testing. He loads the top at the bridge, and then measures the deflection at a grid of points across the whole lower bout, looking for smooth contour lines. Again, it's about how the stiffness is distributed, not just how stiff it is.
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