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Old 09-13-2021, 12:31 PM
Alan Carruth Alan Carruth is offline
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Join Date: Feb 2011
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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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