I've seen wood being burned for spears in movies. I've also seen people burn wood for Sho Sugi Ban finishing technique. But....
Does burning wood increase any mechanical proprieties?
Are there any other advantages to burning wood?
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2Related to: woodworking.stackexchange.com/q/2175/135 Burning (or rather heating) wood changes its chemical and physical properties, for the most part making it harder.– DamonCommented Feb 26, 2016 at 12:38
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I would say this is the inverse of the proposed duplicate, asking for advantages rather than disadvantages. Similar vein, but not a duplicate imo.– Daniel B.Commented Feb 26, 2016 at 22:13
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1Regardless, anybody interested in this question ought to read the answers in the linked question, which answer this very nicely and more. If anything I'm inclined to edit the linked question to cover advantages as well and let it be canonical, since it wouldnt invalidate any of its answers. shrug @DanielB– Jason CCommented Feb 27, 2016 at 23:06
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Another thing that just came to my mind: If you have ever watched any of "In the Outback with Malcolm Douglas" or "Follow the Sun" (worth doing if you haven't), you will notice that they use fire to straighten spears (that is, bend wiggly wood into a shape that it normally doesn't have). Interestingly, the wood first becomes very pliable, and only later becomes hard, and retains its form when cooling down. Apparently the chemical reactions that harden the wood take their time.– DamonCommented Mar 1, 2016 at 9:51
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2 Answers
It's not the burning of wood, it's the heating of it.
By heating it one of the things that happens is you remove the moisture from the wood. This makes it more physically stable. Letting a stick dry out in the sun will have similar properties (as long as it isn't in the rain).
Moisture helps keep wood supple and when you want to push a stick into someone/thing that is not a good quality to have. So heating removes moisture and sets the cellular structure.
answered Feb 26, 2016 at 15:20
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It's not the burning I agree, but I think there are some chemical changes in the structure of the wood when the wood is singed (turned brown by the heat, but not turned to charcoal). These lead to an increase in the hardness and stiffness of the wood. These changes are more than just "drying out" the wood, although the chemical changes I refer to, do produce water as a reaction product. Commented Feb 27, 2016 at 17:53
Not burning or charring the wood, but heating it to a high temperature.
In this article dealing with the "Strength properties of thermally modified softwoods..." that deals specifically with three different softwoods the authors present information about distinct changes that occur in the mechanical properties brought about by heat-treating.
First, the authors discuss the reasons for heat-treating (a well accepted and common two step process that takes the temperature up to the range of 200 C. - think baking a pizza):
- Increase resistance to fungal attack, i.e. slower to rot
- More dimensional stability due to changes in ambient humidity
Then they discuss the specimens, testing methods and mechanical test results that reveal these changes (plus others):
- The tension strength parallel to the grain is dramatically reduced.
- The compression strength parallel to the grain is increased.
- The bending strength (requiring compression and tension strength) is increased slightly.
- Modulus of elasticity is increased.
- Impact strength is greatly reduced (more likely to snap instead gradually failing)
- Reduced water adsorption (hence, increased stability)
Finally, there is discussion about the likely causes of these changes which are caused by activity at the cellular and molecular level.
Surely, we should not extrapolate these particular results to apply to all woods (no hardwood testing here) but I think it's safe to say that the extreme heating of wood does permanently change mechanical characteristics of wood. (Increase or decrease? - some one, some the other.)