Primitive Archer
Main Discussion Area => Bows => Topic started by: joachimM on March 21, 2015, 08:52:41 pm
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Hi folks,
I've been reading a lot on the change in physical properties of wood after heat treatment and why they change. Heat treatment is increasingly used to modify the properties of wood, mostly to make it more durable (and less prone to dimensional instability). This is because heat treatment degrades simple and complex sugars in the wood, which are the main food sources for insects and fungi. Heat-treated wood has hardly any nutritional value left for these critters.
As for structural properties, heat treatment affects wood as well. Nearly all studies report a loss of about all structural properties, such as elasticity, tension and compression strength. The longer the wood is treated and the higher the temperature, the greater the structural losses. This seems remarkable, since for bows heat treatment seems to increase the compression strength!
Still, one of the most important changes that happen during heat-treatment, is that hydrophilic and hygroscopic molecules (with high affinity for water) such as hemicellulose (a molecule binding structural cellulose molecules together) become modified, and lose some of their water-binding properties.
In other words, heat-treated wood will have a lower equilibrium moisture content than non-treated wood for the same ambient moisture content.
Since compression strength increases with decreasing moisture content of the wood (at 5% MC woods is nearly 50% stronger in compression than at 15% MC), heat-treatment will increase compression strength of the belly of a bow because it changes the equilibrium moisture content of the belly, but not of the back. So heat-treated bellies have a lower moisture content than the backs of the same bow, because the cell walls of the belly wood have been chemically modified by the heat to have less affinity for moisture.
In reverse, tension strength has a peak in most woods at about 12% MC, reducing at higher and lower MC. The result of heat treatment of the belly is that the belly properties are closer to the maximum for compression strength, whereas the back remains safe at its usual MC.
Fore example, the equilibrium MC of wood at 66% ambient moisture content is c. 10%. Heat treatment of beech between 200°C and 260° reduces the eMC of the wood at 66% ambient moisture to only 5%, increasing its compression strength dramatically.
This improves cast in two ways: it increases compression strength, raising draw weight for the same wood mass. Next, it lowers the mass of the belly by removing water (lower equilibrium moisture content). Although this may be limited, a few % in mass near the tips can make an important difference in cast.
As for the dimensional stability: heat-treated wood works less than non-treated wood, for the same reason: since its MC changes less in response to changes in ambient moisture, it warps and shrinks/expands less.
A review paper on the topic can be found here:
https://www.ncsu.edu/bioresources/BioRes_04/BioRes_04_1_0370_Esteves_P_Wood_Mod_Heat_Treatment_Rev_367.pdf
Joachim
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That's great info Joachim. Thanks for sharing. I like knowing the hows and whys.
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That makes good sense.
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Awesome. I've been studying this myself lately, although the chemistry is a little challenging. I want to see if thermally treating ponderosa pine can increase its value and make restoration thinnings more economical. Except along with all of its sawmills, Arizona has dismantled all its kilns as well so I have to send samples out :( :(
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Interesting stuff there - thank you.
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I didn't have time to read it all but are they not talking about kiln drying? Iwill go through it after church.
If yes, that's different from the heat treating Marc started.
Jawge
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It's only really different in that the whole piece is being uniformly heated. The change in the wood is the point, not so much how it's done.
This is more than kiln drying.
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So does that mean heat should only be applied to the belly side of the bow when making adjustments? and now im thinking steam bending should be avoided with any unbacked bow.
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So does that mean heat should only be applied to the belly side of the bow when making adjustments? and now im thinking steam bending should be avoided with any unbacked bow.
Good point, maybe. Joachim, is there any mention of what the temp has to be before changes take place? Also, I think it's been beat on before that steam dries wood, but does the moisture in the steam stop/postpone these changes?
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Thermal modification is done once the wood is at 0% MC and is done typically around 400F for a number of hours. I believe this is hotter than kilns get. I'm not entirely sure its the same thing as when heat treating a belly but it sounds to be.
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I don't know if it is the same as kiln drying either now that I read most of the paper.
But after reading it it would appear that wood going through this process is probably not suitable for bows. I hate to be negative but...
Just read the conclusion and you will see this.
Here's one.
11.
The downside of the treatment is the degradation of mechanical properties. The effect
on MOE is small, whereas static and dynamic bending strength and tensile strength
decrease. Brittleness of wood
increases with the deterioration of fracture properties
due to the loss of amorphous polysaccharid
es. The degradation of hemicelluloses has
been identified as the major factor for the
loss of mechanical strength, but also the
crystallization of amorphous cellulose might play an important role. "
Decrease in bending strength and the presence of cracks doesn't bode well for the bowyer.
Jawge
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I think you missed the point of Joachim posting that study, Jawge.
I don't think anyone was saying to use commercially heat treated wood for bows. It's just letting us know what's going on in the wood when we use our specific localized treatment.
If people are still arguing that this is a bad thing I don't know what to say.
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Pat, I understand that but heat treating the belly with a heat gun and these processes are totally different.
Joachim said,
"As for structural properties, heat treatment affects wood as well. Nearly all studies report a loss of about all structural properties, such as elasticity, tension and compression strength. The longer the wood is treated and the higher the temperature, the greater the structural losses. This seems remarkable, since for bows heat treatment seems to increase the compression strength!"
Yes, heat treatment as we know it does seem to increase compression strength.
The reason why it doesn't with the processes in the paper is because they are unrelated to heat treating as we know it.
Jawge
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Interesting information thanks Joachim
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I would read the whole article a bit slower. There is much evidence as to why heat treating the belly works liberally scattered through the study.
Clearly the commercial process does so at the expense of strength but the article mentions several times that strength properties increase during the initial phases of treating in ways that will be beneficial to bow wood.
Particularly noted when treating a wood (Birch) that has much in common with our typical heat treated woods.
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My personal belief is it is quite similar to heating rock for flintknapping, heat treatment for metals. Heat treatment realigns molecules if different ways, depending on the material you are using. Heat treatment on wood I have always thought of as a realignment of sugars, starches, cellulose in the wood....just my beliefs. :)
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Thought I might try to clarify a few things on the post I wrote.
For those who think all of this is just "spielerei":
Knowing why something works can help us to optimize it.
What's called heat-treating when lumber is concerned, and when bows are concerned is obviously different: the former affects the entire piece of wood or lumber, and typically takes more than two hours at high temperatures (note that I was talking about 200-260 degrees Celcius, not Fahrenheit), whereas the latter just affects the belly and takes maybe fifteen minutes and with a more superficial effect. So the structural degradation when heat-treating bows is probably limited. We also pay attention not to heat the back, as that would affect its tension properties when combustion kicks in. (it's english heat-treatment: toasting on one side only).
But essentially, for the surface of the belly wood, the chemical transformation is very similar: it's (the start of) a combustion process caused by high temperatures. This was already discussed once before on this forum: http://www.primitivearcher.com/smf/index.php/topic,35327.msg465115.html
The longer you expose the wood, the more it is combusted and the lighter in weight and darker in color it becomes. Logically, it also loses its strength, because you are burning wood in a very controlled way. This is not the case for kiln-drying, or steaming or boiling or ammonia-bending: molecular structure remains largely the same as there is no combustion of wood molecules. (note that in kiln-drying modest modifications probably take place, also affecting hygroscopicity).
The very first molecules to start combusting (at the lowest temperatures), are the hemicelluloses, of which the hydroxyl groups (which determine the affinity of these molecules with water) are split off first. “In thermal wood modification … the reduction of accessible OH-groups (hydroxyl) leads to a limited interaction with water compared to untreated wood” http://www.thermotreatedwood.com/researches/3-5-1.pdf
When the belly wood starts to turn brown and emits a toasted whiff, that's when we typically move on to the next section. This actually means we’re already breaking down the hemicellulose more than by just removing the hydroxyl groups. The smell is caused by the pyrolysis (combustion) of sugar units split off the hemicellulose molecule by heat, a process known as caramelization, also causing the brown color. If we don’t stop, carbonification sets in, or charring.
On the belly, this start of the combustion process reduces hygroscopicity (affinity for water), thereby reducing the equilibrium moisture content of the belly wood, but not the back wood, as we carefully avoid heating the back. The following two figures should clarify what that does to bow properties:
the first graph shows how tension parallel to grain (A ) changes with moisture content, peaking at 12%. The compression parallel to the grain (C) is highest when the wood is driest. But below 5% MC, any bow is dangerously close to breaking in tension (with spectacular bow explosions).
The second graph shows the equilibrium MC of Scots pine with and without heat treatment (or chemical treatment that also removes hydroxyl groups from hemicellulose in cell walls). Untreated wood has a much higher MC than heated wood at the same relative humidity. (figure from http://www.thermotreatedwood.com/researches/3-5-1.pdf)
Back to the first graph: a heat treated bow in typical humidity conditions has the compression properties of the dot in red on line C, and the tension properties of the green dot on line A: Heat treating allows the bow back to have a MC of 12%, and the belly a MC of 5%.
“The main advantages of heat-treated wood are a reduction in hygroscopicity and an improved dimensional stability, generally believed to be due to the degradation of hygroscopic hemicelluloses”( http://www.metla.fi/dissertationes/df134.pdf)
Marc St-Louis first observed that the belly shrinks during heat treatment. Any wood losing moisture shrinks (see for example http://www.wolfgangbrinck.com/boats/woodlore/shrinkagewp.jpg), but heat-treated wood regains less moisture than it lost (because it has lost its appetite for moisture during the process), so it experiences a net shrinkage. It seems like the density has increased. In some way, that’s right: the wood has shrunk, so for the same relative humidity the volume is smaller and there’s still (nearly) the same amount of dry matter of wood. But the shrinkage, so I think, could maybe be ascribed entirely to a loss of moisture.
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I think it would be better to focus on bow woods since there are noted differences in how various woods react to heat.
There is probably a considerable margin for enough wood being toasted just right to take the compression even if some surface charring is along for the ride.