Why heat-treatment makes stronger bellies

[joachimM]

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
 

[Drewster]

That's great info Joachim.  Thanks for sharing.  I like knowing the hows and whys.

[Badger]

  That makes good sense.

[E. Jensen]

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   

[mikekeswick]

Interesting stuff there - thank you.

[George Tsoukalas]

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

[PatM]

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.

[carpholeo]

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.

[DC]

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?

[E. Jensen]

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.

[George Tsoukalas]

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

[PatM]

 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.

[George Tsoukalas]

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

[Peacebow_Coos]

Interesting information thanks Joachim

[PatM]

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.