Why does heat treating work the way it does?

[Ryan Jacob]

So I’ve heard people say it adds compression strength or draw weight or both, but I never understood why charring the belly would cause that. Can someone please explain this to me?

[Strichev]

Heat treating among other effects supposedly dehydrates (removes -OH groups) hemicellulose and thus lowers wood equilibrium moisture content. The lower the moisture, the higher the compressive strength.

https://www.researchgate.net/publication/279900105_Wood_modification_by_heat_treatment_A_review

[simk]

in my unscientific imagination it's a similar process to caramelizing sugar in a pan - and as far as i know wood also contains sugars that are being procesed i a similar way by heat trearing. but sure there are further effects...more density and less h2o and more....curious about more knowledge from all of you. cheers

[Ryan Jacob]

But then wouldn’t the wood pick up moisture and turn the belly into soggy charcoal?

[PatM]

The wood is not burned to charcoal.     Fire harden an arrow or spear point and you can get a better idea of what happens to the wood.

[bjrogg]

Like pat says. You don't turn wood to charcoal, or if you do you remove that layer tillering. I'm not exactly sure why it works but it sure makes a difference on certain woods. When I heat treat HHB and start to tiller again it's much harder to scrap than before heat treatment.
Bjrogg

[Morgan]

It seems to change the density of the wood, definitely makes it harder. When finish sanding, I run a damp rag over the wood to raise the grain and on belly wood that has been toasted, the grain don’t seem to raise like wood that hasn’t been heated. I would love to know the exact temp that the changes take place, and throw together a hotbox that will reach those temps to see how a bow reacts to a total heat treat..

[Strichev]

In this context dehydrate just means that the -OH functional groups are removed. The wood is then less hydrophilic and has a lower equlibrium moisture content. That's the way I understand the article. Basically it makes the wood less prone to absorbing moisture and since dry wood has more compressive strength that's beneficial for the belly.

I guess this could be tested by weighting a bow before heat treatment and after it has re-hydrated and stabilized afterwards. Some mass loss could probably be attributed to the loss of extractives and some perhaps to the lower equilibrium moisture content of the now modified belly. So then we wouldn't prove anything. They write about checking this stuff in the article though.

That damp cloth not raising the grain could be because it doesn't absorb as much water as untreated wood.

[PatM]

This discussion comes up every few years and it goes in circles.  The effects of heat on wood in various degrees has been posted several times.
 It's more than just "the wood is drier".