Elm recurve

[Badger]

   I would look really closely to the fades to see if I could get a little more bend in that area, it doesn't take much to get another 6" of draw, even if you might have to shorten the handle just a tad. I would also consider dropping the weight to 50# at this point.

[bownarra]

Bownara this is where it starts happening to me. Set that is. Do you think some just doesn’t hold? Then when I get to 23” is where I think the set starts really happening. Arvin

Ain't that the truth! Haha...easy to get to this point but the fun starts now. We will see what happpens, I'm definately out of practise with wood recurves.

[bownarra]

   I would look really closely to the fades to see if I could get a little more bend in that area, it doesn't take much to get another 6" of draw, even if you might have to shorten the handle just a tad. I would also consider dropping the weight to 50# at this point.

Thanks for the advise. I will consider what you've said and have a look when I get back to it tonight. I feel you are right about the weight.

[simk]

this is where it starts happening to me. Set that is.

dumb question from the comfty couch: why not heat treat again at this point...or a bit later, when the set really has kicked in?

(I myself have done this again and again to keep a desired profile with some bows,... but finally also had some bows broken exaggerating it...)

What are your guidelines?

Merrx Xmas everybody

[stuckinthemud]

Oh, I thought you couldn't heat out set as it is permanent compression damage? The once or twice I tried heating out an inch or so of set, it came back as soon as I strung the bow

[Digital Caveman]

I think it depends on how far gone the wood integrity is and how good the tempering is.

[bownarra]

Yes I will be heat treating it again today. I'm getting close to a finished bw now, only a little wood to come off the limbs. Yes I should heat treat before much set turns up. Waiting until you have set and trying to heat it out isn't ideal. It is possible but the belly cells can't be tourtured too much  If you were trying to do that Stuckin the mud I'd make sure the treatment was to dark brown/black.

[stuckinthemud]

That explains it then, I usually treat a bow until I can feel the heat on the back but with no real colour change on the belly.

[Digital Caveman]

Isn't black too much?  I think brown is when the air is forced out of the cells (hence hardening), but black is when they start to break down and burn.

[Marc St Louis]

Brown is good black is bad, unless you have enough material to tiller that black away

[scp]

We definitely need input from wood scientists or material engineers.

Anyhow, according to my "primitive" speculation,
it appears that deep heat treatment should be
around 175°C (347°F) at the minimum and
about 230°C (446°F) at the maximum.
But charring the wood will damaged the stave.

Cf. https://bioresources.cnr.ncsu.edu/wp-content/uploads/2019/02/1965.1.555.pdf
Synopsis - The thermal softening of isolated samples of lignin, hemicellulose
and cellulose has been investigated by observation of the thermally induced
collapse of a column of powder under constant gravitational load. Softening
temperatures of lignins ranged from 127-193 °C. Birch xylan and pine glucoman-
nan softened at 167 ° and 181'C, respectively . Sorption of water by lignin and
hemicellulose caused pronounced decrease of the softening temperature-in
some cases, to as low as 54°C. Softening points of both dry and moist lignins or
hemicelluloses have been shown to correlate with the temperature at which the
sample develops adhesive properties. The softening and adhesive behaviour has
been explained in terms of the concept of the glass transition for amorphous
polymers. Sorbed water is considered to act as a low molecular weight diluent
in plasticising the polymer chains and lowering the glass transition temperature .
Celluloses were found to soften at temperatures greater than 230 °C. In contrast
to lignin and hemicellulose, sorption of water by the cellulose had negligible
effect on the softening temperature . This difference was probably due to the
crystalline nature of cellulose and indicated that water did not plasticise individual
cellulose chains at the molecular level .

[simk]

in my limited experience: a little charring isn't so bad. off course it's got to  be removed. the heat treating will always give room to tiller down...and if only the surface is charred a little, its no problem; very little scraping will leave the perfectly hardened wood.

our friend druid as well likes the belly slightly dark

https://www.youtube.com/watch?v=aeDnw0-moDs
https://www.youtube.com/watch?v=cmqFuhw6-LE

 

[HH~]

one limb bow wonder why the other blew up, Heavy crown flat belly?

HH~

[RyanY]

We definitely need input from wood scientists or material engineers.

Anyhow, according to my "primitive" speculation,
it appears that deep heat treatment should be
around 175°C (347°F) at the minimum and
about 230°C (446°F) at the maximum.
But charring the wood will damaged the stave.

Cf. https://bioresources.cnr.ncsu.edu/wp-content/uploads/2019/02/1965.1.555.pdf
Synopsis - The thermal softening of isolated samples of lignin, hemicellulose
and cellulose has been investigated by observation of the thermally induced
collapse of a column of powder under constant gravitational load. Softening
temperatures of lignins ranged from 127-193 °C. Birch xylan and pine glucoman-
nan softened at 167 ° and 181'C, respectively . Sorption of water by lignin and
hemicellulose caused pronounced decrease of the softening temperature-in
some cases, to as low as 54°C. Softening points of both dry and moist lignins or
hemicelluloses have been shown to correlate with the temperature at which the
sample develops adhesive properties. The softening and adhesive behaviour has
been explained in terms of the concept of the glass transition for amorphous
polymers. Sorbed water is considered to act as a low molecular weight diluent
in plasticising the polymer chains and lowering the glass transition temperature .
Celluloses were found to soften at temperatures greater than 230 °C. In contrast
to lignin and hemicellulose, sorption of water by the cellulose had negligible
effect on the softening temperature . This difference was probably due to the
crystalline nature of cellulose and indicated that water did not plasticise individual
cellulose chains at the molecular level .

If this paper relates to heat treating, is it that the heat changes the properties of lignin and hemicellulose to cause them to be more advantageous to compression or possibly reduce how much moisture the wood holds? And to not heat so high as to damage the cellulose that does the majority of the work in compression anyways? Interesting stuff for sure.

[scp]

in my limited experience: a little charring isn't so bad. Of course it's got to  be removed. the heat treating will always give room to tiller down...and if only the surface is charred a little, its no problem; very little scraping will leave the perfectly hardened wood.

I read wood can be charred even at lower than 175°C, if it gets dry enough. What matters is how deep it is charred. Blackened surface is not a big deal, if you don't let it smoke too long. I doubt anyone here would let it smoke knowingly. But I did it once unknowingly and broke the bow eventually. The thermostat was set at 175°C, but I left the silicon heating pad on for over an hour and the bow was smoking. Luckily the pad still works.