Heat treating/ limb thickness

[DC]

I'm trying to improve some on my best bow. The top limb(in the picture) is the existing bow and the bottom is the blank I'm working on. It's boo backed Yew and the new one will be too. I want to do a really deep heat treat and I think I'll treat it from both sides before gluing the boo on. In order to get a nice even treat I think the blank should be as thin as possible. In the picture the existing bow's yew is about 1/2" thick and the blank is about 3/4". The new bow is 2" shorter at 62" and will(hopefully) have a bit more reflex so there will probably be a little less than 1/2" yew on the new one. Figuring back from the old bow and allowing for wood vagueness, how much do you think I can reduce the thickness of the blank? Or how thick should I leave the blank for conventional tillering? I want to leave enough so I can have room for tillering. I don't want to try and hit the weight right out of the glue up. I ain't that talented

[DC]

Maybe another way of looking at this is how deep does a good heat treat go? And if I heat from both sides how thick can I get thoroughly heat treated?

[aznboi3644]

Have you read Mark St Louis’ chapter in TTB vol. 4 on heat treating?  He does state that a good heat treat goes deep into the wood

[IrishJay]

I'm far from an expert, but in my experience conventional heat treating doesn't go very deep at all. I'm working on one right now that I have been toasting periodically throughout the tillering process in an attempt to control set and keep the profile I shaped into it after floor tiller.  Each time I put it on the tree I end up scraping through the toast anywhere that I take off more than 4 or 5 scrapes. My bow isn't backed so I just retoast it and then wait a few days to put it back on the tree.

Wood is an excellent thermal insultor, so with a heat gun you're going to blacken the extrior before you get 1/4" penetration of the heat.

I think if you want to get an even heat treat the whole way through your blank you're going to need to heat it at a much lower temp than a heat gun produces for much longer time. Think of a piece of meat 🍖 if try to cook it fast over high heat youll burn the outside without getting the inside fully cooked.

I'd say you'd need a kiln. Start it at 350~400 degrees and let it go 24hrs or longer to get the whole blank heated evenly. Then increase temp 50~100 degrees and wait. Wash rinse repeat until you get the whole deal up to 800~900 degrees. Then verrrrry long wait for the MC to normalize enough to work it, because its going to be drier than a popcorn fart when it comes out of that kiln.

[DC]

I've thought of the kiln thing but that's more than I want to do. I think 400-500 would be the final temp. 800 or more is past the ignition temp of wood I think.

I also suspect that I have tillered through the heat treat a few times but more like 1/16" or more, not just a few scrapes.

PS let me modify that last bit. I have had occasion while tillering when a heat treated bow seemed to suddenly start taking more set for no obvious reason. I have suspected that I have tillered into the softer wood. Just because you have scraped off the dark wood doesn't mean you have gone through the heat treat. It goes deeper than that.

[IrishJay]

Good call on the ignition temp. Just looked it up, auto ignition os almost immediate at 700, will auto ignite over time at 450.

[Stick Bender]

I just heat treated & corected some serious lateral bend  over the last week but I only treated to 375f using a infrared thermometer and held the temp for atleast a minute the limb average was 1/2" I don't know how deep it goes but deep enough to hold the lateral correction & reflex ,Im sure some of the reflex will come out after tiller but exspect the lateral to stay , in your case if you could put a ruff pre taper based on your previous builds you could probably go deeper ,wood usaualy plastisises at 350 F  so 350-375 F is probably good ,but Im sure every wood is slightly different ,Im not sure testing and cuting a test peace would tell the whole story !

[Eric Garza]

Do conifers benefit from heat treating? It seems to me that I read somewhere that hardwoods are the ones that benefit from this, more so than softwoods. It seems to me that the resins in conifers would cause them to ignite well below their wood's spontaneous ignition point.

[DC]

I have an infrared thermometer. I usually treat to 400f+ and I keep the gun 3 1/2"(2x4) away. I do my heating on a caul though, so I can't take the temp of the back. Maybe if I cut a notch in the caul big enough to get the thermometer in. Hmmm I have some scraps

PS Or just use a test piece that doesn't need a caul Duuhhhh

Eric- I have treated Yew to 425 before and it does help. It starts to turn dark just over 400.

[PatM]

I've cut up a  few heat treated bows.  The heat goes very deep if you take your time.  In fact the cut cross section will look remarkably similar to a good sapwwod heart ratio or typical backed configuration.    The color change being noticeable to a depth of about 3/4 to 7/8 of the stave thickness.

[DC]

I just did a test on a 7/16" piece of Yew. After about 15 min in one spot the belly was 425-430° and the back was 205-210°. Interesting thing is the back got grey at about the 7-8 min mark and it never got darker. The belly temp first hit 420 about then and the back was only 160ish. That must be the limit of my gun so maybe going by the color is not the best idea. I'd hate to have to start timing this. It's boring enough as it is.
I'll try a thinner piece

[Stick Bender]

I was thinking that to ,maybe color doesn't tell the whole story maybe the wood is effected beyound the color ?  Really only one way to know make the bow and see especially sense you have a similar one to compare to !

[DC]

A thinner piece wasn't a lot different. I used a 5/16 piece. The back got about 10° hotter. What is surprising me is that this time I left it run for over a half hour and it never got more than grey. That's at 4". If I move the gun down to 1"(to see if the air is hot) it chars immediately. I guess at 4" from the gun the air has cooled to close to 400°

[Stick Bender]

Don sense your trying to heat a lam vs stave where your actualy trying to heat each side I wonder if you put some heat reflecting material like tin foil under the lam if it would bring the temp up on the opposite side ?

[IrishJay]

Don, have you considered a spit type setup using fire or hot coals?