Heat Treating Experiments

[mmattockx]

I am doing a sequence of experiments to try and quantify some of the effects of heat treating belly wood. The first experiment showed no changes in the bending stiffness of a 2 lam glue up with red oak lams when a heat treated belly lam was used. Since I was expecting something to be different, I am thinking I didn't heat treat it for long enough or hot enough to change the properties of the wood.

The first test lam I cooked was 0.107" thick and was heated in the oven at 350F for 10 minutes, then at 400F for 10 minutes because there was no visible difference after the 350F session. It changed colour significantly at 400F. That appears to be all that happened, however, as bending stiffness was unchanged. A pic comparing the heat treated lam with an untouched one:




What temps and soak times would our members suggest for future testing? The next tests will be done with maple because that is what I have more of to work with, the red oak was just available as scraps off a bow I am working on so I figured I would start there.


Thanks,
Mark

[Selfbowman]

If don’t know 400 degrees for 15 minutes . Did you preheat the oven? Just asking. Arvin

[Digital Caveman]

Paper burns at 451, so 400 makes sense for your experiment. 

How long can you leave the wood at that temperature before it turns black or smokes?  In an oven you may be able to get a longer and more through temper than you could do with a  heat gun.

I understand a dark brown is best.

Regards,

[mmattockx]

If don’t know 400 degrees for 15 minutes . Did you preheat the oven? Just asking. Arvin

Yes, it was hot from running at 350F for a while before jumping to 400F. I didn't put the lam in until it had sat at 400F for a few minutes as well.

Paper burns at 451, so 400 makes sense for your experiment. 

That's kind of my feeling as well. Anything higher than 400F seems like it would roast the wood pretty bad.

How long can you leave the wood at that temperature before it turns black or smokes?  In an oven you may be able to get a longer and more through temper than you could do with a  heat gun.

A more even, controlled treatment was my hope for this. The colour difference in the pic is the result of 10 minutes, I am sure it could go 20-30 minutes at 400F before turning into charcoal.


Mark

[willie]

Since I was expecting something to be different, I am thinking I didn't heat treat it for long enough or hot enough to change the properties of the wood.

How much stiffer was it after it came out of the oven, but before you glued it up?


my kitchen oven has a terrible differential between off and on. 50 degrees F. I would be more inclined to observe "on" time of the element during the treatment period to access radiant input. the color test is actually more likely to be a more representavive measure than a  timer. Testing with birch and a laser thermometer seems to agree with your results. a mild cooking odor @< 400, while color change really starts happening above 400

[Morgan]

Will be following with interest regarding temps and soak times. Sometime this winter I am going to build a heat treating “oven”. Will be an A frame made out of thin sheet metal that is six feet long, a foot or so wide on bottom tapering up with a 3” wide opening at top. Plan to have a 6’ long black pipe burner in bottom with racks to bring pipe closer or further from the bow belly that is suspended in the top opening. Should be able to control precise temps at belly and hold times.

[mmattockx]

How much stiffer was it after it came out of the oven, but before you glued it up?

Just bending it by hand there was no perceptible difference from the untreated lams.

my kitchen oven has a terrible differential between off and on. 50 degrees F. I would be more inclined to observe "on" time of the element during the treatment period to access radiant input. the color test is actually more likely to be a more representavive measure than a  timer. Testing with birch and a laser thermometer seems to agree with your results. a mild cooking odor @< 400, while color change really starts happening above 400

My oven is probably no better than yours on that count. I didn't mention it before, but I could definitely smell the oak cooking. Not burning, but similar to the scent you get off it while cutting with a table saw or similar.

Does anybody know if red oak normally responds well to a heat treat? I used it because the scrap was available, but it may not have been the best choice of material. I expect more from the maple when I get to it as I know it is considered to heat treat pretty well.

Will be following with interest regarding temps and soak times. Sometime this winter I am going to build a heat treating “oven”. Will be an A frame made out of thin sheet metal that is six feet long, a foot or so wide on bottom tapering up with a 3” wide opening at top. Plan to have a 6’ long black pipe burner in bottom with racks to bring pipe closer or further from the bow belly that is suspended in the top opening. Should be able to control precise temps at belly and hold times.

I'm curious to hear how this works. One of my biggest problems with this will be figuring out how to cook whole belly lams to 400+F temps in a controllable fashion. I can get about 24" long pieces in the oven, but I need much closer to 36" for this to be useful.


Mark

[Del the cat]

Been there done that. 
One of the (many) times I made an ass of myself (usually involving a woman!) was doing something similar to your eperiment with a short length of Hazel...
I posted my "results" on here and was kindly and patiently told that I was doing it too quick and just charring the surface. I was pointed towards TBB vol IV.
Once I'd heat treated a bow successfully, I posted it and ate humble pie...
I'm still willing to listen and learn.
The moral is, you are probably just doing it too quick and not hot enough.
Subsequent experiments, once I knew what I was doing showed that 200 degrees C  (390F) is needed for a sustained time to do the job.
Del

[Eric Garza]

Folks who fire harden wood recommend getting it up to 270 F and keeping it there for at least 2 hours. Brings up the question of which is better: hot and fast or somewhat less hot and slow.

[PatM]

Keep in mind that it seems to be white woods that benefit most.  Red  Oak lumber is not white wood.

[mmattockx]

Subsequent experiments, once I knew what I was doing showed that 200 degrees C  (390F) is needed for a sustained time to do the job.

I'm trying to learn as well. What would you say is a 'sustained time' in this context? I went with colour as the indicator, same as willie is thinking, and I didn't want to overcook it. I only had one shot with this wood, I have more maple and will be able to do multiple tests at various temperatures and time to see how that effects the results.

Brings up the question of which is better: hot and fast or somewhat less hot and slow.

That is much of the question here, all right. DC was the one who started me down this road and he was thinking something like 300F for 30 minutes. I suspect slow and less hot is the better way to go to drive out the moisture and really harden the cells of the wood.

Red  Oak lumber is not white wood.

I have seen people lump it in with the white woods and some not. This is the root of the lack of results I expect.


Mark

[Del the cat]

I reckon 45 minutes per limb. If you're not bored rigid you are not doing it slow enough  .
https://bowyersdiary.blogspot.com/2014/08/heat-treating-belly-billets.html
Del

[bjrogg]

I reckon 45 minutes per limb. If you bored rigid you are not doing it slow enough  .
https://bowyersdiary.blogspot.com/2014/08/heat-treating-belly-billets.html
Del

Well said Mr. Cat. And your arm should be about ready to fall off.

Bjrogg

[willie]

Brings up the question of which is better: hot and fast or somewhat less hot and slow.

That is much of the question here, all right. DC was the one who started me down this road and he was thinking something like 300F for 30 minutes. I suspect slow and less hot is the better way to go to drive out the moisture and really harden the cells of the wood.

why not 150 F for a month?  lignins seem to harden as they dry and age, but is there evidence to support any theory that some transformation occurs at a certain temp? or is it all just a matter of time and temp, and beyond some relative state of dryness/stiffness,  there are just diminishing returns.

[bjrogg]

I kinda think it is temp and penetration. I have never done any measuring. I just know from observations. If I heat HHB till it’s smoking, black and hot to touch. It changes. It becomes very hard which is very noticeable with a rasp, file or scraper. As far as speed of bow I don’t have chrono. I do however have a scales and tiller tree. I can always see a increase in weight even after retillering and scraping off the charcoal
Bjrogg