Heat treating questions

[snowplow]

Hey fellas, the first little vine maple bow for my little guy is almost done. Its down to finish sanding. If I was to be picky, I would like to bend it so the string was a bit more centered in the handle and so the slight twist in one limb tip was gone.

So I was just curious, at what point do you do heat correction and stuff? Am I correct in assuming that it is way too late or I would be risking screwing up the poundage/tiller?

As I was thinking about this I also got to thinking about how I hear about heat treating. Can any of you point me to a how to or explanation of it? I dont know anything about it other than you use a heat gun 

Thank you guys!

[Pat B]

Snowplow, You can make corrections anytime with heat. I've done it throughout the process. Generally small corrections don't take much heat anyway. I also use oil on the belly when making corrections. At this point I do not want to scorch the wood.
 When I heat treat a belly(which in different from heat corrections) I clamp the handle securely to a form and start at the fade on one limb and begin heating about the first 6" until I get the color I want. Here I do not use oil because I do want to scorch the wood. I hold the heat gun with my hand about 2" above the belly, moving it back and forth until the color is achieved(about chocolate brown). As this area get hot I clamp it to the form.Then I move out to the next 6" and do the same(adding clamps as I go) but continue to revisit the area previously heated. I keep doing this until I reach the end of the working limb then start on the other limb. Sometimes, with uneven limbs I'll get the inner limb on both limbs worked with the heat and clamped so I don't overstress one limb or the other. Once everything is done I allow the bow to rest on the form until tomorrow. I like to be sure that the wood is completely cooled before stressing the limbs anymore. Even when the limb feels cool on the outside there can still be residual heat internally.

[snowplow]

Thanks Pat. So what is heat treating for? Just the color?

Can you extrapolate on how to use oil?

Thanks again!

[Pat B]

The heat treating(tempering) is to increase compression strength in mostly whitewoods but I use it on osage too.
 I use olive oil(any cooking oil will work) and coat the area where the correction will be. It helps hold the heat and distribute the heat when using dry heat(not steam) plus it helps prevent scorching. I use oil when I want to make corrections. When I add reflex with heat I'll not use oil but temper(heat treat) the belly(only) as I clamp the stave to the form.

[bowandarrow473]

When you make heat corrections, you have to be sure to heat the wood fibers up through the entire bow, this requires some time over the heat. For heat treating, you must do the same, only at a higher temperature,  this usually gives the belly a dark brown color but as Pat said, it also increases compression strength significantly on some woods, a few woods that greatly benefit  from heat treating are elm, osage, hickory, mulberry, yew, vine maple, and black locust (although some people report tension failures with it). For major corrections I use steam but for more minor ones I almost always use heat.

[snowplow]

Ok cool. So when heating or steaming minor things do you always use a form and clamps or can you just tweak it a little?

What does the heat treating do? How does the increased compression strength help? Does it increase the draw weight?

[Pat B]

You don't have to use a form. You can tweak by hand for minor repairs. Some folks will use the fork of a tree to make corrections or clamp the bow in a vise and hang a weight on it to remove twists or offsets. Whatever works best for you. I like using a form and leave the stave in it until the wood cools completely so I can be working on another project. You just don't want to stress the repaired area until the wood has cooled completely.
 In TBBIV, Marc St Louis explains heat treating in his chapter "Heat Treating Bows" and how and why he does it. I can tell you it works but I doubt I can explain how or why.

[Carson (CMB)]

I am not sure how little your little guy is, but it is quite easy to tweak/make minor corrections by hand on kids bows. Much less mass = much less heat/time required.  Vine maple tends to respond better to steam heat than dry, but I have gotten away with dry heat for corrections and recurve on smaller kids bows.

[bowandarrow473]

I use the fork of a tree like pat said for both heating and steaming, although for recurves I use a form, the fork of a tree works very well and I like using it.

I have heard several theories on what heat treatment on the belly of the bow does but it fore sure increases the compression strength of the wood, balancing the forces of compression and tension in very tension strong woods like hickory or elm. My idea on the actual effects that the heat has on the cells are: the heat squeezes all the water out of the vacuoles of the plant cells, and then causes the air to leave as well. Reducing the amount of empty space in the wood cells and thus increasing the density and compression strength.

[snowplow]

I did a little tweaking to the bow last night. One limb got really weird. But I think I got it fixed. I used a  mix of steam and dry heat.

How in the heck do you know when the bow is heat treated? How do you know you're done?

[DC]

When it turns brown. If you hold the gun 4" away by the time it turns light brown it should be heated correctly. It takes about 30_45 min a limb. If you're not bored stiff you're not finished

[Pat B]

I don't think you can have a tempered belly without at lease a little carmelization color. I guess you could cook a bow at low temp until the M/C is reduced enough to increase the compression strengths of the belly but that would also reduce the viability of the bow in general. IMO  Also, I don't believe you can temper with steam.

[Springbuck]

I don't think you can have a tempered belly without at lease a little carmelization color. I guess you could cook a bow at low temp until the M/C is reduced enough to increase the compression strengths of the belly but that would also reduce the viability of the bow in general.

  I think you are right, Pat, but for Snowplow's benefit I'll say WHY I would be very careful with low, slow heat.

  Heat transfer thought materials is a funny thing.  If you have a very high heat source, like a torch, You can burn the first millimeter thickness black in a couple seconds, while three mm down into the wood is still ice cold (well, room temp, but you get the idea).  Likewise, if you started with low slow heat and allowed the heat time to transfer from the surface down deep into the wood, the heat distribution through the thickness is much more uniform.  In tempering bows we have to hit a balance.  We want that scorching to go deep into the belly, but if the time and temp are too high, that dryness and even cooking level of heat can transfer all the way through to the back.  A tempered BACK is a terrible idea, whether it comes from blowback due to a bad form design, or direct heat on the back, or even if allowed to come from the belly and cook all the way through.

So, when we are doing this, you have to find that balance between temperature, wood thickness, distance, and time. @ 5 minutes is often suggested, but if the limb is already only 3/8" thick, maybe less time but with the hotplate closer.  I also keep hearing about people putting a wet strip of towel or something between the caul and back to protect it from blowback or overheating, and that seems like a good idea.