"Zatara," bamboo backed osage bow, part 3 - heat treating

As I learned from my experience with trying to heat my yew "fence post", you don't heat treat an already glued bow. So I'm heat treating the rough-tapered osage limb sections before I put the bamboo backing on. This should give me an opportunity to straighten the deflex out of them as well as hopefully improve the performance of these marginal leftovers.

All of the heat treating I've done before was done by simply holding the gun and waving it back and forth over the limbs until they changed color. It's tedious and time consuming. I expected the latter, but the former has got to be changed. So, I built a holding jig to keep the gun about 5 inches from the limbs, as you can see in the photo below. So as to evenly temper it, I'm heating only one inch of limb at a time. The first limb took about 5 minutes. At 29" of limb to heat, this is about 2.5 hours of time per limb! Fortunately, I had other things to do (clean the workshop), so it was not too big a deal to multi-task. Set the timer, then move the whole limb one inch to work the next section. Repeat.

 5 minutes seemed like a long time, but I was going by the color change. At some points 4 minutes seemed more appropriate, so I switched to that as needed. I think part of the reason it took so long was that the doors to the workshop were open, allowing some draft to pass through and rob the limb of its heat. However, I didn't want to overdo it, so I limited the time. The resulting limb looked somewhat splotchy, so I ended up treating it in a separate session later, at about 2-3 minutes per inch. I wanted the treating to really permeate the limb, and the result was a nice even deep brown.

Untreated handle on the left; treated limbs on the right.

For the second limb, I did the same routine of 4-5 minutes per limb -- but with the garage door closed, limiting the draft. Apparently this made a significant difference. After I nearly finished the limb I noticed that there were several small cracks throughout the wood! These appear to be small drying or stress cracks. Were they already there, exacerbated by the heating; or were they the direct result of heating?

Cracks can be seen in the limb on the right.

Upon inspecting the first limb, I discovered a few of those there as well, but localized to one small area as opposed to throughout the limb, as they are on the second one. As a result, I've decided that I should go back to the manual method of heating, or reducing the time if using the jig. I've also designated the "crackled" one as the upper (weaker) limb.

I don't know if these cracks will have much effect on the final bow. The cracks shouldn't pose a problem in tension, as the bamboo back will be doing the work there, but they may very well be a sign that the limb got over-treated and will crumble or fret in use. Or maybe not.  I could stop, but I've got nothing to lose but my time at this point, and I'll learn something in the process. I figure that once the bamboo back is glued on, I can hit the cracked areas with low viscosity super glue as a hedge against problems.

Time (and experience) will tell.