In South Texas (and other places I'm sure) the Indians buried imperfect stone with hot coals in a heat treatment method to make the rock easier to knap. I would guess they used hot coals to heat treat and bend wood for bows too. We know about them heating arrows for straightening too. I would think that burying a near complete bow , back side down, in damp sand/dirt and then covering (belly up) with dirt and hot coals would be a good heat-treat method and not damage the back. Tom