Lonbow my experience is if treating over coals its best to start high and check color every few minutes and lower as needed. Really depends on how hot your coal bed is, outside temperature and wind are also variables that can effect the process. Hickory can take a lot of heat, osage can be ruined quickly. Even if you char the belly on hickory I have scrapped it down to good wood and still made a good bow out of it.
I have burnt up a few osage staves and cauls as it is very different from hickory but plan on trying again. These are the only woods I habe tried so far.