OK, that is part of my issue. At this time I have no way to make dedicated "pre-biochar". I'm stuck harvesting out of our wood stove any bones or charcoal that hasn't turned into ashes. Some of it is probably the crunchy carbon I'm looking for, but some of it is probably still has some wood +/- some volatile oils.
You want all of the burnable stuff completed, and you want the volatile oils and other products out so that it is just crunchy carbon.