Pretty sure the whole point of his charcoal production was to use the pottery kiln to make charcoal at the same time as baking his pots. Two birds with one stone; however, there likely was too much open space in the kiln to efficiently make charcoal. (From those videos we see the important of removing airspace. In the video on reusable charcoal mounds the 2nd use of the mound did not yield as much charcoal and the guy who made it claimed that was likely because he had to stack the wood in a way that left more open space. Thus, more of the wood burnt rather than converted to charcoal. Pretty neat the author may be hinting to that.
#themoreuknow