It is inconceivable to me that a small block of charcoal would
create enough dust to be a problem.
Ron, I have always used an oxy/propane torch and done all of my
polishing offsite, so I have to disagree with you. I am self-taught
but enjoy producing metal objects that are pleasing to people.
I used charcoal blocks with and without the borax treatment and had
real problems with black dust. The borax treatment helped somewhat.
Since becoming more proficient using my torch and switching to
firebrick the black dust problem is no longer a problem. Perhaps the
improvement in my soldering techniques was responsible, but I think
getting away from using a charcoal block was the real answer.