I’ve annealed a lot of copper, since it is one of my primary metals.
I do not use butane in my shop, but oxy/acetylene. That may make my
experience irrelevant to yours, although it may figure in in terms
of how hot butane gets, which I do not know, never having used it.
I don’t even put my copper on a soldering block to anneal. I just
put it on my bench top (which is covered in firebrick) and move my
torch over the piece evenly until it glows a dull red/orange and
then (using a pair of pliers!), pick it up and quench in it water. I
never get any sort of buildup on it, or none that pickle or
abrasives would not completely remove. In fact, annealing copper can
turn it a beautiful red color, and this can be used to advantage.
I’m stumped about the black buildup issue. But of course I am also
fairly new to this. Again, I do not know how hot butane gets as
opposed to acetylene, but I would ask you: Are you using a large
enough torch tip? You can’t anneal using the same size tip as when
you’re doing soldering. I don’t even bother with any of my Little
Torch tips for annealing, and my pieces are small - I grab the big
Victor torch and put a small tip on that (I use the OO mostly). You
need sufficient heat to anneal. Annealing should be a very fast
process with jewelry size pieces of copper. A matter of seconds.
The only experience I have with black stuff is when I have the
acetylene turned on way too high. It produces a lamp-black type
stuff. Nothing like you describe, I don’t think.
Others may have more experience with butane, if that’s a piece of
the puzzle.
Rachel