Carbon and electric arcs seem to go together. The more current in the arc, the more carbon. I've always wondered where that carbon came from. I know steel and some other metal alloys contain some carbon but when I worked for the telephone company as a central office technician, even gold relay contacts would get coated in carbon after a while.
I used to have all sorts of high voltage toys to play with. Normally to jump any significant distance you needed a moderately high voltage. However, years ago I discovered a trick. Take a direct current power supply of several hundred volts and a big inductor in series with sparc gap that is spaced far enough that normally it would have taken 10Kv or so to jump.
Now when you turn the power on an arc won't spontaneously start, but if you start one by shorting the gap with say a screwdriver and then drawing it back, it will continue. This does not work without the inductor. I think what happens is every time it tries to extinguish, the magnetic field in th inductor collapses generating an inductive kick which gets it going again.
Anyway, in this device I used brass balls for the electrodes that threaded onto screws thus making it easy for me to adjust the gap. Brass, to the best of my knowledge, is a alloy primarily of copper and zinc, though silicon, aluminum, tin, manganese, iron, but I have found no mention of carbon being a component of brass, yet, after a very short period of operation a black powder having the appearance of carbon would deposit on the electrodes.
I wondered if it might be carbon separated out from carbon dioxide in the atmosphere, but at 350 parts per million that was in the atmosphere back then (now it is more than 380 ppm) I could see where that could build up over time but I discovered that discharging large capacitors through the gap would create large deposits instantly so I don't know if that explanation makes sense.