gemini2810
Sorry for the delay in responding.
The glass is electrostatic and somewhat porous in nature. This means it attracts some of the silver fromt the wires. It is called "plating out."
It does not affect the quality of the ionic silver you are producing or the ppm. You can clean this black off, but it isn't necessary. I usually just make sure the wires are at least 1-2
inches from the bottom, and if black appears I simply clean it after I have made my batch.
cheers, Vicki