High voltage and fire... what more could anyone want...???
I knocked up this very "quick & dirty" 125KV marx generator to test some new solid-carbon resistors I'd bought to replace the failing ones in my big 1MV "Marx Three".
After some initial testing, I started playing, looking for possible ways to capture the patterns of sparks as they tracked across surfaces.
I tried a few things, like a wax coating, but with pretty poor results.
Then I tried some laser printer toner, and got an unexpected, although not entirely unwelcome result...
I think what happens is that the shockwave of the spark throws the toner into a fine dustcloud, which then ignites, in a rather impressive flame-burst.
The remains of the toner on the plastic surface - charred central trail, surrounded by clean area cleared by the shock wave from the spark and subsequently burnt.