Arcs are very hot; this an be uses to melt glass. For example, you could procduce an arc with the 4kV 2000VA mains transformer, fix a piece of glass to an isolating handle and hold it into the arc. This works very well :-)
A more fascinating effect is shown in this experiment:
You can see the current going actually through the glass. This is possible because glass becomes increasingly conductive the hotter it is. So if you heat up glass, you can then have a current flow actually through it and produce so much heat that it keeps the glass on a high enough temperature (at least locally). That's what I've done here: I ignited an arc between the two plugs, pulled it over the glass so that the surface got heated, and after a few seconds, the arc over the glass disappeared. Instead, you can see a very dimly glowing, very narrow band of molten glass through which the current flows.
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