The bar is clamped in a metal vise, so the vise is very slightly heated and that side of the bar is much cooler.
Taken using the Topdon Android attachment.
You can drill through glass but you need to water cool it (just run water over it as you're drilling).
I used similar water cooling when I drilled a hole through my stone countertop to add a new tap, though I'm not sure it was necessary.