In my search for knowledge I recently went through a big thrash with
antenna tuners... Because of cost I built my own variable caps... To
keep the size down I experimented with various insulators between the
plates, from air up to glass...

Being that the tuners on 40 and 20 were end matching a half wave
antenna element the circulating tank current was considerable... On
20 meters where the capacitor plates are 1.5" square for resonance the
glass would simply shatter at about 800W for 5 seconds... Plexiglass
would boil internally in roughly 20 seconds leaving this interesting
rippled piece with 'steam' bubbles in the center of the plexi...
Ceramic tiles seemed to work OK, but I didn't find any thin ones
locally and the thick pieces resulted in plates being awkwardly
large...

On 80 meters the glass plates are 4" X 6" and the glass runs barely
above ambient at 2KW...

denny / k8do