Ancient_Hacker wrote:
Recall that paper is basically wood, and paper has been used for over a
century as an insulator in transformers and capacitors. In capacitors
it's like a few thousandths of an inch for each 100 volts, so it's not
too bad.

So it's likely that DRY wood is a pretty good insulator.

I have not seen paper capacitors that were just sheets of paper and
foil stacked or wound up. And I have dissected lots of capacitors.

Most paper capacitors are actually paper-oil capacitors, where a
highly refined tissue paper web acts as a spacer (similar to what the
spacers between the plates of a lead acid battery ), while the oil (or
wax) soaked into all the pores performs most of the dielectric
function. The paper is also vacuum dried as part of the process of
getting all the pores filled with the hydrocarbon filling. Any air
bubbles left unfilled would be places where the E-field would be
concentrated by the abrupt drop in permitivity, causing corona that
would degrade both the paper and the oil.