[exray]

Guess we need to talk about construction of caps.

In the older days there were mica caps. Leaves of real mica material interleaved with a thin metallic foil not unlike any other textbook cap.
Later on they began to 'spray' the metal onto the mica dielectric instead of having metallic leaves. "Silvered" has to be taken with a grain of salt but a silver compound seems to have been the popular choice.

Silver tends to wander away due to natural electrolysis. (If someone can describe that better feel free to do so). Sort of like that photo that you printed on an ink-jet printer ten years ago that has gotten blurry! On an individual component that's not such a big deal although I have seen open-air trimmers where the 'silver' has simply oxidized away leaving an open circuit.

The most common problem we find nowadays is in IF transformers that were constructed with a mica sheet and little pads of "silvering" to construct two or more capacitors into the circuit. The silvering bleeds across microscopically and creates havoc.