The old silver mica caps were used in lots of antique AM broadcast radios (1930s -1960s), particularly in the IF stages. They were known to cause a phenomenon called "thunderstorming", which, even when receiving a strong station, caused a random loud static that sounded exactly like a nearby thunderstorm. This was caused by the silver deposits on the surfaces of the mica layers actually migrating toward the edges of the mica slabs. The high voltages used in those old tube radios drew the silver atoms toward the edges of the mica, and when enough silver migrated to the edges, there was enough current leakage in the capacitor to cause the "static" noise.
Of course, the only solution was to replace the old micas with new ones and retune the IF stages.
Spoken from experience {:>)
Dave M
